External confinement by the wrapping of fiber-reinforced polymer (FRP) composites sheets provides a very effective method for the strengthening and retrofit of round concrete columns. New formulas to estimate the ultimate axial strain and second tangent modulus E2 above the unconfined concrete strength are proposed for concrete columns confined by the FRP jacket in which the fibers are oriented in the hoop direction. These formulas are based on the use of the asymptotic value of the differential Poisson ratio (the first derivative of the lateral - axial strain curve). Comparison with the experimental data available in the published literature is done. The formulas are shown to be able to provide satisfactory estimation of the ultimate axial strain and the second tangent modulus E2 of stress strain relation above unconfined column strengt
