Studies On Flexural Behavior And Debonding Mechanism Of RC Beams FRP-Strengthened At Different Preloaded States

A number of failure modes in reinforced concrete (RC) members strengthened with fiber reinforced polymer (FRP) laminates are directly caused by debonding of the FRP from the concrete, therefore, the performance of the FRP-concrete interface in providing an effective stress transfer is of crucial importance. Almost all existing analytical models for debonding failures are erected without preload and there are few investigations on debonding mechanism of beams strengthened with FRP laminates at different preloaded states.In this thesis, a systematic investigation is carried out on flexural behavior and debonding mechanism of RC beams strengthened with FRP laminates at different preloaded states. Based on the theory of concrete structure, the FEM results and the test data of nine six-meter long flexural-strengthened beams, new expressions are derived for lagged strain and characteristic flexural capacity (including cracking moment, yielding moment and ultimate moment). Furthermore, new analytical models of interfacial stress and debonding capacity are established respectively on the basis of a sound understanding of laminate end cover debonding and critical flexural crack debonding. The proposed models represent substantial improvements to existing models as verified by compared to existing test data.The major contributions of the work in this thesis are listed as follows:(1) The test results of nine FRP-strengthened beams indicate that the nominal strain (namely the total of actual strain and lagged strain) of FRP and the average strain of steel and concrete accord with the linear law. It is also shown that the interfacial stress concentration can be effectively reduced with U-type FRP sheets at the laminate-ends. And the main reason of critical flexural crack debonding is that the stress of FRP increases rapidly after the tensile bars yield.(2) The errors of existing procedures fluctuate substantially. Further study on lagged strain is carded out, in which the effects of different preloaded states and nonlinear stress-strain relationship of concrete are fully considered. The nonlinear numerical results from self-edited programme by C language are verified and combined with available experimental results to establish the design methods of characteristic flexural capacity for beams FRP-strengthened at different preloaded states. Next, the influence ofpreloaded states on the capacity is also studied.(3) Analytical model of interfacial stress is proposed for the uncracked FRP-strengthened beam based on the shear-slip deformation of adhesive. Effects of properties of concrete, FRP and adhesive are all considered in this approach. Then a parametric study is performed to investigate the influence of design parameters such as elastic modulus and thickness of FRP and adhesive, the distance from the laminate end to the nearer support on interfacial stress concentration near FRP ends. New formulae of interfacial stress near laminate end and between adjacent critical cracks are deduced for RC beams FRP- strengthened at different preloaded states respectively, and then the detailed expressions are listed under different load cases. A theoretical derivation is also made to investigate the interfacial stress at releasing of the pretension in beams strengthened with prestressed FRP and under combination of prestressed stress and external loads. Comparison of analytical predictions with test results and FEM results indicates satisfactory accuracy of the procedures.(4) Existing models for debonding failures are only set up without preload. Based on the detailed understanding of the debonding failure mechanism and the effect of preloaded state on the interfacial stress, new design models respectively for laminate end cover debonding and critical flexural crack debonding are put forward. The predictions of the proposed models are-shown to be in close agreement with the test data, and especially useful in practice.(5) The criterion is proposed for RC beams FRP-strengthened at different preloaded states to predict failure models. For the safe and economic design of externally bonded FRP system, some measures such as the choice of materials, anchorage length of FRP, an'angement of anchoring system and quality control of processing are proposed to avoid the debonding failure.