Center Installs Environmentally Friendly Driveway

10/01/03 by Staff Writer

Kurt Cribb explains permeable concrete to students.

October 2003

Special projects coordinator Kurt Cribb pours a five-gallon bucket of water on the driveway behind the Catawba College Center for the Environment, and the water soaks in as fast as if the driveway were thirsty soil.

The driveway, which was installed in October 2003, is a pervious concrete surface, which allows water to percolate through it into the earth. “The Catawba Center for the Environment has worked to integrate lots of green aspects into both its sustainable building and the land surrounding it,” says John Wear, the Center’s director. “We are always looking for ways to demonstrate green techniques, and pervious concrete is one of the products we want to explore.”

Wear notes that thousands of people visit the Center each year to learn about sustainable practices. “This gives us a good opportunity to demonstrate to others methods that appear to help protect our water resources,” he says.

Ten companies associated with the Carolinas Ready Mixed Concrete Association installed the 94-foot pervious concrete driveway, according to Cribb. The companies donated labor and materials worth about $5,000 for the project.

Pervious concrete is made of coarse aggregate, cement and water with no sand. It is considered an environmentally friendly alternative to asphalt and impervious concrete surfaces because it has an open cell structure which allows water to pass through it.

“If rain falls on asphalt pavement, it grabs all the oils and antifreeze and other pollutants and sends them down into our stormwater system,” says Finley Messick, director of education and marketing for the Concrete Association. Pervious concrete, on the other hand, lets the earth naturally filter the pollutants. Bacteria that live within the pervious concrete even break down the pollutants before they reach the earth.

The association lists a number of environmental benefits associated with the porous pavement:
1) Vegetation is watered, reducing the need for irrigation;
2) ground water is recharged;
3) water resources are preserved;
4) stormwater runoff is reduced; and
5) stormwater runoff quality is improved.

Developers and companies use pervious concrete to reduce or eliminate the need for retention ponds to capture the stormwater runoff that occurs with conventional surfaces. South Carolina has approved the product on a limited basis as a stormwater management system, according to Messick. “It means you don’t
have to put in the huge retention ponds that you see in new commercial buildings,” he says.

Pervious concrete also stores less heat than conventional parking lots. The open structure allows the cooler earth temperature from below to cool the pavement. It has been used in the United States for about 20 years and in North Carolina for about 10 years.

Dr. Michael Leming, an associate professor in the Department of Civil, Construction and Environmental Engineering at North Carolina State University, has done considerable research on the product and finds it promising. “We are always looking for new ways to do things a little bit better and smarter,” he says, “and I think this really has a lot of potential.”

Leming’s studies suggest that the product has good frost durability. “Even when we try hard to saturate it, it takes a lot to get every single one of the pores filled,” he says. He found that the existing water which would typically freeze is sometimes below the frost line. Even if it’s above the frost line, the research indicates that there is room to allow the ice to expand without causing disruption.

The parking lot at the Friday Center at UNC Chapel Hill, which is made of pervious concrete, has weathered freezing rain and snow well, according to Leming. However, he recommends that if people use a snow plow on pervious concrete surfaces, they don’t plow very deep because the material is more sensitive to shearing forces than regular concrete.

Some have voiced concern about the pores in the pervious concrete clogging if the surface experiences a heavy rain. “In most areas, we don’t see a problem with that,” Leming says. He and others recommend putting a filter fabric and stone base under the pervious concrete on clay soil to mitigate clogging.

Another option is to drill through the clay layer and create a well. “You fill it up with stone and then the water will move through the well into the underlying layers,” Leming says. Messick notes that if the soil will perk for a septic tank, it will handle pervious concrete.

Maintenance of the surface is an issue that hasn’t been formally settled. Leming recommends vacuum sweeping or hosing down the surface. “You can have the fire department come out and let them practice on the surface,” he says. “As long as you don’t spray high pressure right at the surface, simple washing will remove a lot of material.”

Placing the material in areas where the ground water table is close to the surface is another cause for concern, according to some critics. “Those are issues that can be addressed by a registered professional engineer,” he says. Good design is critical to the performance of the material.

Leming offers two bits of advice: 1) Placement of the pervious concrete should be done by a person who knows what he is doing. “If you go out and get the low-bid contractor, there are going to be problems,” he says. 2) Contractors must take special care if they place the product on a slope. “We have seen some situations where people didn’t think through the process, and the water just poured out the bottom of the driveway,” he says. The solution is to dig deeper on the down-drain side and use clean stone to fill up the hole.

Cribb notes that the pervious surface seems to be doing well, and he is hopeful that it will perform well over time. If so, Catawba’s driveway can be used as a teaching tool for engineers all over the country.

“It will also reduce the amount of stormwater runoff we have in the creeks,” he says. “The water will actually go back into the ground like it’s supposed to and be purified and cleaned.”
The environment is the beneficiary, according to Cribb: “It’s good for our environment and it’s good for our water quality.”

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