While many large cities are greening the sunny rooftops of skyscrapers, Chicago is branching out into its nearly 2,000 miles of dark, dank alleyways.

Chicago Dept. of Transportation ‘Thirsty’ materials on Chicago’s green alleyways help drain rainwater into ground.

That is where the city is experimenting with pervious pavement, a mix that allows water to seep through the material. “We were one of the first agencies to try it,” says Cindy Williams, quality assurance manager for Chicago Dept. of Transportation, which oversees the city’s 1,900 miles of alleys. One of the main objectives of the city’s “Green Alleys” project is to divert from strained sewers stormwater that normally lands on 3,500 acres of alleys.

Material producers are helping the city experiment with pervious concrete, asphalt and paving stone. Brian Lutey, vice president of green building for Mokena-based Ozinga Illinois, became convinced of the advantages during his time as promotions manager for the Indiana Ready Mixed Concrete Association, a leading proponent of pervious concrete.

Chicago’s $900,000 pilot program fully reconstructed five alleys in 2006 with pervious pavements. In 2007, more green products were applied to the rehab of 22 alleys. Because rainwater drains through pervious material, city officials, who have standing-water complaints in more than 500 alleys, think the material will help.

Chicago Dept. of Transportation Pilot program tries concrete, asphalt (above) and stone.

Pervious concrete contains little or no sand so that voids are created. Sufficient paste to coat and bind the aggregate particles creates a system of highly permeable, interconnected voids that drain quickly. The rocky, low-slump mixture is placed like asphalt. “It’s like the gravel is the peanut and the cement is the chocolate coating,” Lutey says.

Pervious pavements haven’t been used much domestically because owners and contractors aren’t familiar with it, and because “nobody believes research that’s done outside the area,” Lutey says. “In the Netherlands, pervious concrete is used on highways. In Japan, it’s used all over roads and parking.”

On the five pilot alleys, a few winters have passed to test the pavement’s freeze-thaw holdup, and plenty of heavy garbage trucks have rumbled across them. So far, there’s been no trouble like rutting or cracking. At the same time, there is no rush to do future projects this way. “We’re still collecting and monitoring data from those five alleys,” Williams says. “We’re going forward slowly and getting some standard specs.”

The initial cost of pervious concrete can be up to 1.5 times higher than for conventional concrete. On parking lots, it is 6 or more inches thicker than the 4-in. standard for conventional concrete. But sources say life-cycle costs can be lower because there is less need for curbs, gutters and detention ponds. Parking lot owners will spend fewer dollars on development, and the materials also promise to reduce the “heat island” effect of traditional pavements.

Still, proponents have hurdles to overcome. “When we first started looking at pervious concrete, [suppliers] were concerned that if anything went wrong it would be a huge setback,” Williams says. “Now, less than two years later, they think it’s the greatest thing since sliced bread. But the industry still doesn’t have a full grasp of its own product.”