Fine texture drums may go all the way down to 1/4-inch spacing. "When you do that, the drum gets congested and can no longer auger the material fast. You have so many more teeth on the drum that it takes more power to run the machine, and the RAP can't be removed fast. At 1/4-inch spacing the drum cannot cut deeper than 2 inches, because the material simply cannot physically be removed. So those drums are used for texturing only."
Wirtgen offers two drums in the 1/4-inch spacing. With one drum, for every revolution, one tooth strikes the pavement; another drum has up to 1,000 teeth at 6 1/2 feet wide, on which teeth strike the pavement twice on every revolution. "This 1/4-inch x 2, 'double-hit' drum will allow a contractor to pick up the ground speed on the milling machine, yet still maintain a good milling pattern," Wiley says.
Fine-toothed drums provide a smoother, more refined pattern for milled surfaces than conventional drums at higher speeds. "If you have the task of cutting out wheel ruts," Wiley says, "a fine texture drum can mill off the ruts and leave a fine texture, and the road can be reopened to traffic — without an overlay — with no vibration apparent to drivers. But with a standard 5/8 drum, there will be vibration which will be felt in the steering column."
These fine-texture drums are ideal for texturing slick asphalt pavements where the liquid asphalt has "bled" to the surface, Wiley says. "If you have an intersection showing slick spots, the fine texture milling drums will remove that slick surface and create a textured pavement," he says. "Cars will not feel the textured surface but it will provide enhanced friction. You've solved your problem at minimal cost."
Similarly, fine-texture drums have the capability to groove polished or rough Portland cement concrete pavement or bridge decks at a fraction of the cost of conventional diamond grinding, but this application is not practiced in North America — yet. "It hasn't happened yet," Wiley says. "We can achieve the same quality product, but there are a lot of specs out there which will have to be changed. It will take a lot more work on our part."
Thin lifts and fine texturing
For today's thin-lift HMA surfacings, a fine-tooth drum is indicated, Wiley says. "With a conventional drum, your 'peaks-and-valleys' patterns are so high and deep, that if you don't have thicker than 1- to 1 1/4-inch asphalt lift, they can reflect through to the surface. But with the 5/16 spacing, you minimize the potential reflection of the peaks and valleys through the thin lift surface."
Also enhancing smoothness is Wirtgen's Multiplex grade control averaging system. "There are three sensors on each side, a total of six," Wiley says, "On each side, there is a wire rope sensor in the center of the machine above the drum that senses off the 6-foot sideplate, and sonic sensors mounted down to the surface in the front and rear of the machine. They accumulate information and average grade over the length of the machine, adjusting the machine in real time, giving you a smoother surface than before while milling. The Multiplex system also helps the paving process, because the paver won't have to work as hard making corrections."
Yet another advantage to fine-tooth milling is that it provides a smaller, more uniform size of reclaimed asphalt pavement (RAP). "In the case of Eubank, the size of the RAP material was minus 1 1/2 inch," Wiley says. "Contractors like that because it does not have to be crushed, only screened, prior to recycling at the plant. The size of the material depends a lot on the speed at which you mill, but a fine-tooth drum always will give you a smaller particle size than a conventional drum."
While there is an initially higher cost for fine texture drums because of the larger number of teeth — from 278 teeth on the 5/8 drum to 460 teeth on the 5/16 drum — tooth consumption remains the same. "Even though there are almost twice as many teeth on the drum, the teeth last longer, and tooth consumption is the same as with the conventional drum," Wiley says. "The teeth don't have to cut as much asphalt because the one next to it is helping. It's a wash as far as the number of teeth used on a job."