Study Of The Mechanical Properties Of FRP-confined Concrete Core Encased Rebar Under Cyclic Loading

In building structures,reinforced concrete columns are important vertical load-bearing elements and major lateral force-resisting elements in buildings.When subjected to severe earthquakes,they will produce large repeated tensile and compressive deformations,and before the fatigue damage of longitudinal reinforcement in reinforced concrete columns,the reinforcement often already has a relatively obvious buckling instability,and plastic hinges are formed at the column ends,leading to a reduction in structural load-bearing capacity and causing serious damage to the structural system,so it is necessary to Research is needed to study the construction of restrained steel buckling to improve the seismic performance of the building.For building structures located in highly corrosive environments such as coastal areas,the mechanical properties of the reinforcement will be affected by seawater corrosion,which requires that the reinforcement in the column should have both good flexural stress resistance and corrosion resistance.Currently,FRP confine concrete core encased rebar（FCCC-R）is a new type of composite member that has been proposed in recent years.This type of member uses the composite tube confine concrete as the confining core,and the concrete is equipped with steel reinforcement inside,using the composite material and concrete to confine the buckling of the steel reinforcement,giving full play to the compressive load bearing capacity of the steel reinforcement,thus enhancing the load bearing capacity of reinforced concrete structures.Therefore,the study of the hysteresis performance of FCCC-R composite members is of great significance for its promotion in seismic and highly corrosive coastal areas.In this paper,we analyse the damage morphology of FCCC-R composite members from the macroscopic level by means of material properties tests,cyclic reciprocal tests with different loading regimes and finite element simulations,and analyse the load carrying capacity,stiffness and ductile mechanical properties of FCCC-R composite members.The simulation results are compared with the experimental results to show the accuracy of the established finite element model.The main research contents and main conclusions of this paper are as follows.（1）The mechanical properties of FCCC-R and ordinary reinforcement were compared and analysed by cyclic reciprocal tests with different loading regimes on FCCC-R and ordinary reinforcement with different length to slenderness ratios and FRP constrained pipe diameters.It was found that the use of fibre tubes with mortar fillers under different loading regimes significantly improved the compressive properties of the reinforcement,and that FCCC-R had better hysteresis properties compared to ordinary reinforcement.The test results show that the smaller the length to slenderness ratio,the longer the secondary reinforcement section of FCCC-R after yielding in compression,the later the buckling time,and the more significant the reinforcement effect;the increase in the diameter of the FRP-constrained tube,the greater the flexural stiffness of the specimen section,and the more obvious the increase in the compressive load capacity of the specimen.The ductility of FCCC-R in the tensile and compressive cycles is reduced compared to that in the tensile and compressive cycles.The load capacity and ductility of FCCC-R in both loading methods were better than those of ordinary reinforcement.（2）The material ontological parameters of FCCC-R were calibrated by the test results and a finite element model of FCCC-R was established.The results show that the model can better represent the hysteresis performance of FCCC-R,and the simulation results reflect the damage process,stress softening due to buckling and other characteristics of FCCC-R in the test.Based on the finite element model of FCCC-R,an extended analysis of the constraint ratio parameters is established,which provides a good basis for the selection of the external constraint material size of FCCC-R in practical engineering.（3）The low circumference fatigue performance of FCCC-R and common steel reinforcement was investigated by conducting low circumference fatigue tests on FCCC-R and common steel reinforcement.The test results show that different strain amplitudes and different FRP restraint diameters affect the fatigue life and energy consumption of FCCC-R.Based on the test results,a low cycle fatigue life model for FCCC-R and ordinary steel bars was developed.The results are used as a reference for the assessment of the seismic performance of building structural systems applying FCCC-R.