In terms of innovation, ACPA’s Voigt points to a concrete overlay project happening this year on U.S. Highway 18 in Chickasaw and Fayette Counties in Iowa. There, contractor Manatt’s, of Brooklyn, Iowa, is building an unbonded overlay on 19 miles of two-lane roadway. This is the first such concrete overlay to be built one lane at a time, under traffic. Stringless concrete paving, by which automated machine controls guide the paver, opens up the project and allows concrete trucks access from the centerline side. A 24-hour pilot car will lead a group of vehicles first in one direction, then in the other.

The existing roadway is a 7-inch-thick concrete pavement topped by a 6-inch asphalt overlay. The asphalt overlay will be milled down by 1 to 1.5 inches, and a 4.5-inch concrete overlay will be applied, says Todd Hanson, P.E., a PCC engineer in the Materials Division of the Iowa DOT.

As evidence of the growth of concrete overlays, Voigt points to Kansas, where the DOT has recently let approximately 1.5 million square yards of concrete overlays for Interstate 70. “Those overlays are what we call a ‘6-by-6-by-6’ overlay – 6 inches thick and 6-foot-by-6-foot panels,” says Voigt.

“There is an upcoming national open house on that,” Voigt says. “It is essentially a mill-and-fill using concrete instead of asphalt. They are milling down and placing concrete to re-establish the grade that was there.”

In Missouri, MoDOT has built some 25 concrete overlay projects over the past 12 years, says John Donahue, P.E, construction and materials liaison engineer. Most of those projects have been built on Interstate highways, but some have been paved on lower-volume roads as well. Donahue says Missouri has placed several unbonded concrete overlays that use a geotextile as the bond breaker between the old concrete and the new.

“And we have done, to a lesser extent, a number of thin bonded concrete overlays,” says Donahue. “Typically those are about 4 inches thick. In that case, we actually try to get a bond with the asphalt below. We have typically used those at intersection locations where we have had historical rutting occurring on the asphalt.”

A third type of overlay in Missouri is what the state calls its “big block design.” It is an unbonded overlay – thinner than the state’s conventional concrete overlay, which is 8 inches thick. “We would construct it at 5 or 6 inches thick, and place it down on an asphalt surface, or even a concrete surface if we use the geotextile interlayer,” says Donahue. “Then we saw the pavement into 6-foot-by-6-foot panels.”

Technology transfer

ACPA and the National Concrete Pavement Technology Center (CP Tech Center) provide instructional information and provide training related to concrete overlays. ACPA’s education and training program, for example, focuses on delivering training on various aspects of design, construction and pavement rehabilitation using concrete overlays.

The CP Tech Center offers workshops to state agencies on concrete overlays through the National Concrete Consortium. The consortium is a group of 21 states that have pooled their resources and joined together to work on concrete pavement issues and solutions, Harrington says. This NC2 group meets annually to hear speakers and discuss concrete pavement issues.

The CP Tech Center, in partnership with the Federal Highway Administration (FHWA), also provides technical support to DOTs that are considering such overlays. “We have been providing technical experts to assist the DOTs with identifying suitable candidate pavements for the overlays, helping them with the specifications and assisting with construction support,” says Tom Cackler, P.E., director of the CP Tech Center.

Harrington, who works full time for the CP Tech Center and is its former director, says concrete overlay expert teams have visited 22 different sites across the nation over the past two-and-a-half years. An example comes from South Dakota, where the DOT requested an expert team’s services.

The project was constructed on South Dakota Route 50 in 2009. “It was an existing 8-inch concrete pavement with 10-inch shoulders,” says Harrington. “The South Dakota DOT placed an asphalt interlayer as a bond breaker, then a 7-inch unbonded concrete overlay.

“The Center’s team, including FHWA, completed a presentation on overlays to the South Dakota DOT and gave them a site evaluation field review of Highway 50. The team provided comments on design and specifications as they were being developed. Finally, the team provided guidance during the construction and sent the Center’s mobile laboratory to the project for a week. The lab completed testing of material properties for South Dakota DOT. The Center’s team assistance was provided at no cost. When the project was complete, we sent them a final report and evaluation.”

Harrington says when the expert team program concludes in mid-2012, the CP Tech Center will publish a report on all the sites that have been visited. FHWA has requested that the full report include lessons learned about concrete overlays during the expert team consultations. That full report will be shared with all state transportation departments, Harrington says. States visited to date include Delaware, Georgia, Illinois, Louisiana, New Mexico and Virginia. Others involved include Maryland, Minnesota, Nevada, North and South Dakota, Pennsylvania, Texas, Washington state and West Virginia.