Experimental investigation of impact behavior of fiber-reinforced lightweight concrete after exposure to elevated temperatures

Post-fire structural members are frequently subjected to accidental impact actions caused by debris fall, partial collapse, or emergency rescue operations. However, the residual impact resistance of lightweight aggregate concrete (LWAC) floor systems after thermal exposure remains insufficiently understood. This study experimentally investigates the thermo-impact coupling behavior of steel fiber–reinforced lightweight expanded clay aggregate (LECA) concrete slabs subjected to elevated temperature exposure and subsequent repeated low-velocity impact loading. Large-scale slab specimens (70×1000×1000 mm) incorporating three fiber volume fractions (0%, 1.0%, and 1.5%) were exposed to temperatures of 25 °C, 300 °C, and 600 °C, followed by repeated free-fall drop-weight impact tests. The results demonstrate that maximum absorbed impact energy reached 18,404 J, while specimens with 1.5% fiber exhibited up to a 500% increase in impact resistance after exposure to 600 °C. A clear transition in failure mode from brittle cracking to progressive fiber-bridging and ductile damage accumulation was observed with increasing fiber content. Based on a multi-criteria performance evaluation, a fiber dosage of 1.0% is identified as optimal for achieving balanced mechanical performance, ductility, and post-fire impact resistance, whereas 1.5% fiber content is recommended for applications requiring maximum post-fire impact resilience.