Residual impact resistance of hybrid fiber-reinforced cementitious composites after thermal degradation

This study evaluated the residual mechanical properties and impact resistance of thermally degraded highstrength cementitious composites (HSCC) exposed to temperatures up to 700 degrees C. These conditions reflect the structural requirements for resisting progressive collapse caused by high-energy explosions or terror attacks. HSCC mixtures were developed using polypropylene (PP) fibers alone and a hybrid of PP and smooth steel fibers (SSF). Laboratory tests primarily assessed the effects of fiber type along with factors such as specimen thickness (40, 60, and 80 mm) and compressive strength (100 and 140 MPa) under both elevated temperatures and projectile impact loading. Consistent with earlier studies, hybrid fiber-reinforced HSCC specimens outperformed those with no fibers or only PP fibers. In high-speed projectile impact tests following thermal degradation, the hybrid fiber specimens exhibited superior resistance to scabbing at elevated temperatures, confirming their viability for use under such combined loading conditions. This performance was further validated using the failure prediction models. The degraded HSCC showed a thickness reduction of up to 47 % compared to room temperature concrete with the same compressive strength. It can also reduce thickness by up to 56 % based on tensile strength. This superior performance allows for a significant reduction in structural element thickness while maintaining structural safety. Thus, the findings provide more accurate predictions of hybrid fiberreinforced HSCC resistance strength and valuable insights for optimizing protective structures. The results demonstrate the potential of hybrid HSCC to meet modern structural safety demands and advance resilient infrastructure in smart cities.