Using image analysis to measure the porosity distribution of a porous pavement

A method for determining the distribution of porosity across a porous pavement sample using image analysis was developed and evaluated. Its measured average porosity compares favorably to other porosity methods including two volumetric based methods and ASTM D7063 which uses a vacuum sealing device. The impact of the representative elemental area (REA), which is the minimum pavement cross sectional area that must be analyzed to have a statistically significant measure of the pavement porosity, was investigated. The REA was experimentally determined to be 83.9 cm2 (13 in.2) for the different gradations of porous pavements evaluated, which were ASTM standard gradations No. 89, No. 78, No. 7, and No. 67. Though this method can be used to measure a one dimensional porosity distribution in different directions (vertically, horizontally, or radially in cylinders), this work focused on its application to vertical porosity distributions. Using the results of the image analysis and the REA, a smoothed profile of the porosity distribution was produced which showed that, for a porous pavement sample compacted in a single lift, the porosity is high at the surface, due to the surface texture, decreases drastically to a minimum at approximately 2–3 cm (∼1 in.) in depth and then linearly increases until approximately two centimeters from the bottom of the sample where the porosity increases drastically due to the wall effects. The porosity distribution produced agrees with previously proposed distributions.