Concern has been growing in recent years toward reducing the pollutants in water supplies and the environment. In the pavement@ pervious??a pavement comprising material with sufficient continuous voids to allow water to pass from the surface to the underlying layers. percolation rate??the rate@ usually expressed as inches per hour or inches per day@ at which water moves through pervious concrete. porosity??the volume of open and connected interstitial void space in pervious concrete. raveling??the wearing away of the concrete surface caused by the dislodging of aggregate particles. runoff??water from rain or snow that is not absorbed into the ground but instead flows over less pervious surfaces into streams and rivers. surface course??the top layer of a concrete pavement structure. void content??the ratio of the volume of voids@ including both entrapped and entrained air@ to the total volume expressed as a percentage.