Effect of NSM end conditions and materials on flexural response of the RC beams strengthened with novel hybrid steel-FRP bars

Abstract

Reinforced concrete (RC) beams often require strengthening due to material deterioration, aging, design errors, and natural disasters. The lack of bonding and ductility in fiber-reinforced polymers (FRP) poses challenges for their use in concrete reinforcement. This study aimed to evaluate the effectiveness of hybrid FRP in enhancing the flexural behavior of RC beams. An experimental program consisted of ten samples tested under four-point loading arrangements until failure. The test parameters included the near surface mounted (NSM) material (Steel, GFRP, CFRP, and hybrid bars), the NSM area, and the NSM end conditions (straight/hook) anchorage system. The load capacity, deflection, failure modes, crack patterns, stiffness, toughness, and ductility of the tested RC beams were investigated. The RC beams strengthened with end conditions straight and one bar of steel, GFRP, or CFRP, showed ultimate load increments of 60.60%, 78.70%, and 81.10%, respectively, while the sample hybrid steel-GFRP and hybrid steel-CFRP bars increased 90.60% and 94.3% compared to the control beam (CB). Doubling the NSM hybrid steel-GFRP and hybrid steel-CFRP areas achieved about 101.70% and 106% load capacity increments, respectively for the RC beams over the CB while using the previous NSM bars with end anchorage (bent ends) increased the load capacity increments to 139.40% and 159.60% over the CB, respectively. End anchorage of NSM hybrid bars allowed RC-strengthened beams to attain higher loads and deflection values, delaying failure compared to straight NSM bars. The deformability index of the sample one-straight bar NSM with hybrid glass and carbon bars was 3.21 and 2.17 compared to the samples with glass and carbon bars, 2.26 and 1.39, representing an improvement of 42% and 56.1%, respectively. The stiffness was significantly improved by 149% of the sample with double bars hybrid carbon compared to the control specimen.