By Hongbin Xie, Quality Services

A Green Concept

In the natural environment, rainfall sinks into soil, filters through it, and eventually finds its way to streams, ponds, lakes, and underground aquifers. In contrast, the built environment seals the surface. Rainwater and snowmelt become runoff which may contribute to flooding. Contaminants are washed from surfaces directly into waterways without undergoing the filtration that nature intended.

For these reasons, managing storm water is a significant issue in land use planning and development. For over 25 years, porous asphalt parking lots have been used to address these concerns. The concept is simple: construct storm water redetention basins under the street and parking structures. These basins are designed to collect the storm water from structures, pavements, and other areas of a development and hold the water until it can percolate into the soil. Porous asphalt pavement is then placed over the top of these basins. These pavements are designed to let the water flow through them and into the detention basin.

If contaminants were on the surface at the time of the storm, they are swept along with the rainfall through the stone bed. From there they infiltrate into the sub-base so that they are subjected to the natural processes that cleanse water. The porous asphalt pavement also keeps the parking lot dry immediately after storm and avoid the water gathering on the pavement surface which can be slip hazards (Figure 1).

Figure 1 Side-by-side comparison Porous Pavement System Design

Porous Pavement System Design

As shown in Figure 2, a typical porous asphalt pavement consists of a porous asphalt course, a top filter course (choke course), a reservoir course (designed for runoff detention frost penetration, and structural capacity), an optional bottom filter course, filter fabric, and existing soil or subgrade material.

Mix designers may use the target gradation range recommended in the NAPA design guides (1) and utilize a PG binder that is stiffer than the binder typically specified in the area. For California projects, these would be PG 70-10, PG 70-22 PM, or PG 76-22 PM. The permeability, asphalt draindown, mix integrity and moisture susceptibility should be checked to ensure the satisfactory performance of porous asphalt mix.

Figure 2: Typical porous asphalt pavement schematic ¹

Construction and Performance

Porous asphalt pavements are fast and easy to construct. With the proper information, most asphalt plants can easily prepare the mix and general paving contractors can install it.

The stone bed, often eighteen to thirty-six inches in depth, provides a tremendous subbase for the pavement. As a result, porous asphalt pavements tend not to exhibit cracking and pothole formation problems. The surface wears well. Under the stone bed is a geotextile which keeps fine particles from moving into the stone bed from below and filling in the spaces.

Figure 3 Geotextile allows water to drain but prevents soil movement into stone

Porous asphalt pavement is not a new concept, and it has been proven to last for decades, even in extreme climates, such as in areas with many freeze-thaw cycles. These early projects that built in the 1980s are still in service today.

Environmental Concerns

Asphalt pavements are compatible with clean water. Studies² show that asphalt pavements and stockpiles of reclaimed asphalt pavement do not leach. Instead, asphalt pavements have been used for many years to enhance water quality. At landfills, asphalt liners and caps keep contaminants from leaking into groundwater. Drinking water reservoirs lined with asphalt pavement have been used in California since the 1950s. Salmon hatcheries and fish rearing ponds in the Pacific Northwest use asphalt liners³.

LEED Points

Porous asphalt can be used towards a LEED point under sustainable sites category (SS credit 6.1) for storm water control. Implementing porous asphalt pavement can help to meet the requirement that post-development discharge equals pre-development discharge.

References

1. Design, Construction and Maintenance Guide for Porous Asphalt Pavements (IS 131), NAPA, 2003.
2. Townsend, Timothy, and Allen Brantley. Leaching Characteristics of Asphalt Road Waste. University of Florida, 1998. (http://www.hinkleycenter.com/publications/townsend_98-2.pdf, accessed March 10, 2008)
3. Environmental Applications for Hot Mix Asphalt (PR-1). Asphalt Institute, Lexington, KY, not dated.