Experimental Research On Seismic Behavior Of Seawater And Sea Sand Concrete Filled RPC-FRP Tube

In order to solve the key problems in the construction of marine engineering,including the poor durability of reinforced concrete structure and the high cost caused by the long-distance transportation of bulk raw materials,a novel composite column named as seawater and sea sand concrete filled RPC-FRP tube(SWSSC-RPC Tube)is proposed.Basic structural form of SWSSC-RPC Tube is precasting RPC thin-walled tube with closed FRP stirrup,and seawater and sea sand concrete(SWSSC)is poured inside to form composite column.Seismic performance of this hybrid system is the foundation for applying SWSSC-RPC Tube in seismic area.However,FRP bar is a typical brittle material without plastic deformation before its tensile failure,and it has an important effect on the seismic behavior of composite columns,especially for ductility.Therefore,it is necessary to carry out the seismic test of this type of composite column.This study focuses on the seismic behavior of the composite columns.Totally six full-scale SWSSC-RPC Tube specimens were designed and performed the quasi-static test under the constant axial load.The experimental parameters include the stirrup ratio of FRP bars,the types of FRP bars,the internal SWSSC strength and axial compression ratio.Failure modes,hysteretic response,lateral bearing capacities of SWSSC-RPC Tube were obtained in current test.These results were also compared with of the similar compsite column Concrete Filled RPC tube(CFRT),in which steel hoops were arranged in RPC tube.Based on the experimental results,the seismic behavior of SWSSC-RPC Tube was analyzed and evaluated,and the main conclusions are drawn as follows:(1)SWSSC-RPC Tube columns exhibited obvious flexure failure mode under low cycle reversed loading.The cracks of the specimens' surface were mainly distributed in the plastic hinge area within the height of 1.5 times the diameter at the foot.No obvious RPC cover spalling or crushing was observed during the test due to the cracking resistance of steel fibers.With the increase of lateral displacement,the FRP stirrups and some FRP longitudinal bars in the plastic hinge area are partially fractured,and their failure shows a certain brittleness compared with that of CFRT composite columns.(2)Three composite columns with different stirrup spacings were tested for investigating the effect of stirrup raio.The results show that the lateral resistance,initial stiffness and ductility of sepcimens increase with the stirrup ratio increases.Increasing stirrup ratio can improve the energy dissipation and reduce the residual deformation of the composite column.This test also indicates that the minimum volume stirrup ratio of FRP bars can be taken as 1.03% for ensuring the lateral confinement action of FRP hoops under the earthquake.(3)Two SWSSC-RPC Tube specimens with CFRP bars and basalt fiber reinforced plastics(BFRP)bars were tested for revealing the influence from mechanical performance difference of FRP bars.The results show that the BFRP bars are better than CFRP bars for improving the ductility,the energy dissipation,and the stability of lateral force resistance of SWSSC-RPC Tube.The main reason is the relatively higher ultimate elongation of BFRP bars.Therefore,the FRP bars with higher ultimate elongation are recommended in SWSSC-RPC Tube system.(4)Two SWSSC-RPC Tube specimens filled with C40 and C60 grade SWSSC were compared to investigate the influence of internal SWSSC strength.The results show that increasing the internal SWSSC strength can effectively improve the peak load,initial stiffness and energy dissipation of the specimens.Although the ductility of SWSSC-RPC Tube is reduced by using high strength SWSSC,the reduction is limited.Therefore,the high strength SWSSC can be used in this hybrid system.(5)Two SWSSC-RPC Tube specimens were carried out the seismic test under the different axial compression ratios,0.2 and 0.3.The results show that the increase of axial compression ratio can increase the lateral resistance and initial stiffness.However,the deformation capacity of composite columns significantly reduces,especially for the ductility.(6)The analysis shows that the ultimate plastic lateral displacement of SWSSC-RPC Tube specimens is between 0.023 and 0.035,which meets the requirement of China's Code for seismic design of buildings(GB 50011--2010)that of structural columns should not be less than 0.02 under rare earthquakes.In addition,compared with the CFRT with steel reinforcement,the residual deformation of SWSSC-RPC Tube is smaller at a lower lateral displacement rate,so the repairability of the latter is better after suffering small or medium earthquakes.SWSSC-RPC Tube exhibits acceptable seismic behavior and its ductility meet the requirements of the current seismic design code.The novel composite columns have good application prospect in offshore reef engineering,considering its outstanding durability,economy and construction convenience.