Computational models for FRP-confined concrete and FRP-confined RC columns

Abstract

The use of FRP jackets to strengthen RC columns has become popular in recent years due to the well-known phenomenon that lateral confinement can significantly enhance the strength and the deformation capacity of concrete. However, the related confinement mechanism of concrete, particularly when under non-uniform confinement, is still inadequately understood. This thesis is thus concerned with the development of a deeper understanding of the confinement mechanism of concrete in FRP-confined RC columns. This thesis first presents a series of axial compression tests on FRP-confined high strength concrete cylinders. These tests are an important supplement of the existing test data. Based on these tests, a stress-strain model applicable to both normal strength concrete and high strength concrete under active confinement is proposed. Moreover, an existing analysis-oriented stress-strain model for FRP-confined concrete is shown to be applicable to concretes of different strength grades. This analysis-oriented stress-strain model served as a basis of the subsequent studies on the numerical modelling of FRP-confined concrete and FRP-confined RC columns presented in the thesis. Attention is then shifted to the performance of plasticity models and plastic-damage models in predicting the stress-strain behaviour of confined concrete. In the plasticity models or the plasticity part of the plastic-damage models, two techniques have been utilized to define the plastic deformation process: the scaling technique in which the hardening law is defined as a function of the confining pressure and the plastic volume strain technique in which the plastic volume strain serves as the hardening variable. While both techniques are shown to lead to accurate predictions for actively-confined concrete, they are shown to be incapable of providing accurate predictions for FRP-confined concrete. This is because both approaches cannot accurately simulate the lateral deformation process of FRP-confined concrete. In addition, the thesis also presents a study of the use of Bazant's micro-plane model in predicting the behaviour of confined concrete; an improved version of the M4 model, referred to as the M4+ model, is presented for the numerical modelling of FRP-confined concrete. Several important parameters of the M4+ model were set to be confinement-dependent. The improved model provides accurate predictions for FRP-confined concrete. The next part of the thesis is on the development and application of advanced finite element models for FRP-confined non-circular columns. Two constitutive models, that is, Yu et al.'s plastic-damage model and the M4+ model, were employed in the finite element models to predict the behaviour of FRP-confined square and elliptical columns. Numerical results from the finite element model show favourable agreement with the experimental results. The final part of the thesis presents a three-dimensional finite element model for FRP-confined RC columns based on Yu et al.'s plastic-damage model. For this finite element model, a local analysis-oriented stress-strain model is proposed for adoption to avoid the double counting of end restraint effects. This finite element model is shown to produce accurate predictions of the stress-strain behaviour of transverse steel-confined concrete columns and FRP-confined RC columns.