Use of Sustainable Concrete Alternative is Emerging
Salt Lake City had a dilemma. In the center of a downtown parking lot of the Wasatch Touring store, a large silver maple tree was slowly dying. Surrounded by impervious concrete, the tree was withering because water could not reach its roots. The city’s Urban Forestry Division approached the civil engineers at Psomas, who agreed to work on a pro bono basis to develop a sustainable solution.
Psomas redesigned the parking lot with pervious concrete that 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 a relatively new practice in the Western United States. It has been used extensively in moist climates like Florida for nearly three decades. About six years ago, pervious concrete started showing up in newly constructed parking lots and alleys in cold-weather 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.
Pervious concrete also reduces stormwater impacts by substantially decreasing hard surface runoff. Pervious pavement can handle a large volume of water, absorbing from 3 to 17 gallons per square foot per minute, depending on the pavement mix design. The visitor center walkway entrances that Psomas designed at Utah’s 1,200-acre Swaner Nature Preserve can hold 10 inches of stormwater in a gravel layer underneath the concrete. Parking lots – and even alleys – can become stormwater management systems. In Chicago, pervious pavement is currently being installed in about 2,000 miles of alleys to manage urban runoff.
Additionally, 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 LEED Platinum projects in Utah.
Pervious concrete also cleans stormwater by filtering it and removing many of its suspended solids such as fine-grained particles, sands, dirt, and grime.
Predominately Light Uses
Lacking the tensile strength of reinforced concrete or asphalt, pervious concrete is currently not used for major roadways. It works best in light parking lots, alleys, and walkways. It can also be used on urban streets that handle cars traveling as fast as 35 mph.
At the Beijing Olympics, however, the aesthetic advantages of pervious concrete were used to the fullest. About 2.7 million square feet of pervious concrete was installed 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 smaller aggregate that resembles Rice Krispie Treats. The top layer can be intensely colorized and stamped, as was done in Beijing.
Design and Installation
Installation Preparation Essential
Pervious concrete is typically more expensive than traditional concrete in terms of preparation work and installation. While the same materials are used, more design work and installation steps are involved.
Pervious concrete can be used to manage stormwater by either re-introducing it to groundwater (retention), or collecting and releasing it to a downstream storm drain infrastructure system at a prescriptive rate (detention). Whether it’s used for retention or detention, the pervious concrete system comprises of two main layers. The top layer consists of the cured pervious concrete slab (usually 4 to 8 inches thick). Underneath the pervious concrete is a gravel layer that provides additional compressive strength and storage space for stormwater.
Mix design is key to a successful pervious concrete layer because the cured slab must achieve a sensitive balance between strength and porosity. Aggregate size is one of the more important components for achieving porosity. Unlike typical concrete, pervious concrete requires a more uniformly sized aggregate mix. For example, no more than 40 perecnt (by weight) of the aggregates should pass the No. 4 sieve. Well-graded aggregates – the aggregate mix for typical concrete – increase workability and compressive strength; however such a gradation decreases the porosity of the pervious pavement.
The amount of cementitious material in the mix is another key component in the pervious concrete mix design. Some might assume that a more poorly graded aggregate mix, as found in pervious concrete, might warrant an increase in cementitous material. However, too much cementitious material may result in a continuous slab of void-less concrete on the bottom of the pavement layer. If this occurs, water will not drain adequately through the cured pavement, which will cause the concrete to break apart during the freeze/thaw cycles. Also, a high water-cement (w/c) ratio may contribute to concrete breakage. Therefore, the w/c ratio for a pervious concrete mix is a little lower than a typical concrete mix design.
Soils are a primary challenge. Unlike traditional concrete applications, engineers must do a great deal of accommodation in the design phase based on the soil conditions defined by geotechnical investigations. The time it takes for stormwater to leave the pervious concrete layer and the gravel layer (drawdown time) is an essential part of the design. It is common for the draw down time to take no longer than one or two days.
Since one goal of pervious concrete is to promote groundwater recharge, clay subsurface soils can pose a problem. Clay soils have a relatively low percolation rate, decreasing the exfiltration rate from the pervious concrete layer. Some designers propose measures that mitigate the effects of slow draw down times. For example, a layer of clay underneath the downtown Salt Lake parking lot prohibited water from percolating into the soil. Trenches and bore holes 15 feet deep and 12 inches in diameter were proposed beneath the gravel and pervious pavement layers so that the stormwater could pass through the clay layer and into a more pervious soil layer, increasing the exfiltration rate and decreasing draw down time.
Water Storage Issues
When pervious systems are installed, stormwater runoff will flow directly through the paving system down to the gravel layer, where it will remain until it infiltrates back into the groundwater system. The amount of water storage needed depends on each site. Therefore, engineers must consider the size of the tributary area.
The Salt Lake City pervious pavement site, for example, collected stormwater only from the footprint of the parking lot itself. Conditions were different at the Sutton Geology and Geophysics Building at the University of Utah. Psomas, in collaboration with civil engineering students, designed the LEED Gold building’s 2,600 square-foot driveway with pervious concrete. The Sutton building’s pervious pavement accepted runoff from other upstream areas, which required a larger gravel storage space beneath the pervious pavement.
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.
As noted above, the w/c ratio for a pervious concrete mix is a little lower than a typical concrete mix design. However, the proper w/c ratio is critical. Every ounce of water added to the mix is needed to glue all aggregate together and therefore provide adequate compressive strength. For this reason, poured pervious concrete is covered and sealed with poly sheeting and left to cure for several days. This way, moisture loss to evaporation is minimized and the compressive strength is not compromised.
Results Well Worth It
Although added costs, extra training, and challenges may be involved, the sustainable rewards of using pervious concrete are well worth it and will be enjoyed for generations to come.
Scott Rocke, PE, head of the Psomas land development team in Utah, has more than 20 years of experience in the AEC industry. Jonathan Bowers, LEED AP, is the Psomas project civil designer on the Wasatch Touring Building in downtown Salt Lake City, the Swaner EcoCenter, and the Sutton Geology Building projects. Rocke and Bowers both work in the firm’s Salt Lake City office.