Stringless Overlays Only Four Inches (102 mm) Thick
Concrete overlays have a long history and a proven success in the state of Iowa. Mitchell and Worth counties in north central Iowa have led the way. In the past three years, they have put down approximately 150 miles (241 km) of four inch (102 mm) concrete overlay. Concrete Foundations Inc. (CFI), based out of New Hampton, Iowa, has slipformed almost 50 miles (80 km) of the counties' overlays, including 32 miles (51.5 km) this paving season near the town of Osage.
CFI has its roots in the state of Iowa and in concrete. Their parent company, Croell Redi-Mix, was created by Roger Croell in 1965 with one concrete plant in Lawler, Iowa. They now have over 65 plants in six states and in 1999, decided to enter into the concrete paving market by purchasing Concrete Foundations Inc. They started out handforming sidewalks and driveways and grew into mainline paving. Today, the company specializes in concrete work, from driveways to interstate paving.
"The four inch (102 mm) overlay is unique to the state of Iowa," Tom Schmitt, General Manager for CFI, said. "The Iowa Concrete Pavement Association has been really involved with the state and with the counties promoting concrete. We've been experimenting with saw cutting, mix design and different things to create an overlay that will last 25 to 30 years."
CFI has slipformed most of the overlays with their GOMACO two-track GP-2600 paver. This season, the company added the Leica Geosystems stringless system and a new GOMACO four-track GHP-2800 paver to their inventory. Both were put to work on the project near Osage.
"I think you have to spend the money and keep up with the industry otherwise you're going to fall behind and you're going to fall behind quick," Schmitt said. "That was part of the reason we wanted the stringless system. The second reason was rideability. This project isn't a smoothness job, but we have ones that are and we wanted something to handle Iowa's zero-blanking band requirement for rideability."
The switch from stringline to stringless and from the two-track GP-2600 to four-track GHP-2800 went flawlessly.
"Our guys were leery of the new stringless system and when I told our mainline paving operator we had purchased it for our pavers, the first thing he said was, 'You know I'm not very good on computers. I don't like them.' I reassured him that I didn't think he would have any problems. Within a day and a half, we were all comfortable with it.
"If you understand paving, profiles and grades, you'll understand stringless paving. It's almost like riding a bike, once you learn it, you don't forget it."
The new unbonded concrete overlay is 22 feet (6.7 m) wide. Concrete is placed directly on the existing asphalt surface of the roadway. The surface only has to be swept clean before the C3WRC20, two aggregate mix is dumped on it. Concrete slump averages 0.5 to 1.5 inch (13 to 38 mm).
"We hire a survey company to build the project profile for our stringless," Schmitt explained. "They give us a good, smooth profile that we load into the computer.
After that, we only have to turn the paver on, hit start and run. Stringless has really simplified things. We don't have to worry about stringline anymore and all the things that can happen to it... guys tripping on the line, trucks backing over the line, guys eyeballing the line for accuracy."
CFI has seen increased production with fewer men needed for stringline maintenance and setup. Five laborers who normally worked with the stringline are now used elsewhere during the paving process. The lack of stringline also allows them to focus more on quality and production rather than stringline maintenance.
"We started out with the Leica on our GP-2600 and actually increased our production by 1000 feet (305 m), paving close to 6000 feet (1829 m) per day," Schmitt said. "Then, when we started up with the GHP-2800, production increased to 6600 feet (2012 m) of paving per day, with our best day reaching 8400 feet (2560 m) in an 11 hour shift."
They also noticed areas where the quality of their pavement could be improved, for instance on their shoulders. A subcontractor built the 32 miles (51.5 km) of shoulder on the overlay project. When the shoulder is weak and trackline a little soft, the profilometer is showing minor deviations in the trace.
"Trackline is a huge variable," Schmitt said. "Once we get onto a hard grade where the trackline is good, our readings are virtually flat. Our four-track machine handles the variables in the trackline a lot better, too."
The GOMACO GHP-2800 is pulling burlap for a burlap drag finish behind the paver. Finishers work behind the paver with bull floats before a texture/cure machine applies a longitudinal tine and white spray cure.
The switch to stringless has been a great decision for CFI, and one without regrets.
"I guess if we had known it was this easy, we would have switched to stringless a couple of years ago," Schmitt said. "I think at first everybody was a little apprehensive, but now they wouldn't ever go back to stringline if they didn't have to. The guys are comfortable with the Leica system and the GHP-2800 and GOMACO has done a really good job of providing support for both."
-- Select Any Photo To Enlarge --
CFI is using four Leica total stations on the overlay project. Production on the project increased by 1000 feet (305 m) of paving per day with the introduction of the stringless system.
The operator inputs information on the touch-screen interface of the Leica computer.
Transverse joints in the new four inch (102 mm) thick, 22 feet (6.7 m) wide overlay are every six feet (1.8 m), and longitudinal joints are every 5.5 feet (1.7 m).
The new overlay was placed on the existing asphalt roadway, four inches (102 mm) thick. For a visual reference, that's the exact height of this picture above.
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