Superthin Concrete Overlays
Better Roads Staff
Spurred forward by some brave new thinking, the concrete pavement industry is working toward thinner concrete overlays these days – down to 3 inches, even 2 inches.
Some Iowa counties have paved a lot of miles of 4-inch overlays that have stood up well for years. And Illinois, Tennessee, and other states have paved many miles of thin concrete overlays (formerly called ultra-thin whitetopping). These applications, as well a stubborn economy is leading the American Concrete Pavement Association (ACPA) to advance overlay technology in new directions.
“Very thin overlays are the next frontier for the concrete pavement industry,” says Jerry Voigt, P.E., ACPA president and CEO. “This is where we have to go,” he says with conviction.
Voigt proposes that the industry reverse-engineer a new overlay material that would have durability, uniform thickness, compressive strength, and all the qualities of conventional concrete overlays. But instead of trying to fit conventional concrete into a thinner section by adjusting panel sizes and joint spacings, Voigt says the industry should rethink and fit the material to thin applications.
The first thinking should be to design the needed characteristics of a good-quality overlay, he says. With specific engineered qualities like durability, flexibility and toughness in mind, the next effort would be to design – or reverse engineer – a version of concrete to fit those qualities. Voigt says the new overlay may be a shorter-term solution – it may not last 30 or 40 years, as people have come to expect from conventional concrete.
Thinner Can Work
“I think we need to strive to achieve at least 3 inches,” says Voigt. “A 2-inch layer would really be pushing the envelope. But if we can get to a 3-inch layer by modifying our material and looking at the concrete differently, then the number of places that could apply that solution will grow much more than it would with a 4-inch overlay.”
Jeffery R. Roesler, PhD, P.E., is an associate engineering professor at University of Illinois, and worked with Amanda C. Bordelon, Ph.D., to develop a new thin concrete wearing surface material. Called flowable fibrous concrete (FFC), the mixture incorporates a hybrid of synthetic fibers to give the concrete toughness and limit the size of cracks. “An objective of the wearing surface was to construct reasonable slab sizes and crack widths while ensuring economic feasibility,” according to a paper by Roesler and Bordelon for the American Society of Civil Engineers.
At the University of Illinois, Roesler and his students cast a 400-foot-long, 2-inch-thick demonstration overlay with FFC to evaluate constructability and concrete material performance including placement issues, crack spacing and width development, and interface bonding conditions. “I think we have something here that we believe works,” says Roesler.
The demonstration overlay was placed with conventional ready-mix concrete trucks using available materials. “We demonstrated that it’s easy to do,” says Roesler. We used all conventional concrete paving equipment that you would normally see on a paving project.” He says the FFC was placed with a vibrating screed, and flows like honey, so placement is a simple matter.
“It basically flows like self-consolidating concrete that has been used over the past ten years in structures to compact under its own weight and flow around rebar,” says Roesler. “You need very little vibration to get it to consolidate.”
At ACPA, Voigt has reviewed the flowable fibrous concrete, and says it shows promise. “I think we could try it very soon,” Voigt says. “We haven’t gone through the reverse engineering analysis to work our way back to it, but I think there are some concepts there that we could try, straight-away. We need to find an agency to take that step.”
Roesler says that FFC is designed to last 10 years on city streets and low-volume roads. The fibers can hold cracks tightly together, “but you have to come with the mindset that we’re going to get cracks eventually,” he says. “Someone has to take the risk and that’s the biggest thing to overcome,” he notes.
Roesler says the cost of FFC probably slightly exceeds that of asphalt in the same thickness, because of the fibers and extra cement in the mixture design. “But we’re thinking that this is a 10-year surfacing and that usually exceeds the life of a city overlay of asphalt,” he says.
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