Development of environmentally friendly flame retardant to achieve low flammability for asphalt binder used in tunnel pavements

Abstract

This paper reports the development of an environmentally friendly flame retardant, referred to as FRC-Si, to improve the fire resistance of asphalt binder without compromising mixture performance. This flame-retardant composite (FRC) contains four halogen-free flame-retardant components: expandable graphite (EG), magnesium hydroxide (MH), calcium hydroxide (CH), and ammonium polyphosphate (APP). These four components have different ranges of thermal reaction temperature so they can effectively absorb heat throughout the entire process of binder combustion. An orthogonal experiment with six binder properties as the influential factors was designed for this study. Then, a multi-criteria decision analysis method was employed to find the optimal proportion of the combined EG, MH, CH, and APP to be 5:5:4:9, respectively. A silane coupling agent was added to modify the surface properties and improve stability of the asphalt binder, thereby transforming the preliminary FRC to FRC-Si. Based on activation index values and oil absorption of the binder, the optimum content of the silane coupling agent was determined to be 1.2% by mass of FRC. In addition, the rheological properties and flammability of styrene-butadiene-styrene (SBS) binder with various dosages of FRC-Si were obtained and the optimum content of FRC-Si was determined to be 8% by mass of binder. Finally, cone calorimeter tests were performed to evaluate the flame retardancy of asphalt mixtures with SBS, FRC, and FRC-Si binders. Results indicate that FRC-Si not only reduced the amounts of heat and carbon monoxide (CO) released, but also delayed the release of CO, thereby supporting the application of FRC-Si as a flame retardant for asphalt pavements in tunnels.