Thin is In
By Mike Anderson
For belt-tightened agencies, thin hot mix asphalt overlay may be among a plethora of strategies to preserve pavement . . .and money. (Please see Tom Kuennen’s “RoadScience Tutorial” on Page 20.)
For hungry mainline paving contractors, the laying of what is today a reliable thin top layer has become a way to preserve – perhaps even increase – their levels of crew activity and equipment utilization.
“I think [contractors] see it as a huge opportunity for them to enter in and to compete in the pavement preservation arena,” says Dave Newcomb, vice president of research and technology with the National Asphalt Pavement Association (NAPA). “Given the fact that there aren’t a lot of major rehabilitation projects going on right now, a lot of the work is in pavement preservation,” he tells Better Roads. “I think contractors understand that this is a way for them to keep their head above water in very tough times.”
It’s difficult to determine if thin overlays themselves have actually saved jobs among the contracting sector, says Cliff Ursich, president and executive director of the industry umbrella association Flexible Pavements of Ohio. “But we’ve seen the move by agencies to try to conserve, so thin hot-mix asphalt overlays certainly become more attractive to them,” he says. “What’s the most attractive part of it is that they know they are going to get a product that’s going to last a lot longer than a conventional material, because of its composition.” A blend of high-quality aggregates with polymer-modified asphalt, Smoothseal is the industry name for the Ohio Department of Transportation (ODOT)-approved fine-graded spec item. “If we had never developed this mix,” says Ursich, “certainly there would be a lot more micro surfaced roads in Ohio.”
Perhaps not as far back as thin-lift technologies in some other parts of the country, a launching point for Smoothseal in Ohio occurred some 20 years ago, says Ursich. “We started off actually with a thin-lift asphalt mix developed in the early 1990s, when our department of transportation was just getting started in the use of preventive maintenance strategies. They had traditionally looked at conventional asphalt as a preventive maintenance strategy, but then they started to look at thinner materials,” he says, “and we saw the necessity to go and develop a product in the 1990s.” ODOT then backed away from the preventive maintenance strategy due, believes Ursich, to concern over use of poor-performing copycat-type materials that weren’t up to standard. But thin lift was suddenly back in political vogue in Ohio in the early 2000s, upon the Federal Highway Administration coming out with an emphasis on preventive maintenance. In the interim, Flexible Pavements of Ohio had kept up its commitment to polymer-modified asphalt, making a recommendation to ODOT to use the technology in its heavy-duty surface courses “for the specific reason of getting greater longevity out of pavements,” says Ursich.
With the market move back to thin lift, the fine-graded (passing through a half-inch sieve) Smoothseal mix “has really varied as to the type of facilities it has been used on, from the lowest-volume road you can imagine all the way up to the highest Interstate traffic that we have in the state,” says Ursich.
Explains Newcomb in the NAPA document Thin Asphalt Overlays for Pavement Preservation (http://www.hotmix.org/images/stories/is-135.pdf): The aggregate must be capable of withstanding the design traffic loads without displacement resulting in rutting. Because of the higher aggregate surface area due to the finer aggregate particles, a higher asphalt content is needed to properly coat and bind the aggregate. However, the asphalt content and asphalt grade must be selected, so that flushing, rutting or shoving does not result.”
Rich in asphalt content at 6.5 to 7 percent liquid asphalt with a heavily polymer-modified binder, Smoothseal can be laid at three-quarters of an inch thick, and increased to an inch for Interstates, Ursich tells Better Roads.
Next door, in Indiana, paving material producer and contractor Rieth-Riley Construction has for the past five years been producing and laying a 4.75 mm mix that, in the view of Regional Vice President Gene Yarkie, has been a long time coming.
“What’s the most attractive part of it [thin hot mix asphalt] is that they know they are going to get a product that’s going to last a lot longer than a conventional material, because of its composition.”
— Cliff Ursich, president and executive director, Flexible Pavements of Ohio
“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.
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.”
He recalls the words of an old boss: “If you just build it right, it pretty much sells itself.”
The time has indeed come for thin asphalt overlays, says Rieth-Riley’s Yarkie. “Just try it, and I don’t think you’ll be disappointed.”v