Behavior of CFRP confined RC columns under axial load and uniaxial cyclic lateral loading

In recent years, the use of FRP wrapping of reinforced concrete (RC) members has been recognized as a reliable method for seismic retrofit of existing structures. The external confinement of existing RC columns with FRPs is a sound technique for increasing the columns ductility, by preventing brittle mechanisms such as bars buckling or post-yielding shear failures and by improving the mechanical properties of concrete due to the lateral confining effect. Several models have been developed for predicting the ultimate displacement capacity and the ductility of FRP confined RC columns, however the accuracy of predictions has not been well established yet. In this study, the experimental behavior of a RC column confined at the plastic hinge region with one ply of carbon fibers (CFRP) is firstly analyzed and compared with the response of the unstrengthened control column. The specimens are cantilevers loaded with a low compressive axial load and subjected to uniaxial cyclic horizontal displacements. The responses of the columns have been analyzed and compared in terms of failure modes and strength and deformation capacity. Particular attention has been focused on the strain distribution recorded on the CFRP strips used to confine the RC column end; the strain profiles along the cross-section perimeter in case of axial load and bending moment interaction are presented and discussed at different load levels. In the second part of the study, the deformation capacity of FRP confined RC column is analytically obtained, by using models for FRP confined square columns available in literature. The comparison between analytical and experimental results showed that currently available models give conservative predictions of ultimate deformation capacity of confined columns.