Project: Shoreville, Minn., street reconstruction
Pervious concrete cost: $86.30/square yard for a total of $731,000 (including 8,470 square yards of concrete placed, excavation, rock for the drainage layer, and fabric)
Total cost: $1,231,000 (includes reconstruction of the road, extension of water and sewer service, and restoration)
Project delivery method: Design-bid-build
For the past couple of years, several concrete associations have marketed pervious concrete as a sustainable product with many benefits. Although used in the Southeast since the 1980s, it is considered a new innovation in other parts of the nation.
As is the case with products tried for the first time, there have been problems and job failures. Pervious concrete is very different than other types of concrete because of its mix design, placement procedures, and curing requirements, and slab replacements can result when best practices aren't followed.
In 2009 a residential street project in Shoreview, Minn. — a suburban area north of St. Paul — received national attention because it was the largest public street project in the country to use pervious concrete in lieu of storm sewers. The city installed about 1 mile of the porous pavement that will allow water to drain straight to the ground below and prevent runoff that can be damaging to waterways. There were no catch basins, pipes, or settling ponds. But this project also deserves a closer look because of the thoughtful and innovative forensic research conducted by the parties involved before the job even began.
This is the second pervious concrete pervious project for Shoreview, the first being an alley installed in 2007. Tom Wesolowski, the city's assistant engineer, says there were a few problems with the work, such as minor spalling and cracking, but it helped the city's engineering staff to have realistic expectations.
When it became clear the asphalt streets, lacking both curbs and storm sewers, in an older residential part of town needed replacing, the engineering department considered several options. The project area, adjacent to Lake Owasso, is considered an environmentally sensitive area. Options that would increase the level of contaminants and silt in the lake couldn't be considered. The region's sandy soil, which afforded good drainage, was ideal for pervious pavement and a factor that led to the city's decision to specify pervious for the project.
Although the pervious concrete option would be more expensive than options that included storm drainage, Wesolowski says the cost wasn't considered significant when compared to the total value of the project.BUILDING A SOLID SPECIFICATION
Some pervious pavements fail as the result of insufficient subsoil drainage, especially in freeze/thaw climates. Saturating pervious concrete must be avoided or frost-wedging can result, breaking the cement bonds holding the aggregate together.
The Shoreview application required 1 to 3 feet of soil excavation, depending on land elevations. Parts of the project were sloped so low, areas would collect more water and needed deeper stone containments to prevent water from ponding in the pervious concrete.
Wesolowski says the selection of aggregate in the subbase is important; it must be open graded and able to support the weight of ready-mix trucks without rutting during concrete placement. As a result, crushed angular 1½-inch top-sized rock was placed on top of soil-separation fabric.
Recognizing the importance of good curing, the city also specified a seven-day wet-curing period using wet-curing blankets.