Better Roads Staff
Make it Last, and Make it Smooth
A Kansas project delivers.
By Dan Brown, Contributing Editor
For more than 30 miles, Interstate 70 in western Kansas was worn out. It was a full-depth asphalt pavement; sections ranged up to 20 inches deep. The most recent asphalt treatments had lasted just five to seven years, says Andrew Gisi, geotechnical engineer with the Kansas Department of Transportation (KDOT).
The state wanted something that would last longer, so KDOT chose a 6-inch bonded concrete overlay – 1.45 million square yards of it throughout nearly 31 miles. Last year, Koss Construction milled out 6 inches and slipformed two projects with concrete on 15.1 miles of four-lane interstate. Including shoulders, the two projects totaled 725,000 square yards and cost $20.1 million. This year, Koss will mill and pave two more similar projects on I-70 for a total of 727,000 square yards at a cost of $21.9 million.
Koss’ smoothness numbers last year were remarkable. Using a Guntert & Zimmerman S850 four-track paver to pave 30 feet wide, the contractor averaged just 8.5 inches of deviation per mile from a zero blanking band over the two projects. What’s more, the Missouri-Kansas Chapter of the American Concrete Pavement Association (ACPA) gave Koss the “Smoothest Day Paving Award” for a 4,500-foot section on which the contractor hit just 5.4 inches of deviation.
On every section of concrete pavement last year, Koss earned a smoothness incentive from the state. To earn the maximum smoothness incentive, Koss had to achieve 6 inches or less of deviation per mile.
The Colorado Department of Transportation (CDOT) is an especially important player in the Kansas situation, because KDOT used Colorado’s pavement design criteria and system in designing its 6-inch overlay on asphalt. The overlay is sawed into panels that are 6 feet square – and such overlays have provided Colorado with 10 to 12 good years on heavily traveled roads and remain in service.
Gisi says that Kansas expects to get 20 years from the “6-by-6-by-6” overlay, possibly with some panel replacement at mid-life. “The road really needed reconstruction, but we couldn’t afford that,” says Gisi.
“We get really good smoothness numbers everywhere we go,” says Robert Kennedy, quality control manager for Koss. “It is not uncommon for us to get single digits.” It is easier to achieve small smoothness numbers on the thinner 6-inch pavement, because the paver is not pushing a big head of material. “You can push the small head of mud and you are not extruding a lot of concrete either,” says Kennedy. “You don’t get a lot of pushing or moving the paver around is what I am saying.”
Kennedy explains how Koss achieves such smooth-riding pavements. The contractor used dual stringlines. “We pay a lot of attention to the stringlines to make sure they are set up right. We make sure our paver is set up right and that our concrete batch plant is right. Then when we are confident that all of those things are set up, you have to be consistent,” says Kennedy. “The plant has to give you consistent slump on the concrete in every single batch after batch. And you have to have consistent delivery so that the paver never stops. Everything has to be in unison, working together.”
On a good day, Kennedy said Koss paved 1 mile a day, working 30 feet wide. “We averaged about 1,800 square yards per hour, or maybe a little better,” says Kennedy. Two belt placers spread concrete in front of the paver, and that helped boost production. That way, Koss could dump two trucks at once. Each project last year had its own Rex Model S batch plant. Typically15 trucks hauled concrete to the site. An aging CMI paver slipformed the 10-foot outside shoulder.
For nearly 2 miles of pavement last year, Koss paved with a stringless automated control system from Leica Geosystems. The automatic paver control system bases its guidance on a digital terrain model – a digitized 3D model of the pavement – that is entered into a computer onboard the paver. The paver also has two prisms, mounted above the machine, to receive signals from the two robotic total stations set up on tripods ahead of the paver. The prisms on the paver have a relation to points on the concrete paver’s pan.
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