Use of a fine-toothed, full-lane milling drum on a classic mill-and-fill project on I-40 on the Cumberland Plateau in eastern Tennessee last season resulted in a super-smooth pavement so fine the contractor planned to enter it in state and national competitions.
For the night project—involving 11 miles of four 12-ft lanes and shoulders near Crossville, Tenn.—Tennessee contractor Highways Inc. used a Wirtgen W 2200 with a 12-ft 6-in. fine-texture drum, a brand new Vogele America 2219W paver with a 40-ft ski and a HR500E screed and Hamm HD 120 HV (78-in. drum) and HD O90V (66-in. drum) rollers to reconstruct I-40.
In addition to required balloon lights that provided bright, diffuse lighting for the night work, the rollers were outfitted with an optional night light package of 10 additional lights: three each across the front top and back top and one on all four corners to light up the edge of the drums. The paver also employed a beefed-up lighting system.
Earlier in the project, unique oscillation compaction was able to get compaction on a cloudburst-cooled mat that otherwise would have resulted in a contractor penalty, saving Highways Inc. big bucks. And the intermittent water system on the rollers were conserving water to the extent that rollers were having to be refilled only twice a night, enhancing productivity.
Mix for the job was coming from a brand new Highways Inc. Gencor 300-tph drum plant located just south of Crossville, about five miles from the project.
"We’re rebuilding 11 miles of I-40, milling 11?4 in., and putting back a 2-in. layer of binder and 11?4-in. layer of surface," said David Hughes, paving superintendent, Highways Inc. "We’re also responsible for guardrail adjustment, ramp repair work and new deck on a bridge on the project."
For both lifts, the durable asphalt mix has a top size of 11?4 down to 1?2 in., and the binder is rated PG 76-22. The surface course has 5.2% liquid asphalt, while the binder course contains 4.8%. Both mixes are polymer-modified, but the binder course (intermediate) contains 26% reclaimed asphalt pavement (RAP) taken directly from the project. Over 30,000 tons of RAP were to be removed, with about 80,000 tons of hot-mixed asphalt placed.
The fine-tooth (fine-texture), full-lane milling drum helps make that possible. "The RAP is coming out at an inch, inch and a quarter in size," Hughes said. "We have a real fine mill and are able to put the millings right back in the mix. The RAP coming off the ‘microhead’ is so fine that we’re not having any trouble directly recycling it through our Gencor plant."
Mill and fill, oohs and aahs
Highways Inc. felt that a full-lane drum was a must if they were to meet new Tennessee smoothness specs and resulting bonuses. "The lanes are 12 ft wide, and with the 12-ft drum we can mill the lane in one pass without having to back up or pick up," Hughes said. "It makes for a better ride, as the microhead mills the whole surface to a smoother texture, and drivers don’t get that ‘rumbling’ effect that you get with other machines. We’re self-conscious about the rideability of our projects and it’s making a big difference."
"We’re in a bonus situation here where the best ride we can achieve will be recognized by the state," said Mark Odom, executive vice president, Highways Inc. "We hope that if the topping rides as well as the binder course, Tennessee DOT will be pleased."
Smoothness starts with the base, Hughes said. "Mill-and-fill is like building a road from scratch," he said. "You’ve got to work from the base up. If you get a good base, you’ll have a good road. The [milling machine] is cutting a good grade for the paving operation and not creating any deviations. That good fine-grind, smooth surface makes asphalt easier to lay. Our milling machine—with its 12-6 drum and electronics—lets us pull a complete lane at a time. With its good electronics and grinder, it’s making our paver not have to work as hard to get a smooth surface behind it."
"This is our first experience with a fine mill," Odom said. "We like to pave by offsetting and staggering our joints. The 12-ft-6 head allows us to do that. But there are other ways to judge that mill. To walk behind it is to see and feel the quality of the milled surface, which is unlike any other milled surface that we’ve followed in the last five to 10 years, during which we subbed the milling out.
"Second, we observe the quality of the RAP that we are obtaining and taking to our asphalt plant. Instead of scabbing or hunks and chunks, we are sending a RAP directly to the plant that is more like graded crusher run, which has allowed us to put 26% recycle back into the binder course and shoulders, sometimes on the same night. Before the maximum we had ever done was 15% and we had to crush and screen it."
At one point the standard teeth set cut 15 miles before needing replacement. "The set of teeth in the drum right now has milled 15 miles, which is unheard of," Hughes said. "We milled 15 miles cumulative, pulling a 12-ft width, without changing the teeth for five different nights. I’ve been around a lot of equipment the last 35 years, and I could not ask for any better production or quality of equipment than what we have here now."
Oscillation’s objective
The oscillation technology exclusive to Highways Inc.’s roller saved time and money for contractors in an unanticipated way for the contractor. The oscillation compaction works by working the HMA from side to side, rather than by forcing it downwards by an up-and-down motion. With oscillation compaction, the drum never leaves the mat. Unlike traditional vibratory compactors that achieve compaction by "bouncing" the drum on the ground, oscillation technology ensures that the roller drums maintain constant contact with the ground for faster, more effective compaction.
In the oscillation drum, two eccentric masses turning in the same direction cause a movement around the drum axle. The movement changes its direction of effect during one turn so it generates an oscillating or rocking movement of the drum. Horizontal forces are transmitted from the drum into the pavement. The result is better compaction in fewer passes, with less vibration-related wear and tear on operators and surroundings.
"Earlier in the project we had a weather-related density problem," Hughes said. "A heavy rain shower passed through and we had to quit for a while for the road to dry up. Back where we had the shower the mat cooled off pretty quick down to about 100°F. The state density man said we had not achieved density, as he was reading 87 on the cooled mat and we were shooting for 92. So I sent the oscillation roller back, and on just one pass it brought it up to 94."
"The mix was chilled down so quickly that we were having trouble with compaction, before we hit it with the ‘Ozzie,’" Odom said. "The Ozzie brought the cold mat up to 94. With conventional equipment we would have attempted to ‘beat it to death’ to get compaction. Had it failed we would have had to take it up or take a penalty, but fortunately we did not have to."
"The oscillation roller surprised me," Hughes said. "To take a regular vibratory roller and try to beat it into density would have broken the aggregate. But that oscillation made it work, and jumped the reading up seven points. The inspector’s eyes got about three times bigger. He couldn’t believe it."
Low on fuel
The low fuel use of the paver and rollers on this project enhanced the contractor’s productivity, and was so significant that it startled Highways Inc.’s service truck operator, who thought the equipment had been putting in partial nights, Hughes said.
"The paver and rollers were using half the fuel that our other machines would use doing the same operation," Hughes said. "The paver and the two rollers were serviced each morning, and the serviceman keeps up with every gallon every piece of equipment uses. At one point he told me that we must not have run much the night before," Hughes said. "I said we had run 2,200 tons and he said the paver had not used much fuel. He tracked it the next few nights and decided the paver was using half the fuel our other machines had been using. Since we’ve been on this project, the paver and the two rollers have cut our fuel use in half. With the cost of diesel that figures up to pretty good savings."
The roller water systems were just as miserly. "We are filling the rollers up only twice a night, doing 2,000 to 2,400 tons per night," Hughes said. "That’s optimal for filling rollers, and it’s unheard of. Our other rollers will be filled three or four times a night. But the rollers have a high-technology water system which keeps the drum good and wet, but uses half the water."
Making things bright
Highways Inc. was providing an extraordinary level of illumination for this night job, both to improve safety and ensure mat smoothness. Both rollers were equipped with an optional outboard lighting system, with three top-mounted lights each forward and rear, plus a downward-aimed light on each corner of the roller with which to view the drum edge.
"The paver was lit on both sides and in the back, shining down on the mat and in the auger chambers," Hughes said. "We also installed 4,000-watt balloon lights powered by generators mounted on the equipment. We have pretty good lighting, as you can see." The balloon lights are required by state specs, but the other lighting was provided by Wirtgen Group.
"We’re giving Tennessee DOT and the taxpayers a milled surface that probably has never been achieved before," Odom said. "We know the capabilities of the paver, the oscillation rollers and the fine mill. But there are other parts of the puzzle: For this job we installed a new 300-tph Gencor asphalt plant which has provided this equipment and crew with an extremely fine product in a timely manner that’s allowed constant movement of the paving train.
"And TDOT is enjoying the benefit of a milling and paving crew that has traditionally won every rideability and smooth—paving award category for Region II in the last decade," Odom said. "While great strides have been made on the equipment side, a lot of times you can have the greatest equipment in the world, but if you put a bunch of cowboys on it, you’ll get a cowboy ride."
