Granger Construction Company is adding yet another “first” to its list of specialty concrete experiences.
Serving as concrete subcontractor to Walsh Construction, Granger will provide 24,000 CY of concrete at 17 different buildings, including 100,000 SF of pervious concrete, as part of the City of Ann Arbor’s Waste Water Treatment Plant Facilities Renovations project.
On the surface, this project may look like any other heavy concrete project, and much of it is, except the part that calls for a 12 inch thick pervious concrete mat beneath the structural concrete slabs within eight different tanks/structures at the plant.
Granger is certainly no stranger to concrete. In fact, the firm’s General Contracting Division pours, on average, about 20,000 CY of concrete a year and is considered one of the best concrete contractors in Michigan. In 2011, they were recognized for turning concrete into an art form after producing one of the first examples in the country of self-consolidating concrete in an architectural application at Michigan State University’s Eli and Edythe Broad Art Museum. That cutting-edge work drew rave reviews from MSU as well as concrete experts from around the country.
But the Ann Arbor WWTP marks the first time Granger has been involved in a project where pervious concrete is used. Another “first” that Granger’s Darryl Massa, Executive Vice President of Operations, is looking forward to adding to the firm’s specialty concrete repertoire.
Pervious concrete (also called porous concrete, permeable concrete and porous pavement) resembles Rice Krispie Treats. It’s a special type of concrete with a high porosity that allows water from precipitation and other sources to pass directly through. Carefully controlled amounts of water and cementitious materials are used to create a paste that forms a thick coating around aggregate particles. A pervious concrete mixture contains little or no sand, creating a substantial void content. Using sufficient paste to coat and bind the aggregate particles together creates a system of highly permeable, interconnected voids that drains quickly.
Pervious concrete is traditionally used in parking areas, residential streets with light traffic and pedestrian walkways. It’s an important application for sustainable construction and is one of many low impact development techniques used to protect water quality since pervious pavements reduce storm runoff and minimize the amount of pollutants (such as motor oil, anti-freeze and other automobile fluids) typically contained in storm water.
At the Ann Arbor WWTP Facility, however, the application for this specialty concrete is not so traditional. Due to the project site’s proximity to the Huron River, the water table is very high, causing elevated concerns about the potential for tank flotation. When tanks are installed in areas with high water tables, or in areas subject to flooding, the installation must include measures that prevent tanks from floating out of the ground.
The owner is banking on the use of pervious concrete to help mitigate that risk during times when tanks are emptied for cleaning, increasing the risk of buoyancy, or float. The idea is that by constructing a layer of pervious concrete beneath the structural concrete base of the tanks/buildings, and then casting pressure relief valves into the structural concrete slabs, the water will be able to flow freely within the pervious layer and, if need be, up through the relief valves and into the tanks. This would reduce water build up around the tank and increase the weight of the tank itself, thereby reducing the risk of float.
Just like any kind of specialty concrete, however, mixing and installing pervious concrete is a balancing act. If the mixture is too wet, the holes will clog up and the concrete won’t drain as well as intended. If the mixture is too dry, it will become like gravel and reduce the structural integrity of the concrete.
Rob Lange, Granger’s Project Manager on the job, says getting pervious in place will also prove to be much more difficult than your typical concrete pour. In fact, because pervious concrete is a low-slump mixture (it will be placed at a 1” slump for the Ann Arbor project), it cannot be poured, or pumped, into place at all. Instead, Granger will utilize a conveyer to take the concrete from the truck to the placement location. To make things even more complicated, pervious, because it is a drier mix, has a much shorter window for placing than traditional concrete. And instead of using floats and trowels as finishing tools, pervious concrete requires screeding and rolling.
Some liken a pervious pour to that of a long assembly line, with the only difference being that the crew members are much closer together. With traditional concrete, the whole slab is placed before the finishing operation begins. With pervious concrete, the placement, finishing and curing is all done at the same time. Typically, there is one crew member near the conveyor directing the concrete to its location. The screed operators are next in line, screeding the pervious concrete to the proper elevation and compacting it. Then, the finishing crew must cross roll the concrete to give it the right continuity (traditional surface finishing is eliminated to prevent filling of surface voids). And all of this must be done within a 15-30 minute window once the concrete leaves the truck.
Granger has its best concrete guy on the job with Steve Hufnagel serving as the Structural Superintendent, but there will still be a learning curve involved with this being Granger’s first experience with pervious concrete.
“There’s no doubt this will be a challenging job,” says Lange. “Having never worked with this type of concrete before, we will have to do our homework.”
Part of that homework may involve test pours, similar to what Granger did prior to performing the architectural concrete work at MSU’s Broad Art Museum. While not as extravagant as building a 37 foot leaning wall with no imperfections, Lange does foresee soliciting the help of the Michigan Concrete Association in Lansing to bring the project team up to speed prior to its first pervious concrete placement, which is currently scheduled for late 2013.
Pervious concrete is only a small portion of the $8.5 million contract Granger holds for the Ann Arbor Waste Water Treatment Plant Renovations project. The majority of the work will involve the use of traditional concrete to construct the 17 buildings/structures. In addition, Granger will provide over one mile of duct banks, which Lee Hude will oversee. All work is part of a $100 million project, managed by Walsh Construction, to rebuild half of the city’s WWTP. The multi-phased project isn’t slated for completion until 2016.