Do the Right Thin: Thin Asphalt Overlays

Thin asphalt overlays: Valuable versatility

For a host of reasons, thin asphalt overlays are proving to be one of the most useful road treatments in the tool boxes of state and local transportation departments. Budget restraints have pressured agencies to make their road dollars cover more square yards, and thin overlays can do that. Plus, agencies want to add the maximum life to a pavement – preserve it for as many years as possible.

Again, thin overlays answer the bell.

“We’ve been pretty steady on thin asphalt overlays,” says Aric Morse, a pavement engineer with the Ohio DOT. “Smoothseal is a tool that our districts like to use very much.” He is referring to a trademarked thin overlay product that comes in two varieties, Type A and Type B. Both of them go down at about 1-inch thick; sometimes Type B is placed a bit thicker, at 1.25 inches. The Type A overlay has 8.5-percent polymer-modified binder and is typically placed on low- and medium-truck-volume roads. The Type B overlay has 6.4-percent polymer-modified binder and is placed on medium- to high-truck-volume roads.

“Thin overlays give us a new surface course; and the Smoothseal overlays are a little richer in binder content,” says Morse. “They should last 12 to 15 years, if they’re placed on the right pavement.

“You get some economy by placing these overlays a little bit thinner. But they cost somewhat more per ton because there is more asphalt in them.”

Ohio’s Smoothseal Type B requires 100-percent two-faced crushed coarse aggregate for mixes used in heavy traffic conditions. The crushed aggregate provides internal friction, leading to greater stability. “We use PG 76-22, and we also allow a blend of that binder with 5-percent latex rubber,” says Cliff Ursich, executive director of Flexible Pavements of Ohio. “The synergy of using crushed aggregate and a polymer-modified binder results in durability superior to conventional fine-graded hot-mix asphalt.”

Many districts in Ohio place a 3/4-inch leveling course of asphalt topped by a 1.25-inch surface course. “That’s been a strategy of many of our districts for years and years,” says Morse. “They have performed very well. And they have been cost-effective for us.”

At the National Asphalt Pavement Association (NAPA), Kent Hansen, P.E., director of engineering, points out that thin asphalt overlays offer:
• some structural improvement;
• improved ride quality;
• the ability to maintain grade and slope with minimal drainage impact;
• an engineered approach to materials selection and design;
• no loose stones after initial construction;
• very little or no dust generation during construction;
• no curing time to delay opening;
• low-noise generation under traffic;
• no binder runoff; and
• the ability to recycle the material.

Thin overlays are superior to surface treatments such as microsurfacing, says Christie Barbee, executive director of the Carolina Asphalt Pavement Association. “A thin asphalt overlay gives an agency more structure than you get with microsurfacing,” says Barbee. “With a thin overlay, you have the opportunity to level and smooth the road surface, which you don’t get with slurry seals.”

Barbee sums it up by saying, “You’re going to get a longer life out of a thin overlay than you will from a surface treatment like microsurfacing.”

New Jersey’s Tool Box

The New Jersey DOT has five thin overlay mixes available to preserve pavements:
• high-performance thin overlay (HPTO);
• ultra-thin friction course;
• stone-matrix asphalt (SMA) 9.5-mm surface course;
• modified open-graded surface course (MOGFC); and
• asphalt rubber open-graded friction course.

Which one is used the most? “The 9.5-mm SMA, placed 1.5 inches thick, is our most popular thin overlay,” says Robert Blight, project engineer in the Bureau of Materials, New Jersey DOT. “For the cost, that mix gives you the most bang for the buck. We’re not using the two open-graded mixes as much because we’ve had some issues with snowplows gouging them.”

The ultra-thin friction course is placed with a special spray paver that applies the polymer-modified emulsion tack coat directly from the paver before the mix is placed. The top-size aggregate is 9.5 mm and most aggregate is larger than 4.75 mm, so it is basically chips. The mix has a binder content of 4.5 to 5.7 percent and no air void requirement.

“We paved a project with the ultra-thin friction course this past year and it seems to be performing very well,” says Blight. “We put it down in the spring, and so far so good. We’ll see how it goes through the winter.”

Blight says New Jersey did not place any HPTO in 2011. “Right now we’re looking at HPTO, the Ultra-thin and the SMA, and we’re evaluating all three of them for performance and cost-effectiveness,” he says.

Blight estimates that in 2011 the DOT placed on the order of 10 thin overlay projects with the 9.5-mm SMA mix. “A lot of them are mill-and-inlay jobs where we place a 1.5-inch surface course,” he says.

HMA Ultra-Thin in Michigan

In Michigan, the DOT uses an HMA ultra-thin surface treatment that is typically placed at 83 pounds per square yard, or approximately 3/4-inch thick, says Kevin Kennedy, a preventive maintenance engineer with the Michigan DOT.

Kennedy says an agency needs to take care in selecting roads for a thin overlay. “The existing pavement should exhibit a good base condition and a uniform cross-section,” he says. “The visible surface distress may include slight raveling, minor surface irregularities and a slightly polished surface. The cross-sections should be free of ruts or distortions.”

Michigan’s HMA ultra-thin surface treatment protects the remaining pavement structure, slows the rate of deterioration, corrects minor surface irregularities, improves skid resistance and improves ride quality, Kennedy says.

Michigan does impose a crushed aggregate requirement for the HMA ultra-thin overlays. For high-volume roads, the minimum crushed content is 95 percent; for medium-volume roads, the requirement is 75 percent; and for low-volume roads, it is 50 percent. The top-size aggregate is 3/8 inch. Similarly, the binder grades vary, depending on traffic volumes. For low-volume roads, Michigan specifies a PG 64-22; for medium volumes, it’s PG 64-28P; and for high-volume roads, the requirement is PG 70-22P.

Thin overlays are succeeding in this country because they work and they’re cost-effective. A road manager just needs to take care in selecting which roads get thin overlays, and choose a well-proven mix design.

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