Ultimate axial strain of FRP-confined concrete

External confinement by the wrapping of FRP sheets (or FRP jacketing) provides a very effective method for the retrofit of reinforced concrete (RC) columns subject to either static or seismic loads. For the reliable and cost-effective design of FRP jackets, an accurate stress-strain model is required for FRP-confined concrete. Central to such a model is the determination of the ultimate condition of FRP-confined concrete which is reached when the FRP ruptures. This ultimate condition is characterized by two parameters: the ultimate axial strain and the corresponding stress level which may or may not be the ultimate strength of the FRP-confined concrete (i.e. the axial compressive strength). This paper presents a new equation for the ultimate axial strain of FRP-confined concrete on which there has been much uncertainty and considerable controversy. Both the stiffness and the actual ultimate condition of the FRP jacket are explicitly accounted for in this new equation. The theoretical basis and performance of the equation are discussed in detail. The paper also suggests confined cylinder tests as a standard type of tests to determine the efficiency factor of a particular FRP product in confinement applications, defined as the ratio between the actual hoop rupture strain of FRP in FRP-confined concrete and the ultimate tensile strain from material tests generally conducted using flat coupons.