By: Allen Zeyher
In milling the open-graded friction course (OGFC) off I-285 in DeKalb County, Ga., the Georgia Department of Transportation (GDOT) set a target of 1.6 mm for the ridge-to-valley distance of the milled surface.
“That was to make sure that we had positive drainage,” Kimbel Stokes, general manager of the Miller Group Inc., Morrow, Ga., told Roads & Bridges. For its cold-planing effort, Miller was awarded a Roads & Bridges/Asphalt Recycling and Reclaiming Association Recycling Award. Miller’s project manager for the I-285 project was Kevin Gamble, general superintendent for operations.
“Anything outside of 3.2 mm had to be redone,” said Stokes, “and at that point you damage the underlying layer and would have had to remove the underlying area for a certain distance and replace that at the contractor’s expense.”
GDOT also set a requirement of 5 mm from the center of one strike point to the adjacent strike point on the milled surface.
Miller planed off 1.25 in. of asphalt over an area of 500,000 sq yd. None of it had to be redone, and the company incurred no penalties.
“Whenever you have to meet the profile grade as well as the vertical ridge-to-valley, it’s quite an accomplishment,” said Stokes. “But it’s good for the taxpayer. It’s good for the state. And it puts the state on the forefront of extending the maintenance dollars.”
The whole idea behind an OGFC is to allow water to drain through the OGFC until it reaches the underlying dense-graded asphalt. Then it drains to the sides of the roadway and off the edge and soaks into the shoulder.
Restricted entry
Miller had to maintain the grade and cross slope of the dense-graded surface underlying the OGFC. The company used a 30-ft ski to help maintain the proper grade. The cross slope was 1.5-2%.
“This interstate was four or five lanes at times and sometimes seven lanes, and we had to make sure that we had a true cross slope from the crown point all the way to the edge of the pavement,” said Stokes, “so the water, when it hits the dense-graded pavement, would flow all the way to the edge of the pavement and get off the roadway, as the open-graded friction course was designed.”
Half a million square yards of pavement amounted to 71 lane-miles of milling on a busy interstate highway. I-285 sees traffic of 371,500 vehicles a day between Ashford Dunwoody Road and Chamblee/Tucker Road. The highway ranges from four to seven lanes along that stretch.
Because of the level of traffic, Miller was restricted to working between Friday night at 10 p.m. and Sunday afternoon. By 5 a.m. Monday, the roadway had to be cleared and restriped and open to traffic.
“We could only mill as much as the paving contractor could pave back and get striped” over the weekend, said Stokes.
Failure to have the roadway open to traffic on time would have resulted in a penalty of $10,000 an hour. Failure to cover each milled area within 24 hours would have cost another $10,000 an hour.
The micromilling removed wheel ruts and other surface irregularities and restored proper grade and transverse slope to the road.
Beat of a special drum
Miller invested in a special drum made specifically for micromilling. The drum has three times as many teeth, or bits, as a conventional drum for cold planing. On the I-285 project, Miller used a CMI PR-1050 full-lane cold planer, a Wirtgen 2200 half-lane cold planer and a Wirtgen W600 mini planer. Bit wear is even more critical in micromilling than in conventional milling.
“We’ve done two of these projects now, and we purchased the drum specifically for this type of work,” said Stokes. “They really do not have any other application due to the excessive bit wear and so forth that you encounter with this type of operation. You don’t get near the tooth wear on this as you do a conventional drum.
“One of the problems is, if one tooth wears, you’ve got a ridge that won’t meet the ridge-to-valley,” Stokes added, “and you’d have to remill the whole section.”
Eternal vigilance
As a result, Miller has an experienced supervisor constantly watching the milled surface pattern for any irregularities. If there is a problem, it has to be fixed right away.
The Miller Group is hoping that micromilling will expand as a method of preserving pavements so the company can see a better return on its equipment investment.
“We think that other agencies really need to consider this. Some of the contiguous states that we have around here should consider it and do some micromilling and help preserve their pavement structures,” Stokes said. “We think if there’s enough work out there it’ll pay off. But this is an emerging market and something new in the pavement-preservation field and it needs to develop.”
The asphalt millings were taken back to the paving contractor’s plant to be incorporated into new mixes, so that will be recycled, too. The paving was not done by the Miller Group; E.R. Snell Contractor Inc. of Snellville, Ga., was the contractor that laid a new surface layer of 1.25 in. of OGFC on I-285.
The old OGFC had deteriorated to the point where it needed to be removed and replaced, but the underlying stone-matrix asphalt pavement structure was still sound. GDOT decided to employ the micromilling and replacement process as a way to make the pavement last as long as possible.
“They are using this to extend the life of existing pavement structures,” said Stokes, “and even if they get three to five years of life out of that without having to do a complete removal and replacement, it saves a tremendous amount of money for the taxpayers. I think the Georgia Department of Transportation really is to be commended on looking after the taxpayers and using this as a pavement preservation tool.”
GDOT expects the mill-and-replace project on I-285 to extend the life of the pavement by three to five years. R&B
