Aug 18, 2011

Porous Asphalt: Beyond the Parking Lot

Porous pavement—an open-graded asphalt surface over a stone reservoir—has gotten a lot of attention over the past 10 years, primarily for parking lots. In addition, a number of projects now are moving from the parking lot onto the road. This article will move across the country to see what engineers and researchers are doing with this versatile pavement/storm water management tool.

Highway Applications

The porous asphalt surface has a long history of use on roads around the world. This surface often is referred to as a porous friction course or an open-graded friction course. In this case, the porous asphalt mix is used as a surface over a dense-graded, impermeable asphalt road to reduce splash and spray, improve wet-weather skid resistance and reduce noise produced by tires running on the pavement at high speeds.

Recently, however, these surfaces have been found to be effective in reducing pollutants in runoff from roads. Studies by the Texas Department of Transportation showed the porous surface course to be effective in reducing total suspended solids (TSS), total metals and phosphorus compared to conventional impermeable asphalt surfaces. In a recent article published by the Netherlands Ministry of Transport, the following was reported: “Porous asphalt traps a portion of the pollution from water runoff, providing a cleansing effect. The rest of the polluted runoff is captured in the top layer of the soil in the green verge close to the road.”

PASR on Roads

Porous asphalt with stone reservoirs (PASR) has been used widely for parking lot pavements over the past 30 years, with some roadway use. The oldest known PASR road was constructed in Chandler, Ariz., in 1986, and it served well for more than 20 years. While little attention was paid to using PASR on roads in the past, it recently has received more attention. The following are three examples of PASR use for roadways.

A Two-Layer Base for Oregon's Pringle Creek Subdivision

Pringle Creek is an environmental award-winning subdivision constructed in Salem, Ore., in 2006. This project was unique in its use of a two-layer porous asphalt surface. Also notable were the techniques used to protect the pavement during the construction of the subdivision.

Usually PASR pavements con-structed in the U.S. use a single fine-graded porous asphalt mix for the entire porous asphalt layer. For Pringle Creek, however, the porous asphalt mix was placed in two layers.

The first layer used an asphalt-treated permeable base (ATPB), commonly used in highway construction below the conventional asphalt pavements. The gradation for the ATPB is coarser than the surface layer. This coarser gradation requires less asphalt binder, which helps reduce costs.

Jim Huddleston, executive director of the Asphalt Pavement Assn. of Oregon, believes in the two-layer system. The large voids in the ATPB and the thinner porous surface, he said, will reduce clogging potential.

Geotextile fabric protected Pringle Creek's porous pavement during construction. (Photo courtesy of Jim Huddleston.)

As often is done during construction of subdivisions and other real-estate developments, a hard-surfaced road was placed before construction of the structures. This is easy for a conventional pavement because an asphalt base layer can be placed for this surface; once the project is completed, the base is cleaned and a surface course is placed.

With a porous asphalt pavement, however, dirt and debris must be kept from plugging the porous pavement during construction. To protect the pavement at Pringle Creek, the contractor placed and anchored a geotextile to the ATPB. Once subdivision construction ended and the site was stabilized, the geotextile was removed, minor cleaning was performed and the finer porous asphalt surface was placed. The pavement is performing very well, according to Huddleston. Water only shows up on the surface at a low point in the road during the heaviest storm, and it flows into a drop inlet.

Salt Reduction in Minnesota's Shingle Creek Watershed

For many years, anecdotal evidence indicated that PASR pavements required little or no salt during snow events. The first documented evidence of this was at the University of New Hampshire Stormwater Center (UNHSC). Its study showed that PASR pavements require 0% to 25% of the salt required for conventional pavements. This observation now is being evaluated again in the extreme cold of Minnesota.

The Shingle Creek Watershed Commission is working under a federal grant to “evaluate whether less or no salt can be applied to a porous pavement and still maintain road safety in winter driving conditions.” The study also will evaluate the performance of the PASR pavement as a road, monitor maintenance requirements and examine water quality.

The road was constructed using a single layer of a porous asphalt mix over an 18-in. stone reservoir. After one year of monitoring, the results are promising. The porous asphalt maintains a warmer profile compared to the control section. No salt has been applied to the test section. When the snow melts, it flows through the surface to the stone bed instead of refreezing on the surface. Performance has been good, with almost no loss of material. Plow scuff marks are evident.

New Street Project in New Hampshire

UNHSC has involved itself in the evaluation of storm water best management practices (BMPs). The center has constructed and monitored many storm water BMPs, including PASR systems. Recently, the center advised SFC Engineering Partnership and Pelham, N.H., town officials on the design and construction of the state’s first porous asphalt road for Boulder Hills, an active adult condominium community.

Initial designs using conventional site development and storm water management were proving too expensive for this site, primarily due to the need to construct two storm water detention basins and lengthy underground drainage lines. While the cost of a PASR was higher than a conventional pavement, the porous asphalt option saved the developers $49,128 on storm water management—nearly 6% of the total cost of construction of the storm water management systems.

About the author

Kent Hansen is director of engineering for the National Asphalt Pavement Assn. For more information, visit www.hotmix.org.

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