Investigation of macroscopic and microscopic void characteristics' effects on flame-retardant performance of porous asphalt mixtures for tunnel pavement

Given the severity of tunnel fires and limitations of flame-retardant modification technologies, porous asphalt mixtures (PAMs) have emerged as a promising solution due to their potential to mitigate large-volume flammable liquid fires and possible synergistic effects with flame-retardant modifiers. Therefore, this study investigated the relationship between the macroscopic gradation design characteristics, microscopic void characteristics, and flame-retardant performance of PAMs through computed tomography scanning, simulated combustion tests, and statistical analyses. Results revealed strong linear relationships between macroscopic gradation design and microscopic void structural parameters, with the designed void ratio (DVR) having the greatest impact, followed by nominal maximum aggregate size (NMAS) and gradation thickness. Simulated combustion tests showed that the DVR (15–27 %) significantly impacted the flame-retardant performance with maximum variations of 29.9–259.7 %, while transitioning to coarser gradations or increasing NMASs enhanced performance by 8.56–17.86 % and 11.43–43.04 %, respectively. Additionally, microscopic void characteristics strongly correlated with flame-retardant performance (correlation coefficients: 0.756–0.901), where the number of voids (N) positively regulated burning time but negatively influenced the percentage of petrol escape, while the mean equivalent diameter (MED) and fractal dimension of voids (FD) exhibited inverse trends. Correlation and stepwise regression analyses further confirmed N as the key factor influencing flame-retardant performance, followed sequentially by void ratio, FD, MED, and NMAS. It suggests that microscopic void structural parameters may better characterize flame-retardant performance compared to DVR alone. These findings provide valuable insights for designing and optimizing flame-retardant PAMs, advancing sustainable and high-resilient tunnel pavement.