Study On Seismic Behavior Of Hybrid Steel And FRP-reinforced Concrete-ECC Composite Columns

Engineered cementitious composites(ECC)is a new building material with super high toughness and multi-crack development mechanism,which can effectively improve the safety,durability and sustainability of the structure.Fiber Reinforced Polymer/Plastic(FRP)is a high-performance material formed by fiber materials and matrix materials in a certain ratio and compounded by a certain process.FRP bars have high strength,non-magnetic properties,low density,corrosion resistance and other merits.The combination of FRP bars and reinforcement steels will significantly improve the seismic bearing ability and post-earthquake repairability with methods of avoiding low elastic modules and poor ductility as well as achieving the higher post-yield stiffness and less residual deformation.A new type of hybrid steel-FRP-reinforced concrete-engineered cementitious composite(ECC)composite column proposed in this study,FRP bars are placed in the corner of the square column while ECC in the plastic hinges area only with aims of taking advantages of FRP bars to the most extent under a reasonably controlled cost.A pseudo-static experiment of 13 columns was conducted to study the seismic behavior of columns.The bearing capacity of columns was investigated by the method of section strip based on the experiment and parameters analysis was also conducted by the ABAQUS.The specific research content is as follows:(1)The design of the scheme is carried out considering the influence of parameters such as matrix material,axial compression ratio.FRP type and reinforcement ratio on the seismic behavior of column members.Low cycle reciprocating test was conducted on 13 cured columns and the results showed that ECC has a better performance of constraining on the internal reinforcement compared to concrete,so the bearing capacity and ductility of the members can be significantly improved when concrete of the bottom of columns is replaced with ECC.the FRP bar-reinforced steel composites columns performed a lower residual deformation and higher post-yield stiffness compared to reinforced concrete columns when experienced low-cycle reciprocation.Under equal axial compressive ratio,the bearing capacity improves as the reinforcement ratio increases while the change of deformation ability is negligible.Under equal reinforcement ratio,the bearing capacity improves as the axial compressive ratio increases within the range of 0.1-0.42,and deformation ability versus.In summary,the hybrid steel-FRP-reinforced concrete-engineered cementitious composite has superior seismic performance,not only achieved higher load carrying capacity and ductility,but also obtained less residual deformation and higher post-yield stiffness.(2)A program was compiled to obtain the moment-curvature curve of the columns through the method of section strip,based on material constitutive relations.The effective thickness of ECC layer,in which all yield bars are contained,is calculated analytically.And the theoretical method can be verified by comparing its bearing ability with experimental results.The simulation results showed that ECC is beneficial to improve the seismic behavior of structures.Experimental values are found to be close to the theoretical values,which demonstrates the effectiveness of the method in simulating the bearing ability of members.(3)The force-displacement curves of columns are investigated by ABAQUS on the basis of tests to verify the accuracy of the model,and then parameter analysis was conducted considering the influences of the strength of ECC,FRP type,the area ratio of FRP bars to steel bars and reinforcement ratio.The analysis results show that the force-displacement curve of the column member can be well simulated by the finite element analysis.It is further found that the improvement of ECC strength has no obvious improvement on the bearing capacity,and has negligible effect on the deformation ability;the CFRP bar has higher strength and elastic modulus than other FRP bars,so CFRP bar reinforced column member has the largest bearing capacity.In terms of FRP reinforced bar area ratio,the bearing capacity and ductility is significantly improved with the increase of FRP bars;the bearing capacity and ductility of the members are greatly improved as the reinforcement ratio of the column members increases.