Fatigue Analysis of one-way RC slabs strengthened in flexure with NSM- Hybrid Steel-FRP bars

This research investigates the fatigue performance of reinforced concrete (RC) slabs strengthened in flexure using the near-surface mounted (NSM) technique with hybrid steel–fiber-reinforced polymer (FRP) bars, compared with slabs strengthened using conventional steel and carbon FRP bars. The NSM technique is an efficient strengthening method for RC members subjected to repeated loading. Hybrid steel–FRP bars were developed to combine the corrosion resistance and high strength of FRP with the ductility of steel, thereby improving fatigue performance.An experimental program was conducted on twelve one-way RC slabs tested under static and cyclic loading conditions. The statically tested strengthened slabs included slabs strengthened with NSM carbon FRP bars (FCS), hybrid carbon FRP bars (HCS), hybrid glass FRP bars (HGS), hybrid basalt FRP bars (HBS), and steel bars (SS). Fatigue specimens were subjected to one million load cycles at 50% of their ultimate static capacity. Structural behavior was evaluated in terms of stiffness degradation, cracking, deflection, energy absorption, and residual capacity.The results showed that NSM strengthening significantly enhanced both flexural strength and fatigue performance. Slabs FCS, HCS, HGS, HBS, and SS exhibited capacity increases of 100%, 101.4%, 83.33%, 73.33%, and 70.83%, respectively. In addition, the strengthened slabs maintained high residual load-bearing capacity after fatigue loading, confirming the effectiveness and reliability of the NSM technique for fatigue-critical RC structures.