MAINTAINING A WORKING PAVEMENTAs with all projects, a great concern of those working for a municipality is the resulting maintenance requirements. Maintenance for a traditional pavement normally consists of patching and sweeping. Maintenance operations for porous pavements are focused on keeping voids in the surface open. The pavement should never be overlaid—some communities install signs near the pavement to prevent placement of an overlay. Crews should never apply a sealant or gravel or sand to the surface; the surface must be protected from the erosion of fines from adjacent properties. All pervious pavements should be swept twice a year with an industrial vacuum.
Design characteristics of pervious pavementAlthough the following guidelines can be used as a starting point in the preparation of a specification for pervious pavement, it is important to use an experienced professional to help finalize the standard.
SITE CONSIDERATIONSSlope of ground cannot exceed 5%Do not use in areas of potential contamination (gas stations)Permeability of underlying soil must be tested and found acceptableWater table must be a minimum of 3 feet below the baseBedrock must be a minimum of 2 feet below the baseBase cannot be placed on any fill material other than stone.SUBGRADESubgrade must be uncompacted and levelTop of the subgrade must be below the frost line.GEOTEXTILE FABRICThe subgrade and sides of the road bed must be covered by a nonwoven geotextile fabricA public draft of Pennsylvania Stormwater Management Manual lists Mirafi 140N, Amoco 4547, and Geotex 451 as acceptable types.AGGREGATE BASEVolume is dependent on runoff volume required and can be handled in hydraulic routing calculations as a detention basin. A rule of thumb is 5 acres of impervious area requires 1 acre of infiltration bedEnsure a void space of 40%Suggested stone is a uniformly graded 1½ to 2½ inch clean, washed No. 3 stone as defined by the American Association of State Highway and Transportation Officials (AASHTO)Strength and durability must be assessedTypically constructed 12- to 36-inch in depth.PIPE COLLECTION SYSTEM (SITE DEPENDENT)Perforated pipe constructed of polyvinyl chloride (PVC), high density polyethylene (HOPE), or corrugated steel.CHOKER COURSEConstruct between surface and base to provide a level paving planeTypically 1 inch thick and comprised of a No. 57 AASHTO stone.ASPHALT SURFACE COURSETypically 2 to 3 inches in depthConstructed of an OGFC with 5.75% to 6% bituminous asphalt by weightRolled once or twice with vibratory rollerNo traffic allowed on newly placed surface for 48 hours.CONCRETE SURFACE COURSEMix with no sand and a No. 89 washed stone with a high percentage of cementTypically has 20% to 25% voidsBuilt to allow a water infiltration rate of 20 inches per hour6 inches or more in depthPlaced with a vibratory screed and rolled with a steel pipe rollerCovered with plastic and cured for a minimum of seven days.PERMEABLE PAVERSInterlocking units, joints filled with pervious materialA public draft of the Pennsylvania Stormwater Management Manual lists Turfstone, UNI Eco-stone, Checkerblock, and GravelPave as acceptable types.
The texture of pervious concrete pavement, similar to that of a Rice Krispies treat, allows water to flow through the concrete and into the underlying base. Photo: Portland Cement Association
Top: This middle school in West Philadelphia includes a pervious asphalt playground placed in 2002. Photo: Cahill Associates. Bottom: The city of Wilton Manors, Fla., used UNI Eco-Stone pavers to construct more than 30,000 square feet of a parking lot. David Archacki, director of public services for the city said, “We use them in all parking and driveway areas where drainage calculations are hard to meet.” Photo: UNI-GROUP U.S.A.