Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/85916
HighwayContractor by Dan Brown, Contributing Editor Renewing Iowa County Roads A variety of 4-inch concrete overlays C oncrete 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 con- tractors 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 us- ing 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 ma- terials, 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 recog- nizes 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. New research shows that Iowa counties can produce composite pavements with 4-inch concrete overlays Going Stringless A contractor called Concrete Foundations Inc. (CFI), New Hamp- ton, Iowa, has slipformed almost 50 miles of 4-inch concrete overlays in Mitchell and Worth Counties, says Tom Schmitt, general manager of the fi rm. (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. CFI has slipformed most of the 4-inch overlays with their GOMA- CO 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 fi le, we get the fi le from the surveyor, then the county approves it. We plug it in and hit the button and we're paving," says Schmitt. "We fi gure 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 fi le, getting the system set up again and starting to pave. He says the benefi ts of the stringless system far outweigh any prob- lems CFI has had with it. "I would have done it two years earlier if we had known all of the benefi ts," Schmitt says. Better Roads May 2012 9