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.

Bordelon is now an assistant engineering professor at the University of Utah. For her, one of the challenges facing thin concrete overlays is to accurately determine the strength of the underlying material.

“If you get an asphalt that is too severely deteriorated and has a lot of cracking, or if the subgrade is unstable, there’s not a lot you can do,” she says. “If you put an overlay on it, the overlay may not last long. To have a good asphalt in terms of moderate to no cracking is probably the best.” Moreover, she says, developing a good bond between the asphalt and concrete overlay is important. One way to do that is by milling the asphalt down to leave a roughened surface.

E. Tom Cackler is director of the National Concrete Pavement Technology Center at Iowa State University. He sees three challenges facing very thin overlays. For one, current design methodologies, such as DARWin ME, don’t address thin sections. “And Dr. Roesler has been very involved with that in his research,” says Cackler. “Are there any differences we need to do with the mix design? So we’re very interested in the topic, and over the next few years we’ll be focusing on thinner sections.”

Laboratory work is needed to overcome the challenge of a mix design methodology, says Cackler. “We need to look at how the stresses are actually distributed in these thinner sections. What are the failure mechanisms? When we learn those things, then we need to correlate that with practice – what we actually observe in the field.”

Secondly, Cackler says better tools are needed to assess the support value of the existing pavement, whether it’s an existing concrete, or asphalt, or a composite. “That’s another area that needs to be quantified, so that you have a consistent approach to your inputs for your support conditions for these overlays,” says Cackler.

A third challenge is to consider what may help to enhance the concrete mixture. “Dr. Roesler is looking at flowable concretes, the use of fibers, and the strength of the mixes – those are things we need to look at to see if these thinner sections perform better with some modifications to the engineering properties of the mix itself,” says Cackler.

Letter From Iowa

Jim Cable, PhD, P.E., is president of Cable Concrete Consultation, and works with the CP Tech Center on the use of thin overlays in Iowa. In the past two years, the Iowa Department of Transportation has asked the Center to consider ways to build thin concrete overlays one lane at a time, under traffic, on a two-lane roadway.

Cable reports success in doing just that. The 4.5-inch-thick overlay was built last year on a 19-mile stretch of U.S. 18 in northeast Iowa. “It was the first official one-lane paving project that the DOT had done,” says Cable. “We came away from it with a report and about 40 recommendations of how we can do it better in the future.

Some years ago Cable led an effort to experiment with ultra-thin overlays on Highway 21 in Iowa. “Some of that pavement is still uncovered,” says Cable. “But when you get down to 2 inches, you have to be very cognizant of depth, because you want to get a uniform thickness of 2 inches. For anything less than 4 inches in depth, I will tell you that for success you need to add fibers to the concrete.”

SPECIAL FEATURE

Roller-Compacted Concrete Comes of Age

By Dan Brown, Contributing Editor

As a result of the strong interest and increased use, the American Concrete Pavement Association (ACPA) in December approved an RCC task force to refine standards and specifications, provide RCC-related training, and promote the technology. By April, ACPA had identified some 20 industry professionals who desired to serve on the task force. By June, that number had almost doubled, which is an indication of the strong interest in both the technology and the goals of the task force.