Study On Axial Compression And Seismic Performace Of FRP Bars Reinforced Concrete Piers Confined With External FRP Jackets

In the coastal or offshore environment,reinforced concrete(RC)bridge structures were usually subjected to chloride salt corrosion which caused corrosion of reinforcing steel bars during its service period,which has a detrimental effect on its durability and safety.Furthermore,RC bridge piers were tended to form large residual deformation after earthquakes,which is not conducive to post-earthquake emergency repair and post-earthquake reconstruction.The application of fiber reinforced polymer(FRP)to substitute reinforcing steel bars could solve steel corrosion in RC bridge piers due to its corrosion resistance,and could reduce the residual deformation after earthquakes.However,FRP bars reinforced concrete structures suffered from bond slip failure in the straight part and had low tensile strength in the curve part,as well as low compressive strength of FRP bars compared with its tensile strength.These shortcomings severely restrict the engineering application of this high-performance composite structures.Motivated by the aforesaid background,based on the innovative closed FRP ties preparation process proposed by the research team and the concept of FRP confined concrete,this study proposed the concept of FRP bars reinforced concrete piers confined with external FRP jackets.Based on experimental research,theoretical analysis,and finite element analysis,this study revealed the influence of main design parameters on axial and seismic performance.Firstly,verified the proposed innovative closed GFRP ties could fully ultilize material properties.The compressive tests of GFRP longitudinal bars were carried out to investigate the influence of the unbraced length of GFRP longitudinal bars on the compressive strength and failure modes.A database of the compressive tests of GFRP longitudinal bars was established to analyze the relationship between the compressive strength and the unbraced length of GFRP longitudinal bars,and a reasonable range of ties spacing was suggested.Secondly,the monotonic compression test was been carried out on twelve FRP bars reinforced concrete square columns confined with external FRP jackets to verify that this structure can improve the overall mechanical performance of the composite columns.The effects of the type of longitudinal reinforcement,the type and spacing of GFRP ties,and the number of FRP layers on the monotonic and cyclic compression performance of twenty FRP bars reinforced concrete rectangular columns confined with external FRP jackets were further studied.Based on the experimental results,a monotonic compressive stress-strain constitutive model considered double confinements of inner GFRP ties and external GFRP jackets was proposed.Thirdly,the pseudo-static tests on eight full-scale FRP bars reinforced concrete rectangular piers confined with external FRP jackets were carried out to verify the effectiveness of the FRP bars reinforced concrete confined with external FRP jackets concept on improving deformation ability and self-centering ability.The effects of the GFRP tie spacing,the number of GFRP layers and the axial compression ratio on the failure mode and the seismic performance of the FRP bars reinforced concrete rectangular piers confined with external FRP jackets,such as bearing capacity,ductility,residual displacement angle and energy dissipation capacity,were investigated.Besides,the strain development process of ties,longitudinal bars and GFRP jackets on each specimens were discussed and studied.Finaly,based on the Open Sees platform,a fiber section finite element model was established to accurately simulate the fracture and strain penetration effect of GFRP longitudinal bars and the dual confining effect of GFRP jackets and GFRP ties.The model could provide a close simulation for the rupture of GFRP reinforcing bars,strain penetrating effects,and dual confining effect of GFRP jackets and GFRP ties.The influence law of the main design parameters on the load-displacement curve was analyzed and discussed,including the number of GFRP layers,the spacing of GFRP ties,GFRP longitudinal reinforcement ratio,axial compression ratio,shear span ratio,concrete strength.A simplified skeleton curve model of FRP bars reinforced concrete rectangular piers confined with external FRP jackets was established to provide a theoretical basis for the design and application of its seismic performance.