Influence of early-age stiffness and thickness of UHPFRC on fatigue behavior of repaired RC slab

Early loading of repaired reinforced concrete (RC) bridge slabs with Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) has highlighted issues with their sufficiency in resisting traffic loads. UHPFRC thickness also plays a critical role in designing effective repairs. This study employs the finite element method, incorporating the bridging stress degradation concept, to investigate the fatigue behavior of repaired RC slabs, considering both the early-age stiffness and thickness of UHPFRC. The original RC slab is preloaded and subsequently repaired with various UHPFRC thicknesses at two different ages. Results based on crack propagation, displacement evolution, and maximum principal strain distribution demonstrate the superior performance of early-age UHPFRC repairs compared to unrepaired slab and repaired slab with conventional concrete. This improvement is further evidenced by an upward shift in the neutral axis, increased bending moment contribution from the UHPFRC layer, and reduced localized strain. The superiority of early-age UHPFRC repairs over conventional concrete at the same overall thickness is revealed by reductions of 0.81–1.32% and 12.81–16.40% in volume of cracked elements and displacement, respectively. However, slight increases of about 0.4–0.52% in volume of cracked elements and 8.84–9.35% in displacement are observed in early-age UHPFRC repairs compared to 28-day-old UHPFRC repairs. Additionally, increasing the UHPFRC thickness by 10 mm results in slight improvements in fatigue behavior, reflected by decreases of 0.44–0.67% and 4.17–4.51% in volume of cracked elements and displacement, correspondingly.