For the past couple of years, several concrete associations have marketed pervious concrete as a sustainable product with many benefits. Although it has existed in the southeastern part of the country since the 1980s, it is considered a new innovation in most other parts of the U.S. But as is the case with products tried by contractors 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. Slab replacements can result when best practices aren't followed.
In 2009 a pervious concrete residential street project for the city of Shoreview, Minn.—a suburban area north of St. Paul, received national attention because it was the largest public street project in the country. 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.
Getting the owner committed
This is the second pervious concrete project for the city of 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. Ready-mix producer Cemstone, Mendota Heights, Minn., was contacted to discuss mix designs for the concrete.
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 11/2-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.
Designing the mix
After gathering information about job failures from around the country, Kevin MacDonald, vice president of engineering services for Cemstone, designed the pervious concrete mix. By understanding the nature of pervious concrete job failures, he was able to avoid others mistakes.