Porous Asphalt


Porous asphalt, also known as pervious, permeable, “popcorn,” or open-graded 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.


Porous asphalt can be used for municipal stormwater management programs and private development applications. The runoff volume and rate control, plus pollutant reductions, allow municipalities to improve the quality of stormwater discharges. Municipal initiatives, such as Portland’s Green Streets program (Figure 2), use porous asphalt to reduce combined sewer overflows by infiltrating and treating stormwater on site. Private development projects use porous asphalt to meet post-construction stormwater quantity and quality requirements. The use of porous asphalt can potentially reduce additional expenditures and land consumption for conventional collection, conveyance, and detention stormwater infrastructure.

Porous asphalt can replace traditional impervious pavement for most pedestrian and vehicular applications. Open-graded asphalt has been used for decades as a friction course over impervious asphalt on highways to reduce noise, spray, and skidding. Highway applications with all porous asphalt surfacing have been used successfully for highway pilot projects in the United States, but, generally, porous asphalt is recommended for low volume and low speed applications (Hossain et al., 1992). Porous asphalt performs well in pedestrian walkways, sidewalks, driveways, parking lots, and low-volume roadways. The environmental benefits from porous asphalt allow it to be incorporated into municipal green infrastructure and low impact development programs. The appearance of porous asphalt and conventional asphalt is very similar. The surface texture of porous asphalt is slightly rougher, providing more traction to vehicles and pedestrians.

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