Effect Of The Bond-Slip Constitutive Relationship Between FRP Bars And Concrete Based On Durability

There are many factors that make the durability of concrete structures decrease,such as the corrosion of steel bar,freezing and thawing,the physicochemical action under erosion environment,etc.The most serious one must be the corrosion of steel bar.As a new composite material,Fiber Reinforced Polymer(FRP)has great potential for the concrete structure because of its great durability,light weight,high strength and convenient construction.The great adhesive performance between FRP bar and concrete is the foundation to ensure the work is going well,and it has been studied widely in civil engineering recently.Nevertheless,FRP bars are usually used in bridges,tunnels and other harsh environments.In these cases,the FRP reinforcement structure does exist some problems,like the decreased bond strength or the deterioration of material.Therefore,it is quite important to carry out the research on bond slip relationship between FRP bars and concrete in the erosion environment.It is also beneficial to the supplement and perfection of FRP–concrete design specification and the improvement of structure durability.This paper is subsidized by National Natural Science Foundation of China The study of design and performance of FRP based on the durability of FRP tendons and ordinary reinforced concrete.Three kinds of fiber reinforced polymer are selected to study the durability and the bond-slip constitutive model of FRPconcrete specimens in erosion environments,namely CFRP(Carbon Fiber Reinforced Polymer),GFRP(Glass Fiber Reinforced Polymer)and BFRP(Basalt Fiber Reinforced Polymer).The main work is as follows:1)The research on degradation of durability of FRP bars and steel bars in erosion environment.FRP bars are immersed in three solutions(alkaline solution,saline solution and clean water)for 45,90,135 and 180 days respectively.For the common steel bars,the wetting-drying experiments are conducted.The concrete with steel bars are immersed in the saline solution to obtain some reinforcing steel bars with corrosion damage.The differences between the damage of FRP bars and steel bars are compared.Some tensile experiments are also made to analyze their fracture properties.It is indicated that the fracture modes of steel bar and FRP bar are quite different.The durability of CFRP is the best among three types of FRP.Additionally,the alkaline solution is more erosive than other two.It is noticeable that the size effect of GFRP can not be ignored.The degradation degree increases when the diameter of specimen becomes smaller.It is further confirmed that Arrhenius equation can be applied to predict the time-variant characteristic of strength retention index.The relationsip between accelerated factor and temperature is analyzed with Litherland method.It is found that the value of accelerated factor decreases with higher temperature.2)The research on adhesive property of FRP-concrete in the corrosion environment.The pull-out FRP concrete specimens are immersed in different solutions(alkaline solution,saline solution and clean water)for 150,210 and 270 days.The characteristic of moisture absorption is measured through out the whole experiment.After that,the immersed specimens are pulled out to find out the time-variant degradation law of their adhesive properties.It is shown that the moisture absorption rate of FRP-concrete specimen changes under different corrosion conditions.Also,it can be seen clearly from the bond-slip curve that the average bonding strength decreases in each environment.3)The research on the FRP-concrete bond-slip constitutive relations.According to the current bond-slip constitutive model,the τ-s curves of FRP-concrete specimens are compared so as to calculate the average bond stress in the ultimate state.There are three main stages of τ-s curve: the ascent stage,the declining stage and the residual stress stage.The ascent stage and the declining stage can nearly satisfy the revised BPE model.The non-linear analysis of residual stress stage is conducted based on the Fourier series theory.The time-varying parameters in the τ-s curve are also modified properly.Thus,the prediction model of residual stress stage can be put forward based on FRP bond-slip constitutive relation in different erosion environments.4)The research on the time-variant model of adhesive property of FRP-concrete in the erosion environment and reliability calculation.The SR(Strength Retention)model is applied to analyze the pull-out experiments of FRP-concrete specimens.In this way,the time-varient degradation model of adhesive property of pull-out FRP bars can be set up.The results show that CFRP pull-out specimen has the lower degradation trend than BFRP and GFRP ones with the same diameter in alkaline or saline solutions.In other words,CFRP has the better durability.Lastly,the Bonding strength reliability is calculated based on the principle of isoprobability transformation to normality.Parameters such as the relative bond length K are calculated and predicted by the iterative operation module in Matlab,which can provide the theoretical reference for the bond length of FRP bar in the concrete structure engineering.