Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/86539
AMERICAN CONCRETE PAVEMENT ASSOCIATION 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 be- lieve works," says Roesler. The demonstration overlay was placed with conventional ready-mix concrete trucks using available materi- als. "We demonstrated that it's easy to do," says Roesler. We used all conven- tional concrete paving equipment that you would normally see on a paving project." He says the FFC was placed with a vibrating screed, and fl ows like honey, so placement is a simple matter. "It basically fl ows like self-consol- idating concrete that has been used over the past ten years in structures to compact under its own weight and fl ow around rebar," says Roesler. "You need very little vibration to get it to consolidate." Shows Promise At ACPA, Voigt has reviewed the fl ow- able fi brous 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 fi nd an agency to take that step." Roesler says that FFC is designed to last 10 years on city streets and low- volume roads. The fi bers 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 prob- ably slightly exceeds that of asphalt in the same thickness, because of the fi bers 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 as- phalt," he says. Bordelon is now an assistant engi- neering professor at the University of Utah. For her, one of the challenges facing thin concrete overlays is to ac- curately 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 un- stable, 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 con- crete 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 re- search," 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 over- come 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 fi eld." 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 quantifi ed, 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 fl ow- able concretes, the use of fi bers, and the strength of the mixes – those are things we need to look at to see if these thinner sections perform better with some modifi cations to the engineer- ing 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 con- crete overlays one lane at a time, un- der traffi c, 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 fi rst offi cial one-lane paving project that the DOT had done," says Cable. "We came away from it with a report and about 40 recommenda- tions 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 fi bers to the concrete." Better Roads October 2012 11