The City of Salt Lake had a dilemma. In the center of a downtown parking lot, a large silver maple tree was slowly dying. Enclosed with impervious concrete, the tree was withering because water could not reach its roots.
The city’s Urban Forestry Division approached our civil engineers who agreed to work on a pro bono basis to develop a sustainable solution to the problem. They redesigned the parking lot with pervious concrete, which allows water to seep through the surface directly into the soil beneath. It was too late to save the silver maple, but the Urban Forestry Division plans to plant several new trees in the parking lot once the project is completed in early 2009.
The use of pervious concrete is relatively new to the Western United States. It has been used extensively in moist climates like Florida for nearly three decades. And about six years ago, pervious concrete started showing up in newly constructed parking lots and alleys in freezing markets like Indiana, Ohio, Illinois, and Wisconsin.
A Sustainable Approach
Pervious concrete brings with it a number of sustainable rewards. The use of pervious concrete helps to recharge the underground aquifer. The parking lot re-design in downtown Salt Lake City is helping to recreate the natural environment that existed before the parking lot was installed by absorbing water and letting it find its way back into the aquifer.
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Pervious concrete also reduces stormwater impacts by eliminating hard surface runoff. The pervious pavement can handle a large volume of water, absorbing from three to 17 gallons per square foot per minute, depending on the pavement mix design. Parking lots can become stormwater management systems. It is currently being installed in about 2,000 miles of alleys in Chicago to accumulate urban runoff. The visitor center walkway entrances Psomas designed at Utah’s 1,200-acre Swaner Nature Preserve can hold 10 inches of stormwater in a detention basin underneath the pervious pavement.
Obviously, pervious concrete’s sustainable attributes help garner LEED points. The 10,000-square-foot, state-of-the-art Swaner EcoCenter is one of the first Platinum LEED projects in Utah.
Pervious concrete also cleans stormwater by filtering it and removing many of the suspended solids in water runoff like fines, sands, dirt, and grime.
Predominately Light Uses
Lacking the tensile strength of reinforced concrete or asphalt, pervious concrete is not used for major roadways. It works best in light parking lots, alleys, walkways and urban streets handling cars up to 35 mph.
At the Beijing Olympics, however, the aesthetic advantages of pervious concrete were used to the fullest. Some 2.7 million square feet of pervious concrete was used in dock frontage for the rowing and sailing venue. An unusual multi-layered approach was used. The first layer, or “lift”, consisted of larger aggregates. A second top lift was mixed with a finer aggregate that resembles Rice Krispie Treats. The top layer can be intensely colorized and stamped.
Installation Preparation Essential
Pervious concrete is typically more expensive than traditional concrete in terms of preparation work and installation. While the supplies are the exact same materials used for traditional concrete, there is more design work entailed and installation steps involved.
Subsurface preparation is crucial. At the Swaner EcoCenter, it was necessary to design a gravel layer holding pond underneath the concrete. A similar approach was employed at the Gold LEED Sutton Geology and Geophysics Building at the University of Utah, where the building’s 2,600 square-foot pervious concrete driveway will allow stormwater runoff to flow directly through the paving system down to the gravel layer, where it will remain until it infiltrates back into the groundwater system.
Soils are a primary challenge. Unlike traditional concrete applications, engineers must do a great deal of accommodation in the design phase based on the geotechnical report. For example, a layer of clay underneath the downtown Salt Lake parking lot prohibited water from percolating into the soil. The design specified bore holes through the clay 15 feet deep and 12 inches in diameter into a more pervious soil layer so water can percolate down.
Training for Success in Arid Climates
The West’s arid climate makes the use of pervious concrete challenging. Quality control during mixing and installation is important and proper training is essential. Pervious concrete requires a major departure from typical rules of thumb for concrete mix design. Production and installation require careful attention to the critical moisture content in the material to preserve humidity. Installation includes a protective covering of plastic sheeting to hold in moisture for several days of curing.
Although there may be added costs involved, extra training needed, and challenges to face, the sustainable rewards of using pervious concrete are well worth it and will be enjoyed for generations to come.
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