Road Science: Thin is In
While milling adds to the cost, fine milling in advance of a thin lift enhances adhesion, removes any significant poor ride quality, and if done right, can provide a very level surface that will provide the super-smooth riding experience that drivers demand.
“We’ve found that for super-smooth thin lift HMA surfacings, cold-milling of the existing, worn surface with a fine-tooth drum is a must,” says Jeff Wiley, senior vice president, sales and marketing, for Wirtgen America, Inc.
“With a conventional drum, and also relative to ground speed, your ‘peaks-and-valleys’ patterns will be relatively high and deep, and if you are not placing a lift that’s thicker than 1 to 1 1/4 inches, the rough surface can reflect through to the paved surface,” Wiley tells Better Roads. “But with 5/16-inch bit spacing (or less) an owner or contractor can minimize the potential reflection of the peaks and valleys through the thin lift surface.”
For an even smoother surface, full-lane milling with a full-lane fine texture drum will help attain an even smoother surface. These 12.5-foot-wide drums remove a full lane at one pass. When the full-lane-width milling head is combined with a fine-tooth or fine-texture drum, the result is very smooth substrates for overlays, and bonuses for meeting stringent smoothness goals, Wiley said.
“In addition to the fine texture from the drum,” he adds, “the full-lane width drum permits extraordinary control over the outfall of the milled surface, which also contributes heavily to a project’s exceeding smoothness specs, and perhaps a bonus.”
While NAPA’s Newcomb recommends milling ahead of a thin lift overlay, unlike Wiley, he doesn’t necessarily recommend fine milling. “A drum with tight bit spacing is not necessarily required, Newcomb says. “Any milling is a good idea, more from the standpoint of being able to provide a good platform on which to compact,” Newcomb says. “If you have that kind of rough texture on the surface on which you are paving, the mix won’t shove out from under the rollers so easily, and compaction will be more effective.”
Fine milling can be used in this application if the owning agency is concerned about pavement roughness, Newcomb says, adding because of their thinness, thin lifts are vulnerable to significant existing poor ride quality, which can reflect through to the new surface.
Experts recommend that the existing surface be broomed or swept prior to placement. The surface should be clean and dry, FHWA suggests, and if required, a tack coat be applied uniformly at the right rate, and cured prior to the placement of the overlay.
For breakdown and intermediate rolling of a thin asphalt overlay, steel-wheeled vibratory, steel-wheeled static or rubber-tired rollers can be used, FHWA says.
For vibration of the thin lift, high frequency and low amplitude are usually used, at a maximum frequency and at a speed that provides a minimum of one impact per inch for thin lifts over an inch in depth, FHWA says. Rollers should be operated in the static mode when lift thickness is 1 inch or less, as with ultra thin HMA overlays.
For finish rolling, steel-wheeled static rollers or vibratory rollers in the static mode should be used, FHWA says.
If density can’t be achieved, it may be the result of the problems of conventional-depth mats, such as aggregate gradation being outside of the target gradation, binder content being too low, or the mix being too cool, FHWA says.
But there are reasons unique to thin lifts that may complicate getting density. “Check the density of the underlying mat, which will influence nuclear gauge readings on thin overlays,” FHWA says in its Pavement Preservation Checklist: Thin Hot-Mix Asphalt Overlay. “If this is the case, a control strip can determine the maximum achievable density.” Also, the mix nominal maximum aggregate size may be too large for the thinness of the lift. In that case, a different mix or lift depth may be indicated, FHWA says. v
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