Feature: Pervious pavementBy G. Carroll Hughes, AIA on 03/22/2010
For your next project, consider installing pervious pavement. Recent test results gathered from various sites, such as universities, prove the validity of using pervious pavement in our area. Since Western North Carolina is considered a cold climate, there are many local concerns and misconceptions about the applicability of pervious pavements in freeze-thaw environments. As a practicing architect, I understand those concerns.
Before we look at some of the new research data, I want to acknowledge that I am also an avid trout fisherman, and as such, I am very concerned about the stormwater runoff from parking lots that enters and pollutes our streams. There are petroleum products, asbestos from brake linings and other automobile pollutants that are washed into detention ponds and underground retention piping under parking lots. Many of these pollutants ultimately wash into our municipal storm piping systems and are discharged in our waterways.
Stormwater management is a very valid and often code-required goal for projects. When land costs are low, open stormwater detention pond areas are the often-chosen design. However, when the availability of land is scarce and land costs have risen, underground retention piping designs are typical solutions. Pervious pavements are not commonly accepted solutions for stormwater management in our region due to the misconception of incompatibility with freezing temperatures, the high initial cost, and the fact that the installation requires a higher level of technical expertise.
Municipalities and states are changing because of federal mandates; therefore, traditional approaches are changing and pervious pavements are able to meet those new criteria. In addition to all of the other benefits of pervious pavements, test results predict that they will last more than 30 years, versus an average service life of 12 to 15 years for standard pavements.
There has been significant research done on the practical applications of pervious paving in cold climates at the University of New Hampshire Stormwater Center in Durham, N.H. Over the last few years, test sites of various Low Impact Development (LID) solutions, including pervious asphalt and pervious concrete, have been installed and evaluated. Both pervious types have been reported to have performed well, even under sub-freezing temperature cycles.
The following lists attempt to summarize and compare some of the data from this and other recent research conducted, specifically, on pervious concrete and pervious asphalt pavements.
Pervious asphalt pavement
Mix production is more difficult
Easier to place
Allows non-certified installers
Absorbs more heat, thus less de-icing required
Absorbs more heat, thus high urban heat island effect
Performs better in winter
More nighttime lighting required
Pervious concrete pavement
Simple to produce
Trickier to install
Requires certified installers
Absorbs less heat, thus more de-icing required
Absorbs less heat, thus low urban heat island effect
Performs less well in winter
Less nighttime lighting required
It is common knowledge in the design community that non-point source pollution is the primary problem with unfiltered stormwater runoff. Thus, there are incentives for using pervious pavement. Following are some valid reasons:
• Meeting EPA requirements
• Public health (i.e., fewer ponds or wetlands)
• These are Low Impact Development (LID) systems
• Lower winter maintenance (i.e., sand, plowing, salting)
• Safety (less standing water to freeze)
• Reduction or elimination of retention/detention areas
• Recharge local aquifers
• Twice the service life of regular pavements
• Reduced freeze-thaw susceptibility
• Greater load bearing capacity
• Cost-efficient approach to treating water quality
• Efficient approach to reducing the volume of runoff
Pervious concrete pavement was installed at a medical facility in Asheville in 2000. The parking area is 22,000 square feet of 6-inch design mix in the parking areas and 7-inch in the heavy traffic lane. The maintenance is low, there is no sign of raveling, and the performance and appearance have not diminished.
Lessons that were learned on this project:
• High quality concrete products must be used in the design mix
• Proper design mix required
• Proper sub-base required
• Proper placement required
• Voids must be 20 percent to 35 percent
• Thickness can vary depending on the traffic loads
• Can accommodate approximately 10 times the amount of runoff in a 100-year storm
• Igloo effect (sub-base warms the surfaces above)
• If designed and installed correctly, pervious concrete can last 25 to 40 years, even in cold environments
Before your next paving project, educate yourself on the options and consider the use of pervious pavement. Like most products, the skill of the installation crew is the key to success.
[G. Carroll Hughes is a practicing architect and CEO of Spaceplan Architecture, Interiors & Planning. He is also managing partner of Kestrel Construction Company and Moisture ID, a building investigation and diagnostic firm. Mr. Hughes is a member of the American Institute of Architects, the National Academy of Forensic Engineers, ASHRAE, International Code Council and past president of the Portland Cement Pervious Association. He resides in Asheville, N.C., and may be reached at (828) 252-9649.]