“When it pours, it’s porous.”
Porous asphalt pavements offer developers and planners a new tool in their toolbox for managing storm water. These pavements, used mostly for parking lots, allow water to drain through the pavement surface into a stone recharge bed and infiltrate into the soils below the pavement. Such pavements have been proving their worth since the mid-1970s, and recent changes in storm water regulations have prompted many consulting engineers and public works officials to seek information about them.
What can porous asphalt do?
Porous asphalt pavements are of great interest to site planners and public-works departments. With the proper design and installation, porous asphalt can provide cost-effective, attractive pavements with a life span of more than twenty years, and at the same time provide storm-water management systems that promote infiltration, improve water quality, and many times eliminate the need for a detention basin. The performance of porous asphalt pavements is similar to that of other asphalt pavements. And, like other asphalt pavements, they can be designed for many situations.
How does it work?
The technology is really quite simple. The secret to success is to provide the water with a place to go, usually in the form of an underlying, open-graded stone bed. As the water drains through the porous asphalt and into the stone bed, it slowly infiltrates into the soil. The stone bed size and depth must be designed so that the water level never rises into the asphalt. This stone bed, often 18 to 36 inches in depth, provides a tremendous subbase for the asphalt paving.
To view a cross section of a porous asphalt pavement, click here.
What does it cost?
Special features such as the underlying stone bed are more expensive than conventional construction, but these costs are more than offset by the elimination of many elements of standard storm-water management systems. On those jobs where unit costs have been compared, a porous asphalt pavement is generally the less-expensive option. The cost advantage is even more dramatic when the value of land that might have been used for a detention basin or other storm-water management features is considered.
How long do these pavements last, and how long do they remain porous?
Even after twenty years, porous pavements show little if any cracking or pothole problems. The surface wears well. Porous asphalt retains its ability to handle rain water for many years. One of the best-known porous parking lots, located at the Walden Pond State Reservation in Massachusetts, was constructed in 1977. While it has never been repaved, it is in good shape and still drains effectively.
In a study of a porous pavement system constructed at the Centre County/Pennsylvania State Visitor center, researchers found that the system had maintained a consistent infiltration rate. During a 25-year precipitation event, there was no surface discharge from the stone beds.
Do these pavements look “different?” Are they smooth?
While slightly coarser than standard asphalt, porous asphalt pavements are attractive and acceptable. Most people parking on a porous asphalt parking lot will not notice (or believe) that it is porous. The surface of a porous asphalt pavement is smooth enough to meet requirements of the Americans with Disabilities Act (ADA).
What special additives or construction techniques are needed?
An added advantage to porous asphalt is that it does not necessitate proprietary ingredients. It does not require the contractor to have special paving equipment or skills. With the proper information, most asphalt plants can easily prepare the mix and general paving contractors can install it.
How does porous asphalt affect water quality?
There has been limited sampling data on the porous pavement systems, although the available data indicate a very high removal rate for total suspended solids, metals, and oil and grease.
Are there other environmental benefits?
Because of the open structure of the pavement, porous asphalt offers a “cooler” pavement choice. By replenishing water tables and aquifers rather than forcing rainfall into storm sewers, porous asphalt also helps to reduce demands on storm sewer systems. In areas where storm-water impact fees are imposed by local governments, such fees may be reduced by using porous asphalt.
Is there a technical publication that covers porous asphalt?
FHWA has released a TechBrief providing an overview of the benefits, limitations and applications of porous asphalt pavements with stone reservoirs, as well as considerations for design, construction, and maintenance. Also, NAPA offers a publication, Porous Asphalt Pavements. It is a design guide that takes the reader from site design through maintenance.