Constitutive Relationship For FRP-confined Concrete And Fracture Process Of FRP-strengthened Concrete Beam

It is well known that a large number of existing structures in practical engineering need rehabilitation or strengthening because of inappropriate design or construction, damage induced by natural disasters, and performance degradation. The urgent need for new and reliable construction systems promotes the development of advanced composite materials. In recent years, external bond of concrete with fiber-reinforced polymer (FRP) composites has served as a popular strengthening method for beam and column, which can significantly enhance the load bearing capacity and ductility. These materials have many advantages, such as light weight, high strength-to-weight ratio, and good corrosion resistance. There is a large amount of experimental and theoretical work on FRP-retrofitting technique over the world. On the base of previous studies, this thesis focuses on the stress-strain relationship of FRP-confined concrete and the fracture and debonding process of FRP-strengthened concrete beam. The following conclusions can be obtained:(1) A modified stress-strain model for FRP-confined concrete is developed. In this method, a compressive strength model is established based on Jefferson’s failure surface. With the proposed strength model, the strength of FRP-confined concrete can be estimated more precisely. The axial strain at the peak stress is then evaluated using a damage-based formula. Finally, a modified stress-strain relationship is derived based on Lam and Teng’s model. Based on the numerical results, it has been found that Rousakis and Karabinis’s model, Wu and Zhou’s model and the proposed model yield the best results for the compressive strength and that Wu et al.’s model, Teng et al.’s model, and proposed model yield the best results for the axial strain at the peak stress. It has also been found that the proposed stress-strain relationship is fully capable of capturing the compressive behavior of FRP-confined concrete. It can be concluded that, as a competitive alternative, the proposed method can be used to predict the compressive behavior of FRP-confined concrete with reasonable accuracy.(2) By utilizing the compressive fracture energy, an analysis-based stress-strain model of FRP confined concrete is developed. In this model, the compressive fracture energy-based active confined concrete stress-strain model developed by Suzuki et al, and Teng et al’s axial-lateral relationship are combined to consider the passive effect of FRP. Comparisons with experimental results reveal that the proposed model can predict the behavior of FRP confined concrete quite well. Meanwhile, the effect of specimen length on stress-strain relationship of three types of FRP-confined concrete is studied. Numerical results reveal that, for strong type FRP-confined concrete, the specimen length has basically no influence on the stress-strain relationship. However, for weak type FRP-confined concrete, the specimen length should be considered in the modeling of stress-strain relationship.(3) A fracture mechanics approach is presented to model the cracking behavior of and interfacial debonding process in FRP-strengthened concrete beams. The K-superposition method and weight function method are adopted to derive the global control equation and the crack opening displacement compatiable equation. After the validity of the proposed approach is verified with experimental results obtained from the literature, the effects of various factors on the load-bearing capacity and crack growth resistance of FRP-reinforced concrete beams are quantitatively evaluated. It is found that the first peak load increases as the beam height and compressive strength of concrete increase or the ratio of the initial crack length to beam height decreases, whereas the second peak load is an increasing function of the FRP sheet thickness and width. It is also found that the crack growth resistance increases with an increase in ratio of the initial crack length to beam height and FRP sheet thickness and width but is seldom influenced by the beam height and compressive strength of concrete.