Tension Stiffening Effect of Reinforced Ultra-high-Performance Concrete Members in Tension with Thin Cover Thickness

This study experimentally investigates and analyzes the key factors influencing the tension stiffening effect of ultra-high-performance concrete (UHPC) with embedded reinforcement under thin cover conditions, focusing on the effective tensile cross section and bond coefficient. Unlike conventional concrete, which loses tensile load-carrying capacity after cracking, UHPC can sustain tensile forces depending on the steel fiber content; therefore, the steel fiber volume fraction was selected as the first experimental variable. To evaluate the effect of reduced cover thickness, the second variable, the effective tensile cross section was examined by varying the reinforcement diameter and section height. Twelve specimens were fabricated, and material tests, including compressive, direct tensile, and rebar tests, were performed to assess mechanical properties. The results indicated that the tension stiffening effect increased with higher steel fiber content, while specimens with larger cross-sectional areas exhibited enhanced stiffness and reduced crack formation regions. Failure patterns revealed that both maximum crack width and average crack spacing decreased as the steel fiber content increased, due to the fiber bridging effect and the corresponding improvement in tensile strength. Comparison with existing design codes showed that the effective tensile cross section and bond coefficient of UHPC were underestimated despite the thin cover; hence, a predictive equation incorporating the steel fiber volume fraction was proposed. The proposed coefficient and effective tensile cross section improved the accuracy of the design formula in predicting the tension stiffening behavior of reinforced UHPC with thin cover.