Ignitability and thermal stability of asphalt binders and mastics for flexible pavements in highway tunnels

Due to their chemical composition, asphalt binders and mixtures are quite flammable and tend to produce smoke and poisonous gases while burning. Therefore, fire reaction of asphalt pavements is a topic of increasingly interest to improve fire safety in highway tunnels. Different asphalt binders and mastics containing aluminium hydroxide and magnesium hydroxide as inorganic flame retardants (FRs) were analysed through Thermo-Gravimetric Analysis (TGA) and Limiting Oxygen Index (LOI) test. Due to the lack of technical specifications for LOI measurements of asphaltic materials, a specific sample geometry and preparation as well as a testing procedure were developed and applied. The comparison between the two techniques indicated that both thermal stability and flammability are strictly related to asphalt composition. Moreover, the Thermo-Gravimetric test can be preventively used to select the most appropriate FR filler, whose decomposition temperature (together with particle size) is a factor of primary importance. Among the different flame retardants tested, the aluminium hydroxide showed the most important increase in LOI with respect to the unmodified binders. On the contrary, the SBS-modified binder, probably due to the unsaturated nature of the polymeric modifier, showed an increased flammability when compared with the base asphalt binders.