Impact of UHPC tensile behavior on the negative flexural performance of steel-UHPC composite bridge deck

The negative flexural performance of steel-UHPC composite bridge decks is governed by the post-cracking tensile behavior of UHPC; however, the structural consequences of adopting strain-hardening versus strain-softening UHPC remain insufficiently quantified. This study experimentally and numerically investigates the influence of UHPC tensile strain characteristics, associated with different steel fiber contents, on the flexural response of steel-UHPC composite bridge decks under negative bending. Two full-scale composite deck specimens incorporating strain-hardening UHPC (JNS) and strain-softening UHPC (JES) were tested under static negative bending to evaluate flexural stiffness evolution, UHPC crack development, interface slip behavior, reinforcement strain progression, and cross-sectional strain distribution. Finite element models were developed using ABAQUS and validated against experimental results to support mechanistic interpretation. The results demonstrate that UHPC tensile strain characteristics primarily control post-cracking behavior while exerting limited influence on ultimate load-carrying capacity. The strain-hardening UHPC specimen exhibited distributed cracking and restrained crack width development, with only about 3% reduction in flexural stiffness after cracking. In contrast, the JES specimen showed earlier crack localization and approximately 11% stiffness degradation at comparable crack widths. Under high loading (1650 kN), both specimens developed comparable crack widths (1.12 and 1.16 mm); however, reinforcement in the JES specimen approached the yield strain, whereas reinforcement in the strain-hardening UHPC specimen remained elastic. Overall, the findings reveal that the UHPC tensile strain behavior controls the post-cracking load redistribution, the reinforcement demand, and the serviceability of steel-UHPC composite bridge decks, thus providing a roadmap towards the performance-based design of composite bridge decks.