Development of a compressive constitutive model for FRP-confined concrete elements

One of the most effective methods for seismic performance improvement of reinforced concrete (RC) elements is the external confinement by the wrapping of FRP sheets. As a result of FRP confinement, the constitutive model of concrete in compression, i.e., the stiffness, compressive strength and ultimate strain of concrete, can be completely changed. In this research, the three-dimensional nonlinear finite element program COM3 (developed at the University of Tokyo) has been used for modelling concrete samples confined with various amounts of FRP. The effect of different parameters (stiffness, strength and thickness of the FRP sheet) on the compression stress–strain curves of confined concrete has been investigated. Through a numerical parametric study, the effect of confinement level on the Elasto-Plastic and Fracture (EPF) model (plastic strain and fracture parameter) is examined and used to develop equations for a constitutive model. In the development of these equations, the effect of clear distance between FRP straps (incomplete confinement) and the twin effect of FRP and lateral reinforcement (stirrups) in the concrete core confinement are also considered. In order to take into account the elastic-fracture behaviour of FRP in the nonlinear analysis, some modifications were made in the COM3 program. The linear-elastic model was assumed in the FRP element for two local perpendicular axes which can have an angle with the global coordinates. After modelling of the FRP-confined concrete specimens and verification of the results with existing experimental samples, effective parameters in the amount of FRP confinement were determined through sensitivity analysis. The effect of confinement level on the plastic strain and fracture parameter was obtained through a parametric study of these parameters. The FRP rupture in the mean compressive stress–strain curve was foreseen by means of developing a relation for the circumferential strain of the concrete specimen in terms of axial strain and confinement level.