Effects of CFRP U-Wraps on the Behavior of NSM GFRP Retrofitted Reinforced Concrete Beams

The near-surface mounted glass fiber–reinforced polymer (NSM GFRP) technique may encounter problems such as peeling-off and premature failure of NSM GFRP. To tackle these issues, this study explores the effects of carbon fiber–reinforced polymer (CFRP) U-wraps on the behavior of NSM GFRP retrofitted RC beams. Six RC beams were tested. One beam served as the control beam, another was retrofitted with two NSM GFRP bars without U-wraps, and four beams were retrofitted with an identical configuration but incorporated 1–4 pairs of U-wraps at each beam end. The experimental results confirmed the efficiency of the NSM GFRP retrofitting technique. The U-wraps successfully prevented peeling-off failure and forced the GFRP bars to work until rupture. NSM GFRP retrofitting moderately extended the elastic branch of the load–deflection curve in comparison to that of the control beam, increased the yield load by 22.4% and the ultimate load by 32.2%. The U-wraps significantly extended the postyield branch, improving the yield load by 28.6%–30.2% and ultimate load by 50.9%–57.5% compared with the control beam. NSM GFRP retrofitting transitioned the high ductility (7.3) to moderate ductility (2.9). Meanwhile, NSM GFRP retrofitting with 1–4 pairs of U-wraps reduced the ductility to 4.9, 4.4, 4.0 (high ductility), and 3.4 (moderate ductility). NSM GFRP retrofitting and U-wrapping negligibly increased the elastic stiffness, whereas NSM GFRP retrofitting enhanced the postyield stiffness by 380.8%–763.6% compared with the control beam. Finally, sectional analyses were performed and accurately predicted the yield moment capacity of RC beams retrofitted with NSM FRP, which can be useful for structural engineers.