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The most commonly used permeable pavement surfaces are pervious concrete, porous asphalt and permeable interlocking pavers.
Pervious concrete is a special type of concrete with a high porosity used for concrete flatwork applications that allows water from precipitation and other sources to pass directly through, thereby reducing the runoff from a site and allowing groundwater recharge. The high porosity is attained by a highly interconnected void content. Typically pervious concrete has little or no fine aggregate and has just enough cementitious paste to coat the coarse aggregate particles while preserving the interconnectivity of the voids. Pervious concrete is traditionally used in parking areas, areas with light traffic, residential streets, pedestrian walkways, and greenhouses.
Porous asphalt is standard hot-mix asphalt with reduced sand or fines and allows water to drain through it. Porous asphalt over an aggregate storage bed will reduce stormwater runoff volume, rate, and pollutants. The reduced fines leave stable air pockets in the asphalt. The interconnected void space allows stormwater to flow through the asphalt as shown in Figure 1, and enter a crushed stone aggregate bedding layer and base that supports the asphalt while providing storage and runoff treatment. When properly constructed, porous asphalt is a durable and cost competitive alternative to conventional asphalt.
Permeable interlocking concrete pavement (PICP) consists of concrete (or stone) paver units with open, permeable spaces between the units. They give an architectural appearance, and can bear both light and heavy traffic, particularly interlocking concrete pavers, excepting high-volume or high-speed roads. Some products are polymer-coated and entirely porous face. Other variants not discussed in the articles on permeable pavement include single-sized aggregate, porous turf, permeable clay brick pavements, resin bound paving, and bound recycled glass porous pavement.
To assist with selection of the type of surface, a general comparison of the properties of the three major permeable pavement types is provided in Table 1. Designers should check with product vendors and the local review authority to determine specific requirements and capabilities of each system. Schematic cross sections of each system are illustrated in the design section for permeable pavement.
Properties of permeable pavements.
|Properties||Pervious concrete||Porous asphalt||PICP|
|Typical pavement surface thickness1||5 to 8 inches||3 to 4 inches (thicker for high wheel load applications)||3 inches|
|Bedding layer1,6||None||1 in. No. 57 stone||2 inches of No. 8 stone|
|Reservoir layer2,6||No. 57 stone or per hydraulic design>||No. 2, 3, or 5 stone||4 inches of No. 57 stone over No. 2, 3 or 4 stone|
|Construction properties||Cast in place, seven day cure, must be covered||Cast in place, 24 hour cure||No cure period; manual or mechanical installation of pre-manufactured units|
|Installed surfacing cost3||$3 to $4/sf||$2/sf||$3 to $4/sf|
|Minimum batch size||None||None||None|
|Longevity4||20 to 30 years||20 to 30 years||20 to 30 years|
|Overflow||Catch basin or overflow edge||Catch basin or overflow edge||Catch basin or overflow edge|
|Runoff temperature reduction||Cooling at the reservoir layer||Cooling at the reservoir layer||Cooling at the reservoir layer|
|Surface colors/texture||Range of light colors and textures||Black or dark grey colors||Wide range of colors, textures and patterns|
|Load bearing capacity5||Handles all vehicle loads with appropriate surface and base/subbase layer material and thickness design||Handles all vehicle loads with appropriate surface and base/subbase layer material and thickness design||Handles all vehicle loads with appropriate surface and base/subbase layer material and thickness design|
|Surface cleaning7||Periodic vacuuming; replace if completely clogged and uncleanable||Periodic vacuuming; replace if completely clogged and uncleanable||Periodic vacuuming; replace jointing stones if completely clogged and uncleanable|
|Other issues||Avoid concentrated deicers and winter sanding||Avoid seal coating and winter sanding||Avoid winter sanding|
|Design reference||ACI Report 522-2010||Hansen 2008 NAPA||Smith 2011 ICPI|