Advances in flame retardancy of asphalt pavement: A review

Asphalt pavement is widely applied to the surface in high-grade highway tunnels due to its prominent preponderance in road performance. However, asphalt is flammable as the binder material to adhere the aggregates and other additives, resulting that a fire in the semi-closed space of the tunnel can ignite and burn asphalt pavement to generate a large amount of heat and smoke. Therefore, further promoting the advance of flame-retardant asphalt pavement is essential to ensure security in tunnels. We gathered the relevant standards or regulations of diverse nations and test methods concerning flame retardancy of asphalt. Then we reviewed the research status of flame-retardant asphalt mixture, including thermal characteristics of the asphalt and four fractions, the flame retardants applicable to asphalt, and effects on other components. This review demonstrated that establishing universal standards and test methods is a research basis specifically for flame-retardant asphalt pavement. To optimize the flame retardancy of asphalt pavement, it should focus on the synergy with diversified aspects such as asphalt binders, multiple flame retardants, aggregates, mineral powders, fibers, and other additives.