Pervious concrete was used on four different neighborhood streets to improve storm water drainage.
October 5, 2009
by Ivy Chang
Cemstone releases pervious concrete from its trucks while the Allen paver waits to lay concrete on the road.
When the city of Shore-view, MN, needed to rebuild streets in a high priority neighborhood, city officials considered options to address construction alternatives that manage storm water in a different way by using pervious concrete.
“We didn't start out on that premise,” said Mark Maloney, city engineer and public works director for Shoreview. “We analyzed construction alternatives and storm water management alternative, then identified an option to use pervious concrete as opposed to building traditional storm drainage infrastructure. The city council made the decision to move ahead using pervious concrete after comparing the costs.”
Storm sewer infrastructure would not be needed when using pervious concrete, which allows water to seep through the concrete to the soil below. Maloney said the net savings from using pervious concrete, which is more expensive than regular concrete, and not building storm water infrastructure were very close.
The neighborhood where this concrete was used did not have any available land to build storm water retention ponds making pervious concrete a highly desirable and sustainable solution.
Maloney and other city officials became familiar with a national procedure for certifying and training people in the use of pervious concrete to prepare specifications and bidder requirements. The Aggregate and Ready-Mix Association of Minnesota (ARM) provided guidelines on material, contract documents and contractors' experiences. With all this information, city officials discovered their streets would be the first public roadways committed to use pervious concrete in the United States.
North Country Concrete crews spread the concrete on the road before the paver packs and levels the mix.
Cemstone, the largest ready-mix provider in the Upper Midwest, had supplied pervious concrete on previous private projects and was selected to develop this mix. John Lee, sales manager for civil and engineered sales at Cemstone, said the company had provided pervious concrete to Shoreview for an alley about two years ago. “Based on the success of that project, Shoreview officials came to us before this project was designed and put out to bid.
“We had meetings to give Shoreview officials some tips of what they should be looking for in a mix design, curing procedures, recommendations on specifications, and contractors who had experience with this mixture and who knew what they're doing. We made city employees more comfortable about using pervious concrete because we had experience with it for five years.”
Veit, a specialty and waste management contractor in Rogers, MN, was selected as general contractor who sub cut the streets and prepared the site. Veit selected North Country Concrete, Ramsey, MN, as its subcontractor because of its previous experience working with pervious concrete.
After the paver levels the concrete, a cross roller, from Lura Enterprises, pushes down the concrete to pack the surface for better driving.
Lee said the mix Cemstone developed was the same pervious concrete mixture it has used since 2004. “We made some changes to make it more workable and to make sure it comes out of the truck. The specifications we developed were performance driven to meet Shoreview's needs,” Lee explained. Pervious concrete uses one size of aggregates and does not contain sand. “We're creating a concrete that's 20 to 30 percent voids. For years we've always tried to keep water out of concrete because water damages concrete. Now, because of the storm water and water quality concerns with sediment and erosion, cities and project owners are looking for a better way to handle storm water runoff.”
Karl Virkus, president of North Country Concrete, said his crews installed new concrete curbs and gutters, concrete driveways and pervious concrete. “After Veit installed the 2-inch aggregate that was 12 to 18 inches deep on filter fabric in the streets, we poured 7 inches of pervious concrete.”
The mixture takes longer to install because it is a stiff concrete with low slump and requires more time to come out of the ready-mix trucks. Lee said the aggregates used has to be freeze-thaw durable because water filtering through the mixture will probably touch every aggregate particle. Admixtures gave this mix more adhesion properties.
North Country Concrete paved the three-fourth mile pavement on four different streets using an Allen screed with triple rollers. “That method seemed to place the concrete better than using a lightweight hand-pulled screed,” said Virkus. “We packed the mix for a more consistent and durable look.” The rollers went over the mixture once unless a problem occurred. Once a problem was repaired, the crew used hand rollers along the edge of the curb and returned to cut joints.
North Country Concrete workers walked across planks over the pavement to cut the blanket which will cover and cure the concrete.
Pre-wet blankets cover a newly-paved road section.
The curing procedure is different for pervious concrete, said Lee. “Because this concrete has such low water content, it can dry out if we don't take precautions. North Country Concrete used blankets that were pre-wet to cure this concrete and to make sure we maintain high moisture content after the concrete is set which takes about seven days. After seven days, the blankets are removed and cars can drive on the surface.
“By using pervious concrete, you can turn a surface into more than a parking or driving surface. You can turn it into a storm water retention system because any water that hits the surface will run through the pavement and naturally filters back into the soil. While the water filters, it's being cleaned of sediments, heavy metal, phosphorus, nitrogen and other materials going through the concrete and aggregates,” said Lee who knows that many cities and project owners see good benefits to using pervious concrete.
Other advantages Lee cited are a rougher surface for better traction, better slip resistance, and reducing the heat island effect with its lighter color.
Cemstone provided 1,800 cubic yards of pervious concrete covering 70,000 square feet on four different streets. On July 22, more than 100 contractors, engineers, city officials and architects attended an open house sponsored by ARM to see the final concrete placement. The group witnessed a product performance demonstration when a Cemstone truck released 2,000 gallons of water that quickly disappeared into a previously poured road section.
Maloney emphasized that a tremendous amount of interest in the Shore-view project has developed among those who build and manage public infrastructure. “There are a lot of people with the same issues nationally and they're interested in this project,” said Maloney. The city built informal partnership with the University of Minnesota, Department of Health, and the Transportation Engineering and Road Research Alliance (TERRA) to monitor ground water, analyze and test pavement course, and evaluate the pavement for design and for water filtration rates.
Residents were skeptical about the project at first, said Maloney. At city council meetings, Shoreview officials explained the benefits of using pervious concrete and cited their previous experience with the mix. “The neighborhood is satisfied with the project now,” Maloney concluded. “The process has been professional, the installation has been top notch, and we definitely had the A team on the project.”
