Dynamic response of ultra-high performance concrete beams with high strength steel bars under impact load

To systematically investigate the impact resistance of ultra-high performance concrete (UHPC) beams reinforced with high-strength steel bars, this study considers three key parameters: longitudinal reinforcement ratio (0.77%, 1.15%, 1.53%), steel fiber volume fraction (1%, 2%, 3%), and water-binder ratio (0.16, 0.18, 0.20). Drop-weight impact tests (impact mass 580 kg, drop height 1.8 m) combined with ABAQUS finite element numerical simulations were conducted to analyze the failure mode, impact force time-history, displacement time-history, strain development in steel bars and concrete, and energy dissipation capacity. A simplified prediction formula for peak displacement and residual displacement was also proposed based on the principle of energy conservation. The main findings are: (1) Rather than prescribing a fixed minimum reinforcement ratio, the design should treat the longitudinal reinforcement ratio as a ductility-control variable coupled with steel fiber content and water-binder ratio. (2) The optimal steel fiber volume fraction is around 2%; when increased to 3%, fiber agglomeration occurs, leading to a decrease in the secondary peak force by 11.3% and a drop in the energy dissipation ratio from 73.2% to 63.0%. (3) A water-binder ratio of 0.18 provides the best overall impact performance, with the smallest crack width. (4) The synergy between high-strength steel bars and UHPC is significantly better than that of ordinary steel bars, with an increase in platform force of about 25% and a reduction in peak displacement of more than 30%. (5) A higher energy dissipation ratio does not necessarily indicate better impact resistance; it must be evaluated together with the failure mode. Based on these conclusions, engineering design recommendations for impact resistance are proposed, which can serve as a reference for code revisions.