Unilock’s Eco-optiloc precast permeable concrete pavers were used to create the largest permeable parking lot on record at Chicago’s U.S. Cellular Field. The park’s drainage capacity is 1.15 million gallons—or 9.25 million 16-oz. sodas. Image courtesy of Rose Paving and Unilock.

One of the most pressing concerns for city planners today is stormwater. In some cities, new regulations dictate the management of stormwater from the minute the first raindrop falls; Chicago, one of the U.S. cities at the forefront of the sustainable movement, enacted a new stormwater management ordinance this year that requires any regulated development to have a city-approved plan in place that manages the rate and volume of runoff, among other things.

That’s a lot to swallow for some, but in many cities pervious concrete pavement is helping meet new stringent stormwater management requirements.

How is pervious concrete produced?

Unlike conventional concrete, pervious concrete mixes contain limited or no fine aggregates, producing concrete with approximately 20 percent voids. Those spaces allow water to flow through the pavement, then a bed of uniformly sized aggregate below, and into the soil—reducing or eliminating stormwater runoff. Average strengths of pervious concrete are from 500 to 4,000 psi, but can go higher depending on the application; percolation rates average from 288 to 770 inches per hour.

Dan Huffman, managing director of national resources for the National Ready Mixed Concrete Association, says pervious concrete is gaining ground fast. “The U.S. Environmental Protection Agency recommends pervious concrete as a Best Management Practice with compliance with federal stormwater law, and that’s a very powerful thing to say,” he says. “We should be using greywater water all over the place; we waste so much water and we just don’t have to,” he argues.

An alley off of Chicago’s Rockwell Avenue, before (top) and after (bottom) pervious pavement was placed by the Chicago Department of Transportation. Images courtesy of Hitchcock Design Group.

Keeping it clean

The material not only helps manage runoff and capture water, but cleans it as well, according to Stew Waller, executive director of the Arizona Cement Association. “Some heavy metals and contaminants you find in the first flush of a rain storm, hydrocarbons…it washes off all the dirt and contaminants from the pavement,” Waller says. In fact many pervious pavements develop a microbial system in the voids that attacks and consumes these pollutants, so water flowing into the soil is cleaner.

Using least-cost analysis to determine value

Waller points to new pervious pavement at the Glendale, Ariz., Park-and-Ride as an example of the technique at its best. While pervious pavement was more expensive than conventional asphalt pavement upfront, a least-cost analysis considering 20-year service life maintenance costs for both systems proved pervious concrete to be equal in cost in (the city chose to also incorporate additional sub-drainage systems and retention ponds). In more typical new paving situations, he says, using pervious to eliminate conventional sub-drainage infrastructure trumps asphalt in both initial cost and extended service life.

Manufacturer puts pavement into action

An early adopter of pervious pavement for industrial use is Quality Block of Phoenix, Ariz. Owner Clem Hellman says his plant is known for its cleanliness — but the land where pallets of block were stored was plain soil, and operations in that area kicked up dust that collected on equipment.

In summer 2007, Hellman worked with Arizona Materials and Phoenix Cement Company to create pervious pavement for 20,000 square feet of plant yard, the first-known industrial application, he says. Eight inches of pervious concrete were placed over 12 inches of aggregate to create pavement strong enough to hold stacks of four cubes of concrete block — each cube weighing 3,000 pounds — and to withstand frequent forklift traffic.

“I have nothing but good things to say about it,” says Hellman, who praises the resulting cleanliness of his yard, the more stable storage area that offers increased flexibility, and the ability to hold stormwater in the pavement — eliminating the need to buy extra land for a retention pond.

Chicago goes green…and pervious

The impact of Chicago’s new stormwater management ordinance is something the city’s department of transportation (CDOT) has to deal with every day, and the Chicago Green Alley Program aims to deal with flooding that impacts much of the city’s 1,900 miles of public alleys.

Created by CDOT and landscape architects at Hitchcock Design Group (along with civil engineering firm Knight E/A, environmental engineers Hey and Associates and material testers S.T.A.T.E. Testing), the program includes four pilot alley designs. There’s also a Chicago Green Alley Handbook full of sustainable principles for residents who live adjacent to alleys, says Bill Schmidt, senior associate with Hitchcock Design Group, whose sketches — based on his own alley — helped form some of the project concepts.

The favorite design of David Leopold, project manager for CDOT, and Janet Attarian, the agency’s project director and sustainability coordinator, uses pervious pavement for a center trench in the alley and conventional pavement on the sides for wheel traffic. “We have a lot of alleys with adjacent basements…the center trench keeps the infiltration away from a building,” explains Attarian. They create a vertical approach by placing approximately five feet of aggregate beneath the pavement, surrounded by a waterproof membrane, to ensure water doesn’t seep into neighboring basements.

The pervious concrete alleys do more than collect and divert stormwater. The high-albedo concrete increases reflectivity and reduces the urban heat island effect (UHI), says Attarian; the concrete incorporates recycled slag cement; and the base course is recycled concrete aggregate. Leopold points out that by reducing the UHI effect, residents save money on air-conditioning costs, and by diverting water from sewer systems the city saves money on energy costs for pumping and cleaning the water before it’s returned to the Chicago River. By the end of 2008, about 60 alleys will be repaved with pervious concrete, and that number will continue to grow.

Even the Chicago sports community is jumping on the pervious bandwagon. U.S. Cellular Field, home to the White Sox, recently placed the largest permeable parking lot in the country: 265,000 square feet of mechanically installed Unilock precast pervious pavers that include recycled content (slag cement). Chuck Sampey, director of development and facilities for the Illinois Sports Facilities Authority (which owns the ballpark) says the cost savings is in the millions, compared to installing underground storage systems and high-maintenance asphalt paving. For future projects, he says, “everything is moving toward a green solution — no doubt we’ll go for permeable pavers.”

Prime Outlets employs pervious

At the Prime Outlets, conventional concrete was used for traffic lanes, while pervious concrete was used for parking stalls. Image courtesy of Titan Virginia Ready Mix.

As part of a mall reconstruction, Prime Outlets in Williamsburg, Va., recently completed a parking lot replacement that employed approximately 7.5 acres of pervious concrete pavement and 3.5 acres of conventional concrete pavement. (Titan Virginia Ready Mix supplied its Envira brand of pervious pavement.) The system diverts water into the soil instead of the mall’s old stormwater management approach: a detention pond. The pond has been paved over, creating much more land for development, and additional water-harvesting technology helps capture water for landscape irrigation.