Getting Smoothness and Density Just Right
Better Roads Staff
By Daniel C. Brown, Contributing Editor
An Iowa-based contractor has won 80 percent of the available bonus payments on a successful 18-mile, two-lane asphalt paving project near Grinnell, Iowa.
The Iowa Department of Transportation awarded Manatts Inc., Newton, a total of approximately $300,000 in incentives for smoothness and density on the State Highway146 project. Preliminary construction on the project began last March and was substantially complete in September. The existing roadway was a deteriorated, full-depth asphalt pavement that averaged 11.5 inches thick. The first step in rehabilitating the pavement was cold-in-place recycling (CIR) to a depth of 4 inches, says Jeff Steinkamp, project manager for Manatts. The recycling process, by WK Construction, a subcontractor from Middleton, Wis., consisted of milling and rejuvenating the pavement, then laying the material back down in a windrow. Using a windrow pickup machine and asphalt paver, the subcontractor completed the cold recycling.
Manatts’ paving work began with widening the 24-foot-wide cold-recycled roadway. With a Wirtgen milling machine working 2 feet wide, the contractor cut a 6-inch-deep trench on each side of the pavement. Next, Manatts followed up with a Weiler Widener W-30 to place two lifts of asphalt into the trench and widen the road to 28 feet. The widening required 12,400 tons of hot-mix asphalt. When that was compacted, the road was ready for the 3-inch overlay.
To place the first 1.5-inch intermediate lift, Manatts ran a Roadtec RP-195 tracked asphalt paver at a speed of about 28 feet per minute. Running the paver at this relatively slow speed definitely helped with smoothness, Steinkamp says. For grade control, the contractor used MOBA dual-grade automatic controls with a non-contact ski on both sides of the paver. A Roadtec Shuttle Buggy fed asphalt to the paver and kept it moving at a consistent rate of speed.
“Smoothness depends on keeping everything consistent.”
– Jeff Steinkamp, project manager, Manatts
“The Shuttle Buggy helps with smoothness because you don’t stop the paver,” says Steinkamp. “The Shuttle Buggy holds one truck load in storage, and remixes the asphalt to prevent both thermal and aggregate segregation. It keeps everything uniform and consistent.
“Once we established a correct rolling pattern, we just never changed anything,” says Steinkamp. “Smoothness depends on keeping everything consistent. That means the asphalt mix, the temperature of the mix, the rate of production, and the rolling pattern. We kept the mix right at 275 degrees at the paver.”
Compaction was accomplished by a triad of rollers. A Sakai breakdown roller led off the compaction train, followed by a Hamm rubber-tired roller in the intermediate role. A Caterpillar roller running in static mode finished the compaction job.
Manatts typically used 10 tandem-axle dump trucks to haul asphalt from a Cedarapids hot mix plant to the job. Steinkamp said production averaged about 320 tons per hour from the portable plant, which was located on the project. Asphalt content in the mix averaged 6.6 percent, and the top-size aggregate was 3/4 inch.
The Iowa DOT awards a maximum 3-percent bonus for meeting the laboratory voids specification, and Manatts won all of that on the intermediate lift. The contractor also won a 4-percent bonus for meeting the field density specification. “We attribute those results to our rolling patterns and consistency,” says Steinkamp.
Asked about the challenge of the project, Steinkamp assures, “It’s all challenging.” One minor issue arose with the absorptive aggregates for the asphalt mix. If the mix sat in a truck too long, the aggregates would soak up liquid asphalt and drive up the voids content in the mix.
“On the two days when we did not achieve full incentive, we were doing some piecework where there was a side road tying in and we had trucks sitting,” Steinkamp says. “So the longer those trucks sat, the more time the rock had to really cure and soak up more asphalt. So our air voids in turn went up.
“We had to manage trucks carefully,” Steinkamp says. “When we had piecework to do, we would eliminate some trucks – just run four trucks until noon, say, and let the other trucks haul rock.”
Another factor that helped with smoothness was the roadway profile left by WK Construction crews when they did the cold recycling. “They gave us a good base to work on,” Steinkamp says. “There were not a lot of deviations in the mat when they finished. They left an accurate profile for us to pave on.”