Keeping the gaps open: here's a practical approach to pervious concrete research

Concrete Producer, The, March, 2005 by Rick Yelton

For years, academics researching concrete have focused on how to create mix designs to improve strength, fire resistance, and durability. And with their support, concrete has become the building material of choice when engineers look for these attributes. But now that concrete applications are branching into new developing markets, these same researchers are going back to school.

One well-known researcher who has adapted to concrete's new marketing opportunities is LK. Crouch, professor of Civil Engineering at Tennessee Technological University (TTU) in Cookeville, Tenn. Crouch has had an interesting career in concrete research since he graduated from the University of Missouri at Rolla. His research covers a range of products and applications, from concrete block to ready-mixed concrete.

Crouch has joined efforts to push other forms of concrete. Earlier this year, he received the promise of a $250,000 federal grant to continue his research on developing a concrete material for rapid highway and airfield repairs.

The grant allows Crouch, graduate research assistant Vernon Dotson (who now works for the Tennessee Valley Authority), and future graduate research assistants Ryan Hewitt and Jordan Pitt, to continue developing ZOOM! CLSM. Crouch's team developed this controlled low-strength material for the Tennessee Department Of Transportation (TnDOT) about two years ago. And it has received the agency's support.

Crouch's CLSM product gains enough strength to support vehicular traffic in three to six hours, depending on ambient temperature. This mixture of admixtures, portland cement, and fine aggregate was recently specified and used to repair Robertson Road in Nashville, Tenn. He will use this new grant to develop a material that will set and develop compressive strength even faster than the original ZOOM! CLSM.

The pervious project

With Crouch's background of developing the right applied research to help launch new products, it's no wonder that Tennessee producers asked for help in promoting the current new product--pervious concrete.

According to Alan Sparkman, executive director of the Tennessee Ready Mixed Concrete Association (TCA), his state's producers feel it's important to have a local researcher directing the effort to providing practical answers to local questions.

"It's more convincing when we supply engineers research results from our state's schools. It's a matter of comfort of who's doing the work," says Sparkman, who believes this local touch is especially important when introducing new products like pervious concrete. "When dealing with a new product that will require team education, it's important to fund the appropriate research that creates a successful product using local materials and construction techniques."

Over the last two years, TCA producers have been practicing this home-schooled approach to research by sponsoring evaluations of pervious portland cement concrete permeability at TTU. Crouch has lead the effort supported by Nathan Smith, a former research graduate who now is employed at L.I. Smith; Adam Walker, who now works for the U.S. Army Corp of Engineers; and Tim Dunn, a research technician at TTU's Center for Electric Power.

Addressing producer questions

Studying pervious concrete has been like taking a new course, says Crouch, who has adopted a different mindset to study pervious concrete. "It's a matter of learning new limits for concrete," he says. If a pervious mix is too wet, all the paste falls to the slab's bottom and reduces the porosity. It it's too dry, it's too difficult to place. "We've been focusing on developing a product that is easy to place and performs well," says Crouch.

In late 2003, Crouch's group had an opportunity to take core drill samples of some of the state's first pervious concrete projects. The survey results were interesting. The success in each pervious project hinged on the contractor's ability to have the concrete porosity be consistent from the upper surface to the lower surface. In some circumstances, the survey discovered the lower portion of the slab was much denser than the upper surface, reducing the effectiveness of the slab to drain.

Crouch noted the consistency variation wasn't caused by a single production or material factor. He attributes the problem to understanding how the contractor and producer teamed on the project. "Producers need to pair the selection of their ,nix design with the manner the contractor places and compacts the fresh pervious concrete," he says. The problems of over/under compaction are more pronounced when placing pervious concrete because it directly affects the effective voids. Producers can plan for a contractor's compaction technique by selecting an appropriate mix design.

If a contractor uses a heavy hand in compaction, the producer supplies a stiffer mix. If the contractor has a lighter touch, a more fluid mix could be selected. It was this need to understand the contractor/producer relationship that lead to TCA's request for more research to develop information on some basic questions on pervious concrete.