BACK TO BASICS: Milling for Quality Milling can be the unsung hero or the root of all evil. What makes the difference? Quality. Milling can fill many roles for an existing pavement ready to be overlaid. It can remove distresses such as cracking, rutting, and bleeding. It can also establish or re-establish a pavement’s transverse and longitudinal profile to improve drainage and ride quality. Milling can be the blank canvas for a paving crew to paint an asphalt masterpiece. Or milling can mar a surface and lead to compaction, smoothness, and bonding issues. The choice is yours! Basics of a Milling Machine Today’s milling machines come in all shapes and sizes. Some machines are designed for narrow uses like pavement patching and widening; others are designed to remove an entire travel lane in one pass. For the sake of this article, the focus will be on machines used to make one or two passes to remove an existing asphalt layer or layers. The cutting drum removes asphalt in depths ranging from ¼" to as much as 14". The depth is a function of the project requirements; not all drums can perform at all depths. Many milling machines will use a standard or fine-cut drum. The primary difference is in the number of teeth and the wrap spacing (See Figure 1). While a standard drum can be used at minimal depths (i.e., less than 2"), a fine drum cannot mill 14" deep due to the design. Therefore, it is important to match the drum to the project. Many carbide-tipped teeth are mounted on the cutting drum. Standard drums have substantially less teeth than fine drums. These teeth cut the asphalt, and as the drum turns, move the cuttings or milling to the machine’s center for removal via a conveyor depth. Milled surfaces, as seen in Figure 2, are an indicator that one or more teeth may be missing or need replacing. This uneven surface impacts the final quality of the asphalt surface. Along with the cutting head, various forms of grade control are available on today’s milling machine. When a milling machine is used for demolition, the side plates ride along the adjacent surface and will reflect the ups and downs on that surface. While acceptable for demolition, using only side plates will copy the existing pavement profile—not improve it. When a project calls for a strict depth for milling, dips and bumps in the old surface will be there with the new surface. Fortunately, much of the grade and profile technology available on pavers is available on milling machines. This can result in a much more uniform paving surface. Slow Down Tiger Ever heard the phrase—speed kills? While driving a car too fast can be dangerous, so can running a milling machine too fast. Milling fast may get the job done quicker, but it leads to quality issues for the paving crew, impacts the size of the millings, and increases equipment costs. As the cutting drum rotates, the teeth remove asphalt. Observing the pattern of the milled surface is a good indicator of the milling speed. Figure 3 shows the differences in surface texture by the same cutting drum operated at two different speeds. The larger chevron pattern indicates a forward speed exceeding 150 feet per minute. This texture produces larger milling particles that must be processed at the asphalt plant and less surface area for bonding the new asphalt overlay. It may also lead to asphalt tonnage quantities being off and excessive wear and tear on the equipment. Instead of racing to finish the shift, slow the process to 90 feet per minute and allow the cutting head to do its job. Conclusion First and foremost, milling machines must be maintained. The cutting or milling teeth should be inspected daily. Because of the temperatures generated from the friction between the tooth and the asphalt surface, inspection and maintenance should occur at the beginning of the shift. Once an abnormal pattern is observed, milling should be halted, and corrective action should be taken. Unlike Ricky Bobby in Talladega Nights, who only “wanted to go fast,” slowing the milling machine while maintaining the cutting drum speed produces higher quality. A slower speed allows the Figure 1 Figure 2 22 FALL/WINTER 2024
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