Ultrathin Bonded Wearing Courses, Explained
BY AsphaltPro Staff
An ultrathin bonded wearing course is a single-process application of an ultra-thin overlay, typically ¾ inches thick or less, designed to improve pavement performance on high-speed, high-traffic roadways.
Initially introduced to the U.S. market from Europe in the early 1990s as NovaChip, the use of bonded wearing courses has evolved alongside advancements in materials, equipment and construction practices, allowing it to be applied effectively across diverse traffic conditions and climates.
“The use of [bonded wearing courses] has really boomed in the past five or 10 years, particularly in the northeast,” said Robert Betsold, director of marketing at All States Materials Group (ASMG), Sunderland, Massachusetts, during a presentation about the process at World of Asphalt/AGG1 2024. “It’s really become a workhorse application for a lot of the interstates throughout the northeast.”
During his presentation, Betsold discussed how the process compares to traditional thin-lift paving; the materials, equipment and construction methods used; and the best practices of selecting projects for this application. Let’s take a closer look.
Ultrathin Bonded Wearing Course vs. Thin Lift Overlays
The key differences between this treatment and a conventional thin lift overlay are the reduced thickness and the higher tack coat application rate. “When we talk about hot mix overlays, we’re usually talking 1.5 inches or less,” he said. “With [bonded wearing courses], we’re talking about ¾ inches or less.”
The application is designed for high-speed, high-volume roadways, Betsold said, attributing bonded wearing courses’ performance in those conditions to the increased tack application. Betsold estimates a typical tack application for an overlay in the northeast to be 0.05 gallons per square yard, while a bonded wearing course sees an average of 0.2 gallons of tack per square yard.
“The tack is the basis for the performance of this treatment and the ability to pave thin without the typical challenge of delamination,” Betsold said. “If the materials are made right and the application is done right, you don’t have that concern. The material is locked in place.”
Although the process looks similar to conventional paving, there are also differences in application. “If you tried to spray out 0.2 gallons of tack per square yard and tried to drive trucks over it and pave over it, you’d have a mess,” Betsold said. That’s why this process relies on the use of a spray paver with an onboard emulsion tank for the tack and strategically placed spray nozzles that apply the tack immediately before the mix is applied atop it. “The tack is never touched by anything other than the mix going on top of it.”
“This is a great process for agencies looking to get into preservation, because it looks like conventional paving to the driving public,” Betsold said. “They see dump trucks, they see a paver, they see rollers and they see a smooth paved surface. The performance of the application is different, but the outward appearance is the same.”
According to Betsold, the tack most commonly used in the northeast is a polymer modified tack (usually CRS1P). “We get the performance of that polymer modified liquid,” he said, “but it’s really the thickness of the application that gets us the adhesion we need without delamination.”
However, such a thick tack application brings concerns about flushing. That’s why Betsold said the process usually uses a gap graded mix. “This falls in between dense graded and open graded mixes, which allows for the wicking of that 0.2 gallons of tack up into the mix to get that really strong bond,” Betsold said.
“If you have a thickness of ¾ of an inch, you’re typically going to get about ⅜ of an inch (about half the cross section) that will be flooded with the tack coat,” he said. “When you take cores and look at a cross section of it, you’ll see that there are pretty much no air voids. By filling those in, you’re sealing it/waterproofing it.”
Betsold said polymer-modified mix is also often used on these projects, particularly higher volume state roads in the northeast, but that he’s also seen asphalt rubber used successfully. “In the local agencies, the traffic doesn’t warrant [polymer-modified mix] in terms of the upcharge, so the majority of the municipalities are still using conventional hot mix with this application.”
The American Association of State Highway and Transportation Officials (AASHTO) is in the process of finalizing specifications for ultrathin bonded wearing courses, Betsold said. In addition to his role at ASMG, Betsold is a member of AASHTO Emulsion Task Force, serves on the board of FP2, and is an industry representative on the board of the Northeast Pavement Preservation Partnership. “Soon, we’ll have a national specification from AASHTO to be a guideline for local agencies,” he said.
One difference a conventional thin lift overlay and a ultrathin bonded wearing course is the higher tack coat application rate, applied immediately before the asphalt using a spray paver like Vogele’s S18003i SprayJet.
Right Treatment, Right Place, Right Time
The ultrathin bonded wearing course put down in 2015 on Massachusetts’ Route 3 has become a poster child of success for this application, Betsold said. Route 3 is the main north-south route from Boston to New Hampshire, seeing more than 100,000 vehicles each day, he added.
The project consisted of nearly 1,000,000 square yards of mainline bonded wearing course application with fog seals on the shoulder as a trial project through the SHARP program for high volume preservation. The project was divided into three test sections, one using conventional hot mix, one polymer-modified mix, and one mix with asphalt rubber.
It was estimated that a typical mill-and-fill application would take two to three years, whereas the application of a bonded wearing course could be completed in one construction season. Ultimately, the project was paved in 51 night shifts, starting the first lane at 10 p.m., the second at midnight, and opening the road to traffic by 5 or 6 a.m.
MassDOT was hoping to get seven to 10 years, and eight years later Betsold said the treatment is performing “far beyond their expectations. This was really the project that springboarded [the use of this application] in the northeast.”
Betsold said that although the performance of the bonded wearing course on Route 3 has been very good across the board, the polymer-modified and asphalt rubber sections of the project have outperformed the conventional hot mix section, “which is what we would expect if we were putting a traditional hot mix overlay out there compared to polymer-modified and rubber.”
The Route 3 project illustrates the types of applications for which ultrathin bonded wearing courses shine: high speed, high volume, demanding applications. “It’s really going to perform when you get into those applications where you’ve got truck traffic or more than 100,000 vehicles per day,” Betsold said.
However, the treatment can be used on a variety of applications, from high volume roads down to low volume municipal roads. “New York is doing a lot of neighborhoods and subdivisions with it,” Betsold said.
In municipal settings, Betsold said, the treatment is most often used in urban environments with higher traffic counts. He provided the city of Newton, Massachusetts, population 100,000 just outside Boston, as an example. “They’ve been using this in a variety of applications with a lot of success,” Betsold said. “It’s really become a workhorse for them in their preservation program.”
Newton’s first use of bonded wearing courses was a standard preservation overlay in 2018. “Based on the performance of that project, they started looking at how they could use it in other applications and it’s really expanded into some unique applications,” Betsold said. For example, to treat approximately 10 miles of exposed concrete roads “they were paying a tremendous amount to maintain and remove.”
Working with the city, a consultant engineer and various contractors, Newton established a process for applying ultrathin bonded wearing courses to those concrete roads. “They’re typically doing a ¾-inch mill of that existing concrete, doing mastic on any joints or larger cracks, patching if necessary, putting down an asphalt rubber chip seal as an interlayer to get that sealing and crack resistance, then surfacing it with that bonded wearing course as the final surface,” Betsold said. “The first was done in 2020 and to date is performing very well with some minor cracking.”
Ultrathin bonded wearing courses can be placed over asphalt or concrete, atop existing surface or a milled surface, with tweaks to tack application based on the existing surface and traffic type. “If you’ve got a drier surface, you may bump that tack application rate up a bit,” Betsold said. “If you have a richer surface, you might back that application down a little.”
Despite the success of ultrathin bonded wearing courses in a variety of applications, Betsold stresses the importance of understanding its benefits and potential drawbacks. “By no means is this the best application for everywhere,” he said. “I’m a big proponent of using the right treatment, in the right place, at the right time.”
For example, as with any polymer-modified mix, it’s important to minimize handwork. “I was on a project in metro Boston a few years back that had 300 utility structures and trying to rake out the polymer-modified mix was not fun for the crew,” Betsold said. The polymer-modified mix also results in a shorter potential construction season.
The gap graded mix design is also less ideal for projects with significant handwork, for example, cul-de-sacs, sharp turns or variable width roads. “There aren’t a lot of fines with that gap graded mix, so raking it too much can result in shadowing,” Betsold said. “Your better bet may be to mill those out and pave them with a traditional hot mix and then do your main line with the bonded wearing course.”
Betsold said candidates for ultrathin bonded wearing courses must still be in good structural condition. “If it’s new to the agency and you use it [on that kind of a pavement] and it fails, it’s going to be a whole lot more difficult to get them to let you do it again. We need to make sure we’re setting our projects up for success.”
Betsold also cautions against what he calls “scope creep” on bonded wearing courses. “This process was developed to be a quick in-and-out application, not a full-blown construction project,” he said. However, he’s seen some projects in the northeast “where there’s been some scope creep and they’ve turned a process that’s meant to save time and money (while extending the roadway) into a full-blown construction project, which drives up the cost tremendously.”
Best Construction Practices
Although the ultrathin bonded wearing course paving process may look similar to conventional paving to the untrained eye, Betsold outlined some differences it’s important to understand. This begins with prep work.
“Ultrathin bonded wearing courses will fix some minor defects in your profile, but this is not a thick overlay,” Betsold said. He recommends filling any cracks ¼ inch or wider. “Crack sealing creates a tremendous increase in the performance of all types of pavement preservation treatments including overlays, because we’re preventing water intrusion for pennies on the dollar.”
In Betsold’s experience, many state projects using bonded wearing courses are micro milling ahead of treatment. Although milling often isn’t necessary because the lift is so thin, he said it isn’t uncommon for bonded wearing course projects to be milled full-width or gutter milled to maintain curb reveal.
The use of a gap graded mix also has benefits in terms of tire spray, Betsold said. “We all know when we drive on a dense graded mix, we get that tire spray. With gap graded mix, you are going to see reduced tire spray due to that water shedding, although not to the extent of what you’d see with open graded mix.”
“A lot of state projects in the Northeast (and even some municipal settings) have utilized a milled fog line,” Betsold said. This is when they apply a fog seal on the shoulder. “That allows them to handle the shoulders with a less expensive treatment…while tying the surfaces right in together so you don’t have any drop off.”
Then comes the spray paver, which applies both the tack and the hot mix in a single pass. “The only thing that goes on top of that [tack] is the mix,” Betsold said. Calibration here is exceedingly important. “Everything is metered and controlled. You need to make sure your equipment is calibrated correctly so you’re actually getting what the system is telling you you’re getting.”
Areas that require extra attention include takeoffs and joints. “I’ve seen the tack be a bit skinny on the takeoff, which is a potential point of failure,” Besold said. “Using the hand hose allows you to get that consistent heavy application of tack.”
“If you don’t have that tack out there or your application is off, and you’re trying to get ¾ of an inch to adhere without that wicking action, you’re setting yourself up for potential issues,” he said.
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Betsold said overlapping at the centerline joint can be a material solution to a persistent industry problem. “A slight overspray as you pave that joint allows for that adhesion and waterproofs that joint to address some of those issues,” Betsold said. However, he added, “we need to be careful we aren’t overspraying too much and causing flushing.” The exact right amount of overspray depends on the balance between the mix design, the gradation of the mix, the existing surface of the roadway and the tack application rate, he said.
The rollers play a significant role in the wicking action Betsold describes. “We aren’t rolling for density, but to bond that material to the tack and to provide a smooth final surface.”
Usually, these projects require just one roller, though Betsold said two may be used on some state projects. Regardless of the number of rollers, Betsold recommends the roller(s) be immediately behind the paver.
“You can cover a lot of ground really fast at ¾ inches thick,” he said, which also increases the speed of the rollers. “Because we don’t need to hit that traditional density, the speed you can get traffic back on the road is certainly a benefit to the application.”
This benefit was put to the test on one of the first bonded wearing course projects Betsold was ever on. It was a municipal paving project in a Vermont subdivision with just three houses. “Not a single car had come out all day,” Betsold said. The paver was ¾ of the way done when an impatient driver in a Porsche rolled up. “The roller took two passes, and the Porsche drove over it, no problem.”
Ultrathin bonded wearing courses can be used successively, Betsold said, but the existing surface must be considered as always. “If we have a bonded thin lift out there and put a second one on seven to 10 years later, there are a couple things that come into play,” he said. “You have extra air voids [with the gap graded mix], so when you put the tack on there, you’re going to lose some of it down into the previous lift, so you aren’t getting as much wicking action up into the successive lift.” This, like many other factors, must be considered when determining the tack application rate.
On another municipal project near a fire station, the paver was ¾ of the way done when the fire alarm went off. “The roller went over it, then the fire trucks immediately after, and that pavement is still there five years later,” Betsold said.
The key to such a successful project, as with any treatment, is quality control. “Quality control is at the forefront of everything that we do,” Betsold said. He stressed the importance of a quality control plan, “especially if an agency is new to the process.”
“There are agencies out there who think they’re inspecting this the same way they’d inspect traditional hot mix,” he said. “Yes, it’s paving, but it isn’t the same as paving. There are nuances you need to understand in terms of materials, gradation, application rates. Those little nuances can have a big impact on the performance of that application. When we’re talking about such a thin pavement layer, if we’re off by a little bit, that can have a significant impact on the long-term performance of that treatment.”