ROADBUILDING & REHABILITATION
TODAY’S TECHNOLOGY BOOSTS COLD MILLING PRODUCTIVITY By Tom Chastain
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sphalt milling, asphalt grinding and asphalt planing describe the same process by which weathered, oxidized asphalt pavement is removed from a driving surface in varying depths, as specified, done in advance of a fresh asphalt overlay or pavement preservation treatment. There are powerful environmental advantages to asphalt milling. The product of asphalt milling – reclaimed asphalt pavement (RAP) – is removed and stockpiled, prior to processing into engineered aggregates that are specified for future asphalt mixes. Instead of sending the RAP to the landfill – as was the case a few decades earlier – reuse of RAP utilizes existing aggregates that have already been surveyed, acquired, permitted, shot, crushed, screened and stockpiled. Use of RAP preserves virgin ag-
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gregates for more critical applications, conserving quarry reserves. RAP also contains residual asphalt which substitutes for virgin asphalt binder in varying amounts, also saving natural resources. Costs to taxpayers are reduced while natural resources are conserved. Not too long ago, the milling process was considered a necessary evil, in which the paving contractor was forced to mill from time to time to correct a curb line, or scarify a bridge deck. In the 1970s, asphalt mills were simply drums mounted on motor graders. The first dedicated milling machine in the United States was developed in 1974. It was derived from a rear load dirt trimmer that had a conical tooth drum installed in lieu of a spade tooth drum, to cut both concrete and asphalt. In Europe "hot" milling was started in the early 1970s by Wirtgen GmbH, in which asphalt was preheated ahead of the milling process. Then, after pre-
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heating, the milling machine would cut the softened, heated asphalt. In the mid-1970s, Wirtgen began the transition to cold milling, and in 1978 adapted carbide tools for mining to cold milling, which launched the modern era. In 1979 the Wirtgen 2200 C on three tracks was introduced, followed in 1980 by the 2600 C, the first track-mounted large cold milling machine with a milling width of up to 2.6 metres, or 8.5 feet. In 1984 the 2000 VC was introduced, the first cold milling machine with front-loading conveyor. A machine that would cut 50 mm (2 inches) in the 1980s at 9 metres per minute (30 fpm) has been replaced by one that can cut up to, and over, 30.5 metres per minute (100 fpm) today. Thirty years ago, you would mill for an hour or two, then have to work on the machine for an hour. Today’s machines are much more reliable, and downtime is minimal. Early-on cold mills were equipped with hydraulic sensors. Although those systems were very reliable, today’s milling – with its need to establish a super-smooth substrate for supersmooth HMA pavements – requires a more sophisticated system. Today you can read out dual grade, grade-andslope, stringline sensing, averaging, laser-reference, and GPS coordinates. One ground man could easily control the grade from either side of the machine, compared to always having two ground men in the past. Today, contractors seek versatility of machine application to leverage productivity and profitability. Added machine flexibility and utilization means more dollars and higher profits. The most significant technology advance has been the new ability to use one machine for multiple applications. For example, today you can buy a machine on which a contractor can quickly change cutter patterns, from perhaps a 16 mm (5/8-inch) spacing to
a 6.35 mm (1/4-inch) spacing to fit particular jobsite requirements. You can also fit smaller cutter widths into the same unit to cut a smaller-width cut instead of having to acquire or rent a whole new machine to do that width.
Tips for productive milling
The right machine is essential to a profitable milling project, but there are other elements of a project that will boost productivity. Here are a few of my observations: • Keep crew and cold mill together. It’s important that an owner keep his crew with a machine as long as he can. It’s not a good idea to send the crew back to the union hall at the end of the year, and get a new operator and crew the next year that has to be trained all over again on that machine. 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. • Truck management. Beyond the cold mill itself, effective truck management will boost productivity of a milling operation. Operators need to be smart when it comes to managing trucks. 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. Therefore the operator needs to balance the trucks, pace himself a bit, and keep the machine running steady. That’s instead of running the machine wide open, then waiting 15 to 30 minutes for trucks to return.