bond between steel reinforcements and crumb rubber
TRANSCRIPT
Concrete 2013 - Final Paper TemplateBond Between Steel
Reinforcements and Crumb Rubber Concrete: An Experimental
Study
Rebecca J. Gravina1,Tianyu Xie1, Yan Zhuge2, Xing Ma2, Osama Youssf2 and Julie Mills2
1 School of Engineering, RMIT University 2 UniSA STEM, University of South Australia
Abstract : This paper shows an experimental study to investigate the bond behavior of reinforced structural crumb rubber concrete. Concentric bond bar pull-out and beam-end block tests are performed to study the bond between steel reinforcement and crumb rubber concrete at the local and global level. In addition to the research on the semi-structural bond behavior, three large-scale reinforced rubberized concrete beams are also tested under flexural bending. Bar embedded length, bar diameter and rubber content are the main factors studied in this research work. Based on the experimental results, it is observed that the higher deformability of rubberized concrete results in altered local and global bond behaviour, and a reduction in peak bond stress and an increase in the slip at the peak bond are observed. The change in bond behaviour together with reduced mechanical properties of crumb rubber concrete subsequently lead to a lower flexural stiffness at the elastic stage and a reduction in ductility at the post-peak stage of reinforced crumb rubber concrete beams.
Keywords: Bond, Crumb rubber, pull-out test, beam-end tests, bending
1. Introduction Crumb rubberized concrete that utilizes rubber particles derived from the end of life (EOL) tyre as an alternative to fine aggregates in structural concrete, is a construction industry focused application that is attracting rising attention from researchers, designers and engineers. Previous studies have experimentally characterized the mechanical properties of the crumb rubberized concrete at the material level including; the compressive strength [1] , flexural strength , modulus of elasticity [2] , fracture energy, fracture toughness [3] , impact resistance [4] , abrasion resistance [5] and the bond behaviour of the rubber particles to the cement matrix [6] . The more deformable nature of rubber particles and the poor bond between the rubber particles and the cementitious matrix due to the relatively smooth surface of the rubber particles adversely affect the mechanical properties of the concrete products containing rubber. Therefore, the current recommended rubber content is suggested to not exceed 20% by volume of fine aggregates in concrete’ to minimize the impacts of rubber inclusion on the mechanical properties of concrete [7].
A careful literature review reveals that the existing studies on reinforced rubberized concrete bond focused on only local bond behaviour, in which the embedded length of a bar in concrete is recommended around five times the bar diameter ( i.e . as reported in [8] ). The bond stress along a bar with a short-embedded length is relatively evenly distributed, representing the bond behaviour only locally, hence is characterized as a property at the material level. At the member level, the local bond behaviour cannot represent the global bond between the concrete and the reinforcements, as the anchored length of the bar in the concrete exceeds five times the bar diameter and the bond stress distribution along the bar has a gradient. Compared with conventional concrete, the difference in the deformability and local bond behaviour could alter the global bond behaviour of the reinforced rubberized concrete. However, the global bond behaviour of reinforced rubberized concrete is yet to be investigated and hence must be explored.
To fill the research gap identified, a systematic experimental program is undertaken in the present study to comprehensively study the local and global bond behaviour of reinforced rubberized concrete. The effect of the global bond on the flexural performance of reinforced rubberized concrete beams is also experimentally studied. A total of 48 concentric pull-out tests, 96 beam-end pull-out tests, six larger-scale beams tests and the ancillary material property tests are presented.
2. Experimental program
2.1 Materials and mix design The raw materials used in this experiment were: General blended cement (GBC), sand, two sizes of coarse aggregates (10 and 20 mm), fine natural aggregate (5 mm), crumb rubber (2-5 mm), “MasterPolyheed 8875” water reducer (BASF), “MasterAir 905” air entraining admixture (BASF) and deformed steel bars D500N
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grade of diameters 12, 16, 20 and 24 mm . Note that aiming to enhance the performance of the rubber in concrete, the rubber particles were pre-soaked in water for 24 hrs as per the pre-treatment method reported in [9] . The mixing procedure, in terms of mixing time and mixing order, of all the concrete followed the approach developed by the authors as reported previously [9] . The prefixes CC and CRC used in the specimens’ designation stand for conventional concrete (control) and crumb rubberized concrete, respectively and the suffix 32 denote s th e targeted strength grades for the original conventional concrete mix designs, respectively, i.e. 32 MPa. Noting that the replacement of fine aggregate with rubber is 20% by volume of sand in each mix. Hence the only difference in the mix designs between conventional concrete and rubberized concrete is the reduction of sand content and addition of rubber content with equal volume. The mix designs of the concretes are reported in Table 1.
Table. 1 Concrete mix design
Mix ID GBC Coarse Aggregates
Sand Crumb rubber w/c Water Water
reducer
Air entraining admixture20mm 10mm
CC32 337 551 459 826 0 0.61 205.6 0.51 0 CRC32 337 551 459 685 63.6 0.61 205.6 0.51 0
2.2 Test method 2.2.1 concentric pull-out test
Figure 1 shows the setup of the concentric pull-out test and the details of the bond test specimen. Note that transparent and flexible polypropylene pipes were used as bond breakers for the un-bonded section of the deformed bars. The dimension of the concrete block was 200 × 200 × 200 mm and the embedded length of the reinforcement bar was kept as five times the diameter (5Ø) of the bar, where this short bond length was used to characterize the local bond-slip relationship with constant bond stress (e.g. at the material level). To keep the pipes in position during sample preparation and casting stage, the front end of the pipe was tied to the rebar using flexible duct tape that grips the rebar and does not dislocate while pouring concrete and vibration. The section of the flexible duct tape that was lapping the bar was cut off after the specimens were de-moulded. Blue tac was used at the rear (internal) end of the pipe to seal any gap between the bar and the pipe and to block any cement paste from entering into the gap. Two linear variable differential transformers (LVDTs) were mounted on the bars, one at the load end and another at the free end of the bar. The pull-out bond test was conducted under a displacement-control mode with a rate of 0.4 mm/s. The parameters studied were bar diameter, concrete strength grade and rubber incorporation.
Figure.1 Concentric pull-out bond test setup
2.2.2 Beam end block pull-out test Figure 2 shows the beam-end block test setup and the details of the test specimens. Installation of the flexible polypropylene pipes, use of LVDTs and the method of testing was the same as that used for the concentric pull-out bond test. The dimension of the concrete block was 200 × 300 × 380 mm and the bonded
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length of the reinforcement bar was kept as 10 times (10Ø) the diameter of the bar, where the longer embedded length was adopted to represent the global bond-slip behaviour (e.g. at the member level). Four bars per block were installed in N12 and N16 bar setups, with 2 bars at top and 2 at bottom of the block, keeping a clear cover from all edges of three times the diameter of the bar. For N20 and N24 bar setups one bar at the top and one at the bottom of the block were installed keeping the clear cover from their respective top and bottom edges as three times the diameter of the bar. Note that the beam-end tests for the top and bottom bars were conducted to evaluate the effect of specimen casting direction on the global bond properties of the reinforced rubberized concrete. The parameters researched were bar diameter, concrete strength, rubber incorporation and specimen casting direction.
Figure. 2 Beam-end block test setup and specimen details (B=Bonded length, UB=Un-bonded length, Ø = Bar diameter, CC = Clear cover, R = Reaction points)
2.2.3 Beam test Following the local and global bond tests, the effect of rubber incorporation on the flexural performance of reinforced concrete beams was also investigated. Figure 3 shows the details of the test setup of reinforced concrete flexural beams. A total of 6 reinforced beams with 3 duplicates each for CC32 and CRC32 series of concrete, respectively, were tested under four-point bending. The beam was 150 mm wide × 220 mm deep × 2400 mm long having a clear span of 2200 mm. Each beam was reinforced by two 16 mm-deformed bars placed at the tension region and two 8 mm-deformed bars at the compression region of the beam, all with a clear cover of 20 mm. To prevent the premature shear failure of the beams, a 6 mm stirrup was used as the transverse reinforcement placed at 165 mm centres. Each beam was loaded at the top side having two equally spaced loading points via a spreader beam with the constant moment region of one third of the clear span. The beam was tested under a load control mode with a loading rate of 5kN/min until failure of the specimen to generate the complete load-deflection relationship. The deflection profile of the beam was established using four laser displacement measuring instruments installed at the rear of the beam with a spacing of a quarter of the span.
Figure.3 Reinforced concrete beam test setup
3. Test results and discussion 3.1 concentric pull-out test results The local bond-slip behaviour of the reinforced CC and CRC were experimentally investigated using concentric pull-out tests, where a short bar embedment was adopted (5 Ø). Note that this test method was applied to characterize the local bond-slip behaviour at the material level as the bond stress distribution along such a short bar length is relatively uniform. Table 2 summaries the results of each concentric pull- out test accompanied by the other corresponding physical and mechanical properties. The majority of the specimens failed due to the bar being pulled out from the concrete matrix, except for some of the specimens
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with 20 mm or 24 mm bar where a splitting failure of their concrete cover was experienced. The splitting failure was an unexpected failure mode and mainly occurred due to the larger bond force induced as the consequence of a larger bonded area of the bar with a larger diameter and the subsequent reduction in the dimension of the concrete cover with an increased bar diameter [10].
Table. 2 Concentric pull-out test results. Concrete series
CC32 CRC32 CC20 CRC20
stress (mm)
0.1 1.2 ± 0.1
0.2 1.8 ± 0.2
0.2 1.5 ± 0.6
0.3 2.4 ± 0.1
Compressive strength (MPa)
2.2 Beam end block pull-out test results
Unlike the local bond-slip behaviour of the reinforced concrete studied by the concentric pull-out tests, the global bond-slip behaviour of the reinforced conventional and rubberized concrete was studied via beam- end tests with each bar embedded length of 10Ø, where the bond stress distribution along the rebar has a gradient. The specimens with 12 mm and 16 mm bars embedded all failed with the bar pulled out from the concrete blocks. During the test, some premature failures of the concrete blocks were observed, in particular for the specimens with a bar of a larger diameter embedded (e.g. specimens with 20 mm- and 24 mm- bars). Of the same mechanism as those of the concentric pull-out tests where the concrete splitting failure occurred, these unexpected failure modes can be attributed to the larger bond force induced as the consequence of a larger bonded area of the bar with a larger diameter although the cover for the beam end tests increased accordingly (e.g. remaining at 3Ø). This may also be attributed to the relatively lower strength concrete used in the present study. It should be noted that the pull-out test results (e.g. their bond-slip curves) of the bottom bars for all the specimens with 20 mm and 24 mm bars are still presented but are not analysed and discussed in the remainder of this study due to these pre-damages of the concrete blocks. The results of the beam-end bond-slip behaviour in terms of the pull-out load-free end slip relationship are illustrated in Figures 4 . It was interestingly observed that for the specimens that failed with the bar pulled out, the reinforcements placed at the bottom of the concrete block required a higher load to be pulled out from the concrete matrix. The shape of the bond-slip curves of the bottom bars was different to those of the top bars but was similar to that of a bar embedded in the heavily-confined concrete as per the specifications in [11] , where a nearly bilinear load-slip relationship was observed. The differences in the bond-slip curves between the top and the bottom bars are mainly caused by the orientation to cast the specimens, where the concrete at the bottom part of the block is more compacted and hence was much denser owing to the settlement of the heavier ingredients and the self-weight of the concrete. This led to an enhanced interaction between the concrete and the bottom bar compared to that of the top bar. The observed effects of the selected parameters on the global bond-slip behaviour obtained from the beam-end tests are the same as those observed from their companion local-bond behaviour established using the concentric pull-out tests,
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confirming the effects of the local-bond slip behaviour on the global bond-slip behaviour of a reinforced concrete member.
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2.2 Beam test results Having the impacts of adding rubbers in concrete on the semi-structural behaviour evaluated using the concentric pull-out tests and the beam end tests, the influences of rubber inclusion on the reinforced concrete are further studied at the member level. B oth of the beams exhibited a typical flexural failure mode, where the concrete crushing occurred at top of the beams with the constant moment region and flexural cracks formed at the bottom of the beam. During the beam tests, it was first observed that the cracks formed on the tension surface of beams within the region between the two loading points (referring to the constant moment region). With increasing applied load, these cracks gradually propagated to the neutral axis, which subsequently caused a series of flexural cracks in the constant bending moment region. Following the tension damage of the beams, the compressive wedge induced by concrete crushing initiated and the depth of the wedge gradually increased. In the meantime, the buckling of the compressive bars occurred. In the end, all the beams completely lost their load carrying capacity due to the crushing of the concrete at the compression region of the specimens.
The load-deflection relationship of each test specimen is shown in Figure 5 . The rubber incorporation led to no significant change in the shape of the load-deflection curve of the beam. The comparisons between the load-deflection curves and the deflection profiles shown in the figures reveal that the CRC32 beams generally exhibited a lower stiffness within the elastic stage compared to that of the CC32 beam. This is mainly caused by the higher deformability and cracking tendency of the CRC32 concrete (e.g. a lower elastic modulus and a significantly lower flexural tensile strength) containing rubber in conjunction with the relatively inferior bond between the rubberized concrete and the rebars at the tension surface of the concrete beams. Owing to the lower compressive strength and the larger deformability of the CRC32 series concrete, the yielding of tension rebars and the crushing of concrete within the compression region of the beams occurred earlier in the CRC32 beams. This led to a slightly lower load carrying capacity of 8% in reduction for beams containing the crumb rubber. This also in turn resulted in a less ductile post-peak behaviour of the CRC32 beams compared to their CC32 counterparts.
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Figure. 5 Load- midspan deflection relationships of a) CC32 and b) CRC32 beams
3. Conclusions This paper presents a systematic investigation of the effects of rubber incorporation in concrete on the global and local bond, as well as the flexural performance of reinforced rubberized concrete beams. An experimental program is undertaken, which contains concentric pull-out tests, beam-end pull-out tests, large-scale beams tests and the ancillary material property tests. Based on the results of this research work, the following conclusions can be drawn:
1. Local bond behaviour is affected by adding rubber in concrete, up to 52% and 59% reduction in the local bond strength are respectively observed for C32 and C20 grade of concrete with 20% fine aggregate replaced by rubber. At the material level, a lower bond strength (up to 30% and 34 % reduction in the peak pull-out load for C32 and C20 grade of concrete respectively) and a larger slip at the peak bond stress are observed for concrete with rubber partially replacing sand. This is due to the increased deformability of concrete with increasing rubber content (for instance the decreased elastic modulus), resulting in a larger relative slip between the concrete and the steel bar, which in turn leads to an inferior bond properties of reinforced rubber concrete.
2 . The altered local bond behaviour, in turn, results in a reduction in the bond-stress and an increased slip at the peak bond stress of reinforced rubberized concrete at the member level. This is attributed to both the higher deformability of the concrete with rubber and the relatively reduced local bond behaviour.
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3. The beam tests indicate that the reduced bond between the rebars and the adjacent rubberized concrete in conjunction with the higher deformation ability in the service limit state of the rubberized concrete lead to a lower flexural stiffness of the beam within the elastic stage. The earlier yielding of the tension rebars and the earlier crushing of the rubberized concrete result in a less ductile post-peak behaviour of the reinforced rubberized concrete beams. Around 8% reduction in the peak load and approximate 6% decrease of mid-span deflection at the peak load are observed for C32 grade of concrete beam with 20% fine aggregate replaced by rubber.
4. Acknowledgement The authors would like to acknowledge the funds provided by the Australian Research Council (ARC- LP160100298) and industry partners for this project. The industry partners are; Tyrecycle Pty Ltd, Tyre Stewardship Australia, ResourceCo Pty Ltd, FMG Engineering Pty Ltd, and Ancon Beton. The materials donations by ResourceCo Pty. Ltd., Adelaide Brighton Cement Pty. Ltd., and Tyrecycle Pty. Ltd., are greatly appreciated. The authors also would like to thank the support of lab staff members in RMIT and University of South Australia to carry out this research
5. References
1. Khaloo, A.R., M. Dehestani, and P. Rahmatabadi, Mechanical properties of concrete containing a high volume of tire–rubber particles. Waste management, 2008. 28(12): p. 2472-2482.
2. Bisht, K. and P. Ramana, Evaluation of mechanical and durability properties of crumb rubber concrete. Construction and Building Materials, 2017. 155: p. 811-817.
3. Reda Taha, M.M., et al., Mechanical, fracture, and microstructural investigations of rubber concrete. Journal of materials in civil engineering, 2008. 20(10): p. 640-649.
4. Gupta, T., R.K. Sharma, and S. Chaudhary, Impact resistance of concrete containing waste rubber fiber and silica fume. International Journal of Impact Engineering, 2015. 83: p. 76-87.
5. Thomas, B.S., et al., Strength, abrasion and permeation characteristics of cement concrete containing discarded rubber fine aggregates. Construction and Building Materials, 2014. 59: p. 204-212.
6. Najim, K.B. and M.R. Hall, Crumb rubber aggregate coatings/pre-treatments and their effects on interfacial bonding, air entrapment and fracture toughness in self-compacting rubberised concrete (SCRC). Materials and structures, 2013. 46(12): p. 2029-2043.
7. Khatib, Z.K. and F.M. Bayomy, Rubberized Portland cement concrete. Journal of materials in civil engineering, 1999. 11(3): p. 206-213.
8. Youssf, O., et al., Development of Crumb Rubber Concrete for Practical Application in the Residential Construction Sector–Design and Processing. Construction and Building Materials, 2020. 260: p. 119813.
9. Youssf, O., et al., Influence of mixing procedures, rubber treatment, and fibre additives on rubcrete performance. Journal of Composites Science, 2019. 3(2): p. 41.
10. Tepfers, R., A theory of bond applied to overlapped tensile reinforcement splices for deformed bars. Division of concrete structures, 1973.
Rebecca J. Gravina1,Tianyu Xie1, Yan Zhuge2, Xing Ma2, Osama Youssf2 and Julie Mills2
1 School of Engineering, RMIT University 2 UniSA STEM, University of South Australia
Abstract : This paper shows an experimental study to investigate the bond behavior of reinforced structural crumb rubber concrete. Concentric bond bar pull-out and beam-end block tests are performed to study the bond between steel reinforcement and crumb rubber concrete at the local and global level. In addition to the research on the semi-structural bond behavior, three large-scale reinforced rubberized concrete beams are also tested under flexural bending. Bar embedded length, bar diameter and rubber content are the main factors studied in this research work. Based on the experimental results, it is observed that the higher deformability of rubberized concrete results in altered local and global bond behaviour, and a reduction in peak bond stress and an increase in the slip at the peak bond are observed. The change in bond behaviour together with reduced mechanical properties of crumb rubber concrete subsequently lead to a lower flexural stiffness at the elastic stage and a reduction in ductility at the post-peak stage of reinforced crumb rubber concrete beams.
Keywords: Bond, Crumb rubber, pull-out test, beam-end tests, bending
1. Introduction Crumb rubberized concrete that utilizes rubber particles derived from the end of life (EOL) tyre as an alternative to fine aggregates in structural concrete, is a construction industry focused application that is attracting rising attention from researchers, designers and engineers. Previous studies have experimentally characterized the mechanical properties of the crumb rubberized concrete at the material level including; the compressive strength [1] , flexural strength , modulus of elasticity [2] , fracture energy, fracture toughness [3] , impact resistance [4] , abrasion resistance [5] and the bond behaviour of the rubber particles to the cement matrix [6] . The more deformable nature of rubber particles and the poor bond between the rubber particles and the cementitious matrix due to the relatively smooth surface of the rubber particles adversely affect the mechanical properties of the concrete products containing rubber. Therefore, the current recommended rubber content is suggested to not exceed 20% by volume of fine aggregates in concrete’ to minimize the impacts of rubber inclusion on the mechanical properties of concrete [7].
A careful literature review reveals that the existing studies on reinforced rubberized concrete bond focused on only local bond behaviour, in which the embedded length of a bar in concrete is recommended around five times the bar diameter ( i.e . as reported in [8] ). The bond stress along a bar with a short-embedded length is relatively evenly distributed, representing the bond behaviour only locally, hence is characterized as a property at the material level. At the member level, the local bond behaviour cannot represent the global bond between the concrete and the reinforcements, as the anchored length of the bar in the concrete exceeds five times the bar diameter and the bond stress distribution along the bar has a gradient. Compared with conventional concrete, the difference in the deformability and local bond behaviour could alter the global bond behaviour of the reinforced rubberized concrete. However, the global bond behaviour of reinforced rubberized concrete is yet to be investigated and hence must be explored.
To fill the research gap identified, a systematic experimental program is undertaken in the present study to comprehensively study the local and global bond behaviour of reinforced rubberized concrete. The effect of the global bond on the flexural performance of reinforced rubberized concrete beams is also experimentally studied. A total of 48 concentric pull-out tests, 96 beam-end pull-out tests, six larger-scale beams tests and the ancillary material property tests are presented.
2. Experimental program
2.1 Materials and mix design The raw materials used in this experiment were: General blended cement (GBC), sand, two sizes of coarse aggregates (10 and 20 mm), fine natural aggregate (5 mm), crumb rubber (2-5 mm), “MasterPolyheed 8875” water reducer (BASF), “MasterAir 905” air entraining admixture (BASF) and deformed steel bars D500N
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grade of diameters 12, 16, 20 and 24 mm . Note that aiming to enhance the performance of the rubber in concrete, the rubber particles were pre-soaked in water for 24 hrs as per the pre-treatment method reported in [9] . The mixing procedure, in terms of mixing time and mixing order, of all the concrete followed the approach developed by the authors as reported previously [9] . The prefixes CC and CRC used in the specimens’ designation stand for conventional concrete (control) and crumb rubberized concrete, respectively and the suffix 32 denote s th e targeted strength grades for the original conventional concrete mix designs, respectively, i.e. 32 MPa. Noting that the replacement of fine aggregate with rubber is 20% by volume of sand in each mix. Hence the only difference in the mix designs between conventional concrete and rubberized concrete is the reduction of sand content and addition of rubber content with equal volume. The mix designs of the concretes are reported in Table 1.
Table. 1 Concrete mix design
Mix ID GBC Coarse Aggregates
Sand Crumb rubber w/c Water Water
reducer
Air entraining admixture20mm 10mm
CC32 337 551 459 826 0 0.61 205.6 0.51 0 CRC32 337 551 459 685 63.6 0.61 205.6 0.51 0
2.2 Test method 2.2.1 concentric pull-out test
Figure 1 shows the setup of the concentric pull-out test and the details of the bond test specimen. Note that transparent and flexible polypropylene pipes were used as bond breakers for the un-bonded section of the deformed bars. The dimension of the concrete block was 200 × 200 × 200 mm and the embedded length of the reinforcement bar was kept as five times the diameter (5Ø) of the bar, where this short bond length was used to characterize the local bond-slip relationship with constant bond stress (e.g. at the material level). To keep the pipes in position during sample preparation and casting stage, the front end of the pipe was tied to the rebar using flexible duct tape that grips the rebar and does not dislocate while pouring concrete and vibration. The section of the flexible duct tape that was lapping the bar was cut off after the specimens were de-moulded. Blue tac was used at the rear (internal) end of the pipe to seal any gap between the bar and the pipe and to block any cement paste from entering into the gap. Two linear variable differential transformers (LVDTs) were mounted on the bars, one at the load end and another at the free end of the bar. The pull-out bond test was conducted under a displacement-control mode with a rate of 0.4 mm/s. The parameters studied were bar diameter, concrete strength grade and rubber incorporation.
Figure.1 Concentric pull-out bond test setup
2.2.2 Beam end block pull-out test Figure 2 shows the beam-end block test setup and the details of the test specimens. Installation of the flexible polypropylene pipes, use of LVDTs and the method of testing was the same as that used for the concentric pull-out bond test. The dimension of the concrete block was 200 × 300 × 380 mm and the bonded
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length of the reinforcement bar was kept as 10 times (10Ø) the diameter of the bar, where the longer embedded length was adopted to represent the global bond-slip behaviour (e.g. at the member level). Four bars per block were installed in N12 and N16 bar setups, with 2 bars at top and 2 at bottom of the block, keeping a clear cover from all edges of three times the diameter of the bar. For N20 and N24 bar setups one bar at the top and one at the bottom of the block were installed keeping the clear cover from their respective top and bottom edges as three times the diameter of the bar. Note that the beam-end tests for the top and bottom bars were conducted to evaluate the effect of specimen casting direction on the global bond properties of the reinforced rubberized concrete. The parameters researched were bar diameter, concrete strength, rubber incorporation and specimen casting direction.
Figure. 2 Beam-end block test setup and specimen details (B=Bonded length, UB=Un-bonded length, Ø = Bar diameter, CC = Clear cover, R = Reaction points)
2.2.3 Beam test Following the local and global bond tests, the effect of rubber incorporation on the flexural performance of reinforced concrete beams was also investigated. Figure 3 shows the details of the test setup of reinforced concrete flexural beams. A total of 6 reinforced beams with 3 duplicates each for CC32 and CRC32 series of concrete, respectively, were tested under four-point bending. The beam was 150 mm wide × 220 mm deep × 2400 mm long having a clear span of 2200 mm. Each beam was reinforced by two 16 mm-deformed bars placed at the tension region and two 8 mm-deformed bars at the compression region of the beam, all with a clear cover of 20 mm. To prevent the premature shear failure of the beams, a 6 mm stirrup was used as the transverse reinforcement placed at 165 mm centres. Each beam was loaded at the top side having two equally spaced loading points via a spreader beam with the constant moment region of one third of the clear span. The beam was tested under a load control mode with a loading rate of 5kN/min until failure of the specimen to generate the complete load-deflection relationship. The deflection profile of the beam was established using four laser displacement measuring instruments installed at the rear of the beam with a spacing of a quarter of the span.
Figure.3 Reinforced concrete beam test setup
3. Test results and discussion 3.1 concentric pull-out test results The local bond-slip behaviour of the reinforced CC and CRC were experimentally investigated using concentric pull-out tests, where a short bar embedment was adopted (5 Ø). Note that this test method was applied to characterize the local bond-slip behaviour at the material level as the bond stress distribution along such a short bar length is relatively uniform. Table 2 summaries the results of each concentric pull- out test accompanied by the other corresponding physical and mechanical properties. The majority of the specimens failed due to the bar being pulled out from the concrete matrix, except for some of the specimens
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with 20 mm or 24 mm bar where a splitting failure of their concrete cover was experienced. The splitting failure was an unexpected failure mode and mainly occurred due to the larger bond force induced as the consequence of a larger bonded area of the bar with a larger diameter and the subsequent reduction in the dimension of the concrete cover with an increased bar diameter [10].
Table. 2 Concentric pull-out test results. Concrete series
CC32 CRC32 CC20 CRC20
stress (mm)
0.1 1.2 ± 0.1
0.2 1.8 ± 0.2
0.2 1.5 ± 0.6
0.3 2.4 ± 0.1
Compressive strength (MPa)
2.2 Beam end block pull-out test results
Unlike the local bond-slip behaviour of the reinforced concrete studied by the concentric pull-out tests, the global bond-slip behaviour of the reinforced conventional and rubberized concrete was studied via beam- end tests with each bar embedded length of 10Ø, where the bond stress distribution along the rebar has a gradient. The specimens with 12 mm and 16 mm bars embedded all failed with the bar pulled out from the concrete blocks. During the test, some premature failures of the concrete blocks were observed, in particular for the specimens with a bar of a larger diameter embedded (e.g. specimens with 20 mm- and 24 mm- bars). Of the same mechanism as those of the concentric pull-out tests where the concrete splitting failure occurred, these unexpected failure modes can be attributed to the larger bond force induced as the consequence of a larger bonded area of the bar with a larger diameter although the cover for the beam end tests increased accordingly (e.g. remaining at 3Ø). This may also be attributed to the relatively lower strength concrete used in the present study. It should be noted that the pull-out test results (e.g. their bond-slip curves) of the bottom bars for all the specimens with 20 mm and 24 mm bars are still presented but are not analysed and discussed in the remainder of this study due to these pre-damages of the concrete blocks. The results of the beam-end bond-slip behaviour in terms of the pull-out load-free end slip relationship are illustrated in Figures 4 . It was interestingly observed that for the specimens that failed with the bar pulled out, the reinforcements placed at the bottom of the concrete block required a higher load to be pulled out from the concrete matrix. The shape of the bond-slip curves of the bottom bars was different to those of the top bars but was similar to that of a bar embedded in the heavily-confined concrete as per the specifications in [11] , where a nearly bilinear load-slip relationship was observed. The differences in the bond-slip curves between the top and the bottom bars are mainly caused by the orientation to cast the specimens, where the concrete at the bottom part of the block is more compacted and hence was much denser owing to the settlement of the heavier ingredients and the self-weight of the concrete. This led to an enhanced interaction between the concrete and the bottom bar compared to that of the top bar. The observed effects of the selected parameters on the global bond-slip behaviour obtained from the beam-end tests are the same as those observed from their companion local-bond behaviour established using the concentric pull-out tests,
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confirming the effects of the local-bond slip behaviour on the global bond-slip behaviour of a reinforced concrete member.
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RMIT Classification: Trusted
2.2 Beam test results Having the impacts of adding rubbers in concrete on the semi-structural behaviour evaluated using the concentric pull-out tests and the beam end tests, the influences of rubber inclusion on the reinforced concrete are further studied at the member level. B oth of the beams exhibited a typical flexural failure mode, where the concrete crushing occurred at top of the beams with the constant moment region and flexural cracks formed at the bottom of the beam. During the beam tests, it was first observed that the cracks formed on the tension surface of beams within the region between the two loading points (referring to the constant moment region). With increasing applied load, these cracks gradually propagated to the neutral axis, which subsequently caused a series of flexural cracks in the constant bending moment region. Following the tension damage of the beams, the compressive wedge induced by concrete crushing initiated and the depth of the wedge gradually increased. In the meantime, the buckling of the compressive bars occurred. In the end, all the beams completely lost their load carrying capacity due to the crushing of the concrete at the compression region of the specimens.
The load-deflection relationship of each test specimen is shown in Figure 5 . The rubber incorporation led to no significant change in the shape of the load-deflection curve of the beam. The comparisons between the load-deflection curves and the deflection profiles shown in the figures reveal that the CRC32 beams generally exhibited a lower stiffness within the elastic stage compared to that of the CC32 beam. This is mainly caused by the higher deformability and cracking tendency of the CRC32 concrete (e.g. a lower elastic modulus and a significantly lower flexural tensile strength) containing rubber in conjunction with the relatively inferior bond between the rubberized concrete and the rebars at the tension surface of the concrete beams. Owing to the lower compressive strength and the larger deformability of the CRC32 series concrete, the yielding of tension rebars and the crushing of concrete within the compression region of the beams occurred earlier in the CRC32 beams. This led to a slightly lower load carrying capacity of 8% in reduction for beams containing the crumb rubber. This also in turn resulted in a less ductile post-peak behaviour of the CRC32 beams compared to their CC32 counterparts.
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Figure. 5 Load- midspan deflection relationships of a) CC32 and b) CRC32 beams
3. Conclusions This paper presents a systematic investigation of the effects of rubber incorporation in concrete on the global and local bond, as well as the flexural performance of reinforced rubberized concrete beams. An experimental program is undertaken, which contains concentric pull-out tests, beam-end pull-out tests, large-scale beams tests and the ancillary material property tests. Based on the results of this research work, the following conclusions can be drawn:
1. Local bond behaviour is affected by adding rubber in concrete, up to 52% and 59% reduction in the local bond strength are respectively observed for C32 and C20 grade of concrete with 20% fine aggregate replaced by rubber. At the material level, a lower bond strength (up to 30% and 34 % reduction in the peak pull-out load for C32 and C20 grade of concrete respectively) and a larger slip at the peak bond stress are observed for concrete with rubber partially replacing sand. This is due to the increased deformability of concrete with increasing rubber content (for instance the decreased elastic modulus), resulting in a larger relative slip between the concrete and the steel bar, which in turn leads to an inferior bond properties of reinforced rubber concrete.
2 . The altered local bond behaviour, in turn, results in a reduction in the bond-stress and an increased slip at the peak bond stress of reinforced rubberized concrete at the member level. This is attributed to both the higher deformability of the concrete with rubber and the relatively reduced local bond behaviour.
RMIT Classification: Trusted
3. The beam tests indicate that the reduced bond between the rebars and the adjacent rubberized concrete in conjunction with the higher deformation ability in the service limit state of the rubberized concrete lead to a lower flexural stiffness of the beam within the elastic stage. The earlier yielding of the tension rebars and the earlier crushing of the rubberized concrete result in a less ductile post-peak behaviour of the reinforced rubberized concrete beams. Around 8% reduction in the peak load and approximate 6% decrease of mid-span deflection at the peak load are observed for C32 grade of concrete beam with 20% fine aggregate replaced by rubber.
4. Acknowledgement The authors would like to acknowledge the funds provided by the Australian Research Council (ARC- LP160100298) and industry partners for this project. The industry partners are; Tyrecycle Pty Ltd, Tyre Stewardship Australia, ResourceCo Pty Ltd, FMG Engineering Pty Ltd, and Ancon Beton. The materials donations by ResourceCo Pty. Ltd., Adelaide Brighton Cement Pty. Ltd., and Tyrecycle Pty. Ltd., are greatly appreciated. The authors also would like to thank the support of lab staff members in RMIT and University of South Australia to carry out this research
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