Achieving Smooth Overlays

A Tennessee contractor recently won a $58,000 smoothness bonus on a thin-lift hot mix asphalt (HMA) overlay by using a fine texture drum the full width of a lane on I-40 in Cheatham and Williamson counties in central Tennessee.

Utilizing a Wirtgen W 2200, Eubank Asphalt Paving fine-milled 40 lane miles of I-40 — a 10-mile interstate mill-and-fill project — to a depth of 1 1/4 inches using a full-lane, fine-texture drum with tooth spacing at 5/16 inches, approximately twice as dense a configuration as the conventional 5/8-inch spacing. The firm met or exceeded smoothness specs of less than 35 inches per mile using the half-car International Roughness Index (IRI) measurement, and Eubank received the bonus on every section.

"The state is doing a lot of thin overlays, in which we take off an inch and a quarter and put an inch and a quarter right back," says Michael Eubank, director of field management, Eubank Asphalt Paving and Sealing, Charlotte, TN. "We found that when this is done with a conventional milling head, the surface will ravel and you can feel the milled surface reflecting through the fresh surface. We know now that the milled surface reflects through that inch and a quarter, and the fine-textured milling drum does not. That's what we were looking to correct."

As the result of this job and a few others, the state of Tennessee is taking a close look at its specs for milling limited access highways. "We are looking at use of full-lane drums, closer teeth spacing and forward speed as it relates to teeth spacing," says Brian Egan, P.E., field operations engineer, Tennessee DOT.

"As we vary from interstate overlays with mill, binder and surface layers to mill-and-fill with thin overlay, we are paying special attention to the quality of cold milling, or milling for smoothness," Egan says. "We like the 12-foot drum because we eliminate the two machines, the overlapping of the joint and the possibility of a mismatched cross slope. We also like the non-contact leveling systems we saw. There's no doubt that the milled surface with the fine textured milling drum is smoother and more uniform than the surface with the conventional milling drum. Although we are not moving ahead with a method spec for cold milling at this time, we will continue to study how to get the best smoothness for our customers."

Thin lift challenge

Stung by the rising popularity of high-performance, polymer-modified chip seals, the asphalt industry has vigorously researched and promoted thin-lift HMA overlays. The definition of thin-lift overlays varies, but the lift usually is from 1/2- to 1 1/2-inch in depth.

The technology of HMA is making thin lifts more applicable. The chemistry of liquid asphalt has been enhanced by a new generation of asphalt modifiers, boosting the performance of Superpave mixes (Superpave Plus), open-graded friction courses, and thin-lift overlays.

In 1995 the National Asphalt Pavement Association (NAPA) reported that thin lift overlays are employed to improve ride smoothness, correct surface defects, improve safety characteristics such as skid resistance and drainage, enhance appearance, and reduce road-tire noise. NAPA has produced technical information on thin lifts.

Thin-lift paving, such as that seen in non-structural overlays, presents several construction concerns. Thin lifts require less HMA per foot of road length than thick lifts, resulting in faster paver speeds (in excess of 70 ft/min) which can make it difficult for rollers to keep up.

The thin lifts cool quickly, resulting in less time available to get density before the overlay reaches the temperature at which rolling must stop. Thin lift construction produces greater screed wear. If the lift depth is less than about twice the maximum aggregate size, the HMA may tear under the paver screed. Very thin lifts (less than 1 inch) can be damaged by the screed dragging large particles.

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."

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."