When setting up the two total stations, a technician back-sights each of them to known control points. That fixes the location of the total stations relative to the runway’s digital model. The total stations can then “see” two prisms on the paver and communicate to the paver – by free-wave radio – the paver’s precise location. The on-board computer then processes the differences between the actual paver location and the digital terrain model. Knowing those differences, the computer controls the paver pan location automatically.

Koss used a total of three robotic total stations, and set two of them 250 feet in front of the paver. One robotic total station was set behind. When the paver advanced close to the two forward stations, a technician would leapfrog the rear total station to a point up ahead.

“The smoothness was good with the stringless system,” says Kennedy. “But those weren’t the two smoothest days we had. We were still learning the equipment. The guys were still on a learning curve. The crew liked the access to the equipment and they liked the access to the slab for finishing. But some of them have been paving for 20 years, and to not have that stringline to check grade was a little concerning for them.”

Kennedy says one advantage to the milling and concrete overlay process was that Koss could correct slopes and transitions into and out of curves. With so many asphalt treatments over 40 years, the slopes had gone awry. “We could go in there and pave back to the exact slope and correct all of those geometric issues,” Kennedy says.

Koss inserted tie bars at 3-foot intervals across the longitudinal joints, but no dowel bars on the transverse joints. An automatic tie bar inserter on the Guntert & Zimmerman paver pushed the bars into the freshly-placed concrete.

The four I-70 projects by Koss are really Kansas’ first such major concrete overlays on asphalt. “We spent a lot of time and effort working with the KDOT people on this overlay product,” says Todd LaTorella, executive director, Missouri-Kansas chapter of ACPA. “KDOT had a need. What they were doing on that stretch of I-70 was not performing to their liking. Their asphalt overlay actions tended to have successively shorter lives. They finally got to the point that they felt comfortable with concrete overlays.”

Renewing Iowa County Roads

A variety of 4-inch concrete overlays

By Dan Brown, Contributing Editor

Concrete overlays have a successful track record in a number of states, including Colorado, Iowa, Illinois, Minnesota, North Carolina and Michigan, to name a few. Even so, some public agencies and contractors have been hesitant to use concrete overlays. Such hesitancy has been based on a number of factors, including the misperception that concrete overlays are expensive or difficult to build.

Concrete overlays have been standard practice in Iowa for years. Various counties in Iowa have been using 6-inch concrete overlays since 1975, says John Cunningham, vice president of the Iowa Concrete Paving Association. “Iowa counties have done hundreds and hundreds of miles of 6-inch overlays on asphalt. We did not count on the bond between the two materials, even though we knew we would have it,” says Cunningham. “They were not designed as composite pavements. We really just counted the old asphalt as a strong base under a new pavement.”

In the past three years, Mitchell and Worth counties in Iowa have paved 150 miles of two-lane county roads with 4 inches of concrete over asphalt. “Today, new research factors in the benefit that you get from the bond between the asphalt and the concrete, and recognizes that the two layers form a composite design,” says Cunningham.

With the newer 4-inch overlays, longitudinal joints are sawed on the centerline of the 22-foot slab, and at the quarter points, or at 5.5-foot spacings. Transverse joints are spaced at 5.5- to 6-foot intervals, Cunningham says.

Whether or not a contractor mills off some of the old asphalt depends on the county. Some counties mill the asphalt to roughen the surface and get a better bond with the concrete. And some counties use milling to remove surface deformations in the asphalt so that they can control the quantity of concrete in the overlay. Other counties simply pave the overlay over the ruts and let the concrete quantity become what it will.

“We have also done many airport runways with 6-inch overlays on asphalt,” says Cunningham.

Going Stringless

A contractor called Concrete Foundations Inc. (CFI), New Hampton, Iowa, has slipformed almost 50 miles of 4-inch concrete overlays in Mitchell and Worth Counties, says Tom Schmitt, general manager of the firm. (CFI’s parent company, Croell Redi-Mix, was created by Roger Croell in 1965 with one concrete plant in Lawler, Iowa. Croell now has more than 65 plants in six states.) And last paving season, CFI paved 32 miles of two-lane county roads near the town of Osage, in Mitchell County.

New research shows that Iowa counties can produce composite pavements with 4-inch concrete overlays.

CFI has slipformed most of the 4-inch overlays with their GOMACO two-track GP-2600 paver. And in the spring of 2011, CFI bought a GOMACO four-track GHP-2800. On both pavers, CFI has used an automatic stringless control system from Leica Geosystems. “We had the 2800 delivered right to the jobsite and it was paving stringless two days after delivery,” says Schmitt.