Constitutive model for FRP and TIE–confined concrete

Confining wraps or jackets to rehabilitate and strengthen existing concrete col-umns has proven to be an efficient technique for seismic retrofit of structures. However, most of the compressive strength models of confined concrete only consider the increased strength and ductility provided by fiber reinforced polymers (FRPs), neglecting the contribution of the existing steel reinforcement inside the column’s section. Even if the existing steel stirrups in a reinforced concrete column are not sufficient to confine the concrete core, they must also con-tribute along with the FRP jacket in confining the section. Therefore, the FRP-confined con-crete model proposal by fib has been enhanced to take into account the confining effect of the already existing steel reinforcement when retrofitting a reinforced concrete column with FRP jacketing. To this end, confining pressures contributed at each step of deformation by the case of existing transverse and longitudinal steel reinforcement have been evaluated considering the stress-strain law of the reinforcing steel. Moreover, compatibility of strain in the lateral direction between the jacketing system and the encased concrete is enforced. Finally, the bili-near stress-strain response of FRP-confined concrete is terminated by jacket rupture owing to hoop strains exceeding the strain capacity of the material or to interaction of the jacket with the buckled longitudinal bars. Correlation with three experimental studies gives promising results.