Better Roads Staff
In 2010, PPS was cold-milling day and night with fine-tooth drums. For example, in March 2010, during the day, the company was micromilling the I-185 expressway just southwest of downtown Greenville, S.C., and at night, was using some of the same equipment to micromill I-85 west of the city.
“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 cold mills with 12.5-foot drums, and a half-lane micromill.”
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 must be 0.2 inches, which translates to 5 mm spacing,” he said. “It means from chevron to chevron, or tooth mark to tooth mark, you have to have 5 mm or less.
“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 feet per minute,” 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.
As standard bids are cut to the bone, these contractors strive to earn smoothness bonuses, which PPS makes easier by its fine milling. “We are setting them up not to fail, but to win, by giving them a superior surface,” Limbaugh says.
Fine-Mill Grooving Substitute?
Micromilling with a fine-tooth drum also can be used on driving surfaces just to restore friction and smoothness.
For example, fine milling or texturing may be appropriate for a county that does not have a lot of money to mill out and pave its roads completely, and they may just want to shape up the roads and remove some rutting and areas where asphalt is shoved in intersections. “A fine milling drum can be put in a cold mill and used to repair that driving surface,” says Jeff Wiley, senior vice president with equipment manufacturer Wirtgen America. “The county will have bought some time while being able to open a road to traffic immediately.”
With a narrower drum on a smaller cold mill, fine milling is also useful for removal of road markings, or for prepping pavements for road-marking application.
But in limited instances – at least when a pavement is prepped for an overlay – micromilling with a fine-tooth drum can replace diamond grinding of existing pavement. For example, not too long ago, micromilling was used instead of the specified diamond grinding of the driving surface of the President George Bush Turnpike north of Dallas.
There, the President George Bush Turnpike is a major east-west route in the northern half of the Dallas Metroplex, and is a 30.5-mile, six-lane, limited-access toll highway that passes through or along the cities of Garland, Richardson, Plano, Dallas, Carrollton, Farmers Branch and Irving.
The turnpike owner – the North Texas Tollway Authority – planned to overlay a section of the expressway with NovaChip proprietary open-graded friction course. But such thin lifts – with maximum 3/8-inch stone – require extremely well-prepared and even surfaces for placement. Any irregularity in the existing pavement will reflect through the thin surfacing immediately.
Conventional surface prep for worn pavements previously involved diamond grinding the pavement to precise tolerances. But cold milling subcontractor TexOp Construction, Roanoke, Tex., in conjunction with its general contractor APAC, proposed to cold-mill using a 2-meter (6.56-foot) fine-tooth drum to prepare and level the aged asphalt pavement surface prior to the thin surfacing.
The project already had been designed with diamond grinding in the contract, so a change order was the means to get the cold mill on the project instead of a diamond grinder. On this particular job, fine-texturing with a cold mill saved the authority a significant sum over diamond grinding. Also, the diamond grinders were asking for a minimum 60 days, and the contractor was on track to finish the same area in 10 to 14 days.
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