Porous (or permeable) pavement options are an excellent method to use stormwater on-site and minimize the environmental impact of pollutants (such as engine oil or antifreeze from parking lots) carried by storm runoff. Their "open matrix" characteristic allows liquids to pass through them, like salt settling to the bottom of a bag of popcorn.
The most commonly used porous material is concrete, where the amount of porous voids can range between 18% and 35% of the total volume. Even with that much empty space, the compressive strength is usually high enough that laying the concrete 5 to 6 inches thick is enough to support a fully loaded garbage truck.
As far as freeze-to-thaw cycle, the key is to get the water to completely pass through the pavement by using a proper sub-base structure. The Northern Arizona University Applied Research and Development building, a LEED-Platinum building in Flagstaff, Ariz., has now gone through two freezing winters monitoring its concrete permeable parking lot and the material has performed very well.
The largest pervious concrete project in the United States was completed in March 2008 in Williamsburg, Va.: a 7-acre parking facility at Prime Outlets shopping mall. Porous pavements had been avoided in Virginia due to freezing and expansion concerns, but proper installation methods have overcome apprehension. Typical freeze-prevention measures include use of fine aggregates, air-entraining the paste, laying a 6- to 18-inch aggregate base, or laying perforated pipe to capture and redirect water.
Porous asphalt may have more niche uses. Rubberized asphalt porous pavement (adding crumb rubber from discarded tires) has fantastic acoustical properties and is being used widely by the Arizona Department of Transportation in repaving highways, particularly in inner-city areas to reduce noise pollution in nearby neighborhoods. Other benefits include a reduced heat island effect because rubber is an insulator and absorbs heat less readily. Porous pavements also reduce the chance of vehicles hydroplaning because the water is removed from the surface. The jury is still out on porous asphalt's ability to weather the freeze-to-thaw cycle well.
Porous concrete costs more per square foot than traditional concrete, but can save big dollars in the big picture. By taking care of stormwater drainage through the surface of a parking lot, less land and materials need to be set aside and dedicated to stormwater management infrastructure. Therefore, a developer can get more parking spaces onto a smaller lot. The ability to shade the parking lot increases, as well, as trees can expand their root zones and canopies because the roots can now get water from everywhere instead of just from tbe immediate tree well.
Other permeable surface treatment options exist as well, each with its ideal uses. They include plastic grid structures through which grass can grow or that can be filled with gravel, pavers spaced with sand as grout, and soil that is stabilized with additives but remains permeable. These options are best-suited for parking or footpaths.
Permeable pavement is an effective, easy way to prevent pollution from stormwater runoff and meet Environmental Protection Agency requirements, and it can help earn LEED credits for both New Construction and Homes. And porous concrete looks like a Rice Krispie treat, so it's got even more going for it!