Shape Effects on the Behavior of Hybrid FRP–Strengthened Rectangular RC Columns under Axial Compression

Abstract

Confinement with external bonding (EB) of fiber-reinforced polymer (FRP) composites is effective for improving the performance of reinforced concrete (RC) columns under axial compression. However, EB confinement is less effective for sections with a side aspect (width/depth) ratio of more than 2. Thus, a hybrid FRP strengthening technique that combines the advantages of EB and near-surface mounting (NSM) of carbon FRP (CFRP) laminates was explored. Twenty-four rectangular RC columns of different aspect ratios (b/d = 1.0, 2.0, and 3.0) were strengthened and tested under axial compression to investigate the effect of shape on the efficiency of hybrid FRP strengthening. Two specimens each for control, EB confinement, NSM, and hybrid FRP strengthening were tested at each aspect ratio to decouple the interaction effects in hybrid FRP strengthening. A three-dimensional nonlinear finite-element (FE) analysis was conducted using commercially available software. Three-dimensional FE models of different RC columns of varying aspect ratios were created with EB confinement, NSM, and hybrid FRP strengthening and were analyzed. FE models were validated with test results and subsequently used for parametric study. A detailed parametric investigation on the effect of the hybrid combination of a varying NSM ratio (0.4%–1.6%) and a constant EB confinement ratio was studied. Both experimental and FE results indicated that the efficiency of confinement reduces with an increase in the aspect ratio. The hybrid FRP technique was effective in improving both the axial strength and ductility of columns with large aspect ratios.