Porous Asphalt is King of the Road
You don’t get to be the king of the road for just looking great or being adored by millions—you have to have soul, too.
After more than 30 years in the business of improving safety, reducing erosion, and mitigating water pollution, porous asphalt pavement is finally getting the recognition it deserves.
Porous asphalt pavements are being used successfully throughout the United States, in every type of climate and geography. They are recognized by the Environmental Protection Agency as a best practice for stormwater management, but we like them because few pavements are giving back more to the planet, making porous asphalt road royalty.
A solution for pollution
Scientists and engineers who observe the complicated relationship between water and roadways have long understood the need for a sustainable method to simulate the natural filtration process occurring outside populated areas.
Rainfall is supposed to sink naturally into soil, filter down through it, and eventually find its way to streams, ponds and lakes. In urban or other high-population areas, rainfall becomes what is called “runoff” and can contribute to flooding on roads and parking lots. Contaminants such as oils and minerals are then washed from surfaces directly into waterways without undergoing the filtration that nature intended.
Porous asphalt is an all-natural material using rocks glued together with asphalt cement, like most standard roads, but with one difference. The secret to this mix is “less is more” because manufacturers withhold the small (fine) aggregate so the remaining large particles leave open spaces that allow water to flow through it. Under this porous asphalt pavement surface is a “recharge bed” built of stones with spaces between them. Rainwater sinks through the pavement surface into the recharge bed, where it is retained until it can slowly filter out into the earth. Just like in your home water filter, the water comes out a little better than when it went in.
More than one way to get porous
When city planners speak of porous asphalt, they often mean the full-depth porous parking lots that are part of the most up-to-date stormwater treatment systems.
Highway engineers also use the same porous asphalt surface on superhighways – but instead of putting it over a stone recharge bed, they put a thin layer of it on top of a conventional road base. Rainwater sinks directly into the surface, then hits the impervious base. This forces the water to drain off to the sides. Even in a driving rainstorm, splash and spray from trucks disappears. Visibility for drivers is so much better that crashes and fatalities can be greatly reduced. Although they are thin, these porous asphalt surfacings also improve water quality.
Safer road surfaces for wintery climates
The only foolproof way to be safe when roads are covered with standing rainwater, ice, or snow is to stay at home. But if you have to leave, you will appreciate the benefits of the king of pavements, especially in the cooler climates where snow is measured in feet and not inches.
Porous pavements show significant reduction in the need for deicing and anti-icing practices common in the north. So while we all enjoy a clear, slip-free road that anti-icing agents can provide, the effects of these chloride-laden treatments are toxic to aquatic life, can degrade drinking water supplies, and cannot be reduced from runoff by treatment with another substance.
Because snow and ice melt faster on porous asphalt (nothing to freeze when the water is beneath the asphalt), less salt is needed to clear roads and parking lots. This benefit is significant both in the potential economic savings for winter maintenance and the environment.
Reduced costs for greener solutions
On a yard-by-yard basis, the asphalt cost is approximately the same as the cost of conventional asphalt. The underlying stone bed where the water filters through is usually thicker, and therefore more expensive, than a conventional one, but this cost difference is generally offset by the significant reduction in stormwater pipes and inlets we’re used to building. Because porous pavement is designed to “fit into” the topography of a site, there is generally less digging into the Earth.
At Walden Pond
Since the 1960s when the concept of porous pavement was proposed to “promote percolation, reduce storm sewer loads, reduce floods, raise water tables, and replenish aquifers” the concept has been discussed and refined.
The concept and application was so appealing that in 1977 the Walden Pond State Reservation in Massachusetts used the material for the parking lot of their visitor center. Since that time it has raised the local water table while reducing erosion, pollution, and the need for storm drains or road salt. They continue to use porous asphalt today to protect a site that serves as one of our most treasured literary muses—Walden Pond.
