15-25% void spaces may be achieved, with an average flow rate of around 480 in/hr. In pervious concrete, the fines are greatly reduced or entirely removed. Portland cement concrete (PCC) is typically made with coarse aggregate (gravel), fine aggregate (sand), water, cement, and optional additives. Porous concrete, much like porous asphalt, has the fines removed in order to create voids. The appearance of porous asphalt is much the same as non-porous asphalt. Polymer-reinforcing fibers assist with cohesion of the mix.
Polymers keep the binder from migrating into the void spaces.
(NCDENR, 2007 Hun-Dorris, 2005 )Īdditives and additional binders are often used to enhance the characteristics of porous asphalt. This has been ameliorated with the use of additives and additional binders. There were problems in the past with early porous asphalt, as the binder would migrate into the higher void spaces, blocking the travel path of the water. The binder remains somewhat soft long after pavement is laid, and a little moves into these voids, which is called migration. Asphalt is typically designed with a small amount of air voids, typically 4% of the total mix volume, in order to allow the binder to migrate a little. Large aggregate is also used to raise the void space. This leaves space for water to flow through and collect. Porous asphalt, developed about 1970, greatly resembles non-porous asphalt except the fines (very fine sand and dust) have been removed, leaving additional air voids where the fines would have been. These limitations are due to the fact that in order to allow water to pass through, permeable pavements are not designed with the structural capacity to support heavy loads (Pennsylvania Stormwater Management Manual, 2005 ). Permeable pavements excel in the following areas: (CRWA, 2008 )įrom left to right: regular HMA, regular PCC, permeable PCC An uncompacted soil base is highly recommended, and construction practices which emphasize this are critical for groundwater recharge. (Hun-Dorris, 2005 )ĭesign of permeable structures generally include a permeable surface such as asphalt or portland cement concrete over a base of fines, which help to filter the water, and uniformly graded gravel, which stores the water as it infiltrates through the ground below the structure. This is especially beneficial in cities which experience extremely high temperatures in summer – traditional “blacktop” temperatures can make some public spaces unusable in warmer weather. (Hun-Dorris, 2005 )Įvaporation of water at or below the surface also produces surface cooling, as opposed to the traditional heating of paved areas. Pervious surface treatments reduce the risk of stormwater runoff, retaining the water sub-surface as it gradually infiltrates the soil holding the storm water in multiple air voids or cells also assists in pollution control through degradation of hydrocarbons into carbon dioxide and water, and retaining metals in the structure keeps them from the groundwater table. (NCDENR, 2007 ) Rain and snow may then recharge the groundwater table, and air may pass through to nourish the roots of grass and trees.
Permeable pavements allow water to infiltrate through surfaces that would normally be impermeable, such as asphalt or portland cement concrete parking lots. Permeable portland cement concrete (PCC) in a residential setting.