Bond Properties For GFRP Bars In Concrete Under Reversed Cyclic Loading

With the long-term use of reinforced concrete structure,the corrosion of steel bar has caused serious damage to the structure,the bearing capacity and service life of the concrete structure can not reach the expectation,and even more serious will directly lead to the damage of the structure.It is one of the effective methods to choose fiber-reinforced polymer(FRP)bars with better corrosion resistance to replace steel bars.Therefore,in recent years,the applicability of FRP bars in structures has been widely concerned.In order to further understand the failure mode and working mechanism of FRP-RC structure,the bond properties between FRP bars and concrete has become the focus of many scholars.The elastic modulus of FRP reinforcement is low,and it shows the characteristics of linear elasticity before failure.These characteristics are different from those of reinforcement,which also attract the researchers’ attention to the seismic performance of FRP-RC structure.At present,it has been found that FRP-RC columns and joints have a large contribution to the deformation of column and beam column joints due to the FRP bar slip at the column base and joints.Therefore,it is necessary to develop the bond slip mechanism of FRP Bars under the reciprocating load,and analyze the seismic performance of gfrp-rc structures from the stress level.Therefore,based on the study of the bond properties of GFRP bars under monotonic load,the bond properties of GFRP bars under cyclic load is studied in this paper:(1)Based on the eccentric pull-out test of GFRP and concrete under monotonic load,the influence of reinforcement diameter,surface treatment form,volume stirrup ratio,concrete cover thickness and concrete strength on the bond properties is studied.The results show that the bond strength of FRP with ribs is higher than that of FRP with sand;the bond strength of FRP with concrete cover thickness and strength is higher than that of FRP with sand The stirrup ratio increases with the increase of degree and volume,and decreases with the increase of reinforcement diameter.(2)Through the eccentric pull-out test of GFRP reinforcement and concrete under the action of reciprocating load,the bond strength under monotonic and reciprocating load is compared,and the factors such as the hysteretic criterion of hysteretic curve,unloading stiffness,reinforcement material diameter,surface treatment form,thickness of concrete protective layer,concrete strength and volume stirrup ratio that affect the bond degradation of GFRP reinforcement are analyzed.The results show that before the peak slip,the reciprocating load will not cause the degradation of bond strength,but after the peak slip,with the increase of cyclic displacement loading amplitude,the bond properties degradation of GFRP reinforcement is obvious.The bond degradation rate decreases with the increase of concrete strength,protective layer thickness and volume stirrup ratio,and increases with the increase of reinforcement diameter.(3)Based on the existing constitutive model of bond stress slip relationship between GFRP Bars and steel bars,combined with the analysis of monotonic and reciprocating test results and theoretical derivation,the bond stress slip relationship constitutive model between GFRP Bars and concrete under monotonic load and bond stress slip hysteresis constitutive model under reciprocating load are established,and the calculation results of the model are combined with the test results The results show that the model can accurately predict the characteristic value of bond properties,and can better reflect the bond degradation law of GFRP Bars under reciprocating load.(4)In this paper,the finite element model of the pull-out specimen is established by the finite element software ABAQUS,and the constitutive model of bond stress slip relationship proposed in this paper is used to define the element properties of nonlinear spring element to simulate the bond slip between GFRP reinforcement and concrete.The feasibility and effectiveness of the constitutive model are verified by comparing the simulation results with the test results.