THE PATH TO PERVIOUS PAVING
By Stephanie Aurora Lewis, RA, LEED
Pervious paving is a green stormwater management system that recharges the water table, cleans water runoff, and improves the heat island effect for a surrounding ecosystem. The RMC Research and Education Foundation (RMCREF) is a central resource for AEC professionals, owners and government entities interested in pervious paving and other sustainable concrete technologies.
Julia Garbini, executive director of the RMCREF, stated, “Concrete offers lots of opportunities to build green.” In fact, concrete may be one of the greatest sustainable materials for its ability to maintain a variety of admixtures (fly ash), aggregates (crushed concrete), and advanced technologies (pervious pavement) that contribute to greener construction practices.
Down the Drain
Intuitively, the term pervious pavement is a composite of what materials it comprises and how it performs. Rather than introducing sand to the cement, water and aggregate mixture, a specialized paste adheres to the aggregate in poured concrete and asphalt. By eliminating sand from traditional concrete mixtures, the pavement solidifies leaving a 15 to 25 percent content void. Water, in the form of rain and snow melt, literally percolates through the mixture, engineered layers of aggregate and fiber mesh to slowly infiltrate the soil sub-base.
As the water drains through the various layers of aggregates and soil, microorganisms clean the water of pollutants—unwanted sediments, phosphorus, nitrogen, zinc, lead, gasoline, automotive oil etc. Most pervious paving mixtures drain rainwater at a rate of three to eight gallons per minute per square foot.
Form and Function
Pervious pavement may take the form of poured-in-place pervious asphalt or concrete, block and concrete modular pavers, or grid pavers. Though the initial material and installation costs are 10 percent to double that of traditional paving methods, the environmental and long-term cost benefits outweigh the upfront investment. The extra costs may be offset by the elimination of a need for curbs, gutters, storm drains and large retention ponds. Depending on the project’s location, a community may reduce stormwater fees as an incentive for decreasing the stormwater infrastructure for that particular site.
For sustainable pavement to function as it was designed, it needs to be installed with precision and later maintained.
Pervious surfaces must be vacuumed regularly and power washed as needed. For example, debris left behind after snow piles melt may clog the void content. The RMCREF partnered with the University of Central Florida to provide a free, downloadable guideline for suggested long-term maintenance.
Street Smart
Aside from its educative purposes, the RMCREF continues pervious paving research for a wider range of climates, soil types and purposes. A research project in partnership with the Cleveland State University shows that pervious paving can be constructed in cold climates with freeze/thaw cycles and in areas where salt and de-icers are used.
At a maximum compressive strength of 500 – 4000psi, pervious paving options cannot yet support large and heavy vehicular traffic. The RMCREF and Iowa State University are working on changing the psi loading factor so pervious paving could be used for heavy vehicular areas and highways. Completion of this research project is still two years away. This and other specific research data and guidelines are free resources through the RMC Education and Research Foundation as well as through the Ready Mixed Concrete Foundation.
“Pervious paving is a new technology, and it uses different types of materials. The goal of the RMCREF is to help the industry construct pervious surfaces that exhibit successful applications and that stand the test of time,” Garbini said. “Since this is a new technology, we do recommend that contractors become certified in pervious pavements.” Certification information is available through the National Ready Mixed Concrete Association.
On the Road to Success
Despite initial glitches during the development phase, pervious paving projects have met with success. “The industry worked through those glitches and the practice of pervious paving is now becoming well recognized for its many environmental benefits—from stormwater management to the ability for large box retailers to build more parking areas to safety issues, such as the elimination of pooling water that turns to ice in the winter,” Garbini concluded.
Resources
1. RMC Research and Education Foundation. 19 May 2008. www.rmc-foundation.org
2. “Pervious Paving.” Portland Cement Association, 21 January 2008. www.concretethinker.com/Papers.aspx?DocId=10
3. “What Can Be Done.” U.S. Environmental Protection Agency, 21 January 2008. www.epa.gov/hiri/strategies/index.html
4. “Pervious Pavement.” Lake Superior Duluth Streams, January 2008. www.duluthstreams.org/stormwater/toolkit/paving.html
5. Pervious Concrete. National Ready Mixed Concrete Association, 19 May 2008. www.perviouspavement.org
6. American Coal Ash Association, 19 May 2008. www.acaa-usa.org
RMC Foundation Research Currently Available
1. Ready Mixed Concrete Industry LEED Reference Guide
2. Construction and Maintenance Assessment of Pervious Concrete Pavements, University of Central Florida, January 2007.
3. Hydraulic Performance Assessment of Pervious Concrete Pavements for Stormwater Management Credit, University of Central Florida, January 2007.
4. Portland Cement Pervious Pavement: Field Performance Investigation on Parking Lot and Roadway Pavements, Cleveland State University, December 2007. (This project studies pervious paving in cold weather climates.)
5. Crushed Returned Concrete as Aggregates for New Concrete, National Ready Mixed Concrete Association Laboratory, September 2007. (If fully utilized, this recycling strategy has the potential to save the industry $300 million annually in materials and disposal costs—a win-win for the environment and industry.)
6. Pervious Concrete Research Compilation, Middle Tennessee State University. (A revision will be available by July 2008.)
7. Pervious Concrete Contractor Certification Craftsman Text, National Ready Mixed Concrete Association. (More information is available at www.nrmca.org/certifications/pervious.)
RMC Foundation Research Currently Underway
1. New Technology-Based Approach to Advance Higher Volume Fly Ash Concrete with Acceptable Performance. (This project was originally funded by the Dept of Energy; RMC funded the remainder. Available July 2008.)
2. Side-by-Side Comparison of Pervious Concrete v. Porous Asphalt, Villanova University. (This project is co-funded with the EPA and Prince George’s County, Md. Available 2010.)
3. Pervious Concrete Mix Design for Wearing Course Applications, The Concrete Pavement Technology Center at Iowa State University. (This project is expected to help take pervious to highway applications in the future. Available 2010.)
4. Effect of Pavement Type on Fuel Consumption and Emissions, University of Texas at Arlington. (Available 2010.)