Effect of Composite Flame Retardants on Combustion Behavior and Pavement Performance of Tunnel SBS Asphalt

This study investigates the development of three composite flame retardants based on a catalytic charring mechanism to enhance the flame resistance and smoke suppression properties of styrene-butadiene-styrene (SBS) asphalt for tunnel applications. A synergistic flame-retardant formulation, comprising a biobased flame retardant, ammonium polyphosphate, zinc borate, and a titanate coupling agent, was designed to optimize condensed-phase interactions. An orthogonal design approach, guided by limiting oxygen index (LOI) analysis, was employed to determine the optimal proportions of these components. The flame-retardant mechanism was further elucidated through scanning electron microscopy characterization of fire residues after cone calorimeter testing. The results show that the maximum LOI of biobased composite flame-retardant asphalt is increased to 29.3% with the addition of 10% by weight composite flame retardant. This flame-retardancy enhancement is attributed to the catalytic carbonation of transition metal compounds and better protection of the formed char layer in the condensed phase as well as to the release of water from magnesium hydroxide that cools the gas-phase combustion zone. Subsequently, the thermal stability, storage stability, and rheological properties of the modified asphalts containing various composite flame retardants are evaluated. The addition of a composite flame-retardant decreases the penetration and ductility of SBS asphalt and increases its softening point and high-temperature stability while significantly enhancing its elasticity and antirutting properties. The synthesized biobased synergistic flame retardants exhibit good application prospects in tunnel flame-retardant asphalt pavements.