FRP Flexural Strengthening Method For Damaged RC Beams Based On Time-varying Material Properties & "Warning Wire" Theory

With the accumulation in service time of reinforced concrete structures,the bearing capacity of a large number of members deteriorates due to the corrosion of the surrounding environment and the load.Therefore,it is urgent to evaluate the bearing capacity and take safe and effective methods for reinforcement and maintenance.As a new reinforcement method,prestressed FRP reinforcement technology has been widely used in engineering owing to its good reinforcement effect.In recent years,researchers at home and abroad have done a lot of experimental research and theoretical analysis on the bearing capacity and deformability of prestressed FRP reinforcement structures.The research shows that:although the characteristic load and rigidity of the reinforced members are significantly improved,their deformability is reduced,and the ductility performance is obviously insufficient.There is no obvious sign before the damage of the members,which results in insufficient safety reserves of the members.Aiming at the above problems,this paper proposes a method for FRP flexural strengthening of damaged RC beams based on time-varying material properties and "warning wire" theory.In view of the stress characteristics of damaged members,prestressed FRP plates are used to strengthen them to solve the problem of insufficient bearing capacity.Besides,a reasonable early warning wire is designed to make the breakpoint as the yield point of the reinforced member,so as to solve the problems of no obvious signs and insufficient safety reserves before the damage of the reinforced members due to insufficient ductility.The main research contents and results of the paper are as follows:(1)A calculation model for the bending capacity of normal cross-section of a damaged component based on time-varying material properties is established.This article based on the determination of the time-varying constitutive model of steel,concrete,and interface bonding,the flexural performance of the damaged member is simulated by finite element method.The results show that:corrosion of steel bars and concrete can increase the amount of slip between the interface,thereby reducing the synergy,which reduces the bearing capacity and the deformation capacity of the member.The failure mode of the component can gradually change from ductile failure to brittle failure.By analyzing the analog data to determine the relationship between the strain of steel bar and the strain of concrete at its position under the yield condition.Combined with the time-varying constitutive model of steel and concrete,a time-varying model of normal section bearing capacity suitable for damaged members is derived.The accuracy of the model is verified by comparing the experimental data from other literatures.(2)Establishing a design method for flexural reinforcement of damaged members based on the "warning wire" theory.According to the mechanical properties of the damaged members,the amount of FRP plate reinforcement required to reach the expected bearing capacity of the damaged members is derived,the failure modes of the reinforced members are analyzed,and the limit reinforcement ratio between each failure mode is calculated.The paper deduces the theory of "warning wire" of the member,obtains the design formula of the initial prestress of early warning wire,analyzes the safety reserve parameters involved in the formula,and determines the value range of the safety reserve parameters involved.By analyzing the application conditions and construction conditions of the reinforced members in actual engineering,the design method is modified to reduce the effects of secondary stress and prestress loss on the performance of the reinforced members.(3)Validation of FRP flexural strengthening method for damaged RC beams based on time-varying material properties&"warning wire" theory through tests and simulation analysisBy analysis of the test and simulation results,it can be seen that the bearing capacity of the members strengthened by the designed method presented in this paper is significantly improved,and the mechanics performance is good,which can meet the design requirements for bearing capacity.With the addition of warning wire,the reinforced members can send out obvious warning signal before the failure.The problem of no obvious warning before the failure of the prestressed FRP reinforced members is successfully solved.The equivalent safety reserve index of reinforced members can meet the requirements of Keq≥2.1,and the basic safety reserve index of bearing capacity can meet the requirements of K0≥1.5,which solves the problem of insufficient safety reserve of reinforced members.