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.

We asked Schmitt why he bought the stringless system. “We were thinking labor savings, smoothness, ease of use, and it cuts down on man-made errors,” comes the reply. “It cuts down on the actual string errors or knocking down of strings. It’s a real advantage to show up on the job Monday morning and not have six miles of string laying on the ground because some vandals came along and cut it or drove through it.”

Schmitt says the stringless system is easy to use and quick to learn. “Once you set up the digital surface model of the job, called a D45 file, we get the file from the surveyor, then the county approves it. We plug it in and hit the button and we’re paving,” says Schmitt.

“We figure that anywhere from four to six guys can do something else besides set string,” says Schmitt. “You don’t have one guy out in front of the paving train checking the stringline and eye-balling it so to speak. The days of eye-balling are gone.”

It does take one worker, sometimes two, to move the robotic total stations. CFI uses four robotic total stations that are set at 500-foot intervals on each side of the paver. But the total stations – really surveying instruments on tripods – are staggered so that one stands 250 feet down the grade from the one on the other side. The robotic total stations read paver position information from two prisms on the paver, then send that position back to the computer on-board the paver. The computer uses that position information to control the paver’s line and grade automatically.

Once an operator is familiar with the stringless system, he can stop the system, move, set up and be ready to pave again in 15 minutes. It’s simply a matter of reloading another D45 file, getting the system set up again and starting to pave.

He says the benefits of the stringless system far outweigh any problems CFI has had with it. “I would have done it two years earlier if we had known all of the benefits,” Schmitt says.