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Rain, Rain, Go Away with Permeable Paving

April showers bring May flowers.

April showers also bring storm water rushing down driveways and across sidewalks, into the streets, picking up contaminants from air pollution, spilled oil, detergents, solvents, de-icing salts, dead leaves, pesticides, fertilizer, and bacteria from pet waste.

Courtesy PCA (www.cement.org).Aaaah, spring.

However, it doesn't have to be this way at your home.

Just as drinking water can be filtered to remove impurities, the earth filters rainwater. Unfortunately, common paving gets in Mother Nature's way, carrying rainwater directly to storm drains and into waterways, if you are lucky. It also can contribute to the erosion due to improper drainage -- on or off your property.

This is why permeable paving is a valuable upgrade. These pavement mixtures and grid systems minimize water runoff problems, provide groundwater recharge, facilitate pollutant removal and improve the aesthetics of your home.

Courtesy PCA (www.cement.org). Installing pervious concrete or asphalt is generally the same as its impervious counterpart. Not surprisingly, the biggest difference is the substrate materials -- the layer below the paving -- that must be designed to handle the water. Installing porous pavers is easier in some ways than installing impermeable pavers.

Make sure you choose the most appropriate type of permeable pavement system for your situation. There are several options available:

Porous Asphalt: A great advantage to porous asphalt is that the same mixing and application equipment is used as for impervious asphalt. Only the formula for the paving material changes. The amount of asphalt binder required is about 6 percent of weight, which is somewhat higher than required for standard impermeable asphalt.

Porous Concrete: Once again, porous concrete uses the same equipment and process as common concrete. The difference is larger pea gravel and a lower water-to-cement ratio to achieve a pebbled, open surface that is roller compacted.

Courtesy PCA (www.cement.org). Plastic Grid Systems: High-strength plastic grids-often made from recycled materials-are placed in traffic areas. Some are designed to be filled with gravel on top of an engineered aggregate material, while others are filled with a sand/soil mixture on top of an aggregate/topsoil mix that allow grass to be planted on the surface. The grids provide a support structure for heavy vehicles, and prevent erosion. After heavy rains, the grids act as mini holding-ponds, and allow water to gradually absorb into the soil below.

Block Pavers: This material can be used to create a porous surface with the aesthetic appeal of brick, stone, or other interlocking paving materials. Just use sand in the joints instead of mortar. They are most often used for driveways, entryways, walkways, or terraces to achieve a more traditional, formal appearance.

Gravel: Yes, that's right, plain old gravel. Lots of driveways used to be surfaced with it. Some types look beautifully elegant. Some just do the job. But talk about pervious. . .and less labor goes into the whole process, including the manufacturing part. So in the right application gravel is the greenest choice.

Permeable paving is most appropriate for sidewalks, patio, driveways or alleys. You know, around your house. Parking lots, too. It is not for high traffic/high speed areas because it has lower load-bearing capacity than conventional pavement.

As for initial costs, permeable paving is competitive with conventional materials; sometimes lower, sometimes higher. Definitely better for the environment.

For your household project, you should run the numbers, look for the one that appeals to you, and decide what's best for you.

Resources:

Center for Watershed Protection

National Pavement Contractor’s Association

Pennsylvania Stormwater Management Manual Porous pavement specification

United States Environmental Protection Agency

Content updated on 4/6/2007

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