Exploring fracture characteristics of carbon fiber-reinforced concrete under variable amplitude flexural fatigue loading

Concrete structures undergo numerous stress cycles driven by traffic, wind, and vibrations, often in combination. Fatigue from repeated cyclic loading often leads to cracks and, eventually, structural failure. Although fiber reinforcement has been widely investigated, the limited standardized guidance for fracture characterization of variable-amplitude fatigue loading represents a critical gap in current practice. This study investigates the fracture and fatigue behaviour of concrete beams containing 0%, 0.5%, and 1.0% carbon fibers (CF), using a specific experimental protocol for variable amplitude fatigue loading. Flexural performance was evaluated under center point loading, and fatigue behaviour was assessed under cyclic loading, at 1 Hz with a 1 kN load increment every 500 cycles, using a closed-loop servo-hydraulic testing system. The analysis also focuses on crack-mouth-opening displacement (CMOD) and vertical mid-span displacement analyses to understand load-carrying capacity and failure mechanisms. Additionally, a computational framework was applied to the load-CMOD histories to assess fatigue damage. The study showed that 0.5% CF dosage provided the most favourable flexural and fatigue performance for the investigated experimental range. The 0.5% CF mix enhanced the static modulus of rupture (MOR) by 123% and cyclic MOR by 74% relative to control concrete, improving flexural strength and post-crack toughness. The analysis of secant stiffness, hysteresis energy, and damage evolution showed that the mix with 0.5% CF provided the most favourable fatigue response, whereas mix with 1.0% CF exhibited greater post-cracking energy dissipation and fracture resistance. These results provide a guide for fiber dosage selection according to the intended flexural fatigue requirements.