Experimental And Theoretical Study On Bond Behavior Between FRP Bar And ECC

Engineered Cementitious Composites (ECC) is a class of advanced composite material which exhibits pseudo strain hardening behavior and super high toughness. ECC with 2% PVA fiber in volume shows an ultimate strain about 300 times of concrete, with crack width of less than 100μm before ultimate damage. Application of ECC can improve load bearing capacity, ductility and energy dissipation ability of structures. However, the bond behavior between FRP bars and ECC is one of the key factors affecting mechanical behavior of ECC members or structures, and is still one of the most important issues which need to be comprehensively studied in the practical application. Therefore, in this paper, the bond behavior between FRP bars and ECC is comprehensively studied through experimental studies, theoretical analysis and numerical simulation.1. Based on the pull-out tests, the bond-slip mechanism between FRP bars and ECC and the influence of various parameters, such as the embedment length, FRP types, FRP diameter and FRP shape, on the bond behavior between FRP bars and ECC had been comprehensively studied. According to the results, the load-slip curves between FRP bars and ECC could be divided into three parts including ascending phase, descending phase and residual phase. At residual phase, the curves would fluctuate and the curves of deformed FRP bars fluctuated stronger than that of mild FRP bars. The ascending phase’s slope of bond-slip of CFRP bar with ECC was larger than that of GFRP bars, and the displacement corresponding to peak bond stress of CFRP bar with ECC was smaller than that of GFRP bar. This is mainly because the elastic modulus of CFRP bar is larger than that of GFRP bar. Under the same bond length conditions, with the increase of diameter, the bond strength between FRP bars and ECC deceased. Under the same diameter conditions, with the increase of bond length, the peak load between FRP bars and ECC increased, while the bond strength between FRP bars and ECC deceased. The bond strength of deformed FRP bars was higher than that of mild FRP bars under the same anchorage conditions, which showed that the surface processing of FRP rebar could largely improve the bond performance between FRP bars and ECC material.2. A number of specimens had been designed for the pull-out tests, and the bond behaviors between CFRP bars and GFRP bars and ECC had been comprehensively investigated. With the strain gauges in the FRP bars, the variations of strains along the anchored FRP bars could be obtained, resulting in the distributions of the bond stress and relative slip displacement along the anchorage length of reinforcing bar. Then, the bond-slip relationships at different points along the anchorage length could be calculated. The bond-slip relationship between ECC and FRP reinforcement at a certain point could be deduced with a typical bond-slip relationship and a position function, which was used to consider the effect of anchorage position. Thus, the bond-slip relationship which considered the positon function could provide the basic reference for the finite element and the mechanical analysis of FRP reinforced ECC structures.3. Based on the test results, three linear bond-slip constitutive model was proposed. By solving the differential equation, the formulas of the relative slip between FRP bars and ECC, the bond stress on the surface of FRP bars and the load at loaded end were obtained. Good agreements had been obtained between the theoretical and experimental curves of load verse displacement at loaded end and free end, indicating that the proposed bond-slip constitutive model was feasible to simulate the bond-slip behavior between FRP bar and ECC. The influence of various parameters, such as the maximum bond stress, the initial bond stress, the slip corresponding to the maximum bond stress, the slip corresponding to the residual bond stress of constitutive model, the diameter of FRP bars, the bond length, the elastic modulus of FRP and the elastic modulus of ECC, on the maximum load at loaded end had been comprehensively studied. Finally the formula of anchorage length was given.4. The bond behavior between FRP bar and ECC was simulated with the software of ATENA. The simulation results showed that good agreements of load-slip curves at loaded end, FRP bar stresses along the anchorage region and bond stresses along the anchorage region, had been obtained between the theoretical and numerical results indicating that the theoretical analysis and numerical simulation were feasible to simulate the bond-slip behavior between FRP bar and ECC. The numerical simulation provided a new way for the study of ECC structure. Finally, the effect of different parameters (including the anchorage length, the diameter of FRP bar and the maximum bond stress) on the bond behavior between FRP bar and ECC had been carried out. The results indicated that under the same conditions, with the increase of anchorage length, the pullout force of FRP bar and ECC were greatly improved and the residual load also increased; with the increase of diameter, the maximum pullout force and residual load increased; with the increase of maximum bond stress, pullout force increased and residual load kept unchanged.