Study on dynamic mechanical properties and damage evolution of steel-polypropylene hybrid fiber reinforced concrete under repeated impact

Concrete structures are frequently subjected to repeated or occasional multiple impact loads during service, and the proper incorporation of hybrid fibers can effectively enhance their impact resistance. In this study, the compressive properties of steel-polypropylene hybrid fiber-reinforced concrete (SPFRC) under multiple impacts were investigated using an electromagnetically driven Split Hopkinson Pressure Bar (SHPB) device. By analyzing the failure modes, stress-strain curves, and damage levels of the specimens, we evaluated the effects of different design parameters on impact resistance. The results demonstrate that as the number of impacts increases, the damage to concrete specimens intensifies, with peak stress decreasing and peak strain increasing progressively. Additionally, the stress reduction coefficient at failure is lower than 0.35, while the strain increase coefficient is higher than 2.0. For the same number of impacts, higher fiber content results in greater dynamic peak stress and strain, and hybrid fibers outperform single fibers. Under multiple impacts, the impact toughness of specimens first increases and then decreases. After more than three impacts, the impact toughness of specimens enters an attenuation stage; in this stage, hybrid fiber specimens exhibit higher peak toughness and more gradual attenuation. As the number of impacts increases, the ultrasonic velocity of specimens decreases gradually, while the damage variable rises. Increasing fiber content helps mitigate wave velocity reduction and damage development, with hybrid fibers achieving the optimal effect.