Study On The Seismic Behavior And Design Method Of Ultra-high Performance Concrete Compressive Members

Ultra-high performance concrete(UHPC)is considered as a new generation of cement-based material due to its excellent mechanical properties and durability.It has a good application prospect in civil engineering.However,seismic performance of UHPC structures still lacks in-depth research,leading to the absence of seismic design provisions in the relevant structural technical specifications.It has restricted the promotion and application of this high-performance material.Thus,with the combination of National Natural Science Foundation(51878262)and National Key Research and Development Project(2017YFC0703008),the seismic performance of UHPC shear walls and columns is studied in this paper.The main research contents are as follows:(1)Quasi-static tests of six full-scale UHPC shear-wall specimens were conducted,and the loading process and failure pattern of the specimens were obtained.The influences of reinforcement type(steel reinforcement,hybrid reinforcement of CFRP and steel bars)and axial-load ratio(0.1,0.3,0.35 and 0.4)on the skeleton curves,characteristic points,reinforcement strain,energy dissipation capacity,self-centering capacity,strength degradation,stiffness degradation,lateral displacement curve and moment-curvature relationship,of the UHPC shear-wall specimens were analyzed.The test results showed that the UHPC shear walls with hybrid reinforcement exhibited high self-centering capacity while maintaining enough energy-dissipation capacity.The ductility of UHPC shear walls decreased gradually as the axial-load ratio increased.Due to the lower modulus of CFRP bars compared to the steel bars,at an axial-load ratio of 0.1,more cracks appeared on the web region of the hybrid shear walls than that of the steel-reinforced shear walls.As the axial-load ratio increased,the tensile strain in the longitudinal bars decreased,and the differences of the failure mode of two kinds of shear walls became limited at high axial-load ratio.(2)Quasi-static tests of nine full-scale UHPC column specimens were conducted,and the loading process and failure pattern of the specimens were obtained.The influences of reinforcement type(steel reinforcement,hybrid reinforcement of CFRP and steel bars),axial-load ratio(0.1,0.3 and 0.4),stirrup ratio(0%,0.25% and 0.5%)and steel fiber fraction(0%,1% and 2%)on the skeleton curves,characteristic points,reinforcement strain,energy dissipation capacity,self-centering capacity,strength degradation,stiffness degradation,lateral displacement curve and moment-curvature relationship,of the UHPC column specimens were analyzed.The test results showed that the UHPC columns with hybrid reinforcement,which could exhibit good selfcentering and energy-dissipation capacity at the same time,had good comprehensive seismic behavior.With the increase of axial-load ratio,the energy-dissipation capacity of the UHPC columns decreased obviously.As the stirrup ratio increased,the ductility and energy-dissipation capacity of the UHPC columns increased.When the steel fiber fraction increased from 0% to 1% and 2%,the ductility factor of the UHPC columns increased by 38% and 45%,respectively,and the cumulative energy dissipation at the ultimate displacement increased by 107% and 142%,respectively.The steel fibers can limit the crack development on the UHPC columns,reduce the crushing and spalling of the concrete,confine the concrete matrix,maintain the integrity of the concrete,delay the buckling of the vertical bars,and therefore increase the ductility and energydissipation capacity.(3)A restoring force model of the UHPC compressive members was established.On the basis of the plane section assumption,the calculation method of the cracking point and yield point of the UHPC compressive members were proposed.On the basis of the summarized experimental data,the calculation methods of plastic hinge length,peak point,ultimate point and unloading point of the UHPC compressive members were proposed.With the combination of the hysteretic rules,the restoring force model of UHPC compressive members was proposed.The restoring force model was used to simulate the hysteretic response of the tested UHPC shear-wall and column specimens.The simulation results agreed well with the experimental results,which verified the applicability of the proposed restoring force model.(4)A finite element model of the UHPC compressive members was established.On the basis of the verified finite element model,the parametric analysis on the seismic performance of the UHPC compressive members was conducted,and design suggestions of the UHPC compressive members were proposed,including the proper location of the carbon fiber-reinforced polymer(CFRP)bars and proper replacement ratio of CFRP bars in the boundary element of the hybrid UHPC shear walls,the proper replacement ratio of CFRP bars in the hybrid UHPC columns,the axial-load ratio limit of the UHPC shear walls and columns,and the equivalent formula of steel fibers and stirrups in the UHPC columns.Based on the double parameter seismic damage model and classification of seismic damage,a damage model was established to evaluate the damage level of the UHPC shear walls and columns.