Gradation Determination of Impermeable Asphalt Mix on Subgrade Surface Layer for Ballastless Track in High-Speed Railway Lines

Based on the general review of the grading characteristics for traditional asphalt mix, this paper summarizes a relevant test on nine groups of asphalt mix, including three nominal maximum sizes (4.75mm, 9.5mm and 13.2mm) and three design grading types (close to the upper, median and lower limit of the grading range recommended by specification). The test results indicate that there is no direct relationship between aggregate nominal maximum size and permeability, while the gradation plays a decisive role. Also, as for surface impermeable asphalt mix, SAMI-10 is the most suitable grading type for anti-water in the subgrade surface layer of ballastless track in high-speed railway lines. Finally, the grading limit of SMAI-10 following the current highway specification is recommended.