Examination of the Material Found in the Pore Spaces of Two Permeable Pavements

Clogging of pore space is the leading cause of failure of permeable pavements. A study conducted at Villanova University on two permeable pavements, porous asphalt and pervious concrete, endeavored to answer the question of what materials were depositing in the pore spaces. In 2007, an existing parking lot at Villanova University was retrofitted with a 30 by 9 m section of permeable pavement. Half of the section (15 by 9 m) consisted of pervious concrete, whereas the other half was porous asphalt. The $270{\mathrm{m}}^2$ porous pavement area had a drainage area of $730{\mathrm{m}}^2$, which was 100% traditional dense-graded (impermeable) asphalt. A stone infiltration bed underlaid the permeable pavements. Intensive vacuuming was performed using an industrial strength vacuum cleaner to extract the material contained in the pore spaces of the pavement surface layer. The material collected was analyzed to determine the grain size distribution, percentage of organic matter, and concentration of metals (copper, lead, and zinc). Most of the material removed during vacuuming was pieces of the pavements that had raveled. This analysis revealed that very little fines or organics were found in the pore spaces of the pavements, which is attributable to the measures taken on-site to reduce the influx of foreign materials into the pavement surfaces. Metals had sorbed onto the materials removed from the pore spaces; over 20.1 g of copper, 1.8 g of lead, and 18.7 g of zinc were retained because of trapping of sediments in the pores of the permeable pavements. These results indicate that permeable pavement infiltration systems not only reduce the volume of runoff by providing a means of infiltration, but that they improve water quality by removing pollutants at several steps along the water’s path. Pollutants can sorb to the sediments in the pore spaces, the rocks in the infiltration bed, and the natural soils beneath the infiltration bed.