Thin Asphalt Overlays

Answer the Call

New research, as well as past performance, proves their value

Research from two separate field sites is proving the effectiveness of thin asphalt overlays under heavy traffic loadings. Meanwhile, thin asphalt overlays continue to succeed as a way to extend pavement lives across the country.

Since 2004, the Michigan DOT has laid approximately 800 lane-miles of HMA-Ultra Thin overlays at ¾-inch thick.

The Mississippi DOT has sponsored a test section at the NCAT Pavement Test Track consisting of a 4.75-mm top-size aggregate mix that was placed as a ¾-inch inlay. Operated by the National Center for Asphalt Technology, the test track uses heavy-duty trucks to apply accelerated traffic loads. Since being laid down in 2003, the MDOT section has withstood more than 24 million 18-kip equivalent single axle loadings (ESALs).

“It’s holding up beautifully, and the rutting performance has been outstanding,” says Buzz Powell, Ph.D., P.E., test track manager and an assistant director at NCAT. Rutting is only about 5 mm and there is no cracking. The International Roughness Index is around 60 inches per mile, which is good.

“It was set up as a 100-percent screenings mix, and the longevity of it has surprised everyone,” says Powell. “It looks much the same today as it did when we placed it; it has not oxidized much at all.”

MDOT decided to place the 4.75-mm mix to test its suitability for low volume road preservation. But all indications are that the mix will carry successfully 30 million ESALs by the end of September 2011, say Powell and Shane Buchanan, Ph.D., P.E., of Vulcan Materials.

The layer underlying the 4.75-mm mixture consists of 3.25 inches of a 12.5-mm nominal maximum aggregate size, slag/limestone, dense-graded Superpave mix. The aggregate blend grading in the 4.75-mm mix is 69.3-percent limestone dry screenings, 18.8-percent crushed gravel and 10.9-percent natural sand, along with 1-percent hydrated lime.

Success at MnRoad

At MnRoad, a field research facility set up on Interstate 94 in Minnesota, several sponsors convened to place a 2-inch asphalt overlay over a 5-inch concrete pavement. The group studying the entire composite pavement structure consists of the University of Minnesota, the University of California, the states of California and Washington, and the Federal Highway Administration (FHWA). A second group including the Minnesota DOT, the University of Minnesota, and NCAT are studying the performance of the asphalt layer.

“For aggregates we used two different taconites, both from the iron range in Minnesota,” says Timothy R. Clyne, P.E., a MnRoad operations engineer. “We used manufactured sand and 7.3-percent asphalt binder.

“The asphalt mix is performing well, but the concrete is falling apart,” says Clyne. “The overlay had reflective cracks in it within a year, but there’s been no rutting to speak of. We used a 15-foot joint spacing in the concrete and just about every joint has reflected up through the asphalt.

“I think the concrete was a little too thin to support the interstate traffic,” says Clyne. “You could just look at it and say the taconite aggregates are to blame, but I don’t think that would be the correct conclusion.” He said the road receives about 600,000 ESALs per year and the test section has been there more than a year, so the section has seen more than 1 million ESALs. “Part of our intent was to build thinner sections so that they fail quickly and we can learn from them,” Clyne says.

Thin overlays: A valuable tool

Thin overlays grew in popularity following FHWA’s 2002 push for preventive maintenance. States such as Ohio, Michigan, and North Carolina have placed thin overlays for years. As well, a number of other states, including Delaware, Georgia, Illinois, New Jersey, Tennessee and West Virginia, have all reported to NCAT that they have a fine mix designation. Other states have probably added 4.75-mm mixes since a nationwide survey was completed.