Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/85916
HighwayContractor 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 replace- ment at mid-life. "The road really needed reconstruction, but we couldn't afford that," says Gisi. Getting Smoothness "W Koss ran stringless for two days on I-70 in western Kansas e 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 confi dent 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. When setting up the two total stations, a technician back- sights each of them to known control points. That fi xes the loca- tion 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 dif- ferences 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 learn- ing curve. The crew liked the access to the equipment and they liked the access to the slab for fi nishing. 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 longi- tudinal 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' fi rst 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 fi nally got to the point that they felt comfortable with concrete overlays." 6 May 2012 Better Roads Photo courtesy of Missouri-Kansas Chapter of ACPA