https://doi.org/10.14311/APP.2022.33.0207 Acta Polytechnica CTU Proceedings 33:207–211, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague EQUIVALENT THERMAL DIFFUSIVITY OF NATURAL AND RECYCLED AGGREGATE CONCRETE AT TEMPERATURE UP TO 350 ◦C Izabela Hagera, ∗, Katarzyna Mróza, Tomasz Tracza, Václav Kočíb, Lukáš Fialab a Chair of Building Materials Engineering, Faculty of Civil Engineering, Cracow University of Technology, 24 Warszawska St. 31-155 Cracow, Poland b Czech Technical University in Prague, Department of Materials Engineering and Chemistry, Faculty of Civil Engineering, Thakurova 7/2077, 16629 Prague 6, Czech Republic ∗ corresponding author: izabela.hager@pk.edu.pl Abstract. The paper presents the evolution of equivalent thermal diffusivity with temperature of concretes with natural coarse aggregates and recycled concrete aggregates. Thermal diffusivity was evaluated based on the temperature measurements made on cylindrical samples using so-called "inverse tech- nique". Thermal diffusivity parameter was estimated for concretes manufactured with different re- placement ratios of natural by recycled coarse aggregate (0, 50 and 100 %) produced by crushing the worn-out concrete pavement that was dismounted after exploitation period. The diffusivity of concrete was tested up to 350 ◦C. The progressive decrease of thermal diffusivity was observed, and the lowest D values were obtained for higher recycled concrete aggregates content. The results were presented against the background of other authors results and referred to the D values proposed in EUROCODE 2. Keywords: Fire, recycled concrete aggregates, temperature, thermal diffusion. 1. Introduction By volume,construction and demolition waste (CDW) are one of the largest streams of waste in Europe with 850 million tons that are produced per year [1]. The waste separation process enables to obtain good quality aggregates and their use in concrete is economically, environmentally and technically feasible with the respect of all technical standards. The progressive replacement of natural aggregates by recycled concrete aggregates con- tributes to the implementation of the strategy for sustainable development of the field of construction through the successful implementation of sustainable building materials with a reduced carbon footprint. The concrete industry scepticism based on concern about poor-quality recycled products needs to change with the research projects showing successful application of recycled concrete aggregates RCA without compromising on quality and contribute to the decrease environmental impact. The detailed recommendations concerning assessing the properties of recycled aggregates are given in [2], however the imitation concerning their replacement of natural aggregate in the concrete mix is imposed [3]. In the present research the properties of concrete with recycled aggregates were investigated in the view of study their behaviour in fire. The fire behaviour of concrete with natural aggregate is well documented in the literature [4–6]. Well known cases of fires in engineering facilities like Channel (1996); Mont Blanc (1999); Gotthard (2001) tunnels has drew attention to one of the disadvantages of concrete that is con- crete spalling in a fire. The sudden loss of concrete cover and concrete element cross-section reduction may jeopardise the overall load-bearing capacity of the structural element [7]. It has been confirmed that the composition of concrete, as well as boundary con- ditions, affects the concrete propensity to spall in a fire. One of the main parameters governing concrete spalling is permeability. In denser and less perme- able concrete, the spalling risk is higher [8]. Concrete permeability is a derivative of the type of aggregate used, transport properties of the cement paste and the permeability of interfacial transition zone ITZ be- tween the cement paste and the aggregate. The per- meability of recycled concrete aggregate differs from natural aggregate, thus transport properties of those concretes may be higher than in traditional concretes [9], thus lowering the risk of spalling behaviour. In view of studying the RCA concrete behaviour in the fire the first step that was undertaking was the ob- servation of thermal properties evolution presented in this paper. The aim of this project is to examine the impact of concrete composition, in particular the recycled con- crete aggregate amount RCA on the thermal prop- erties of concrete. As the heat transfer in concrete plays a crucial role in fire resistance evaluation, the 207 https://doi.org/10.14311/APP.2022.33.0207 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en I. Hager, K. Mróz, T. Tracz et al. Acta Polytechnica CTU Proceedings Figure 1. Thermocouples arrangement in the concrete cylinder, and the furnace heating rate. thermal properties that change with the temperature are essential and need to be provided to the calcu- lation model. Thermal diffusivity (D) is a specific property of the material and allows you to determine how quickly a given material reacts to temperature changes. This is a property that measures the rate of heat transfer from the exposed surface of the material to the inner layers. The higher the value of thermal diffusivity, the faster the temperature increase at a certain depth. To predict the behaviour of concrete during a fire and carry out simulations determining the temperature development in a heated element, one need to know the value of thermal diffusivity. Thermal diffusivity can be evaluated directly from its definition or its evaluation can be based on the tem- perature measurements made on cylindrical samples, called an "inverse technique". 2. Experimental method The thermal diffusivity (D) of concrete depends on its thermal conductivity (λ), density (ρ) and specific heat of the material (cp). Thermal diffusivity can be determined (Eq. 1) by directly determining these physical properties by experimental methods or by using an inverse method leading to the determination of D. The unit of thermal diffusivity is [J/kgK], but [mm2/s] or [m2/s] are also used in the literature. D = λ ρ cp ! m2 s " (1) The method determining the equivalent thermal diffusivity of concrete was recently described by Fe- licetti [10, 11] showing good compliance with exper- imental results from standard fire tests. The main advantage of the "inverse technique" is the determina- tion of thermal diffusivity without the need for cum- bersome measurements of thermal conductivity and specific heat values as well as density change evalua- tion, knowing that all those values vary with temper- ature increase. The inverse method is quite sensitive to moisture presence in concrete and other physicochemical pro- cesses that occur when the material is exposed to high temperature. It consists in heating the cylin- drical sample (diameter 100 mm; height 200 mm) in the electric furnace and measuring temperature in- crease in the centre of the sample (Tc - centre) and on 2 mm from cylinder lateral surface (Te - exterior) using K-type thermocouples, Figure 1. The temper- ature differences are measured on the central cross- section where the field of temperature is considered dependant only to distance from the cylinder centre. The temperature increases with the constant heating rate 1 ◦C/min up to 350 ◦C. The test result is the temperature difference Te − Tc in function of time. during the test. In the determination of thermal diffusivity by the inverse method, the initial assump- tions is adopted that the cylinder is axis-symmetric and infinitely long, so the analyse is related to a dif- ferential equation for transient heat conduction. To determine the diffusivity that characterises the tran- sient process of heat exchange inside a constant mass we use Fourier’s Law (Eq. 2): ρ cp ∂T ∂t + div (q) = 0 (2) Assuming a cylindrical coordinate system (θ, r, z) (Eq. 3): ∂T ∂t = D ! d2T dr2 + 1 r ∂T ∂t " (3) and using Laplace transformation, we get a formula for the temperature difference between the edge and the centre of the sample (Eq. 4): 208 vol. 33/2022 Natural and Recycled Aggregate Concrete Thermal Diffusivity Component Units OC RAC 50% RAC 100% CEM I 42.5 R kg/m3 414.2 414.2 414.2 water kg/m3 186.4 186.4 186.4 w/c − 0.45 0.45 0.45 river sand 0/4 mm Dwudniaki kg/m3 615.1 280.6 569.2 coarse agg. 2/8 mm Dwudniaki kg/m3 579.9 280.6 − coarse agg. 8/16 mm Dwudniaki kg/m3 562.3 264.1 − recycled concrete agg. 4/16 mm kg/m3 − 825.3 1057.2 plasticizer BASF BV 18 % mc 0.9 0.9 0.9 Basf Glenium SKY 591 % mc 1.4 1.4 1.4 cement paste content dm3/m3 320 320 320 mortar content dm3/m3 542 511 540 density kg/m3 2248.5 2236.8 2221.3 water content % 5.2 5.8 6.7 water absorption % 6.9 7.3 7.8 28 days compressive strength MPa 46.1 56.8 54.5 28 days tensile splitting strength MPa 3.3 3.6 3.4 Table 1. Mix design and properties. T (r, t) = R ! t − a2 − r2 4D " (4) From here we get the formula for D - equivalent thermal diffusivity (Eq. 5): D = Ra2 4∆T (5) where: • R - heating rate [◦C/min], • a - sample radius [m], • ∆T - temperature gradient [◦C/cm], • D -thermal diffusivity [mm2/s]. 3. Materials The measurements were carried out for concrete made of CEM I 42.5 cement, riverbed coarse aggre- gate Dwudniaki from Dunajec River southern Poland (grain size 2/8 mm and 8/16 mm) and crushed con- crete from the worn-out concrete pavement with grain size - 4/16 mm. Before being crushed the mechanical properties of concrete from the pavement was deter- mined on drills. The compressive strength of 66.8 MPa, modulus of elasticity 41.2 GPa and apparent density of 2290 kg/m3 characterised the recycled con- crete. For all 3 concretes the same water-cement ratio (w/c) and cement paste volume was maintained. Ad- ditionally, a plasticiser was used to obtain good work- ability and water content reduction. The mix compo- sition of the concretes is shown in Table 1. The con- cretes were labelled: the reference ordinary concrete (OC) made of natural riverbed aggregates; RAC 50% is concrete made of recycled pavement aggregate with 50% of natural coarse aggregate replacement with re- cycled one; RAC 100% recycled aggregate concrete made of recycled concrete coarse aggregates. In all concretes - OC, RAC 50% and RAC 100% riverbed quartz sand was used. The concretes specimens were cast and stored in plastic cubic or cylindrical moulds for the first 24 hours. After preliminary curing, water evaporation was prevented by covering them with plastic lids for 7 days. During the next 28 days, they were stored in natural air-drying conditions at the temperature of 20 ± 5 ◦C and relative humidity of 50 ± 5 %. The com- pressive strength and splitting tensile strength of con- cretes after 28 days were determined on 10 cm3 cubes. All basic physical (density, water content, water ab- sorption) and mechanical (compressive and splitting tensile strength) performances are summarised in Ta- ble 1. 4. Results and observations The temperature evolution in time measured with type K thermocouples was presented in Figure 2. The solid line is showing the heating programme. The pre- sented results were used to determine effective ther- mal diffusivity as a function of temperature, obtained from the Eq. 5 and expressed in [mm2/s]. The results are shown in Figure 3. Due to thermal gradient value close to zero at the beginning of the test, when the furnace starts to heat, the effective thermal diffusivity values are very high. This phenomenon was previously observed by [10], and the correction factor was proposed to reduce this unwanted effect so that the suggestion to compute the instantaneous heating rate was proposed [11]. The equivalent thermal diffusivity decreases with increas- ing temperature in the range up to 350 ◦C. In heated 209 I. Hager, K. Mróz, T. Tracz et al. Acta Polytechnica CTU Proceedings Figure 2. Time - temperature curves for OC, RAC 50% and RAC 100%. Figure 3. Equivalent thermal diffusivity for OC, RAC 50% and RAC 100%compared with the results of Felicetti [11] and EC 2 proposed values. with 1 ◦C/min cylinders the values of equivalent ther- mal diffusivity reaches at 100 ◦C 0.76 mm2/s, 0.61 mm2/s and 0.45 mm2/s respectively for OC, RAC 50% and RAC 100%. The highest value obtained for OC results from the density of this material. With an increasing proportion of recycled concrete aggregates, the D values are reduced. The similar tendencies are observed at 200 ◦C and 300 ◦C. From the graph, we can see the water migration ef- fect occurring in the lower temperature in case of con- cretes with recycled concrete aggregates. It confirms that those materials enable faster water migration due to their higher permeability. The observed wa- ter migration perturbance on Figure 3 is linked with specific heat significant peak at a temperature close to 200 ◦C. The comparison with the D range proposed by EU- ROCODE 2 (EC2) [12] for both samples with re- cycled aggregates RAC 50% and RAC 100% shows thatthe thermal diffusivity values are mostly below both the upper and lower limits given in EC2. The results remain similar to the experimental results of Felicetti [10, 11]. 5. Conclusions Determination of thermal diffusivity is a vital param- eter for the correct modelling of heat flow through heated concrete. Thanks to the inverse method, we determine diffusivity for much higher temperatures than other stationary and non-stationary methods. Non-stationary transient temperature methods, de- spite their broad application, require the use of spe- cialised and often costly measuring equipment com- plex in mathematical and numerical terms. In the inverse method, we can use widely available and easy- to-use laboratory equipment to perform the tests. Al- though the technique does not allow for specific pa- rameters to established: ρ, cp and λ but their ratio 210 vol. 33/2022 Natural and Recycled Aggregate Concrete Thermal Diffusivity expressed in diffusivity definition D = λ/ρ cpis as- sessed. Acknowledgements This research was supported and funded in part from the EMMAT project E-mobility and sustainable mate- rials and technologies PPI/APM/2018/1/00027 financed by the Polish National Agency for Academic Exchange (NAWA). References [1] Global Recycling. Recycled Construction Waste in Europe. https://global-recycling.info/archives/2964. [2] RILEM TC 121-DRG. Specifications for concrete with recycled aggregates. 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Role of Polypropylene Fibres in Concrete Spalling Risk Mitigation in Fire and Test Methods of Fibres Effectiveness Evaluation. Materials 12(23), 2019. https://doi.org/10.3390/ma12233869. [9] M. Choinska, A. Khelidj, G. Chatzigeorgiou, et al. Effects and interactions of temperature and stress-level related damage on permeability of concrete. Cement and Concrete Research 37(1):79-88, 2007. https: //doi.org/10.1016/j.cemconres.2006.09.015. [10] R. Felicetti. Thermal Diffusivity and Residual Strength of High-Performance Light-Weight Concrete Exposed to High Temperature. Proc. 6th Int. Symposium on the Utilisation of High-Strength/High Performance Concrete 2, Leipzig (Germany) 935-48, 2002. [11] R. Felicetti. Assessment of the equivalent thermal diffusivity for fire analysis of concrete structures, Proc. Fib Task Group 4.3 Workshop Fire Design of Concrete Structures, Coimbra 149-58, 2007. [12] EN 1992-1-2. 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