Research on Ablation Performance and Evaluation of Flame-Retardant Asphalt Mixtures for Tunnel Pavement

Tunnel fires often cause significant damage to asphalt pavements. Flame-retardant asphalt is used to mitigate this damage, but effective evaluation indicators for assessing the extent of damage are currently lacking. In this study, a series of methods such as ablation, rutting, semicircular bending, and immersion Marshall tests were used to investigate the influence of ammonium polyphosphate (APP) and Ca-Al hydrotalcite (CAHC) composite flame retardants on asphalt mixture performance. Additionally, the study assesses changes in the properties of flame-retardant asphalt mixtures before and after simulated fire. Based on the indoor test results, ablation depth is proposed as a novel index for evaluating the performance degradation of flame-retardant asphalt mixtures post-combustion. The findings indicate that the addition of flame retardants enhances the fire-extinguishing speed of the asphalt mixture by 21.36% and improves high-temperature performance by 25.83%. While water damage resistance stability and fatigue performance show slight improvements, low-temperature cracking resistance is reduced. Compared with ordinary asphalt mixtures, flame-retardant asphalt mixtures demonstrate a 30.30% increase in minimum dynamic modulus at high temperatures and a 17.22% decrease in maximum value at low temperatures, indicating superior temperature adaptability. Despite a decline in post-combustion performance, the flame-retardant asphalt mixture maintains satisfactory pavement performance, with an ablation depth calculated at 34.7 mm. Field application of the flame-retardant asphalt pavement shows good pavement performance. Field sampling and simulated fire tests indicate that the ablation depth, as the thickness of the flame-retardant layer, effectively protects the asphalt pavement, validating the effectiveness of ablation depth as an evaluation metric.