Pervious Concrete 101

By Chere Peterson, President of Petrus UTR, Inc.

Complete Concrete?EUR??,,????'???s Highway McNeillus concrete truck with its wide throat cuts normal discharge time to 15 minutes for a high-dosed admixture porous load. Photo by Chere Peterson

Pervious (porous, permeable) concrete was developed to prevent stormwater runoff more than 50 years ago in Sweden, Switzerland, Germany and Belgium. Sunbelt population explosions in the early 1980?EUR??,,????'???s and incumbent high-density development raised water quality issues, particularly in Florida with its high water table. Pervious concrete was an excellent solution to allow rainwater to infiltrate back into surface water recharge areas ?EUR??,,????'??? almost where it fell ?EUR??,,????'??? like it used to do before we ?EUR??,,????'??paved over paradise and put in a parking lot?EUR??,,????'?? (acknowledgements to Jimmy Buffett and Joanie Mitchell).

Some 15 years ago, the Portland Cement Pervious Association, a confederation of Florida?EUR??,,????'???s early process developers, devised the first visual inspection standards: an installation could have no more than 2 percent raveling (loose stones) and/or consolidation (no holes), and it had to be hose tested (a garden hose at ambient water pressure) to see if it actually drained.

Roads and sidewalks were designed to go around trees. Pervious concrete and pavers allow water and air to flow through and nurture them?EUR??,,????'???a model of Low Impact Development. Photo by Chere Peterson

PCPA promotes, through education, the appropriate use of pervious concrete in North America. It seeks to establish standards, guidelines, specifications and installer certification standards to insure quality installations, and its continued use for stormwater management and tree preservation. They created the first entirely new set of Design Guidelines and Specifications, written by design professionals with a working knowledge of appropriately installed permeable concrete. The board and voting membership is comprised of professional engineers, architects, landscape architects, stormwater designers, academicians, environmentalists, and PCPA certified installers. Sales engineers, product representatives and their respective marketing organizations are welcomed as supporters of PCPA?EUR??,,????'???s high standards as nonvoting associates.

Is it for tree preservation or stormwater management? If the application is to protect and preserve mature trees, excavation depth can be critical, since many trees have relatively shallow feeder or stanchion roots. Dr. Don Gardner, former city of Savannah, Ga., Park & Tree Director (for 16 years) and an early advocate of pervious, cautions that damaging or destroying these roots could risk killing the trees the designer (or regulator) wishes to preserve.

A wide sweep of porous concrete sidewalk and parking captures stormwater and protects the thickly vegetated, treed areas that separate the parking from the river near Stephen Foster State Park Education Building in Fargo, Ga. Pervious pavers capture much of the watershed runoff before it can harm the river?EUR??,,????'???s water quality. This is the state?EUR??,,????'???s first Gold LEED Certified Public building. Photo by Chere Peterson

If the design is to prevent stormwater runoff, Professor Bruce K. Ferguson, landscape architect and director of the University of Georgia?EUR??,,????'???s School of Environmental Design, advises that pervious concrete should be considered as conveyance; for vehicles or pedestrians over it and water through it. If the installer is knowledgeable, the mix design may be changed as frequently as from one truckload to the next to accommodate numerous variable conditions. Consequently void structure can vary. Consider water storage as a bonus.

Thomas and Hutton Engineers (Savannah) designed an elegantly simple stormwater management system for a Hilton Head Island, antebellum style and residential development ?EUR??,,????'??? The Paddocks on Jarvis Creek. With salt marsh and Jarvis Creek meandering along two sides and the relatively high water table of most East Coast barrier islands, water quality issues were critical considering NPDES Phase II compliance. Thomas & Hutton devised a plan consisting of an asphalt road canted to cause first flush stormwater to sheet-flow over grassed tree lawns, into 1.5 miles of pervious concrete sidewalks?EUR??,,????'??? with a dry well for detention. Additional storage was provided in the form of a catch basin on the lowest corner (.03 percent slope from high point to basin) of each four-block square.

Due to windy conditions, almost 9,000 linear feet of rebar was secured to hold the 6 mil poly in place for seven days. Much credit for this great pervious goes to the batch plant manager?EUR??,,????'???s tight quality control of the installation at the Walmart Supercenter in Fitzgerald, Ga. Photo by Chere Peterson

Maintenance could be an issue in a porous concrete sidewalk with abundant hardwood and pine trees, because organic matter could temporarily clog the surface voids; but Hilton Head?EUR??,,????'???s toney cache requires manicured grounds and the budgets to maintain them. If pervious does become clogged, Do not use a high pressure water hose. All that does is relocate a surface problem into the matrix. Use a commercial vacuum to unclog it.

Water quality issues were obviously a consideration for Georgia?EUR??,,????'???s first public sector Gold LEED Certified building, at Stephen Foster State Park. The modern rustic education building overlooks the tannin-black, pristine headwaters of the Suwannee River as it flows out from the Western edge of the Great Okeefenokee Swamp. David Freedman, PE is the director of construction for owner Georgia Department of Natural Resources?EUR??,,????'??? Park and Historic Sites ?EUR??,,????'??? and responsible for five public LEED Certified Structures within Georgia?EUR??,,????'???s state park system, more than any state. Stephen Foster?EUR??,,????'???s almost 20,000 square feet of pervious concrete parking and sidewalks met the additional challenge of being an hour and 20 minutes travel distance from Scruggs Concrete in Valdosta, Ga.

Copper downspouts collect roof stormwater which is piped to a catch basin at the Western Carolina Retinal Associates in Asheville, N.C. The 22,000 sf of pervious concrete serves as detention for 12,000 gallons of stormwater, which infiltrates after the storm event. Pervious concrete becomes the soft grey matte which frames the building and compliments the dry-stacked stone and pebble-dash stucco. Photo by G. Carroll Hughes, AIA

Porous is also called fine-free concrete, because fines like rock dust, fly ash, sand and slag can clog or lump and create fracture points in this ultra low water mix. A typical washed granite mix design is 600+/- lbs of Type 1 Portland Cement, 2,445+/- pounds of washed 3/8-inch granite, and approximately 20+/- gallons of water per cubic yard. When these elements combine, the chemical process of becoming concrete is called hydration, which produces heat. Pervious concrete?EUR??,,????'???s large amount of cement and low amount of water accelerates hydration and produces an extremely hot mix, which is why it is important to try to work with the closest, reliable ready mix plant. The solution came via engineers from Euclid and W. R Grace, companies that manufacture concrete additives. They understood that this is precisely why pervious concrete is so unlike conventional concrete, because it requires unconventionally high additive dosing. It takes an experienced porous concrete installer to regulate the quantities successfully ?EUR??,,????'??? and a compliant, quality driven dispatcher at the ready mix plant. The Stephen Foster project was most likely the first time pervious has been successfully placed with this kind of travel time.

Here stormwater flows directly from the street into the pervious sidewalk; separated by a flush mounted header curb to prevent asphalt from underwashing and degrading. Mature Live Oaks (Evergreen Diciduous trees) were protected by the pervious. Photo by Chere Peterson

Porous paving is also more difficult to deal with when the weather is hot, which is the time most developers want to have it installed. A perfect example of this was the Fitzgerald, Ga., Wal-Mart SuperCenter designed by Freeland and Kauffman Engineers in Greenville, S.C.; general contractor DBS Construction (Chattanooga, Tenn.) represented Atlanta, Georgia Developer Bright Meyers, LLC.

More than 9,000 lineal feet of steel rebar was used to anchor the 6-mil poly covering to prevent moisture loss. The 65,000 square foot parking area accepts stormwater runoff from the asphalt, which starts treatment the minute it enters the pervious. Hydrocarbon-consuming bacteria lives within its 7-inch deep matrix and 6-inch base of washed half-inch stone; the sandy clay sub base slopes to three separate catch basins to handle overflow during heavy storm events. When high admixture dosing is implemented, coring should wait 21 days, in order to achieve accurate ASTM C 140 (entire core) test results. Traffic may be allowed at that point. Scheduling for exterior lighting placement is best done prior to pervious installation, unless design is a minimum 7?EUR??,,????'????EUR??,,????'??? deep, over 6?EUR??,,????'????EUR??,,????'??? of washed stone base.

Jointing is important even with pervious concrete. Jagged expansion cracks appear if the concrete isn?EUR??,,????'???t properly jointed. The water and the land intimately relate to each other. The preservation of both is dependent on Low Impact Development. Photo by Chere Peterson

The type of locally available stone used in the mix can yield different void percentages: lime rock produces a durable void structure range of 15 percent to 22 percent; granite?EUR??,,????'???s range is between 11 percent and 16 percent and requires four gallons less water per cubic yard.

Following PCPA design guidelines, specifications, installer certifications, and most importantly, its visual inspection standards, offers the best and quickest way to a successful pervious concrete installation.

Stormwater flows into the concrete horizontally from the tree lawn and vertically, from the top into the stone dry well. View full Size Image