Journal of Sustainable Architecture and Civil Engineering 2015/2/11 72 JSACE 2/11 Journal of Sustainable Architecture and Civil Engineering Vol. 2 / No. 11 / 2015 pp. 72-78 DOI 10.5755/j01.sace.11.2.12568 © Kaunas University of Technology Research of Local Compression Concrete Reinforced by Steel Fibres Received 2015/03/02 Accepted after revision 2015/06/01 Research of Local Compression Concrete Reinforced by Steel Fibres Valerijus Keras, Mindaugas Augonis*, Nerijus Adamukaitis, Eglė Vaitekūnaitė Kaunas University of Technology, Faculty of Civil Engineering and Architecture Studentu st. 48, LT-51367 Kaunas, Lithuania The investigation was performed for estimation of influence of steel fibers to local compression of concrete. Evaluation the fact that the reinforced concrete structures quite a lot could be loaded by local impact, the research of local zone strengthening with steel fibers were performed. It is known that the state of stress and strain in local zone of concrete loaded by concentrated forces is quite difficult. For that reason the experiments carried out to obtain the main parameters of local zone described by design standards and the results were compared with theoretical. There are investigated the stress- strain mode of fracture, increasing of local compression strength and the influence of fibers content. KEYWORDS: concrete, local compression, steel fibres, mode of fracture. *Corresponding author: mindaugas.augonis@ktu.lt http://dx.doi.org/10.5755/j01.sace.11.2.12568 The investigation was provided for estimation of influence of steel fibers to local compression of concrete because such kind research is not quite popular. In practice there a lot situation when the reinforced concrete structures are loaded by local force, so, it’s important to investigate the strengthening possibilities of such sections. Nowadays more popular became concrete reinforced by steel fibres. After concrete consist review, we can notice that several decades more and more fibres reinforcement is used for various building structures even for load bearing structures. As- suming that steel fibres can to increase the resistance of local concrete area, it is possible to avoid the built-in parts in reinforced concrete structures. Analyzing the behaviour of flexural reinforced concrete elements, we can notice that steel fibres are quite important factor not only for devel- opment of cracks but for increase of strength also (Kaklauskas G., et. al., 2012, Di Prisco M., et.al. 2009, Jones P. A. et. al., 2008, Timinskas E., Jakubovskis R., 2011). But that factor is not described in design standards. The EC2, STR and SNiP standards are based on data of Bach and Bauschinger experiments. Also, the classic Flaman, Mitchell, Boussinesq and Hertz solutions could be used. Each method has some limitations and some advantages (Venckevičius V., Keras V., 1974). The standards are updating from time to time, but there is no presented algorithm of local compres- sion strength calculation for concrete reinforced by steel fibres. As before, such algorithm could be prepared after performing a lot of experiments and summarizing of obtained results. The author’s takes it into account and performed some experiments with short term loading. In such investiga- tions was used only one kind of concrete and steel fibre, but different content of fibres. Introduction 73 Journal of Sustainable Architecture and Civil Engineering 2015/2/11 For local compression testing were prepared 36 concrete specimens reinforced by diff erent con- tent of fi bres. The dimension of specimens was 150x150x150. All specimens were hardened in the same conditions. As concrete aggregate were used the granite break stone by 5/20 fraction and sand by 0/4 fraction. The length of steel fi bre with hooked ends (Fig.1) was 50 mm, diame- Methods Results and discussions Fig. 1 The steel fibres with hooked ends were used for test Materials in 1 m3 Ratio of water/cement Granite break stone, kg Sand, kg Cement (CEM II-42.5N), kg 1180 590 425 0.48 Table 1 The composition of concrete the brittle failure character and let to develop quite high residual strain because of plasticity of steel and micro cracking in concrete (Kaklauskas G., et. al., 2012). The fi bres quite eff ective oper- ate to restrict the development of normal cracks so, it’s should be eff ective for inclined cracks in local zone also. Concrete with high content of fibres Concrete with low content of fibres Concrete without fibres Strain Stress Fig. 2. The stress-strain relationship of concrete reinforced by steel fibres. Results and discussions The compression strength of concrete reinforced by steel fibres is presented in table 2. The scatter of concrete compression strength result is quite a small except the concrete with 35 kg content of fibres. The content of fibres can differently evaluate the compression strength of concrete. It depends on fibres type, bond with concrete, mortar fraction and so on (Neves R. D. Fernandes de Almeida J.C.O, 2005). Table 2. The compression strength of concrete reinforced by steel fibres Content of fibres, kg/m3 Strength, MPa 25 35.32 30 34.58 35 28.15 40 37.08 Although the both area of loading was small, the tendency of results obtained different and it could be seen in figures 2, 3 and 4. In Fig. 2 presented the results obtained when the plate 30x30 were used and in Fig. 3 – when the 53x53 plate were used. In that figures the influence of fibres cannot be strictly estimated, and the arising of fibres content differently causes the local concrete strength. The distribution of local strength average values is presented in Fig. 4 where could be noticed some tendency of fibres influence. When the specimens were loaded by 30x30 plate the maximum strength obtained with 25 kg/m3 fibres content and when by 53x53 plate – with 35 kg/m3. So, for specimens with small loaded area the influence of fibres is no significant and it hides in scatter of result data. For specimens with bigger load contact area, the influence becomes noticeable. Reinforcing the concrete with fibres almost is no possibilities to control the direction and distribution. So, the possibility of stratification is real. In most cases the direction of fibres is not optimal to restrict the development of cracks at least for flexural and compressed members (Kaklauskas G., et. al., 2012, Di Prisco M., et.al. 2009, Jones P. A. et. al., 2008). According to this we can state that fibres with different properties (for ter – 1 mm and tensile strength 1150 MPa. The specimens were formed by 4 series with diff erent content of fi bres: in the fi rst series were mixed 0.32 % (25 kg/m3), in the second – 0.38 % (30 kg/m3), in the third – 0.44 % (35 kg/m3) and in the fourth – 0.50 % (40 kg/m3). Each series includes 9 specimens. The composition of concrete presented in table 1. The specimens of each series were loaded by polished steel plates with two diff erent dimensions 53x53x23 and 30x30x20mm because it aff ect the load bearing of con- crete (Ince R., Arici E., 2004, Hawkins N., M., 1974). The plates were quite small for rea- son to evaluate the confi nement eff ect. The specimens were loaded uniformly until local zone failure. Tests were performed by test hydraulic press machine with force velocity control. The use of fi bres should strengthen the local zone because according to the ordi- nary stress-strain diagram of concrete re- inforced by steel fi bres, presented in Fig. 2, can be seen that the steel fi bres decrease The compression strength of concrete reinforced by steel fi bres is presented in table 2. The scatter of concrete compression strength result is quite a small except the concrete with 35 kg content of fi bres. The content of fi bres can diff erently evaluate the compression strength of concrete. It depends on fi bres type, bond with concrete, mortar fraction and so on (Neves R. D. Fernandes de Almeida J.C.O, 2005). Fig. 2 The stress-strain relationship of concrete reinforced by steel fibres Journal of Sustainable Architecture and Civil Engineering 2015/2/11 74 Although the both area of load- ing was small, the tendency of results obtained different and it could be seen in figures 2, 3 and 4. In Fig. 2 presented the results obtained when the plate 30x30 were used and in Fig. 3 – when the 53x53 plate were used. In that figures the influence of fibres cannot be strictly esti- mated, and the arising of fibres content differently causes the local concrete strength. The dis- tribution of local strength aver- age values is presented in Fig. 4 where could be noticed some tendency of fibres influence. When the specimens were loaded by 30x30 plate the max- imum strength obtained with 25 kg/m3 fibres content and when by 53x53 plate – with 35 kg/m3. So, for specimens with small loaded area the influence of fibres is no significant and it hides in scatter of result data. For specimens with bigger load contact area, the influence be- comes noticeable. Reinforcing the concrete with fibres almost is no possibilities to control the direction and distribution. So, the possibility of stratification is real. In most cases the di- rection of fibres is not optimal to restrict the development of cracks at least for flexural and compressed members (Kak- lauskas G., et. al., 2012, Di Pris- co M., et.al. 2009, Jones P. A. et. al., 2008). According to this we can state that fibres with differ- ent properties (for example with less length) can be more effec- tive to resist the local stress. It maybe could be more effective for concrete under cyclic load (Breitenbucher R., 2007) Table 2 The compression strength of concrete reinforced by steel fibres Content of fibres, kg/m3 Strength, MPa 25 35.32 30 34.58 35 28.15 40 37.08example with less length) can be more effective to resist the local stress. It maybe could be more effective for concrete under cyclic load (Breitenbucher R., 2007) Fig. 2. The distribution of the local compression strength Fig. 3. The distribution of the local compression for specimens loaded by 30x30 plate. strength for specimens loaded by 53x53 plate. Fig. 4. The distribution of the average local compression strength values according to different loaded area. In spite of loaded area and content of fibres the failure character was similar to concrete without any fibres. The initial micro cracks appear near corners of steel plate. At that time the steel plate are slumped in concrete. Such stamp in concrete remains and the initial cracks become wider and develop until specimen failures. The failure state is presented in Fig. 5,6. The load when the first crack appears was not estimated. The failure of the plain concrete is brittle (Keras V., 1972) and happens suddenly. The fibres change the failure to plastic (Fig.2) and failure process becomes longer what is positive to position of structural design. example with less length) can be more effective to resist the local stress. It maybe could be more effective for concrete under cyclic load (Breitenbucher R., 2007) Fig. 2. The distribution of the local compression strength Fig. 3. The distribution of the local compression for specimens loaded by 30x30 plate. strength for specimens loaded by 53x53 plate. Fig. 4. The distribution of the average local compression strength values according to different loaded area. In spite of loaded area and content of fibres the failure character was similar to concrete without any fibres. The initial micro cracks appear near corners of steel plate. At that time the steel plate are slumped in concrete. Such stamp in concrete remains and the initial cracks become wider and develop until specimen failures. The failure state is presented in Fig. 5,6. The load when the first crack appears was not estimated. The failure of the plain concrete is brittle (Keras V., 1972) and happens suddenly. The fibres change the failure to plastic (Fig.2) and failure process becomes longer what is positive to position of structural design. example with less length) can be more effective to resist the local stress. It maybe could be more effective for concrete under cyclic load (Breitenbucher R., 2007) Fig. 2. The distribution of the local compression strength Fig. 3. The distribution of the local compression for specimens loaded by 30x30 plate. strength for specimens loaded by 53x53 plate. Fig. 4. The distribution of the average local compression strength values according to different loaded area. In spite of loaded area and content of fibres the failure character was similar to concrete without any fibres. The initial micro cracks appear near corners of steel plate. At that time the steel plate are slumped in concrete. Such stamp in concrete remains and the initial cracks become wider and develop until specimen failures. The failure state is presented in Fig. 5,6. The load when the first crack appears was not estimated. The failure of the plain concrete is brittle (Keras V., 1972) and happens suddenly. The fibres change the failure to plastic (Fig.2) and failure process becomes longer what is positive to position of structural design. Fig. 2 The distribution of the local compression strength for specimens loaded by 30x30 plate Fig. 3 The distribution of the local compression strength for specimens loaded by 53x53 plate Fig. 4 The distribution of the average local compression strength values according to different loaded area 75 Journal of Sustainable Architecture and Civil Engineering 2015/2/11 In spite of loaded area and content of fi bres the failure character was similar to concrete without any fi bres. The initial micro cracks appear near corners of steel plate. At that time the steel plate are slumped in concrete. Such stamp in concrete remains and the initial cracks become wider and develop until specimen failures. The failure state is presented in Fig. 5, 6. The load when the fi rst crack appears was not estimated. The failure of the plain concrete is brittle (Keras V., 1972) and happens suddenly. The fi bres change the failure to plastic (Fig.2) and failure process becomes longer what is positive to position of structural design. Fig. 5 The failure view of specimen with 30x30 load area Fig. 6 The failure view of specimen with 53x53 load area When the loading areas are small, the initial cracks are brighter and wider, but when the areas are bigger there are more cracks which develop in various directions. Some cracks are very narrow and remain such until fully failure. But failure character is similar for both series specimens. The reviewed character of failure can be distinguished in several stages (Fig. 7, 8). In the fi rst stage (1-2) can be seen the process of compaction and the modulus of deformation uniformly increase (Fig.8). In the second stage (2-3) the modulus of deformation remains constant and only the elas- tic strain develops in specimen. In the third stage (4-5) the micro cracks begin appears and the deformation modulus begins decrease. Such micro cracks begin to connect into the macro cracks and the maximum stress is reached. Of course the distribution of cracks should be a little bit dif- ferent because of diff erent boundary zones. When the plate 30x30 was used the eff ective area at the specimen bottom not exceeds the specimen area, but if the 53x53 mm plate were used the eff ective area exceeds the area of specimen bottom. Journal of Sustainable Architecture and Civil Engineering 2015/2/11 76 Fig. 8 The stress-strain relationship of specimen with 53x53 load area Fig. 7 The stress-strain relationship of specimen with 30x30 load area Aft er comparing such two diff erent cases we can conclude that the scatter of results could not so diff erently aff ect the relation curves. To substantiate it the mean square deviation (MSD) and coef- fi cient of variation (VC) was calculated (table 3). The variation of results is not high if to compare with fl exural members. It’s interesting to compare experimental results with theoretical, calculated according to EC2, STR and SNiP codes. The load area is not signifi cant to the local concrete strength at all. The local strength with 30x30 area plates is 141.3, 60.0 and 62.6 MPa according to EC2, STR and SNiP. With 53x53 plate the local strength is 80.0, 60.0 and 60.2 MPa respectively. In calculations were assumed the cylindrical strength of con- crete. Also, calculating the local strength by EC2 was assumed that eff ective area is limited by edg- es of specimens. According to STR such area should not exceed 90x90 if the loaded area is 30x30. So, theoretically according to STR and SNiP codes the diff erence of such two series specimens is not signifi cant, but according to EC2 diff erence is ~40%. Such diff erence exceeds the variation of results and the fi bres could aff ect that. Comparing theoretical results with experimental, the EC2 gives best coincidence for both cases. The various factors infl uent the strength of concrete under concentrated loads are investigated in other articles (Venckevičius V., Lukoševičius K., 2007, Venckevičius V., 2005) Table 3 The statistical parameters of specimens Content of fi bres, kg/m3 Compression with plate 30x30 Compression with plate 53x53 MSD VC, % MSD VC, % 25 16.49 9.6 12.42 15.18 30 16.10 11.28 2.18 2.48 35 11.73 7.42 2.93 3.05 40 16.29 10.15 3.76 4.05 77 Journal of Sustainable Architecture and Civil Engineering 2015/2/11 The failure character is similar in spite of partially loaded area 30x30 or 53x53 and the failure char- acter is like for concrete without fibres. Solving by influence of content of fibres to local strength we can summarize that arising of fibres content does not have influence for specimens loaded by 30x30 steel plates, but for specimens with 53x53 plates the influence is significant and effective until 35kg/m3 then begin to decrease. In such case the resistance to local stress increase until 15% comparing with plain concrete. The fibres effect depend on specimens dimensions, content and fibres properties and the rationally area of concentrated load. It, can be seen when the local strength values obtained by different codes is compared, because not all parameters are evaluated when the fibre concrete specimens is use. The best coincidence was obtained by EC2 code although not evaluating the fibres influence. Conclusions STR 2.05.05:2005. Betoninių ir gelžbetoninių kons- trukcijų projektavimas [Design of concrete and re- inforced concrete structures]. LST EN 1992-1-1 Gelžbetoninių konstrukcijų pro- jektavimas. 1-1 dalis [Eurocode 2: Design of con- crete structures. Part 1-1] Breitenbucher R. Ibuk H., Alowieh H. Influence of cyclic loading on the degradation of mechanical concrete properties. In Conference Edition Ad- vance in construction materials 2007. Heidelberg, Berlin. Part V: 317-324. Hawkins N. M. The bearing strength of concrete loaded through rigid plates. Journal of the struc- tural division, 1974; 10(8): 1685-1702. Keras V. Apie betono sąlyginai trapias savybes. De- formacijų koncentracija mikro objektuose, detalė- se, konstrukcijose [About relatively brittle charac- teristics of concrete. The concentration of strain in micro-objects, components, constructions]. Kon- ferencijų medžiaga, Kaunas, 1972: 114-118. Neves R. D. Fernandes de Almeida J.C.O. Com- pressive behaviour of steel fibre reinforced con- crete, Structural concrete, Journal of the fib 2005, Vol.6 No.1 Ince R., Arici E. Size effect in bearing strength of concrete cubes. Construction and Building Ma- terials, 2004; Vol. 18: 603–609. http://dx.doi. org/10.1016/j.conbuildmat.2004.04.002 Venckevičius V., Keras V. Apie betoninių elemen- tų stiprumo apskaičiavimo metodikų patikimumą (histogramų) vietinio gniuždymo atveju. Defor- macijų koncentracija mikro objektuose, detalė- se, konstrukcijose [About reliability of calculation procedures of strength of concrete elements (his- tograms) of local compression case. The concen- tration of strain in micro-objects, components, Referencesconstructions]. Konferencijų medžiaga, Kaunas, 1974: 51-58. Venckevičius V., Lukoševičius K. Apie betoninių elementų skaičiavimą esant kraštiniam ir kampi- niam vietiniam gniuždymui [About the calculation of concrete elements subjected to edge and corner local compression]. Pažangioji statyba. Konferen- cijų medžiaga, Kaunas, 2007: 53-60. Kaklauskas G., Bačinskas D., Gribniak V. Kompozi- tais armuotos betoninės konstrukcijos [Composite reinforced concrete structures]. Vilnius, Technika, 2012; 301. Di Prisco M., Plizzari G., Vandewalle L. Fibre rein- forced concrete: new design perspectives. Materi- als and Structures, 2009; 42(9): 1261-1281. http:// dx.doi.org/10.1617/s11527-009-9529-4 Jones P. A., Austin S. A., Robins P. J. Predicting the flexural load–deflection response of steel fi- bre reinforced concrete from strain, crack-width, fibre pull-out and distribution data. Materials and Structures, 2008; 41(3): 449-463. http://dx.doi. org/10.1617/s11527-007-9327-9 Timinskas E., Jakubovskis R. Stiklo pluošto arma- tūra armuotų betoninių elementų eksperimentinis ir skaitinis tyrimas [Experimental and analytical research of concrete beams reinforced with glass fiber reinforcement bars]. Konferencijų medžiaga, Vilnius, 2011: 1-5. Venckevičius V. About the calculation of concrete elements subjected to local compression. Journal of civil engineering and management, 2005; 11(3): 243-248. http://dx.doi.org/10.1080/13923730.200 5.9636355 СНиП 2.03. 01-84 Бетонные и железобетонные конструкции [Concrete and reinforced concrete structures]. Москва 1989 Journal of Sustainable Architecture and Civil Engineering 2015/2/11 78 VALERIJUS KERAS Assoc. Professor Kaunas University of Technology, Faculty of civil engineering and architecture, Department of Building Structures Main research area Fracrure mechanics, concentration of stresses and strains, durability, destruction Address Studentų st. 48, LT- 51367 Kaunas, Lithuania About the authors MINDAUGAS AUGONIS Assoc. Professor Kaunas University of Technology, Faculty of civil engineering and architecture, Department of Building Structures Main research area Durability of Engineering Structures, Strength and Stability of Reinforced Concrete Structures Address Studentų st. 48, LT- 51367 Kaunas, Lithuania, e-mail: mindaugas.augonis@ ktu.lt NERIJUS ADAMUKAITIS Dr. Lecturer Kaunas University of Technology, Faculty of civil engineering and architecture, Department of Building Structures Main research area Durability of Engineering Structures, Strength and Stability of Steel Structures Address Studentų st. 48, LT- 51367 Kaunas, Lithuania, e-mail: nerijus.adamukaitis@ ktu.lt EGLĖ VAITEKŪNAITĖ Student Kaunas University of Technology, Faculty of civil engineering and architecture, Department of Building Structures Main research area Engineering Structures, Partially Loaded Reinforced Concrete Members Address Studentų st. 48, LT- 51367 Kaunas, Lithuania