Preparation and characterization of low flammable asphalt for tunnel pavements

Abstract

In this study, a silane modified flame retardant blend named FRB-SC was developed to combat combustion risks faced in tunnel asphalts. FRB-SC is a composite flame retardant principally made up of four components: Expandable Graphite (EG), Magnesium Hydroxide (MH), Calcium Hydroxide (CH) and Ammonium Polyphosphate (APP). The orthogonal design and the technique for order of preference by similarity to ideal solution were used to determine the optimal dosages of these four components. KH550 silane coupling agent was then added to modify the composite flame retardant and improve its functionality and stability in asphalt. Subsequently, the basic performance, rheological properties and flame retardancy of modified asphalts containing various dosages of FRB-SC (i.e., 2, 4, 6, 8, 10 and 12%) were evaluated, and thus the optimum dosage of FRB-SC was determined. The results showed that the asphalt's limiting oxygen index increased to 24.7% with the addition of 8% FRB-SC. Also, after the addition of FRB-SC, the asphalt's penetration and ductility decreased, its softening point increased, its high temperature stability increased, and its low temperature anti-cracking performance decreased. Meanwhile, based on the oil absorption and activation index of the FRB-SC, the optimal dosages of EG, MH, CH and APP were found to be 5%, 5%, 4%, and 9% by mass, respectively, and that of the silane coupling agent determined as 1.2%. Finally, the cone calorimeter test conducted on FRB-SC modified asphalt showed that carbon monoxide emissions were significantly reduced when combustion occurred, and the amount of heat and smoke released were also noticeably decreased, establishing the flame retardant efficacy of the FRB-SC. Now, although this study’s findings have not been applied in actual tunnel mixtures, this however provides an insight to the feasibility of its use for tunnel asphalt pavements, a move which could significantly reduce the hazards of tunnel fires and accidents.