Study On Fatigue Properties Of RC Beams Strenthened With Pre-tensioned Prestressed CFL

Fatigue performance and durability of the reinforced concrete (RC) structurestrengthened using fiber reinforced polymer (FRP) is a frontier topic in the field of civilengineering. Strengthening technique with prestressed CFRP has been increasing usedrecently in engineering fields,which can offer the benefits such as giving full play to thecharacters of light weight and high tensile strength of FRP, increasing the cracking load,decrease the width of crack, delays yielding and implicates a better exploitation of themechanical performances. However, limited researchers studied on the fatigue behavior of RCbeams strengthened using prestressed FRP reinforcement. Therefore, considerableinvestigation of the long-term performance of prestressed FRP strengthening systems isnecessary before they gain full acceptance in civil infrastructure applications.Thus, this study attempts to investigate the long-term performance of RC beamsstrengthened using pre-tensioned prestressed Carbon Fiber Laminate (CFL) which inventedby the authors’ research group under service fatigue loading. The main research contents andconclusions in the paper are listed as below:1) CFL was pre-tensioned with the level of prestress15%and22%by using owndeveloped anchorage, grip and prestressing bed. The experimental research and theoreticalanalysis on the prestress loss of the prestressed reinforced concrete structures were studied inthis paper. The prestress loss during the reinforced period contained three parts:1) prestressloss caused by the relative slip between CFL and anchorage device;2) prestress loss causedby the elastic compression deformation of concrete;3) prestress loss caused by the stressrelaxation of CFRP. The formula to calculate the prestress loss was proposed based on theexperimental results, which would lay a good foundation for the later study on the prestressloss of the prestressed FRP reinforced concrete structures.2) Through the three point bending fatigue test of RC beams strengthened withprestressed CFL, fatigue mechanism of the strengthened beams and propagation law of mainfatigue crack were explored, and S~N curves of the reinforced beams under cyclic loadingwere obtained in the paper. Experimental results showed that fatigue life of the beamsstrengthened with prestressed CFL increased considerably comparing to those without prestress. The fatigue life of reinforced beam improved more with higher level of prestress. Asemi-empirical for estimating the fatigue life of beams strengthened with prestressed CFL wasput forward according to the experimental and theoretical analysis. It’s convenient to estimatethe fatigue limit of strengthened beams with given level of prestress and load conditions byusing the formula.3) Theoretical and experimental studies were performed to investigate the progression ofprimary fatigue crack of beams strengthened with prestressed CFL. A theoretical equation ofI-type crack stress intensity factor of strengthened beams was proposed and validated byexperimental results. The progression pattern of primary fatigue crack was extensivelyinvestigated based on the experimental results. A semi-emperical formula, in a similar form tothe Paris formula, of crack progression rate in the second progression period of primaryfatigue crack of strengthened beams was further proposed based on the theoretical equationand the experimental data previously mentioned. The proposed simple semi-empiricalformula was believed to estimate the fatigue life of strengthened members with prestressedCFL more accurately than the formulas in the current practice.4) Theoretical analysis and experimental study were performed to investigate theevolution law of dynamic stiffness of beams strengthened with prestressed CFL under fatigueloading. A semi-emperical formula of dynamic stiffness of strengthened beams was proposed,and a theoretical model for predicting fatigue life of strengthened beams was established withdynamic stiffness being the input parameter. Within the practical prestress range studied, theinvestigation indicated that damage quantity was approximately0.70at the stage of fatiguedamage of strengthened beams. The fatigue life prediction of strengthened RC beams basedon this model are reasonbly close to the test results, indicating that the dynamic stiffness wasa valid and effective input parameter of a theoretical model in predicting the fatigue life ofbeams strengthened with prestressed CFL.