Thin lifts are more sensitive to vibratory rolling. Incorrectly chosen amplitude, frequency or roller speed can result in aggregate breakage and damage of the bond between the overlay and the existing pavement. Finally, density control is difficult. Thin lifts provide fewer options for aggregate particles to rearrange under compaction, thus, mat densities will tend to be less uniform than those associated with a thicker lift. This should be recognized if pay is in any way tied to mat density.
With these difficult variables to overcome, Eubanks found it greatly enhanced its chances of success on a thin-lift overlay by using a fine-texture drum on a full-lane paver.
Quality through technology
"We're always trying to keep up with the current technology and improve our overall quality," Eubank says. "Safety is our No. 1 priority but we strive to maintain very high quality as well. If we can't do a very high quality job, we'd rather not do it."
Eubank went to the state and asked if it could use a fine-tooth drum on the I-40 project and got the go-ahead. "Wirtgen has the most innovative products in our line of work, and we decided to use a W 2200 with 12 1/2-foot fine textured drum to work on I-40," Eubank says. The machine and drum also was used to correct heavy rutting on Saturn Parkway in Murray County, TN. There, the machine cut from a half-inch to two inches of ruts off the pavement, followed by placement of a 1 1/2-inch base layer, followed by 1 1/4 of surface mix.
"On I-40 they cut the inside shoulder with a W 2200 with standard cutting drum, and followed with the W 2200 with a 12-foot drum cutting 1 1/4 inches with the fine texture drum," says Jeff Wiley, Wirtgen America vice president, sales and marketing. "By making one full-lane pass with the 12-foot machine, you don't have to bother with using two mills and matching the joint at the center. The 12-foot machine offers better rideability and smoothness — with a clean cut from edge to edge — when you're out on the interstate, especially with thin lift HMA." On I-40 the milled surface was replaced with a single lift of 1 1/4-inch PG 76-22 topping with 5/8-inch aggregate.
In addition to the fine texture from the drum, the full-lane width drum permitted extraordinary control over the outfall of the milled surface and contributed heavily to the project exceeding smoothness specs and the bonus. Eubank also used Wirtgen's Multiplex grade averaging system to achieve a level base.
While the W 2200 was rented from Wirtgen in 2004, Eubank is a repeat customer. "We previously bought a 1900 DC in 1996, and in 2002 traded it in for a W 2000 with 6 1/2-foot drum, a half-lane machine," Eubank says. "We also own a W 50 DC to trim near concrete and a Hamm HK 90, a vibratory, steel-wheel roller with four pneumatic tires in back."
Operator of the W 2200 with fine-texture drum was Charbon Contracting, which also owned the W 2200 with conventional drum used in shoulder removal.
Fine milling, tighter teeth
A conventional drum has standard 5/8-inch spacing, with a triple-wrap of teeth flights from edge of drum to center of cutter. "The teeth are staggered," Wiley says, and the reason is that the flighting is angled toward the center of the drum, so as the drum rotates, it augers the cut material toward the center of the drum so it can be ejected onto the conveyor belt. The conventional drums are good at cutting deep — down to 13 inches — and they're also good at cutting shallow."
At a minimum a fine-milling or fine-texture drum has spacing exactly half the conventional drum, that is, 5/16 inches. "With the fine-mill drum, another cutter tooth fills in the gap between the teeth on the conventional drum," Wiley says. "The 5/16 drum is twice as 'tight' as the conventional 5/8 drum, so it provides a fine texture. It's not the finest texture drum you can cut with, but it will allow you to use our Type III bolt-on tooth holder system, so if you want to keep that drum in top condition, you won't have to weld new holders on; you can unbolt and bolt them on. Long delays in the field or trips to the shop are avoided."
"We went with the 5/16 teeth because you can still use bolt-on, quick-change tooth holders," Eubank says. "The fine drums have their applications. For thin lift applications of 2 inches or less it's the only way to go."