Evaluation of Rheological Properties of Warm Mix Flame-Retardant Asphalt (WMFRA) Binder Suitable for Tunnel Area

This study aimed to systematically evaluate the rheological properties of warm mix flame-retardant asphalt (WMFRA). First, conventional performance tests were conducted on the prepared warm mix rubberized asphalt (WMRA), incorporating different warm mix agents in order to screen out an agent with optimum performance. Subsequently, limestone power (LP), aluminum trihydrate (ATH), OA composed of ATH and organically modified montmorillonite (OMMT), and zinc borate (ZK) were employed in the oxygen index (OI) test of WMFRA to determine the optimal dosage of flame retardants. Finally, a dynamic shear rheometer (DSR) and a bending beam rheometer (BBR) were used to evaluate the rheological properties of WMFRA. The results showed that the R-Type warm mix agent was superior to S-Type in reducing consistency and improving low-temperature cracking resistance but slightly weakened high-temperature stability. The OA composite flame retardant could enhance the OI from 20.16% to 24% at 15wt% dosage, thereby meeting the specified flame-retardant requirement. Furthermore, OA could markedly boost the high-temperature performance of WMFRA, exhibiting significantly higher complex modulus (G*) and rutting factor (G*/sinδ) compared to WMFRA with other flame retardants. In general, all flame retardants reduced the temperature sensitivity of WMFRA, with ZK being the most effective at 12.6%. Regarding low-temperature performance, LP and ATH improved stress relaxation of WMFRA, while ZK and OA impaired this capability. All flame retardants reduced low-temperature flexibility, but the low-temperature behavior was still dominated by the S(t). For fatigue performance, LP and ATH degraded the fatigue performance by advancing the damage time by 958.9 s and 669.7 s, respectively. In contrast, ZK improved fatigue performance by increasing the complex shear modulus, thereby extending the fatigue life (<i>N_f50</i>) by 3.2%. This study provided a theoretical basis for the formulation optimization of WMFRA.