Effect of Curing Age and Fiber Dosage on High Strain Rate Compressive Behavior of Ultrahigh-Performance Concrete

Ultrahigh-performance concrete (UHPC) is increasingly used in critical infrastructure exposed to dynamic loads due to its exceptional strength, toughness, and durability. While its mechanical behavior under static and high-strain-rate conditions after full curing is well studied, to our best knowledge, limited attention has been paid to its early-age performance, despite its frequent application in bridges and military structures. This study investigates the dynamic compressive behavior of UHPC incorporating 0%, 1%, and 2% (all expressed as volume percentage) steel fibers at different curing ages (1 to 56 days) under strain rates ranging from 40 to 170 s − 1 using a split Hopkinson pressure bar. Key mechanical parameters were evaluated, including compressive strength, peak strain, elastic modulus, and energy absorption. Results show that strain rate sensitivity decreases with curing age but improves significantly with increased fiber content. Higher internal moisture and lower stiffness lead to enhanced strain rate effects at early ages. A new empirical model for the dynamic increase factor, incorporating age, fiber dosage, and strain rate, is proposed and validated, demonstrating improved predictive capability compared to existing models. Fractal dimension (FD) analysis of postfailure surfaces revealed increased FD values with higher strain rates and fiber volumes. This corresponds to finer fragmentation patterns, indicating enhanced crack branching and energy dissipation. The trend reflects improved dynamic toughness due to fiber reinforcement under impact loading. These findings provide critical insights for the early-age design of UHPC elements in high-impact applications.