“We always have the worry that the heavy diamond pattern from the conventional milling drum will reflect up through a thin overlay as traffic works it in further,” says McPhilmy. “So the fine tooth milling creates an almost ideal bond surface but at the same time, it has a finer pattern that won’t push up through the thin overlay.”
McPhilmy says the drum that cut the New York project had more than 900 teeth. “Our spec required three lines of teeth wrapping around the drum and a 5-millimeter spacing between the cuts,” he says. “That may have been a little finer than we needed, but the surface was beautiful.”
McPhilmy says the glass bead test can be misleading, because the smoothness or roughness of the milled surface depends on the aggregate gradation of the asphalt mixture that you cut into. “If you mill through a top course to an old binder course, a coarser intermediate lift, it will be very rough and shaggy,” he says. “And you can use the same drum to mill through a top course to an old top course, and the new surface will be smoother.
A “messy thing”
“So it sounds like a messy thing to put into a contract,” McPhilmy says. “You could end up with a bunch of arguments, or perhaps rejecting a machine with nothing wrong with it because the lower lift is giving you what it can give you. So some of this is not the machine’s fault and it isn’t the contractor’s fault – it is the existing lift that you are uncovering.”
Does specifying the tooth pattern restrict bidding to only contractors who have that type of drum? “Yes, that’s a concern,” he said. “For the future, we would definitely try to make the spec a little more liberal because there are some drums that come with teeth configured a different way. So it may not have three wraps of teeth but it may yield a perfectly good surface. I would talk to my advisor based on what we both saw and try to open up the spec because we definitely want competition.”
Late in 2010, Nebraska contractor Werner Construction micromilled all four lanes of Interstate 80 for 14 miles west of Sidney, NE. On a cutter drum that was 12.5 feet wide, Roadtec fitted 840 teeth, compared to 271 teeth on a conventional drum. On one day of high production, the crew milled more than five miles, working one lane wide, says Ken Kalvoda, Werner’s project superintendent.
That section of I-80 had some minor rutting, which the micromill removed by cutting up to ½-inch deep, Kalvoda says. The Nebraska Department of Roads chose not to place an overlay on the highway, but did do some crack cleaning and sealing.
Working on a spec
Tennessee is testing the micromilling process and working on a specification for it. Last summer a Tennessee contractor micromilled a 5.5-mile stretch of S.R. 96 in Dixon County. The micromill cut 5/8-inch deep at most and averaged between 3/8 and a half-inch deep, said Shay Deason, a project engineer with the Tennessee Department of Transportation. The state followed up with a thin lift overlay, applied at 85 pounds per square yard.
Deason was pleased with the ride numbers that resulted from the test. The existing surface, pre-milling, had a half-car index of 50. After micromilling, the surface dropped to 41 on the index, which is positive. Following the thin overlay application, the half-car index dropped to 21. The state awards bonus money for any ride under a 35 on major highways. And a contractor can win the maximum bonus for a ride of 25.
“We are working with our headquarters division to come up with a set of specs to determine when we are going to do these thin level mills,” says Deason. “We are trying to look at ride numbers versus rutting. There are several indices they can use to combine to come up with these numbers. So we are in the process of reviewing that to come up with the best alternative.”
Deason likes the fact that micromilling leaves nearly all of a roadway’s asphalt structure in place. “You are milling at a thinner rate, a cheaper rate,” he says. “You are not removing a lot of your structure the way you do when you take off an inch and a quarter or an inch and a half.”