Seismic behaviour of shear critical square RC columns strengthened by large rupture strain FRP

This paper presents an experimental study on the seismic behaviour of shear critical square columns strengthened by large rupture strain (LRS) FRP under reversed cyclic lateral load together with constant axial load. The experimental program included 2 control columns, 6 LRS FRP-strengthened columns, and 2 CFRP-strengthened columns. The tested specimens’ failure modes, hysteretic responses, ductility, and cumulative energy dissipation were carefully analyzed. The influences of axial load ratio, FRP layers, and FRP type on the seismic performance of these columns were investigated. Compared with the control column, the ductility ratio and energy dissipation of the column wrapped by 3-layer LRS FRP at a low axial load ratio were raised by 491.7 % and 6498.3 %, respectively. The advantage of LRS FRP over CFRP in seismic retrofitting for shear critical RC columns was demonstrated through comparative analysis for the first time. Although the tensile stiffness of 1-layer PET FRP is much smaller than 1-layer CFRP, the ductility ratio and cumulative energy dissipation of the 1-layer PET FRP-strengthened column under a low axial load ratio were still slightly larger than that of the column wrapped by 1-layer CFRP. The CFRP-strengthened column under a high axial load ratio showed sudden explosive failure, while the LRS FRP-strengthened columns exhibited progressive failure. In addition, the axial flexure shear interaction (ASFI) method considering buckling of longitudinal reinforcement in FRP-confined concrete was developed and verified by comparing test results and predicted envelope curves.