Flexural behaviour of thin-walled UHPC members reinforced with unprestressed steel strands

Ultra-high-performance concrete (UHPC) provides lightweight, durable solutions for thin-walled structures in marine environments; however, conventional prestressing introduces considerable challenges in construction and long-term durability. This study proposes a novel composite system using unprestressed steel strands as passive reinforcement, benefiting from the exceptional properties of UHPC to enable the full utilisation of the strand tensile strength without prestressing, thereby simplifying construction. To validate this concept, the flexural performance of six beams and eight slabs was evaluated under four-point bending to investigate the effects of steel fibre content and reinforcement ratio. The results indicate that steel fibre content is the primary determinant of performance at the serviceability limit states (SLS), whilst the reinforcement ratio governs the ultimate limit states (ULS). An increased steel fibre content significantly enhanced crack control, a critical factor for durability in corrosive marine environments. Although higher reinforcement ratios generally reduced material efficiency, over-reinforced members exhibited superior crack control. A near-linear relationship was observed between the reinforcement ratio and peak load; however, excessive reinforcement altered the failure mode from strand rupture to concrete crushing, thereby reducing deformation capacity. Finally, a deflection-based theoretical model was developed that accurately predicts the complete flexural response, with its predictions showing excellent agreement with the experimental observations.