High-viscosity modified asphalt mixtures for double-layer porous asphalt pavement: Design optimization and evaluation metrics

•Two types of high-viscosity modified asphalt mixtures with different air voids are designed for the double-layer porous asphalt pavement.•High-viscosity asphalt is formulated by incorporating high-viscosity modifier into base and rubber-modified asphalt.•Orthogonal design method facilitates determination of aggregate gradations.•The performance metrics of drainage, high- and low-temperature performance as well as moisture susceptibility is evaluated on porous asphalt mixtures. Porous asphalt mixture has been extensively exploited as a surfacing layer on highways because it contributes to controlling pavement runoff, enhancing driving safety, mitigating urban-heat island, and reducing tire-pavement noise. This study proposes a methodology to design two types of high-viscosity modified porous asphalt mixtures with target air voids of 20% and 22% (PAC-10 and PAC-16) for the double-layer porous asphalt pavement, involving in fabricating high-viscosity asphalt, formulating aggregate gradations, and optimizing asphalt-aggregate ratio. Firstly, high-viscosity asphalt was manufactured by incorporating high-viscosity modifier into the base asphalt by 12% and 15% (Base-12, Base-15) and crumb rubber-modified asphalt by 8% (CR-8). The physical properties, 60 °C dynamic viscosity, rotational viscosity and viscoelastic properties of high-viscosity asphalt were characterized. Next, based on orthogonal design method, two aggregate gradations were optimized for PAC-10 and PAC-16 mixtures. Furthermore, the Draindown and Cantabro tests were carried to determine the optimum asphalt-aggregate ratios. Finally, the performance metrics of air void content, permeability coefficient, high-temperature and low-temperature performance, and moisture susceptibility was evaluated on PAC-10 and PAC-16 mixtures. The results show that with the addition of high-viscosity modifier into asphalt binder, the softening point, ductility, 60 °C dynamic viscosity, rotational viscosity, complex shear modulus, phase angle, and rutting factor of the binder are increased while the penetration is decreased. The formula of voids volume of the mixture as a function of aggregate gradation is developed, which facilitates the determination of aggregate gradation. Furthermore, the optimum asphalt-aggregate ratio is found to be 4.7% for PAC-10 mixture and 4.1–4.4% for PAC-16 mixture. Both PAC-16 and PAC-10 mixtures with CR-8 are recommended to construct the double-layer porous asphalt pavement.