Micromilling with a full-lane drum provides paving contractors a more-even substrate on which to place asphalt lifts, essential for winning smoothness bonuses. For bonus-winning thin-lift hot mix asphalt layers micromilling is a must.
South Carolina is taking a leadership position on micromilling and it shows in the increase of fine milling jobs undertaken by the contractor.
"In 2009 we had a couple of micromill projects, but this year we have seven so far within South Carolina alone," says Douglas E. Limbaugh, operations, Pavement Products & Services, Inc., Piedmont, SC.
Limbaugh attributes this in part to the fact that the state is correcting cross slopes. "South Carolina has a lot of 'flat' roads," he says. "The cross slopes are out of balance. So we will surface-plane or micromill an inch or two off the road, to get the aged asphalt off, and once that's done, variable surface-plane or cross-slope correct the main lines or roadways."
By doing the planing in two stages, PP&S eliminates the drop-offs that would result from doing the work in one pass.
"Motorists don't have to drive with more than a 1-inch drop, and in the meantime they get a better surface to drive on," Limbaugh says.
To do this work, PP&S was utilizing its Wirtgen cold mills both day and night. In March, during the day, PP&S was micromilling the I-185 expressway just southwest of downtown Greenville, and at night, was using some of the same equipment to micromill I-85 west of the city.
PP&S is a major customer of Wirtgen, with four W 2000s, and three W 2200s. One of the W 2200s was only three weeks old when it was put to work on I-185. "Using our fine-tooth drums, we are removing an inch from I-185, then coming back the same day and fine-milling a second inch," Limbaugh says. "We then will go to isolated areas and cross-slope correct there."
That project represented two 50,000-square-yard lifts, 100,000 total.
The I-85 work - three lanes both north and south - that night was part of a total 1.4 million square yards of 1-inch micromilling, and 295,000 square yards of variable surface planing for cross slope correction.
"We will be using two full-lane W 2200s with 12.5-foot drum, and a half-lane micromill," Limbaugh says.
Fine milling = surface planning
For super-smooth thin-lift HMA surfacings, cold-milling of the existing, worn surface with a fine-tooth drum is a must, says Jeff Wiley, senior vice president, Wirtgen America Inc.
"With a conventional drum, and relative to ground speed, your 'peaks-and-valleys' patterns will be relatively high and deep, and if you are not placing a lift that's thicker than 1 to 1 1/4 inches, the rough surface can reflect through to the paved surface," Wiley says. "But with 5/16-inch bit spacing (or less), the definition of a fine-toothed drum, an owner or contractor can minimize the potential reflection of the peaks and valleys through the thin lift surface."
The South Carolina DOT uses the terms "surface planing" but the same term applies to "fine milling" and "micromilling," Limbaugh says.
"The spacing on the cutter drum is very tight, with restrictions from chevron to chevron, or tooth mark to tooth mark, that necessitate the use of tightly wrapped surface planing drums," he explains.
"The state also requires tolerances of 1/8-inch from high to low on the tooth marks, so you can't just put a thousand-tooth drum out there and run 80 fpm," Limbaugh says. "You have to go at a steady pace because that will leave as smooth a surface as you can. In ride quality it's comparable to a porous friction course."
On top of that, for cross slope correction, the state imposes a rideability spec.
"That's where our Wirtgen 'triplex' or multiplex systems come in to play," he says. "We have been very successful with them here. And for paving, when you do a final cross slope correction, they are much more demanding in depths. Technology from Wirtgen is helping get us there. When you combine the multiplex averaging system, the Level Pro slope system, and the fine-milling capability, all three are giving us the product that's keeping us at the top."