Controlling stormwater runoff from paved areas is a source of sustainability points under either LEED or Green Globes. But more importantly, capturing this runoff at the source can actually make sites viable that might otherwise be ineligible for development and can eliminate the need for stormwater retention ponds. Permeable or pervious pavements allow rainwater and snowmelt to be collected where it falls and directed below the surface where it is stored and percolated into the soil or collected.

Pervious concrete allows surface drainage at rates that far exceed any conceivable storm. Designed and constructed properly, a pervious concrete parking lot, driveway, or street reduces heat-island effects, is durable, and can even have integral color or stamped patterns.

The Mix

An essential part of successful pervious concrete is the mix. The main requirement is that there be no fines so that the resulting concrete has many voids through which water can easily drain. It is important to work with a local concrete producer and contractor who have expertise in the best local practices for placing pervious concrete. Although pervious concrete has often been designed without admixtures, those mixes can be very stiff and difficult to place. The most common admixtures used are air-entraining agent (in freeze-thaw areas), water-reducers (plasticizer), and a viscosity-modifying admixture. The water-reducer and VMA help with the placeability of the mix and also tend to result in longer set time, giving the contractor a bigger window of workability.

Design

Since pervious concrete is intended to drain, the designer needs to determine where the water is going to go. Most soils will allow the runoff to percolate into the water table. Any soil that would be suitable for septic tanks is suitable for pervious concrete, therefore, the water table should be at least three feet below the base and any bedrock at least two feet. For very impermeable soils, a way to drain the water (such as a system of perforated pipes) would be needed, but still make pervious concrete a good choice.

Atop the soil layer, the contractor will place a non-woven geotextile to prevent the subbase from being contaminated and clogged by fines. The depth of the aggregate subbase that is then placed is dependent on the amount of runoff it is designed to store, but typically is 6 to 12 inches. A properly sized and lightly compacted subbase, with about 40 percent voids, is critical to the success of a pervious installation.

Construction

Photo courtesy of Bill Palmer.

To maintain a high void ratio in the pervious concrete, it is important not to compact it too much, while still getting it down to the 20 percent void level. This is accomplished in a couple of different ways. The ideal is to place the concrete and then use a spinning roller screed to first strike the surface off to the proper grade, and then to go over it again to get the concrete to final compaction and finish. The advantage of this method is that you can move quickly to the critical curing stage.

Other contractors will strike the surface off with a straight edge leaving the concrete about 1/4 inch higher than final elevation. Heavy steel pipe rollers will then be used manually to complete the compaction to the final elevation. No floating or troweling is needed or desirable, since that might serve to close up the surface. A roller with a 1.5-inch fin, like a pizza cutter, is used to tool in contraction joints.

Since the water content of pervious concrete is so low, any loss of water, especially at the surface, can be fatal to a successful installation. And since the concrete matrix is so open, air can pass through the surface concrete, drying it out very quickly. Most failures can be traced back to poor curing. Evaporation retarders can be sprayed onto the surface prior to getting plastic sheeting down, but the plastic should be used to cover the concrete surface as soon as possible—within 20 minutes of placing and rolling the concrete. Since the surface is already rather rough, and the concrete is so stiff to start with, there is little danger of damaging the surface finish by placing plastic film. The film should be wide enough to allow proper sealing around the edges and should stay in place for seven days—longer in cooler weather or when fly ash is used in the mix.

Durability

Pervious concrete is relatively simple, but there is a real potential for problems without experienced designers and contractors. A list of certified technicians and craftsmen is available at www.nrmca.org/certifications/pervious.

One typical concern is that pervious concrete will be unable to withstand freeze-thaw action. Experience has proven that with a properly designed subbase, pervious is very durable in cold climates. There are hundreds of successful installations in severe winter states like Ohio and Minnesota.

Another concern is clogging of the concrete matrix, but with normal maintenance, and a few common sense measures, this has not proven to be a problem. For example, stockpiling landscaping soil or sand on the pervious surface, even for a short time, is not wise. If the surface does become slightly clogged, power washing and vacuum sweeping have been successful.

Advantages

Pervious concrete can help to reduce urban sprawl, since less land area is needed for stormwater detention and retention—this can add up on larger retail and commercial projects. The Environmental Protection Agency has accepted pervious pavement as a Best Management Practice (BMP) for treating stormwater runoff. Studies in Maryland and Virginia have shown that stormwater that percolates through the concrete and subbase ends up with high reductions in suspended solids, phosphorus and nitrogen. Even oil dripped onto the surface by parked cars tends to be degraded by bacteria living within the concrete matrix. This cleaned water then serves to recharge local aquifers, rather than sending stormwater into storm drains or local surface waters. Plant life growing near pervious parking lots also benefits, since water is available to the root systems.

Another advantage occurs in winter weather. Melted snow, rather than pooling on the surface, drains away immediately, leaving a dry surface behind.

A recent success story was at the Stratford Place project in Sultan, Wash. By using pervious concrete for driveways and streets in this residential community, the developer was able to eliminate the stormwater collection system, resulting in savings of about $260,000. Since retention ponds were eliminated, he was also able to sell two additional homesites for about $100,000 each.

Pervious concrete may not be right for every outdoor surface. But, for sustainable construction, it is worth a serious look.