Nonlinear Micromechanics-Based Finite Element Analysis of FRP-Rapped Concrete Columns Subjected to Axial Load

Abstract

In this research work, new modifications for the microplane constitutive law are first presented to precisely simulate concrete behaviour under various levels of lateral confinement pressure. A key feature of the suggested formula is to make the microplane theory valid to represent concrete behaviour under both low and high lateral confining pressures. An analysis is then carried out using two different numerical procedures. The first procedure is the use of a three-dimensional finite element analysis with the modified microplane formulations as a user-supplied subroutine into the FE commercial software ADINA to model the concrete behaviour under various stress and strain histories. Here, the sub-objective is to validate the formulated microplane approach that is proposed in the first phase of the research program. In the second analysis; the confinement behaviour is simulated using an in-house code. The code uses the proposed formulations for the microplane theory to represent the concrete characteristics to predict the stress-strain relationships of FRP-wrapped concrete columns up to failure. An accurate equation correlating the axial stiffness of the FRP laminates and the lateral strain of the concrete columns to the confining pressure is incorporated in the in-house code. The two analyses give almost the same predictions, with minor discrepancies due to some numerical aspects. An experimental program consisting of testing thirty eight concrete cylinders under various lateral confinement pressures is carried out to assess the accuracy of the numerical predictions. In the experimental program, the lateral pressure is designed to give almost the same lateral confinement behaviour as that resulting from applying FRP sheets. The numerical predictions are finally compared to experimental results for FRP-wrapped and un-wrapped concrete columns under various levels of lateral pressure. This paper gives an overview for the whole research program with a special emphasis on the theoretical part.