Nonlinear Finite Element Analysis of FRP-reinforced Concrete Beams under Ambient and Elevated Temperatures

摘要

Fibre reinforced polymers (FRPs) have been widely used in reinforced concrete beams as a substitute for traditional steel reinforcements due to their superior material properties. However, FRP-reinforced concrete beams are not without deficiencies, such as bond-slip behaviour and the vulnerability to high temperatures. Although research on the structural behaviour of FRP-reinforced concrete beams under ambient conditions has been reported, studies on these beams affected by bond-slip and elevated temperatures are inadequate. A series of simple one-dimensional composite beam elements are developed in this thesis to investigate the structural performance of FRP-reinforced concrete beams under both ambient and fire conditions, as well as the effect of bond-slip on the structural behaviour. Timoshenko's beam functions are employed to construct the new elements, giving a unified formulation for slender to moderately deep beams, and the shear-locking problem is avoided naturally. Layered approach is used to characterize nonlinear material properties accurately, and an integrated beam element can be achieved without having to model concrete and reinforcements separately.A two-node beam element is first developed for analysis of laminated beams, and is further extended for analysis of steel/FRP-reinforced concrete beams at ambient conditions, considering both geometric and material nonlinearities. This element is then developed to model steel/FRP-reinforced concrete beams at elevated temperatures. Two-dimensional nonlinear finite element analysis based on heat transfer theory is performed to determine the temperature distribution throughout the cross-section of the beam. Nonlinear finite element analysis procedures for the coupled heat transfer analysis under thermal effects and structural analysis under mechanical loading are established. A new beam element is developed with additional nodal degrees of freedom being introduced to account for bond-slip. No separate bond elements are required by using this element and it is demonstrated to model the bond-slip effect successfully. Experimental studies on the flexural behaviour and bond-slip behaviour of FRP-reinforced concrete beams are also conducted. The proposed elements are validated against reported results and experimental data, and they are shown to be accurate and computationally efficient. Parametric studies are carried out to investigate the influence of various parameters on the structural behaviour.