Laboratory characterization of directional dependence of permeability for porous asphalt mixtures

Abstract

Water permeability is an important property for porous asphalt mixtures. Previous numerical modeling showed that the permeability of the porous asphalt mixtures varies in different directions and a single permeability cannot accurately evaluate the mixture's directional permeability. To investigate the direction-dependent water permeability of the porous asphalt mixtures, a unidirectional permeameter was used to measure the permeability in twelve directions in the vertical plane (parallel to compaction direction) and twelve directions in the horizontal plane (perpendicular to the compaction direction) on two open graded friction course (OGFC) mixtures with different nominal maximum aggregate sizes, i.e., OGFC-13 and OGFC-10. Furthermore, a new multidirectional permeameter was designed which can control the rainfall intensity and adjust transverse slope to simulate the actual water flow process in pavement. The multidirectional permeability and void saturation of eight porous asphalt mixtures were determined by the multidirectional permeameter. Results show that the porous asphalt mixtures demonstrate direction-dependent permeability properties in both vertical and horizontal planes, whereas the dependence is less in the horizontal plane than that in the vertical plane. In the vertical plane, the minimum permeability occurs in the vertical direction and the maximum value occurs in the horizontal direction. In the horizontal plane, the permeability differs in different directions, but has no obvious relationship with directions. Increasing the air void content and the nominal maximum aggregate size of the mixtures can reduce the directional difference of the permeability. The void inside porous mixture cannot be entirely occupied by water when surface runoff occurs. Increasing the air void content and aggregate particle size can lead to an increase of the permeability and the void saturation in the porous asphalt mixtures.