Analytical model for the effective strain in FRP-wrapped circular RC columns

The available models for predicting the ultimate capacity and stress–strain relationships of confined concrete rely on an assumed FRP ultimate strain value. It is commonly assumed that FRP fails when hoop strain in the jacket reaches its ultimate tensile strain determined according to flat coupon tests. However, experimental results of FRP confined concrete showed that in most cases, FRP experimental ultimate tensile strain is clearly not reached at the rupture of the FRP jacket. The discrepancies may include the shape and conditions of concrete substrate as well as defects or stress concentrations in FRP jackets (in particular, the multiaxial stress state in FRP wrapping due to the transfer of loads through the bond with concrete). The present paper covers only issues related to this latter aspect. Since the average absolute error of all confinement models showed a remarkable decrease when the effective FRP hoop strain (smaller than ultimate flat coupon strain) was inserted in the equations, an analytical model to directly evaluate the FRP strain efficiency factor as the strain ratio between the effective FRP hoop strain at failure and the flat coupon test outcomes has been formulated. Multiaxial failure criteria have been adopted (i.e. 3D Tsai–Hill criterion for FRP) in the model considering axial, circumferential and radial stresses. Results of theoretical analyses and experimental tests (experimental data available in literature) have been compared showing a good agreement.