On the representation of stress-strain behaviour of concrete under combined FRP and steel confinement

A large number of stress-strain models have been developed for FRP-confined concrete (FCC) based on axial compression tests on FRP-confined plain concrete cylinders. These models are thus applicable only to concrete in FRP-confined reinforced concrete (RC) columns with a negligible amount of transverse steel reinforcement. Existing test results of FRP-confined RC columns have shown that the stress-strain response of concrete under combined FRP and steel confinement (referred to as FRP-steel-confined concrete or FSCC hereafter) possesses the following characteristics: (1) it consists of two approximately linear portions connected by a curved transition portion; and (2) the transition portion is significantly longer than that for the corresponding FCC. Due to the presence of a long transition portion, the algebraic expressions used in existing stress-strain models for FCC may not be suitable for FSCC. This paper examines this issue to identify a suitable form of algebraic expressions for the stress-strain behaviour of FSCC. Four possible options are proposed based on the characteristics of FSCC and then examined against existing test results. All four options are shown to capture the stress-strain response of FSCC reasonably well. One of them, which strikes a good balance between accuracy of prediction and simplicity in form, is recommended for use in design.