A test on fire-induced damage of concrete with multiple parameters analysis—Based on tunnel linings of China

Fires in highway tunnels will have a significant impact on the structural performance of lining materials. A ablation test was conducted on 288 concrete specimens (1500 ×1500 ×1500 mm) with four commonly used grades of C20, C25, C30, and C35 at temperature levels of 300 ℃, 600 ℃, 750 ℃, and 900 ℃ for constant periods of 1 h, 2 h, and 4 h. The experiment revealed that: (1) From low to high temperatures, the specimens experienced cracking, spalling, explosion and eventually complete collapse; all grades of concrete remained intact or mostly intact at 600 ℃ but suffered varying degrees of damage above 750 ℃. (2) At the same fire temperature, the damage rate increased linearly with longer ablation time; while at the same ablation time, the damage rate increased quadratically with higher fire temperature. (3) The ultrasonic wave velocity ratio decreased exponentially as the ablation time increased; higher fire temperatures resulted in lower wave velocity ratios within the same ablation time period and there was a turning point at a two-hour ablation time where specimen integrity was lost. (4) For concrete specimens of the same grade and under identical exposure times to fire, as the fire temperature increased,the rate of compressive strength reduction accelerated significantly; sudden drops in compressive strength occurred at temperatures of 600 °C，750 °C，and900°C,and there was a turning point for compressive strength reduction loss after one hour's ablation to fire.(5) When subjected to high ablation temperatures,the decrease rate in concrete strength exceeded that in wave velocity,and changes in strength were more sensitive than changes in wave velocity.