Effect of stirrup ratio on response of ultra-high performance concrete beams subjected to low-velocity impact loadings

Ultra-high performance concrete (UHPC) has experienced rapid advancements and is now increasingly being employed in a variety of engineering applications where impact resistance is crucial. Understanding the dynamic behaviour of UHPC structural members with stirrups provides insights into the effectiveness of reinforcement strategies. In this study, reinforced concrete (RC) beams and UHPC beams with different stirrup ratios were designed and assessed through low-velocity drop hammer impact tests. The impact scenario was realised by a freely falling drop hammer weighing 411 kg at different drop heights. The test results implied that elevating the stirrup ratio from 0.0047 to 0.0093 significantly improved the ductility of RC beams, shifting from catastrophic shear failure to severe flexural failure, while an increased stirrup ratio from 0 to 0.0047 marginally decreased the maximum deflection of UHPC beams by 5.3%. To further evaluate the influence resulting from the stirrup ratio on UHPC beams exposed to impact loads, refined numerical modelling was employed utilising the plasticity based constitutive model for UHPC. The stirrup ratio was found to have a limited effect at lower impact energies, whereas under higher impact energies, an augmentation in the stirrup ratio activated more stirrups to form reinforcement cages and thereby enhancing the impact resistance. Furthermore, a simplified semi-empirical approach was proposed to evaluate the maximum impact deflection at the midspan of UHPC beams.