A lesson in pervious

The art of placing pervious concrete definitely can be learned

When scheduling gets turned upside down, it isn’t always a bad thing. I was trying to get out to cover a one-day pervious concrete placement in August and things worked that way. 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.

It was great going through the NRMCA class one day and watching a crew the next day as they learned about and got comfortable with this material on what was, for most of them, their first pervious project.

THE PROJECT

This pavement was being placed as a donation on the campus of the Morton Arboretum (www.mortonarb.org) 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. (www.ozingagreenbuilding.com/filtercrete). Naperville, Ill.-based Builders Concrete (www.builders-concrete.com) 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 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 amounts 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” in the October 2008 issue of Concrete Construction or on the Web at www.concreteconstruction.net.

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.

On this project the arboretum staff did the excavation and base work. The Builders Concrete crew set the side forms the day before the concrete placement. Note that the stakes are cut flush with the top of the form so the roller can pass by. The arboretum staff placed and compacted about 6 inches of typical base material for the area atop a non-woven filter fabric.

Shortly before the concrete arrived, the forms were sprayed with soybean-based oil developed by C2 Products, Cicero, Ind. (www.c2products.com), which is also applied to the concrete as it is placed. It helps in removing and cleaning the forms as well as helping keep the moisture in the concrete while it cures.

The filter fabric is placed with extra at each end of the placement and wrapped up around the end of the base material. This helps keep fines out of the permeable base over the long haul. After the concrete is in place, the remaining filter fabric is simply cut off with a utility knife.

It’s important to have all the equipment and materials that will be needed in place before the placement begins. That includes the plastic to cover it, finishing rollers, sprayers and extra bean oil, materials for fastening and weighting down the plastic, and of course the hydraulic roller screed. The first time you see it you may be surprised how rapidly the steps follow in succession. The idea is to get it placed and covered and as quickly as possible.

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The mix should arrive a little dry from the supplier -- it’s easy to add a little water at the site, but if it comes in too wet that can be a real problem. If you make a ball of the concrete in your hand it should hold together; this is referred to as the NRMCA test. However, Lutey says he has found that a mix that’s stiff but a bit sticky results in better placements than a drier mix

At the training session the day before the Morton Arboretum job, he explained how to judge the mix’s consistency by letting it fall from your hand.

Of the two open-handed tests shown here, the upper shows too little paste left on the hand, which is an indication that it is too dry. The lower has left a good coating of paste on the hand, even though the ball itself stuck together.

We also saw an example of a wet mix. One rule of thumb: When you can see bubbles in the mix, you know it’s too wet. Spinning it in the drum sometimes can drive off enough of the excess moisture to proceed.

When a crew first works with pervious concrete, there’s a need to learn a few new techniques. First, you want to keep a head or one to two inches in front of the roller screed. If it were standard concrete, that would be too much, and first-timers instinctively rake it away.

But when they find themselves constantly shoveling to fill in behind the roller, and then making another pass, they learn pretty quickly. By the time the second truck arrived for this project, they had a good feel for what they were doing.

Using a heavy roller screed is an effective way to flatten and compact the fresh pervious concrete in a single operation. Developed by Bunyan Industries (www.bunyanusa.com) and consisting of a steel pipe weighted with pea gravel, this two-person screed’s rotation comes from a hydraulic motor, while the personnel on each end provide the forward movement.

Overcharge the material along the edge forms and press down with your foot ahead of the roller screed. This densifies the edges, making them more durable.

To control hydration and limit evaporation in fresh pervious concrete, “The Bean” is sprayed on as soon as the concrete has been placed, leveled and compacted. Jim Miller of C2 Products developed the soybean-based, biodegradable liquid specifically for use with pervious concrete.

Cross rolling further flattens the surface. It can begin before, and continue after, the plastic covering is put in place.

It’s very important to completely cover the concrete with plastic. The concrete’s open structure allows both water and air to pass freely through it, which is good once it has hardened but makes it very susceptible to drying out before the cement has hydrated. If that happens, the concrete has no strength.

On this project, the Builders Concrete crew used cap nails to fasten the plastic directly to forms. The large plastic washer allows it to hold the plastic without tearing through.

Any holes that are made in the plastic must be patched with tape. Seams should be overlapped 6-12 inches and sealed -- a good spray adhesive can do this effectively.

Because the mix also includes some polypropylene fibers, balls sometimes form. They cause no problems for the concrete structure, but should be pushed down into the mat to avoid anything strange on the surface.

The project included placing a 4-foot-diameter expanded polystyrene plug which will later be removed and replaced by a stone compass to match several others within the arboretum.

This 168-foot long, 12-foot wide, 5-inch-thick project took about 32 cubic yards and was finished in about three hours. Leaving the forms and plastic in place -- secured on all edges and held down on the concrete surface -- throughout the seven-day curing period ensures that the mix water remains available for hydration.