Effect of Slenderness on Hybrid FRP-Strengthened RC Columns under Eccentric Compression: An Experimental and Analytical Study

RC columns are commonly strengthened using external bonding (EB) of carbon fiber–reinforced polymer (CFRP) materials. The confinement effect provided by the EB of CFRP fabric is highly effective for short columns under axial compression. However, the effectiveness of the EB confinement reduces drastically for slender columns particularly under eccentric compression. Bending demand increases due to lateral deformations from second-order effects when slender columns deform under eccentric compression. The near-surface mounting (NSM) of CFRP laminates is ineffective under axial compression because NSM does not increase axial rigidity compared with flexural rigidity. However, NSM strengthening is highly effective in resisting the bending dominant loads, acting as an additional tension reinforcement. Hybrid (HYB) fiber–reinforced polymer (FRP) strengthening combines the advantages of EB confinement and improved flexural rigidity from NSM laminates. Control and FRP-strengthened columns with different techniques, such as EB, NSM, and HYB, are tested at eccentricity ratios of 0.42 and 0.85. Columns of different lengths are tested to understand the slenderness effect. The test results are compared with analytical predictions, considering material and geometric nonlinearities. The results indicate that the efficiency of EB confinement reduces with an increase in slenderness and eccentricity ratio. NSM laminate strengthening is ineffective for strength improvement of short columns but effective for slender columns. Hybrid FRP strengthening resulted in the best performance improvement for both short and slender columns under eccentric compression.