State of the Art: Permeability of Asphalt Concrete

Abstract

The findings of an extensive literature review on the permeability of hot-mix asphalt concrete are detailed in a state-of-the-art report on the measurement and interpretation of asphalt concrete permeability data. The permeability of asphalt concrete is affected by a range of factors with various levels of importance, which are reviewed along with their impact on the coefficient of permeability. Many theoretical, empirical, semiempirical, and numerical models have been developed to predict permeability, using a range of indicators. Some of these models are reviewed and their advantages and shortcomings discussed. Recent advances in X-ray tomography studies are also summarized. The review reveals that field permeability measurements are not reported to match numerically well with laboratory measurements, though there is some correlation. The reviewed test methods for permeability all rely on an assumption of laminar flow that is unlikely in more porous mixtures. Attempts to measure connected air voids improve the chances of obtaining more meaningful correlations between permeability and air voids, regardless of the mathematical model used to link the two quantities. The lift-thickness to nominal maximum aggregate size (NMAS) ratio and/or changes in the binder content has a less significant effect on permeability than changes in the porosity and/or mix gradation. Air void gradients and distributions in compacted asphalt concrete mixtures can now be assessed with X-ray techniques: the distribution of air voids appears to be nonuniform in laboratory-prepared specimens.