Experimental Investigation of the Fatigue Behavior of RC Beams Strengthened with CFRP Grid–PCM Composite After Freeze–Thaw Cycles

To investigate the effects of freeze–thaw cycles on the fatigue performance of reinforced concrete beams strengthened with carbon fiber reinforced polymer (CFRP) grid-polymer modified cement mortar (PCM) composites, this study conducted experimental research under combined freeze–thaw and fatigue loading on beams with two reinforcement ratios (0.84% and 1.31%). The evolution of failure modes, variations in fatigue life, accumulation of residual deformation, and the development of strains in various materials were analyzed. Experimental results show that CFRP grid–PCM strengthening can significantly improve the fatigue performance of beams. The fatigue life of beams with a low reinforcement ratio increased by approximately 275% after strengthening; even after undergoing freeze–thaw cycles, beams with a high reinforcement ratio could withstand over 3 million fatigue load cycles, demonstrating excellent long-term fatigue resistance. Under combined freeze–thaw and fatigue loading, the crack development in strengthened beams exhibited a typical three-stage characteristic, and the failure mode transitioned from fatigue fracture of steel reinforcement to a composite form involving fiber pull-out of the CFRP grid or interfacial debonding. Based on experimental data, a cumulative evolution model considering the synergistic damage of concrete, CFRP grid, and interfacial bonding was established, which effectively describes the stiffness degradation and damage accumulation process under combined freeze–thaw and fatigue action. The research findings provide a theoretical basis for the fatigue performance evaluation and life prediction of CFRP grid-strengthened RC structures in cold regions.