Granulometric and Pore-Based Index Properties of Porous Asphalt Mixes

When properly designed for urban source areas such as roadways, porous asphalt functions to provide hydrologic restoration while functioning as a filtration interface. This study examined the role of aggregate granulometry on the pore-based indices of porous asphalt, specifically permeability (hydraulic conductivity), void ratio and porosity. Three different groups of porous asphalt gradations comprised of 12 series of samples and separated based on gradation size indices have been examined. Two groups represent common mix designs in Italy while the third group represents micro-draining mixes which are less common. For all samples the vertical (kv) and horizontal (kh) permeability were measured with the commensurate total void contents (e) and total porosity (n). Marshall tests were performed in order to control the stability of the mixes. The directional permeability values were strongly correlated with kh as approximately 60% of kv across the range of permeability tested. There was a strong relationship between the permeability and the void content, and as the void content increases, the permeability also increases linearly. For a given aggregate gradation, each increment of asphalt content increase caused a decrement in permeability and porosity. Results indicated that in addition to the coefficient of uniformity (U) and the total void content, a size-based granulometric index (P5·P2·Dmax) provided a relationship with vertical permeability and the total void content that were very consistent for the entire range of porous asphalt mixes. Results of this study allow the role of mix design on the permeability to be examined. These mix design results are a necessary precursor to examination of in-situ permeability after placement, and degradation of such permeability as a function of time and rainfall-runoff hydraulic and particulate loadings.