Pervious concrete may eliminate need for storm drains
nicely positioned on Lake Owasso, and currently the storm water from the street just washes directly into the lake,” said Mayor Sandy Martin. “For years, some of the people who live near the discharge have asked the city to create a holding pond or a way to divert the water before it goes into the lake, carrying salt and sand and oil and all those things. But there is no land available in this neighborhood to create any kind of a holding pond.”
Because the soil under the street has the high sand content that would promote the desired drainage, “We thought this would be the perfect opportunity to use the pervious concrete,” Martin said.
Blake writes that pervious concrete has been in use in Minnesota on a limited basis for about five years, said John Lee, an engineer and sales manager for Cemstone, a Mendota Heights, Minnesota-based concrete supplier for the Midwest, which will supply pervious concrete to Shoreview.
The Minnesota Department of Transportation, working with the Aggregate Ready Mix Association, began studying pervious concrete in 2005 on a driveway, sidewalk and road test strips at its MnROAD research center at Albertville. Although MnDOT is still evaluating its performance, “We think, based on the tests at Albertville, that it can be made freeze-thaw durable,” Lee said.
The original theory was that snow and ice would build up in the pores of the pervious concrete and tear it apart during the freeze-thaw cycle, Lee said. But the pavement design includes a layer of aggregate under the pervious layer that stores water while it filters into the ground. “The whole point is that you don’t have ice and water building up in the pervious concrete itself. It is designed to be free-draining.” Making it work also requires using an aggregate that does not break down with freezing and thawing and a cement paste that also stands up to the cold, he said.
Because of these requirements, pervious concrete is a third to 50 percent more expensive than regular concrete, Lee said. “But now you have a pavement that is not only a pavement but also a storm-water management system.”
Pervious concrete requires special installation and maintenance.
Keeping its pores open for optimum draining requires monthly vacuuming with a special air-brush street-sweeper. Shoreview plans to buy one to handle the job. The concrete installation also requires special skill and experience because the concrete comes from the mixer stiff and dry, and must be worked into place with minimal handling to keep the voids open. It also must be allowed to cure slowly under a membrane that holds in moisture. “If it dries out too quickly and doesn’t gain the strength it needs, then you get little pieces of rock popping off,” Maloney said.
Shoreview set rigid requirements for installation and rejected three of six of bids for the contract because the companies lacked experience with pervious concrete, Maloney said. The winning bid was slightly more than $1 million.
Work is expected to be completed by fall. Representatives are going to be watching the project as well, making sure it is installed properly and using it as a training ground, Maloney said. The Ramsey Conservation District also will be watching. Four wells will be sunk when the streets are built, so it can monitor how drainage through the pavement affects the level and purity of the area’s groundwater.
