April 29, 2010


Experimental Lot 122 in north campus is half regular concrete (bottom) and half pervious concrete (top). Photo by Bob Elbert.

Iowa State's pervious concrete experiment is working

by Anne Krapfl

After three years of use, weather extremes and data collection, the university's experiment with earth-friendly pervious (porous) concrete has been positive by most accounts. However, the higher cost and strict requirements for the underlying soil for pervious concrete to be effective have deterred planners from installing a second one.

In pervious concrete, what looks like a highly textured concrete is only the top layer of a system that also includes (bottom to top) a fabric barrier, two inches of sand and a layer of aggregate (12 inches in half the ISU test lot, 18 inches in the other half).

Rainfall trickles through the "pervious" surface layer and is stored temporarily in the aggregate layer before soaking into the soil. The soil layer helps further remove pollutants before the water reaches the water table. The ultimate goal with pervious surfaces is to reduce the volume of rainfall reaching local storm sewers and pollutants reaching local streams.

The experiment

The experimental lot (122) is in the far north part of campus, between the railroad tracks and the environmental health and safety department's building. The 34-stall lot was constructed in October 2006; half is standard concrete, half is pervious concrete. Standard concrete is made from cement paste, limestone aggregate of an inch or less and medium-coarse sand. Pervious concrete differs in that the limestone pieces are smaller and more uniform in size and most of the sand is removed, creating an 18 to 25 percent void space. The higher end of this range works in the southern states, but the repeat freeze-thaw pattern in Iowa's winters requires more sand in the pervious mix for greater strength and durability in freeze-thaw cycles.

Extension civil engineer Stephen Jones and his team from Iowa State's Concrete Pavement Technology Center began studying pervious concrete for northern climates about six years ago and developed the "recipe" for the version used in lot 122. Through this fall, his team is tracking the performance of the lot. The test lot is featured in their pervious concrete recommendations in the Iowa Stormwater Management Manual. Here's some of what they learned so far:

"It can take a lot of water."

--Stephen Jones

Strength through the Iowa winters

Jones said the pervious concrete is holding up well.

"There are some scrapes and stains, but the structure of it is doing just fine," he said. There have been no incidents of cracking or buckling.

Regular (Portland) concrete can withstand 3,500-4,000 pounds of compression per square inch (psi); the pervious sample in lot 122 has a compressive strength of 3,200-3,500 psi. By contrast, Southern varieties of pervious concrete, containing virtually no sand, can withstand about 2,000 pounds per square inch. The mix designed for lot 122 includes 6-7 percent (by total weight) sand to make the concrete stronger, but also diminishing its permeability.

Jones' team placed six temperature sensors in the various layers of the pervious system. He said they believe that the air voids act as an insulator against winter surface temperatures. They were surprised that the measured temperatures in the pervious concrete typically are 5-6 degrees warmer than regular concrete. He said they also haven't seen freezing and thawing in the soil below the pervious concrete lot.

Rain and snow runoff

"It can take a lot of water," Jones said.

Once a year his team conducts permeability tests, using large buckets of water, to see if the void spaces in pervious concrete are staying clear. The lot is designed to handle 300 inches of water in an hour. In tests, Jones said the middle of the lot took as much as 900-1,000 inches of water per hour. In the 40 months that Jones has been monitoring the lot, he said Ames received one major storm -- four inches in 24 hours. Most rains delivered 1.25 inches of water, or less, in 24 hours.

Trapping surface pollutants

Jones said his team still is working on this piece of the research. They expect to have data later in the fall.

Lot maintenance

A key drawback to pervious concrete in northern climates is that sand can't be applied on icy patches because it would plug the voids in the concrete. As with all concrete, salt can be applied after the first year; it dissolves in the melting ice.

Les Lawson, manager of campus services, said that with the exception of sand, his staff has been able to maintain lot 122 much like other parking lots. He said ice is rare on the pervious concrete because the water drains before ice forms or as it melts. His team uses the same street sweeper in lot 122 it uses elsewhere (a combination of brushes and a vacuum). All lots get cleaned two to three times each year; lot 122 typically gets an extra cleaning, Lawson said.

Success is not guaranteed

"To be fair, there are pervious concrete projects in Iowa that haven't worked," Jones said.

The failures typically are due to poor installation (teams not well trained in this method), soils below that contain too much silt or clay and thus are impermeable and not suited to this system, or a bad concrete mix (such as too much water in the concrete, too much sand or fine gravels in the aggregate layer).

For example, Jones said using a pervious system for the lots at the Iowa State Center wouldn't work. The land is in a flood plain and the soil contains a lot of silt.

"We could get the water through the pervious pavement, but then where would it go?" he said.

Jones said 80 percent of pervious concrete failures are due to ineffective erosion control adjacent to the pavement that, during storms, allows sediment to wash onto its surface and plug the voids.

Always a consideration

Mark Miller, manager of the parking division in the department of public safety, said there aren't any plans yet for a second pervious concrete use on campus. But, nearly every time a lot needs to be repaved "we take a look at whether this is a good place to do a pervious pavement, either concrete or asphalt." His considerations include the cost and whether there are grants available to try something new, and the financial return on the lot.

Jones said pervious concrete still costs 15 percent more per square yard than standard concrete. Part of that is labor, because not all concrete crews are trained in the technique yet. And part of it is because pervious concrete requires greater size consistency in the materials that go into it.