Thin is In

Across the country, paving material suppliers andcontractors alike mining new opportunities with thin hot mix asphalt overlay technologies.

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.”

Proven to hold up, thin hot mix asphalt overlay as experimented with and developed over the past two generations is now an accepted -- and growing -- technique for pavement preservation. As road and highway agencies struggle to do more work with less money, thin overlays are increasingly answering the need.

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.

A thin overlay approach to pavement preservation does have its inherit challenges. A higher asphalt content is needed to properly coat and bind the aggregate, because of the higher aggregate surface area due to the finer aggregate particles, advises the National Asphalt Pavement Association.

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.

‘Bout Time

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