Strengthening of eccentrically loaded RC columns using NSM steel and GFRP bars confined with CFRP sheets

In this paper, an experimental investigation has been conducted on the behavior and failure modes of RC columns strengthened using near surface mounted (NSM) steel and glass fiber-reinforced polymer (GFRP) bars. Thirteen RC column specimens were considered in the experimental tests with dimensions of 150 × 150 × 1200 mm (width × depth × height). Three of the RC column specimens are unstrengthened and considered as reference columns, while the other specimens were strengthened by NSM steel and GFRP bars. However, two of the NSM strengthened column specimens were wrapped by carbon fiber reinforced polymers (CFRP) sheet. All column specimens were tested up to failure under concentric and eccentric loads at eccentricity distances of 37.5 mm and 75 mm. The experimental tests considered many parameters, including: material of NSM bars (steel or hybrid steel and GFRP), number of NSM bars, type of adhesive material, confinement effects using CFRP sheets, and the eccentricity ratio. Experimental results in terms of crack patterns, failure modes, and load–displacement curves were all monitored, recorded, and evaluated. The experimental test results have demonstrated an improvement in the ultimate load capacity of all NSM strengthened column, especially when the strengthened column is wrapped by CFRP sheets. Results have also revealed that, under concentric loads, no considerable improvement in the strength of NSM strengthened RC column specimens can be achieved. However, the ultimate load capacity of eccentrically loaded NSM strengthened columns can be increased up to 79.5% compared to that of the control column. Finally, using adhesive material consisting of 50% epoxy and 50% cement mortar as bonding agent, decreases the ultimate load capacity of the strengthened columns by about 35.6% compared with that using 100% epoxy. © 2023, The Author(s), under exclusive licence to Springer Nature Switzerland AG.