Effect of Stress Ratio on the Fatigue Behavior of Ribbed Glass Fiber–Reinforced Polymer Bars for Concrete Reinforcement

The fatigue behavior of glass fiber–reinforced polymer (GFRP) bars, a promising alternative to steel reinforcement in corrosive environments, remains inadequately understood, particularly under varying stress ratios the stress ratio is the minimum stress divided by the maximum stress (R). This study investigates the effect of stress ratios on the fatigue life of ribbed GFRP bars through an extensive experimental program tests were conducted under tension–tension cyclic loading at three stress ratios (0.1, 0.4, and 0.6), with an analysis of failure modes, fatigue life, and performance trends. Bars tested at a stress ratio of 0.6 demonstrated a fatigue life approximately 40 times higher than those at 0.1, revealing the significant influence of the stress ratio on material performance. To complement the bar-level investigation, a parallel fatigue testing program was conducted on full-scale concrete beams reinforced with the same ribbed GFRP bars. These beams were tested under a stress ratio of 0.4 and maximum fatigue stress levels of 30% and 35% of the bar’s ultimate tensile strength. While the beams exhibited flexural cracking and some stiffness degradation during cycling, they retained 87%–100% of their static capacity after 2 million cycles, confirming that the endurance threshold observed in the bar tests also holds in structural applications. A proposed fatigue life equation and updated S-N (fatigue stress – number of cycles to failure) curves were developed, incorporating probabilistic models calibrated using the experimental results to improve predictive accuracy. The results highlight limitations in existing design standards and support the development of improved fatigue stress limits for GFRP-reinforced structures. By addressing the relationship between stress ratio and fatigue performance, this work provides new insights into enhancing the durability and reliability of GFRP-reinforced concrete under cyclic loading, bridging a crucial knowledge gap for structural applications.