Thin asphalt overlays offer a number of benefits, especially as a technique to extend the life of a good pavement, says Dave Newcomb, vice president for research and technology at the National Asphalt Pavement Association. For example, thin overlays offer:

A long service life and low life cycle cost;

Improved ride quality;

An ability to quiet a noisy pavement;

An engineered approach to materials selection and design;

No loose stones after initial construction;

Little or no dust generated during construction; and

Ability to recycle, requires no curing time, and can be easily maintained.

High performance in New Jersey

New Jersey uses five mixes for thin overlays. They are named:

High Performance Thin Overlay (HPTO), with a 4.75- mm NMAS;

Ultra-Thin Friction Course (or Novachip, a patented product);

Stone-Matrix Asphalt (SMA), with a 9.5-mm NMAS;

Modified Open-Graded Friction Course (MOGFC); and

Asphalt Rubber Open-Graded Friction Course (AR-OGFC).

The HPTO, or High-Performance Thin Overlay, does not allow any reclaimed asphalt pavement (RAP), requires 100 percent crushed aggregate and PG 76-22 binder, says Robert J. Blight, project engineer, Materials, New Jersey DOT. The minimum binder content is 7 percent, but mix designs are usually closer to 8 percent.

The HPTO is designed for high-volume traffic roads in New Jersey – typically greater than 30 million ESALs. In-place allowable air voids in HPTO range from 2 to 7 percent. In addition to passing volumetric requirements, the mix is required to pass a rutting requirement of 8,000 cycles in the APA Rut Tester and not exceed a maximum of 4 mm (0.16 inch) of rutting.

Blight says HPTO was designed as a 1-inch thick wearing course, but has been used as a leveling course on a couple of projects. “Our first two pilot projects were constructed in 2008, both as wearing course preventive maintenance mixes,” says Blight. The total quantity placed for three projects in 2008 reached 16,545 tons. Costs ranged between $125 and $145 per ton.

“In 2009 we placed approximately 10,000 tons of HPTO as a leveling course on one project,” says Blight. “So far the sections look good and we are adding the HPTO to our standard specifications.”

SMART overlays

Illinois has an asphalt overlay program called SMART – for Surface Maintenance at the Right Time. Overlay thicknesses range from 1.25 to 2 inches, and candidate pavements have to pass a set of criteria before being eligible for a SMART overlay, says Jeff South, bureau chief of statewide program planning for the Illinois DOT.

In recent years, many more Michigan cities and counties are starting to use thin overlays.

“Pavements have to be in fairly decent condition, and we don’t put a SMART overlay on bare concrete,” says South. “We only consider roads that get fewer than 500 trucks per day. Only minimal patching can be needed. These overlays can be used on all marked and unmarked roads but not on the interstate.

“We’re only trying to get seven to 10 years more out of the pavement,” says South. “In general, the program has been successful. If we miss on the criteria, we might get less time – or if the road is in downstate Illinois where the weather is not so harsh and traffic is less, we can get more than seven to 10 years.”

Back in the ‘90s, Illinois averaged nearly 350 centerline miles of two-lane roads per year with SMART overlays. Last year, the state placed close to 250 centerline miles. “We are still hoping to place about 250 miles this year,” says South. The cost? “Our planning estimate is about $200,000 per mile, so over 250 miles, that is approximately $50 million,” South says.

Experimental overlays

At a 3.27-mile stretch of Illinois SR 72, an arterial in suburban Chicago, the Illinois DOT is running an experimental overlay program aimed at discovering new ways to apply an overlay without using the friction aggregate in the entire 1.5- or 2-inch asphalt surface lift. The stated goal of the study, conducted by IDOT and the University of Illinois, is: “To investigate and develop a new surface cross section that utilizes locally-available aggregates as much as possible, while being effective relative to friction, durability, noise and life-cycle costs.”

Steel slag, a commonly-used friction aggregate in Illinois and Indiana, is in limited supply and the shipping costs can be high depending on the proximity of the project to the steel mills. “Currently, friction aggregate is included in the whole surface lift, but the friction aggregates are really only needed on the pavement surface,” says Tom Zehr, HMA implementation engineer with the Illinois DOT’s Central Bureau of Materials. “So, that prompted this research to better use our friction aggregates on the surfaces and use our more locally-available aggregates that are less expensive, have lower shipping costs, and are more plentiful, in the rest of the lifts and the structural layers.”

Hot Springs Village, a community in Arkansas, uses 4.75-mm nominal maximum aggregate size mixes for all of their overlays.

Construction on the experimental project is scheduled to begin and end this year. Four experimental surface mixes will be tested. One will have quartzite as the friction aggregate, and a second will have steel slag and polyolefin/aramid reinforcing fibers in it for strength and rigidity. The third mix, says Zehr, will be a 4.75-mm stone-matrix asphalt (SMA) with terminal-blended ground tire rubber asphalt. The 4.75-mm SMA mix will have an N-design compactive effort of 80 gyrations and the other three mixes will have an N-design of 90.