Road Science: Secrets to Successful Milling
“We measure how far that 200 milliliters of beads spreads out,” says Doug Jones, general manager of Denver-based Alpha Milling Co. “It has to be within a certain range, over 9.5 inches, which is less than 0.17 macrotexture, thickness of the glass beads displaced. Anything less than 9.5 inches indicates the pavement is too rough and requires a re-mill.”So-called “fine milling” now is getting much more attention as thin surfacings and overlays become more popular, but fine milling also can be used on driving surfaces to restore friction and smoothness.
“Fine milling is an advantage if the surface is going to be turned back over to traffic right away,” Baker says. “With fine milling, you are adding more teeth and wear components, so that’s a drawback, but there are applications for it.”
While thin surfacings such as micro surfacing – and thin-lift, hot-mix asphalt (HMA) – provide a durable driving surface, their thinness makes them very vulnerable to variations in the pavement substrate on which they are placed. The evenness and smoothness of these thin surface treatments will depend mostly on the smoothness of the prepared surface, but that can be ensured by cold milling of the existing, worn surface with a fine-tooth drum.
“With a conventional drum, and relative to ground speed, the ‘peaks-and-valleys’ patterns will be relatively high and deep,” Wiley says. “If you are not placing a lift that’s thicker than 1 inch to 1-1/4 inches, the rough surface can reflect through to the paved surface. But with 5/16-in. 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.”
Cutter drums are continually undergoing analysis and improvement to enhance their performance in their abrasive environments. For example, in spring 2009, Roadtec introduced improvements to its line of cold planer cutter drums, including redesign of the drum end ring configuration, for better match-cutting and increased tool life; the impact angle of all the tools on the drum was adjusted to optimize the life of the consumable bit; and the “lacing” pattern was adjusted to provide a better texture on the milled surface, as well as a more balanced impact when the cutting tools strike the surface.
Grade and slope controls
The variety of grade and slope controls available to operators of cold mills is wide, but they all assist operators in achieving smooth, predictable cuts.
“Every job is different, and the operator today has the opportunity to select the type of grade control system he wants to use,” Wiley says. “It could be a laser system, running off stringline, GPS, dual-grade, grade-and-slope, averaging, averaging with three sensor heads, or seven sensor heads. The owner can have anything and everything he or she needs to perform the most precise job that he needs to.”
In addition to balancing trucks, speed of forward movement can impact the quality and evenness of the cut. “As long as the bid is all about production – that’s the way all milling contractors make money these days, by the square yard and tonnage that they achieve in a day – the machine has to run at a high rate of speed,” Wiley says. “But very high speeds don’t necessarily mean a high-quality cut. It’s better for a milling contractor to run at a somewhat slower speed, but at a steady pace, 40 to 45 minutes an hour, and he still will have achieved the production he needs at the end of the day, but with a superior pattern.
This applies especially to critical applications such as the milling of professional racetracks. Slower milling, and use of a sonic averaging system, was the ticket to success on a recent track project, says Mike McElroy, co-owner of Fonseca/McElroy Grinding Co. Inc. (FMG).
“It was one of our slowest [jobs],” McElroy says. “Details mattered a lot on this job, and the best way to take care of them was to go slow, very, very slow. But when we were done, there wasn’t a bump in sight, which was the goal.”
The firm was hired to take 2 inches off the Laguna Seca Raceway in California’s Monterey Peninsula.
The general contractor stressed the need for the milled surface to be smooth to increase the likelihood that the finished mat also would be level. To ensure smoothness on this high-profile job, McElroy used a Caterpillar Paving Products’ PM200 with a Topcon sonic averaging system.
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