Study On Debonding Failure In CFRP Flexural Strengthened RC Beams

During the recent decade, due to the development of material industry, the application of CFRP (Carbon Fiber Reinforced Polymer) in strengthening RC structures is developed dramatically. CFRP have advantages such as high specific strength, ease of construction, good durability, etc. However, the performance of strengthened members may always be compromised by debonding of CFRP. So the characteristics of debonding failure should be studied thoroughly. The main content and achievement of this paper are summarized as follows:Black-box prediction on the debonding load and failure modes is carried out. On the prediction of debonding load, a genetic algorithm based selective wavelet network ensemble named GAWNSEN is proposed. Experimental results show that GAWNSEN have advantages such as good generalization, stabilization and accuracy. It outperforms popular methods including BP, RBF, GRNN and GASEN. Decision tree is utilized to extract attribute importance and decision rules from the experiment data. Support vector machine (SVM) is adopted to accurate classify failure modes.Based on sine and cosine series and principle of least work, a closed-from solution on shear and normal interfacial stress between FRP and concrete is presented and compared with Roberts'solution. Parametric analysis results are obtained: the most important factor on interface stress is anchorage length. Interface stress decreases with the increase of anchorage length, and is proportional to FRP modulus and FRP thickness.Auto crack propagation in single material and interface of bi-material is programmed. Concrete cover delamination failure near FRP plate end is studied. Crack propagation process is simulated and related fracture parameters are obtained: It is mix-mode fracture when crack propagated along the flexural reinforcement and mode I fracture is dominate; It is mix-mode fracture when crack propagated along the interface and mode II fracture is dominate, but the interface between concrete and adhesive is not the weak link. Based on results of numerical simulation and theory of elastic fracture mechanics, a new analytical model is proposed. the compute formulas are deduced, and calibrated with experimental data. It is found outperforming other models.When bond length is adequate, the concrete form of bond stress-slip is proven to has no influence on the debonding load of direct shear specimen. The debonding load is only affected by mode II fracture energy, modulus, width and thickness of FRP sheet. An experimental study on the bond behavior between FRP and concrete in a double shear test is undertaken. The Electronic Speckle Pattern Interferometry (ESPI) technique was used to measure the deformation field in the bond area. The measured deformation field was used to infer the local bond stress and slip relationship. A total of 44 CFRP plate and CFRP sheet specimens were tested. Through analyzing the test result of these specimens, an empirical bond stress-slip model has been proposed.When adopting the formula of direct shear specimen into beam specimen, the influence of confinement stress and flexural crack should be considered. Through numerical simulation and experimental data calibration, the formula of debond load of mid-span flexural crack induced deboding is established and is compared with other existed formulas.Finally, the design flow chart on CFRP flexural strengthened RC beams considering debonding failure is proposed and some problems to be further solved are presented.