“I’ve been in the business for 24 years now,” he says, “and when I started, we used to talk about a finer hot mix – minus-three-eighths – but the conventional wisdom was that smaller aggregate just didn’t create a mix that was tough enough, that it just didn’t have the stability of the larger aggregate. The conventional surface mixes are a minus-half-inch limestone chip.” However, he explains, quality assurance wasn’t at the level then that it is today, and earlier nonconventional mixes were rudimentary. “This new 4.75 mm is designed volumetrically and it’s got the same properties that the courser mix has, that the larger-aggregate-type mixes have, and it works every bit as well in our experiences.”

By having a three-quarter-inch overlay compared to an inch, county agencies are sold on the fact they can cut yield by 25 percent and still have their needed rehabilitation and preservation projects completed, says Yarkie, who is based at the South Bend office of Rieth-Riley, which has operations throughout Indiana and Michigan. “The cost per ton is a little higher due to the higher asphalt content required, but it is more than offset by the lower amount of tons that are required on the job.”

Alan Selner, area manager at the company’s LaPorte, Indiana, plant, recalls how Rieth-Riley introduced the thin mix to the local county agency there. Rieth-Riley was to put the intermediate course down on a county road that was to be topped by chip-and-seal, but instead convinced the agency that the reduced tonnage required for thin lay would make a smooth top affordable. “They pay us by the ton,” says Selner, whose plant is now into its third season of 4.75 mm. “By using the less tonnage, they could get the price down, and plus not anger the people in the county as much. Some people wait a long time to get their road paved,” he says, “and the intermediate pave is laid down and then all of a sudden they get chip-and-seal on top of that, and there’s a little bit of a letdown.” With thin lift, the resulting surface is considerably more aesthetically pleasing, says Selner. “Everyone wins.”

While still early, results so far are good, he says. “It’s held up very well. The key is you’ve got to do it in warmer weather, and you’ve got to make sure the material you are placing is warm. It is very temperature sensitive. If you get it too cold through the paver, it just doesn’t seal up as nicely and it doesn’t look as good. You definitely want to keep it warm. Since you’re only laying somewhere around 80 pounds a square yard, it cools quick, so you’ve got to make sure you work it fast and take care of it quickly.” Material temperature from about 280 degrees F up to approaching 300 turns out the best result, recommends Selner. Down into the 270s, “it cools fast and doesn’t look as good aesthetically,” he says, “and that’s what you’re trying to sell.”

A Little History

Rieth-Riley’s foray into thin asphalt overlays on LaPorte County roads in Indiana as detailed by Selner is similar to earlier experiences of contractors in Maryland, relays Brian Dolan, president of the Maryland Asphalt Association. The roads being worked on had worn and broken off on the outer edges.

An objective he can recall dating back a couple of generations was “to treat as much roadway as you possibly could for the least amount of dollars with something that would hold up,” says Dolan. “Many of those roads were what we refer to as high-crown roadways. The center of the road would have a fairly normal cross-slope, and the outside of the lanes would fall off at 5 percent. So what we would do is hold the crown and drag, and wedge up the outside edge of the road to maybe a 2-percent cross slope, and then we would cover everything with an inch of overlay.” By having the contractor place much of the material only where it was needed, and finishing with an overlay across the road, the result for the owner was an efficient use of resources (which, in today’s reality, may be a key factor in agencies determining if a job, or how much of a job, is actually put to tender). “Some of those roads lasted 30 to 40 years that way.”

Moving forward, thin was in. Dolan recalls a job about 15 years ago on a significant Maryland rural highway – averaging 15,000-18,000 vehicles per day, including more than five-percent trucks – where about 20,000 tons of 4.75-mm mix was laid down at three-quarters of an inch thick.

During a time when public agencies need to stretch their dollars, a thin hot mix asphalt overlay solution to pavement preservation offers a number of advantages, including correcting roughness, notes Dr. Dave Newcomb, National Asphalt Pavement Association vice president of research and technology.

Prior to his 15 years at the state’s asphalt pavement association, Dolan spent 30 years with the Federal Highway Administration, retiring as district engineer in charge of metropolitan Baltimore. He recounts a much earlier assignment, working for a district engineer in the southern part of the state “who was just a huge advocate of thin overlays.” The idea is not new; the improved technologies are.

“In an era when the agencies are looking to stretch their dollars as far as they can and do the best job that they can, they are evaluating means of extending pavement life through pavement preservation techniques. A thin overlay offers a number of advantages,” says NAPA’s Newcomb, noting that the technology is not constrained to Superpave mixes only, but also applicable to Marshall mix designs. “It allows you to correct some things that other types of treatments don’t, for instance roughness. With a thin overlay, you can smooth out the bumps that are in the existing pavements. It can be used to treat a wide variety of pavement surface defects, anything from minor cracking to raveling. If you go in and mill ahead of the overlay, you can even correct things like minor rutting.

“A thin overlay gives you greater versatility in terms of addressing problems in the roadway, and it does it without having to jeopardize things like roadway geometrics.”

Recommends Selner at Rieth-Riley: “It’ll definitely hold up fine, but in order to lay that thin lift, you have to have a road that’s in good shape to start with. You can’t try to lay three-quarters of an inch of material on a road that’s rough, and up and down. You’ll be breaking stone, even as little as the stone is.”

If rutting or shoving is present in the roadway, NAPA recommends the origin of the distortion be ascertained. “If it is present only in the surface, then it may be possible to remove the surface and replace it with a thin overlay. If the distortion is deeper in the pavement, then a more extensive rehabilitation is required.” It is imperative, the association stresses, that a thin overlay not be used to correct widespread structural distresses, such as alligator or longitudinal cracking in the wheel path that originate deep in the pavement.

“When we developed Smoothseal,” says Ursich, “we wanted it to look the same everywhere and perform the same everywhere in the state.” With the diversity of aggregate within the Buckeye State, the standards set were tough. “The leadership in the industry understood that for the industry to maintain its market share, it needed to understand what was it that the customer was wanting,” says Ursich, “and we set out to meet that desire.”