POROUS ASPHALT
INSIDE THE FLOODPLAIN | OUTSIDE THE FLOODPLAIN

What Is It?

Cost: $0.50-$2.50 per sq ft. (EPA), typically 10-15% more expensive than conventional asphalt (FHWA).
Applications: All applications as alternative to conventional asphalt, including highways, parking lots, and streets.
Service Life: 7-20 years

Porous asphalt is open-graded asphalt mixed with less sand and fine particles than conventional asphalt and then laid above crushed stone aggregate layer to allow for water infiltration. Water is able to permeate the asphalt due to increased void space of approx. 16% compared to 2-3% for conventional asphalt. In order to maintain infiltration, porous asphalt should not be used on slopes of greater than 5%.

The infiltration rate ranges from hundreds of inches per hour when freshly paved to more than one inch per hour when the void spaces are clogged (EPA). Porous asphalt requires proper maintenance (primarily vacuum sweeping) to maintain high rates of infiltration. Potholes and cracks can be fixed with patching mixes unless >10% of the surface needs replacement. Infiltration rates are also highly dependent on the subgrade soil. Sandy soils have higher infiltration rates, but lower treatment capacity. In contrast, clay soils capture more pollutants, but have lower infiltration rates.

Benefits

  • Enhances groundwater infiltration while reducing stormwater runoff volume, rate, and pollutants.
  • Contributes to lower urban air temperatures when moist due to evaporative cooling.
  • Reduces occurrence of black ice/freezing puddles in cold climates; requires fewer applied deicers.
  • Higher frictional resistance allows for better traction than conventional asphalt in cold climates.

Drawbacks

  • Pollutants and deicing salts can infiltrate groundwater. Should not be installed in areas where hazardous material spills are possible.
  • Plowed snow piles and improper deicing treatment can clog void spaces and reduce infiltration rate (sand should not be used). Snow plow blades can damage surface.
  • Can contribute to higher urban daytime temperatures due to lower reflectivity.

Regulatory Impacts and Requirements

A summary of potential regulatory touchpoints follows below:
Porous-Asphalt

  • Nearby buildings: When installing permeable pavement, it is critical to conduct subsurface investigation. There are many old vaults with direct access to building basements underneath Boston sidewalks that could potentially leak water into the building if permeable pavement is installed at the surface. Many of these vaults are unmapped and there are legal issues around who owns the vault and the sidewalk/sub-base above it. In addition, rubble foundations and other old building foundations nearby may begin to leak from increased hydrostatic pressure.
  • Groundwater Trust: Installation of permeable pavements in the Groundwater Conservation Overlay District (GCOD) may require consultation with the Groundwater Trust. In particular, permeable pavements cannot be installed over polluted sites within GCOD.

Financing Options, Incentives, And Rebates

Additional Resources

Project Examples

SAMPLE OF SUPPLIERS

*Indicates Boston-area supplier

SOURCES

Photo credit: VBW-Asfalt on Wikimedia Commons