Road Science: Thin is In
“People are looking for a way to keep their road system in as good a shape as possible, for as little money as possible,” says David E. Newcomb, P.E., vice president, research and technology, National Asphalt Pavement Association. “Thin lift overlays are less expensive than thicker overlays, while lasting longer than other surface treatments.”
Thin asphalt overlays have been studied intensively, and their design has become accepted in recent years. Some agencies, like Maryland and Ohio, have been using them for decades.
“Other agencies have adopted them more recently as a pavement preservation technique,” Newcomb tells Better Roads. “Some were skeptical about using such fine, or small aggregate size mix on the surface, for fear it would rut, but research at the National Center for Asphalt Technology (NCAT) has shown that fine mixes are no more prone to rutting than coarse mixes.”
Like their thicker “big brothers,” thin asphalt overlays can be dense-graded, open-graded or stone matrix asphalt. Open-graded overlays allow surface water to drain away quickly, preventing hydroplaning and improving visibility in wet weather.
For thin lifts, polymer modification can help bring neat asphalt up to performance grade, adding durability, and resistance to rutting at high temperatures, and to cracking at low temperatures. Elastomeric polymers in the binder will result in a thin overlay that will be more elastic under traffic, and less sensitive to temperature fluctuations. Polymer-added asphalts also will fight raveling on conventional and open-graded mixes.
While surface treatments for pavement preservation include chip seals, slurry seals and enhanced chip seals, micro surfacing is the biggest rival to thin lift HMA overlays, due to its durability and ability to repair surface ills like rutting.
Some states specs such as Missouri’s describe micro surfacing and call it a thin lift asphalt overlay, but there’s actually a big difference.
Micro surfacing – which is defined by the Foundation for Pavement Preservation (FP2) as a mixture of polymer modified asphalt emulsion, mineral aggregate, mineral filler, water, and other additives, properly proportioned, mixed, and spread on a paved surface – must be applied by a truck modified with proprietary equipment. A conventional asphalt paver places thin lift asphalt overlays, just in a thinner section.
“Thin lift overlays offer a better opportunity, in my opinion, to provide a smooth surface, as opposed to micro surfacing,” Newcomb says. “Micro surfacing can be relatively noisy, while thin lift overlays generally have a dense surface – one that’s not quite so open – so tire noise is not quite so pronounced in most cases, research has shown. They provide a quieter riding surface. And thin lifts do provide a small, but measureable, amount of structural benefit.”
Unlike slurry seals, micro surfacing usually is placed at more than a single stone thickness. But like thin lift asphalt overlays, micro surfacing can be used on high-volume roadways, to correct wheel path rutting, and provide a skid-resistant pavement surface.
Ohio sold on thin asphalt lifts
Ohio took a close look at thin asphalt lifts, and in 2008 released an important study that justifies them cost-wise.