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Permeable Surfaces: A Hot New Installation Segment02-29-08 | News

Permeable Surfaces: A Hot New Installation Segment

By Anne Balogh, ConcreteNetwork.com, for LCN

This plantable concrete system is delivered in pallets stacked with 2 x 2 ft. panels. A single worker can place panels on the ground. Irrigation lines should be installed before the pavers?EUR??,,????'?????< Photo courtesy of Kevin Weemers, www.soilretention.com

1. An attractive, alternate kind of permeable surface can be created using concrete grid pavers (CGPs) that allow turfgrass or other groundcover to fill in the open space. To accommodate the plant material, organic matter can be added to the sand that will make up the subgrade. Photo courtesy of Kevin Weemers, www.soilretention.com

Three Types of Permeable Surfaces:

The numbered boxes illustrate the most common types of paver and aggregate surfaces that allow water to percolate.

2. Like conventional concrete, pervious concrete (PC) is made from a mixture of cement, coarse aggregates, and water. However, it contains little or no sand, which results in a porous open-cell structure that water passes through readily. Photo by Khara Dizmon, ConcreteNetwork.com

3. This view from above shows how sand fills the gaps and secures permeable interlocking concrete pavers (PICP). A 2007 study by University of Florida researchers suggests that these kind of pavers not be installed next to areas of disturbed soil, which tends to blow over and clog the openings. Photo courtesy of Pavestone Company

Permeable pavers and permeable concrete are set to take off with the new wave of green-themed products that are sweeping the county. While not appropriate for all applications, the new pervious surfaces let water percolate through, filtering out impurities and limiting runoff. With a bit of research and practice, contractors who are used to installing traditional concrete and pavers can expand their offerings to include the porous varieties.

Paved surfaces are so ubiquitous in urban areas today that most of us give little thought to the impact they have on water quality and the health of the environment. But here?EUR??,,????'?????<

A simple solution to avoiding these problems is to stop installing the impervious surfaces that block natural water infiltration into the soil. But few of us are ready to give up our paved roads, driveways, and parking lots. Rather than building them with conventional concrete or asphalt, more and more communities, municipalities, and businesses are switching to pervious concrete, a material that offers the inherent durability and low life-cycle costs of a typical concrete pavement while retaining stormwater runoff and replenishing local watershed systems.

Pervious concrete is delivered by conventional cement trucks and placed in standard forms. Because pervious concrete is thicker in consistency than regular concrete, a vibrating mechanical screed is often used to level it off. Vibration is followed by compaction with a heavy steel roller (seen here) to attain greater strength.

Manufacturers have designed another variety of paver for use in parking lots?EUR??,,????'?????<

Let Water Percolate

Instead of preventing infiltration of water into the soil, pervious concrete assists the process by capturing rainwater in a network of voids and allowing it to percolate into the underlying soil. In many cases, pervious concrete roadways and parking lots can double as water retention structures, reducing or eliminating the need for traditional stormwater management systems such as retention ponds and sewer tie-ins.

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Pervious concrete can be tinted using the same stains that are used on other concrete. This walkway along the Mississippi River illustrates (in blue Tintura stain) the convergence of the Mississippi and Minnesota Rivers. Photo courtesy of L.M. Scofield Co.

Workers use a hand stamp and a vibrating compactor to finish freshly-poured pervious concrete in a confined area. One great advantage of using pervious concrete in an enclosed spot like this is its built-in drainage.

Installing Pervious Concrete

Pervious concrete is delivered to the jobsite by conventional ready-mix trucks and placed within standard forms. Because pervious concrete is thicker in consistency than regular concrete, a vibrating mechanical screed is used to level it off. Vibration is followed by compaction with a heavy steel roller to attain greater strength.

Because pervious concrete has a low water content, curing is especially critical. After placement, the concrete is misted with water and then covered with plastic sheeting and kept damp for at least 7 days to allow full hydration of the cement.

Often paving crews can complete pervious concrete jobs faster than when installing regular concrete. That?EUR??,,????'?????<

Essentially, pervious concrete is a structural concrete pavement with a large volume (15 to 35 percent) of interconnected voids. Like conventional concrete, its made from a mixture of cement, coarse aggregates, and water. However, it contains little or no sand, which results in a porous open-cell structure that water passes through readily.

While this thirsty pavement is an excellent option for certain situations, it may not always be a viable choice.

Rain that hits this pervious concrete driveway in Northern California will percolate through instead of puddling and flowing off the surface. This surface was created with dark-colored aggregate, which yields a color close to traditional asphalt. Photos courtesy of Pervious Concrete California

The crew that completed this pervious concrete job used a brown-colored aggregate to produce a finished surface that closely matches the on-site soil.

Limits on Use

Because pervious concrete has a rough-textured, honeycombed surface, moderate amounts of surface raveling are normal. This can be a problem on heavily traveled roadways.

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Special attention must also be given to the overall design of the pavement system in order for pervious concrete to perform as intended. Proper engineering of the substrate beneath the pavement is essential, since it must be able to temporarily store the water while it percolates into the soil. An initial soils site survey and site-specific stormwater calculations should be performed by a stormwater management engineer

This view of a traditional brick driveway shows how grout traps water on the surface after a rain or hosing down. Despite the lack of permeability, brick?EUR??,,????'?????< Photo courtesy of landcraft inc.

A crew in Tustin, Calif. finishes a stained and stamped concrete driveway constructed using traditional concrete. An informal survey shows that pervious concrete is less used in low-precipitation Southern California, but its popularity is growing there too.

Accommodating Peak Flow

When pervious concrete is used for paving, it can take in stormwater at a rapid rate of 3 to 5 gallons per minute per square foot of surface area, which exceeds the flow rate needed to prevent runoff in most rain events. The rainwater may be stored in a coarse gravel layer underneath the pavement or allowed to percolate into the underlying soil. Because the pavement itself acts as a retention area, it helps to prevent much of the polluted runoff that normally occurs with impervious pavements. The filtration process also helps to purify the water. As the water percolates through the open cells of the pavement, aerobic bacteria in the voids help to break down harmful pollutants and chemicals.

While pervious concrete pavements may be new to some areas of the country, Florida and other southeastern states have been installing them since the 1970s to control runoff, erosion, and flooding. Florida has been a leader in the construction of pavements using pervious concrete, according to the Florida Concrete & Products Association. Hundreds of projects have been completed statewide, with many pavements in service for more than 10 years.

The worker at left is placing perimeter blocks that will enclose the patio and create a clean edge. The next step at this Califon, N.J. home will be the pouring of a concrete base?EUR??,,????'?????<

Workers place interlocking concrete pavers on screeded and compacted sand in this view. One rule stays the same regardless of paver type?EUR??,,????'?????< Photos courtesy of Jeffrey D. Robinson, Landcraft Inc.

More recently, pervious concrete has been embraced by the West Coast for its environmental benefits. For example, pervious concrete is helping communities in California and Washington restore groundwater supplies and reduce pollution of coastal waters, which can endanger fragile aquatic ecosystems and even swimmers. In a study conducted by researchers at the University of California at Irvine, contaminated runoff has been linked with waterborne illnesses in surfers in urban areas.

In the Pacific Northwest, most of the annual precipitation comes from rainfall events of less than an inch. A stormwater management system using pervious concrete can be very effective at reducing total runoff and increasing the amount of filtered groundwater, according to Glacier Northwest, the areas largest concrete supplier.

In California, pervious concrete is primarily being used to pave parking lots, says Youngs. But he notes that its also becoming a practical alternative for subdivision streets, sidewalks, and golf cart paths.

More homeowners are using pervious concrete as well, to eliminate puddling, prevent erosion, and save the expense of tying into local storm sewer systems. Typical applications include driveways, walkways, pool decks, and patios.

Below are links to projects across the country that have used pervious concrete for purposes ranging from controlling water runoff, to protecting sensitive fish habitats, to irrigating land.

City officials in Cerritos, Calif., overcame pollution runoff problems by paving a 10-acre overflow lot for a huge auto mall with 90,000 square feet of pervious concrete. This is the second largest pervious installation in the nation and the second lot in the state to use pervious concrete paving:https://www.scrpa.com/mportal.aspx

The Washington State Aggregates and Concrete Association investigates the use of pervious pavements to manage stormwater and to protect endangered salmon habitats in Puget Sound: www.washingtonconcrete.org

A list of pervious concrete paving projects compiled by the Pacific Southwest Concrete Alliance.

Sustainable landscaping activists Owen Dell and Melanie Yanke of Santa Barbara, Calif., install a pervious concrete driveway to prevent water runoff: www.cncpc.org

Finley Stadium in Chattanooga, Tenn., installs pervious concrete in the parking area to collect stormwater runoff and use it to irrigate the playing field: www.nrmca.org/aboutconcrete/commercial/chat.asp

The City of Santa Monica is installing pervious concrete gutters on several streets to prevent polluted runoff that eventually makes its way into Santa Monica Bay: www.smgov.net

Oregon Zoo?EUR??,,????'?????<

www.northwest.construction.com/features/archive/www.northwest.construction.com

Shredded rubber creates a permeable, cushioned surface in the play zone at foreground here. Decomposed granite forms another permeable surface around the garden planters. The crew at work in the background are preparing the subgrade for the site?EUR??,,????'?????< Photo Courtesy of Pacific Concrete California

Economic Benefits

In general, initial costs for pervious concrete pavements are higher than those for conventional concrete or asphalt paving. But total costs can be substantially lower.

The material itself is only a little more expensive, but we tend to install pervious concrete thicker than regular concrete, said stormwater expert Andy Youngs. The reason is that we know the water is going to go through and saturate the subgrade underneath. So we have to design for a weaker subgrade. With a pervious parking lot, we may go 6 inches thick versus 4 inches for conventional concrete.

But he adds that when you compare overall installation and life-cycle costs, pervious concrete is the clear winner. You just cant look at per square foot costs. You have to look at overall system costs, he says. For parking lot owners, pervious concrete is a sustainable product that actually saves them money. It ends up being less expensive than a conventional parking lot.

From Field Survey of Permeable Pavement Surface Infiltration Rates

By Eban Z. Bean, William F. Hunt, and David A. Bidelspach, University of Florida, Gainesville (2007).

A recent study completed at the University of Florida at Gainesville found that regular maintenance is critical for permeable concrete and pavers. Vacuum sweepers should be operated periodically to sweep away dust and grime that accumulates and clogs surfaces. This BMC Pro 522 was photographed on a cobble street in Turkey. Push-operated sweep-vacs are also available for purchase or rental.

Two key observations were drawn from this field study: 1.) maintenance was key to sustaining high surface infiltration rates for concrete grid pavers (CGPs); 2.) the siting of permeable pavement applications, including permeable interlocking concrete pavers (PICPs) and permeable concrete (PC), away from disturbed soil areas was a significant factor in preserving high surface infiltration rates; and permeable pavement that was installed in sandy soil environments maintained relatively high surface infiltration rates, without regard to pavement age or type.

As a result of this study, suggested siting and maintenance guidelines are as follows:

1. For CGP sites filled with sand: To sustain higher surface infiltration rates, maintenance, using a vacuum sweeper, should be performed at regular intervals Balades et al. 1995; Hunt et al. 2002. Removal of the top 13?EUR??,,????'?????<

2. For PICP/PC sites: PICP and PC sites installed for infiltration purposes should not be located adjacent to areas with disturbed soils as accumulations of fine particles have been shown to significantly and dramatically decrease surface infiltration rates. Maintenance should include regular use of a vacuum sweeper, or as needed, for sediment accumulation on the surface Balades et al. 1995; Hunt et al. 2002. Problems with fines should be addressed before the fines are either compacted into void spaces or migrate to lower, harder to maintain depths within the pavement void profile. Permeable pavements installed in stable watersheds will function substantially better than those constructed in unstable watersheds.

Source: Journal of Irrigation and Drainage Engineering

The Concrete Network

www.ConcreteNetwork.com

Established in 1999, The Concrete Network?EUR??,,????'?????<

The web site excels at connecting property owners and contractors with local sub-contractors through its Find a Concrete Contractor service: www.concretenetwork.com/contractors.

The service provides visitors with a list of decorative concrete contractors throughout the U.S. and Canada, and is fully searchable by 23 types of decorative concrete work and 202 regional areas throughout North America.