The Growth of Micromilling

It’s no secret that thin asphalt overlays are growing in popularity as states’ highway budgets have diminished. To remove shallow ruts and improve the bond between these thin overlays and the existing roadway, many states have turned to micromilling. It is shallow milling (1/4 to 1 inch) that gives the roadway a finer texture and leaves it smoother than conventional milling does.

Micromilling is poised for rapid, nationwide growth over the next five years, says Jeff Rule Sr., Roadtec Cutter Drums. That’s partly because state and federal budgets likely will continue to favor thin asphalt overlays, which fit with micromilling like a glove on your hand. Micromilling offers several advantages that conventional milling does not. For one, a micromilled surface will not reflect upward into a thin overlay, as an aggressive, conventionally-milled surface can do. Micromilling is also less expensive, because the cutter drum doesn’t go as deep and there is less recycled asphalt to haul away.

Micromilling drums have approximately 700 to 1,000 or more teeth on them, compared to 264 teeth on a conventional 12-foot 6-inch-wide cutter drum. The additional teeth create a smoother surface, which is what states want. Several states have been busy writing specifications for micromilling, including New York, Rhode Island, Connecticut, Georgia, Alabama, Virginia, California, Indiana and South Carolina, says Rule.

“A number of the New England states are doing micromilling, including Massachusetts and New Hampshire,” he says. “Tennessee is using it now, and so is Georgia. Alabama has a spec and has done some micromilling. Washington State is doing a lot of micro work.”

With a common specification, used in Rhode Island among others, a technician tests the smoothness of a milled surface by pouring a premeasured amount of glass beads in one place on the surface. A technician then takes a hockey puck and with a circular motion, spreads the beads out until they come to rest. The smoother the surface is, the further out the beads will spread, says Frank Corrao, a deputy chief engineer in the Rhode Island Department of Transportation.

So the technician measures the diameter of the circle covered by the spread-out beads. “If that circle is greater than 6 inches, then you have a fine corduroy micromilled surface that meets our specs,” says Corrao. “But if you spread the beads out and the bead circle average becomes less than 6 inches, it’s not considered an acceptable micromilled surface.”

Better bonds for thin overlays

Suit-Kote Corp., a contractor based in Cortland, NY, micromilled an entire 10-mile stretch of two-lane roadway without changing teeth on the drum. Working 12.5 feet wide on Route 281 near Cortland, the crew’s milling machine averaged around 50 feet per minute, says Mike Bush, recycling superintendent for Suit-Kote. The mill was cutting between a quarter-inch to one inch deep.

Jim Fitzpatrick, equipment manager for Suit-Kote, says he is very pleased with the new cutter drum. With a total of nine milling machines, Suit-Kote works over much of New York State and into Pennsylvania.

Earlier in 2011, the New York state Department of Transportation micromilled 10 lane miles of Interstate 81 near the Syracuse Airport. The milling machine cut the surface 5/8 inches deep, and the state put back a 1-inch overlay. Thomas McPhilmy, regional materials engineer for the state, says micromilling enables a road manager to place a thin overlay and still reuse the shoulders, because milling avoids the 1-inch drop-off at the edge of the driving lane.

McPhilmy says he is certain that micromilling creates a better bond with the overlay than to simply use the existing asphalt surface. “Paving on an existing road is sometimes a slippery pain in the neck,” says McPhilmy. “The mat moves around on you instead of grabbing hold of the lower lift. And sometimes the rollers break the bond between the new overlay and the existing surface. When you create a milled surface, it gives something for the overlay to grab hold of.