Numerical Modeling of Asphalt Mixture Site Permeability (05-0226)
Gordon Airey, University of Nottingham, United Kingdom
Alistair Hunter, Scott Wilson Pavement Engineering Ltd

The permeability of an asphalt pavement is an important mechanical parameter. For the case of a hot mix asphalt (HMA), it is often desirable to limit the permeability to a minimum value in an effort to reduce the damage to the pavement resulting from moisture damage and oxidation of the asphalt binder. Conversely, for a porous asphalt a minimum value for permeability may be specified to allow for surface water drainage. The measurement of permeability may be undertaken either in a laboratory or on site, using either a falling head or constant head arrangement. One of the main inaccuracies in undertaking site permeability testing is the assumption that the water flows directly downwards from one layer to the next. In reality the direction of water flow is determined by the gradient of the potential field which in turn is dependent on the pressure head applied, saturation of the layer, radius of the permeameter and permeability of the layer. This paper quantifies the inaccuracy involved in assuming direct vertical flow, utilizing numerical techniques to model the flow, and provides correction factors. It also presents a procedure enabling permeability to be determined from a site permeability test for a layer of infinite depth, suitable for the sub-grade or a thick asphalt layer. The hydrostatic potential plots and resulting flow paths which can be expected from typical scenarios are investigated. The aim of the paper is to improve the understanding of the basic flow mechanism involved in site permeability testing and the correlation between laboratory and site results.