Permeability of Asphalt Mixtures with Bailey and Conventional Aggregate Gradations

Highly permeable asphalt mixtures are prone to moisture damage leading to reduction in pavement service life. This study correlates dense-graded asphalt mixture’s air voids with its permeability. As permeability initiates due to significant interconnectivity of air voids, the relation between total air voids (TAVs) and connected air voids (CAVs) requires a critical review. Permeability of laboratory-compacted asphalt specimens was measured in the laboratory using four different permeameters. Conventional and Baily methods were utilized to develop aggregate gradations for asphalt mixtures. Different testing techniques were also used to ascertain the amount of CAVs, as aggregate gradation affects the airvoids significantly. An equation, based upon Darcy’s law, while assuming equilibrium of inflow/outflow of water to and from asphalt sample was proposed to determine the CAVs in the asphalt mix. The digital images of asphalt specimens were captured, and digital computed tomography (CT) technique was employed using ImageJ software for validation of the CAVs estimated from the relationship. It was observed that air voids, especially CAVs, and type of aggregate gradation have significant effects on permeability of asphalt mixture. The Baily modified mixes exhibited lower permeability. The permeability also increased with an increase in nominal maximum aggregate size. The interconnected pores, determined with the help of digital CT technique, validate the values of air voids estimated from the proposed relationship.