Road Science: Secrets to Successful Milling

 In a little more than 30 years, the asphalt cold mill or cold planer – also called a “grinder” – has gone from being a curiosity to a commonplace piece of equipment on the nation’s roadways.
roadscienceRemote mounted control boxes (rear of machine) on Caterpillar PM-201 allow manual or automatic grade and slope control operation from either the operator's station or at ground level.

Cold milling or planing is routinely used for “mill and fill” projects, where milling is followed sooner or later by a lift or lifts of hot mix asphalt which provide a new driving surface. But milling is much more than that.

Cold milling or planing can remove aged or damaged bridge decks, even those composed of tough, latex modified concrete; remove wheel ruts and improve friction coefficients; excavate damaged shoulders quickly and with minimal risk to workers or motorists; refine butt joints; restore road profiles; reveal curbs to improve storm water drainage; remove asphalt pavements full-depth; trim road bases prior to asphalt or concrete placement; and pulverize asphalt pavements.

“In recent years, the machines themselves have become a lot more productive with better technology, but the biggest thing that has happened is that they have become more reliable,” says Eric Baker, marketing manager for Roadtec, Inc. “With the first machines in the mid-70s you would work on them almost much as they would work for you. They were almost self-destructive. But with improved horsepower in the engines, improved component life, and better cutter technology, they’ve really evolved into a very reliable unit.”

The growth of asphalt milling in the context of both “green” construction and the burgeoning pavement preservation movement has resulted in a near-explosion of machines available to United States owners. The result has been an influx of new models.

For example, earlier this year at World of Asphalt, Volvo Construction Equipment introduced its MT2000 and MW500 milling machines. The MT2000 is a four-track, front-load, half-lane milling machine powered by a 610-horsepower Tier 3 Cummins engine. The MW500 is a four-wheel, rear-loading, utility-class milling machine that offers a standard cutting width of just less than 20 inches.

In 2006, BOMAG plunged into the North American market with its BM 2000 series, and the BM1000/30 and BM1300/30 models. They join offerings from long-standing manufacturers like Wirtgen America, Inc., Roadtec Inc., Terex Roadbuilding (formerly CMI Corp.), and Caterpillar Paving Products. 

Planning for productivity
Planning a job each day is a key component of higher productivity, and that will mean planning the efficient use of trucks.

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“For operators, the biggest thing they can do to enhance productivity is to think through the project the way a paver operator does,” Baker tells

“Operators need to be smart when it comes to managing trucks,” says Jeff Wiley, senior vice president, Wirtgen America, Inc. “Today’s machines have high productivity, and if you don’t have the right number of trucks for that that machine to feed and keep it running steady, you will be inefficient. The milling operation should actually mill and load trucks at least 40 to 45 minutes out of the hour. If you are not doing that, you are not efficient. Therefore the operator needs to balance the trucks, pace himself a bit, and keep the machine running steady. That’s in place of running the machine wide open, then waiting 15 to 30 minutes for trucks to return.”

How truck use can boost productivity was illustrated by a project this spring at Las Vegas’ McCarran International Airport. There, Las Vegas Paving faced a $500,000-per-day disincentive for not completing the project on time when it began a six-month, $75 million milling and paving project on the airport’s busiest runway, 25L/7R, on Nov. 1, 2008.

In part the project involved removing asphalt from the entire 10,525-foot-length of the runway and an 11,000-foot-long taxiway, using two Terex PR800 7-12s and a three-track Terex PR950. Las Vegas Paving’s crews worked 24 hours per day, 7 days a week for the first month, and the mills ran 20 to 22 hours per day, stopping only for fuel and daily maintenance between the two twelve-hour crew shifts.

Las Vegas Paving owns its own fleet of trucks, so it was able to balance truck capacity and milling productivity without communications errors. The firm dedicated 20 sets of doubles – capable of holding up to 38 tons of millings each set – to the job. The crew kept a trailer underneath the mill’s 42-inch discharge conveyor belt. The larger PR950 mill was able to load a set of doubles in about four minutes, and another set always was ready, keeping the project going and on schedule to the point that the contractor met the May 1 completion deadline.

Another way of keeping a cold mill moving without stops is replenishing water (used to cool the teeth and suppress dust) “on-the-fly”, without stopping. While the mill moves forward, a water truck moves with the machine, filling its water tank.

The human element is a big part of productive milling, and keeping crews with a single machine over time will boost performance.

“It’s important that an owner keep his crew with a machine as long as he can,” Wirtgen’s Wiley said. When crews stay on a machine month after month, year after year, they understand it, they know what to do on the machine to keep it up and running, they have memory and records of any issues or problems with the machine. Having new crews all the time is not good for a milling operation. The best crews are those that have been with the machine for the life of the machine, as they treat it as their own.”

Posen Construction, Inc., Utica, Mich., knows the need to keep crews on one machine for heightened productivity and machine longevity. Posen recently purchased a half-lane BOMAG BM 2000/60 cold mill. Posen Construction primarily is a bridge building contractor, but is using the cold mill to expand its capabilities and diversify its customer base.

Posen makes sure it hires people who are equally dedicated to excellence and quality. Posen keeps an all-female crew – including a 21-year-old chief operator – to operate the BM 2000/60. Though it is unusual for an all-woman crew to run such a large piece of equipment, it’s the perfect arrangement, Posen maintains. They are often chosen to run Posen’s most expensive and complex machines, Mike Schook, vice president of Posen says. “They pay attention and they’re conscious of what they’re doing,” he says. “They take pride in the machine and keep it spotless.”

Wirtgen’s Wiley says it doesn’t matter how old a machine is, how many hours are on it, or what model it is. “What matters is the condition of the machine and how it’s been maintained,” Wiley points out. A good, well-maintained cutter system is crucial. Also, If a machine is ‘tight’ and not ‘loose’, every time the grade control calls for an adjustment, the milling machine will respond correctly and not create ups-and-downs due to the way its been maintained, he says.

Better Roads. “You have to plan the production for the day. You don’t want to go a quarter-mile, and then have to wait on trucks. Lining up the day’s work, and making sure you have enough trucks, is essential for a full day’s production. A machine will fill a truck up very fast these days, and any downtime is money just waiting to be claimed.” 

All about the cutter drum
The cutter drum is the axis around which the entire milling process revolves. A well-designed and well-maintained cutter drum will help the operator in his mission.

Aside from breakdowns, the proof of success will be in the cutter pattern. Variables in control of the cutter pattern include the condition of track pads, the cutter drum condition and tooth spacing, the cutter tooth and holder condition, the cutter rotation speed, the cutter “wrap” (tooth pattern on the drum) and tooth spacing, available grade control system, the existing pavement condition, the ground speed, the availability of water for dust control and tooth rotation, and the overall condition of the machine.

Milling pattern performance is so critical that the Colorado DOT has instigated a new milled surface spec that must be tested on the job for performance.

The spec determines the roughness of a milled pavement by placement of glass beads on a surface, which are then carefully spread out using a clear plastic disk. By the time the disk comes to rest on the peaks of the milled surface, the bead pile must have spread at least 9.5 inches in diameter, indicating a fine pattern; if they spill too quickly, not achieving the required diameter, the pavement is too rough and must be re-milled.

For the test, from a maximum 4-inches height, 200 milliliters of glass beads used for retroreflectivity of lane striping are poured onto the milled surface, then distributed evenly on the surface using a slow rotating motion with a plastic disk, until the disk rests on the points of the milled surface.

“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.

The sonic averaging system has a sensor in the front, back and middle of the PM200. “Going slowly gives the machine adequate time to compensate for grade changes,” McElroy says. “Eventually, we should be going 80 to 90 fpm per minute with the PM200. It should mill smoothly at that pace, but this was an exceptionally demanding job as far as smoothness.”