When scheduling gets turned upside down, it isn't always a bad thing; things still can work out. I was trying to get out to cover a one-day pervious concrete placement in August. The date was pushed back several times and finally leapfrogged to the day after I was going to sit in on a pervious training session. How perfect.
It was great going through the National Ready Mixed Concrete Association (NRMCA) class one day and watching a crew the next learn about and get comfortable with this material on what was, for most of them, their first pervious project.
The pavement was being placed as a donation to the campus of the Morton Arboretum in Lisle, Ill. In addition to being a horticultural reserve, the arboretum serves as an educational showcase for landscape architects and homeowners. The facility repaved its main parking lot in pervious pavers some time ago, and this project provides a second example of pervious paving technology.
The material was provided by Ozinga, based in Mokena, Ill. Naperville, Ill.-based Builders Concrete provided the workforce. Ozinga's resident expert on pervious concrete, Brian Lutey, was on hand to help the crew with the new methods they would need. He also brought along the hydraulic roller screed used to flatten and compact the concrete.
Lutey has been working with pervious for more than seven years, first in Indiana with the Indiana Ready Mixed Concrete Association, and since early 2007 in northern Illinois with concrete producer Ozinga. He says he has put down more experimental pervious than anyone he knows, which is how he has come up with mix designs that work. As Ozinga's main man in the field, he has been on every pervious placement they have supplied since his arrival training crews in how to work with the material.
The pervious concrete mix design needs to be adjusted for northern climates, Lutey says, to hold up to freezing and thawing. Primarily this means adding about 100 pounds of sand to a mix that's otherwise known as "no fines."
Hydration retarder is one critical component in any pervious concrete mix. The dosage depends on the mixing and transportation schedule, as well as the temperature, but it should be enough to "put the concrete to sleep" until it has been placed and compacted. Only then do you want the hydration process to begin.
The amount of water and cement also are critical. Unlike standard concrete, where as a general rule more cement means more strength, pervious concrete doesn't necessarily get stronger as more cement is added, simply because of the geometry dictated by using gap-graded aggregate. For more information, see Learning to Do Pervious.
Placing the pervious
Pervious concrete pavement is very much a system. The pavement part can be perfect, but it relies on a well-designed and well-placed base coarse to function properly.