High-Cycle Fatigue Behavior of SD690 High-Strength Reinforcing Bars in New RC Beams: Effects of Bar Diameter

The increasing use of high-strength reinforcing steel in reinforced concrete structures highlights the need to understand its fatigue behavior under repeated loading. However, experimental evidence on the high-cycle fatigue behavior of New RC beams reinforced with large-diameter SD690 steel bars remains limited, particularly regarding the effects of bar diameter on fatigue life and member-level performance. This study investigated 14 New RC beams subjected to four-point bending fatigue tests using SD690 bars of three diameters (D25, D32, and D36), with SD420W-D25 reinforcement as a reference. Fatigue life, deformation, strain development, crack propagation, stiffness degradation, and fracture characteristics were examined. The results showed that increasing bar diameter reduced fatigue strength and fatigue life under comparable stress conditions. The proposed diameter-adjusted stress–life (S–N) relationship predicted stress ranges of approximately 241, 184, and 169 MPa at 1.25 million cycles for D25, D32, and D36 reinforcement, respectively, corresponding to reductions of approximately 24% and 30% for D32 and D36 relative to D25. Experimentally, specimen D36HFL5 failed after approximately 2.17 million cycles at a stress range of 171 MPa, whereas D36HFL7 failed after approximately 2.68 million cycles at 182 MPa. In contrast, D32HFL3 survived 3.0 million cycles without fatigue fracture, representing a run-out observation. The findings demonstrate the importance of accounting for bar diameter in fatigue-life prediction and provide an experimental basis for improving the fatigue assessment and design of high-strength reinforced concrete members under high-cycle loading.