Wanting to be thin

May 4, 2015

Asphalt overlays gaining in popularity in Oregon

Thin-lift overlays are quickly emerging as a preferred alternative to other road-surface preservation treatments.

In Oregon, thin lifts are typically made up of 38-in. (9.5-mm) mix installed at a thickness of 1 to 1.5 in. These overlays are proving to be more economical than other surface treatments when compared on a life-cycle cost basis. They also provide the unique benefit of actually adding structure to the pavement rather than simply covering or sealing specific problem areas. 

Thin-lift overlays are a common preservation treatment by many agencies. Such overlays have been constructed in Oregon for several years and use of them is growing in the state. Washington County, which encompasses a few large cities just west of Portland, applied its first thin-lift overlay treatment in 2000. That overlay was expected to last seven to 10 years, but was still performing well in 2011 and is expected to last another four years or more. Washington County has specified several thin-lift overlays since then, and all are still in good to excellent condition.

Thin-lift overlays are not the solution for every maintenance situation, but there are specific applications where they’ve proven most effective. They are excellent as a preventive-maintenance tool for pavements that are still in fair to good condition, meaning the pavement’s condition index (PCI) is 70 or better. They are ideal in cases where the pavement still possesses structural integrity but the asphalt surface is worn or damaged. And because they cool quickly, it is recommended that thin-lift overlays be installed in warm, dry weather for best compaction and performance. They will not be effective as a preventive-maintenance tool in cases where more significant structural repair or rehabilitation is warranted. 

The initial cost of a thin-lift overlay is typically higher than that of other surface-maintenance treatments, but the overlay can last two to three times longer. When costs are averaged over the life of the thin-lift overlay and compared with the life-cycle cost of a chip seal or slurry seal, for example, the thin-lift overlay often costs less.  

Beyond cost benefits, thin-lift overlays provide other advantages as well. They can improve ride quality—which is always popular with the driving public—and they can restore skid resistance. Thin-lift overlays also add more structural strength to the pavement, while other surface treatments like chip seals and slurry seals do not. 

The need for speed

The thin-lift overlay project cited previously was not done on just any ordinary road. It was installed on Murray Boulevard, right in front of the entrance to Nike’s world headquarters in Beaverton. Average daily traffic at this location is 25,000 vehicles. 

Prior to installation of the thin-lift overlay, the road exhibited signs of oxidation, showed some raveling and had minor cracking. Because of the road’s condition, and the fact that a lengthy closure was highly undesirable, this project was a prime candidate for a thin-lift overlay. 

Other options, including a micro surfacing and a slurry seal, were considered. A micro surfacing was expected to last about seven years, and a slurry seal had potential for an even shorter lifespan. In addition, the slurry seal would have required a four-hour cure time that was not favorable for such a busy road. 

Washington County opted for the thin-lift overlay treatment, and the construction crew completed the four-lane overlay in just one day, closing the road one lane at a time for about two hours per lane. As mentioned earlier, this treatment was initially projected to last seven to 10 years but is now serving well into its 11th year and is expected to last another four years or more. 

Buying time 

Walnut Boulevard is one of the busiest streets in Corvallis. When an eight-year-old layer of micro surfacing began to delaminate on that street, ruts and poor rideability were the result. A chip and fog seal project was let for bid. 

“The intent was to provide a five- to eight-year surface treatment that would keep the surface intact until we could do a more significant grind and inlay,” said Bruce Moser, transportation services supervisor for the city’s public works department. 

After conducting a review of the relative cost-to-benefit ratios for switching to a thin-lift overlay, and discussing with the contractor, North Santiam Paving Co., potential performance and construction problems with a chip seal, the city of Corvallis agreed to convert the project specification to a thin-lift overlay. 

“We promoted the choice of the overlay and felt it would provide a far superior product in terms of ride quality and structural capacity,” said Pete Sipos of North Santiam Paving Co. “We were also very concerned about chip loss and potential windshield damage since this is a very high-traffic urban street.”

The cost for the paving portion of the thin-lift overlay project was about 33% higher than for the chip and fog seal treatment, but the difference was much smaller when evaluated as a component of the entire project. Reducing the overall cost further is the fact that the overlay is expected to last approximately nine years compared with the six-and-a-half-year life expectancy of the chip and fog seal originally specified. Moser said cost-per-year calculations came to $29,200 for the chip and fog seal, versus $28,900 for the thin-lift overlay.

“We see this approach as a way to prolong the point at which a more costly repair is required, especially for streets that are failing from the top down,” Moser said. “The Walnut Boulevard project was the largest grind/thin-lift overlay project the city has ever done, and the larger an area for which you can prolong more extensive repairs, the greater the return on your short-term investment,” he added.

Moser noted other benefits of the grind/thin-lift overlay (mill-and-fill) approach to maintenance: It keeps the project parameters simple; less surveying and design are required; grades can remain unchanged; and construction impacts to users are minimized. 

“The thin-lift overlay construction on the Walnut Boulevard project provided additional depth of strength, improved ride quality and less road noise,” Sipos said. “And the public perception is that we actually improved the road rather than just sealing it.”

What works where 

“We were always big on chip seals for maintenance,” said Todd Whitaker, director of Polk County Public Works, “but we think thin-lift overlay technology has real promise.”

Polk County conducted several trials with thin-lift overlays as an alternative to chip-seal treatments. The trials included roads with various traffic volumes, condition states and locations. “We weren’t trying to fix roads that were structurally degraded,” Whitaker said, “but rather to get a feel for what works and where.”

Whitaker reported that the county has been very happy with its thin-lift overlay projects and pleasantly surprised by the degree of ride-quality improvement that is achievable with thin mixes. “The thin mixes that we tried (1.25- to 1.5-in. lifts) proved to have great workability, smooth appearance and exceptional ride quality,” he said. “The ultra-thin mix (1-in. lift or less) was a little more finicky, but we only placed a small amount and it went down in tougher conditions.”

Whitaker noted that thin-lift overlays require more attention in order to apply the tack-coat uniformly and at the proper shot rate. “The thinner the lift, the more important it is to establish a good bond with the existing surface,” he explained. 

Ultimately, maintenance in Polk County will likely be a mix of thin-lift overlays and chip seals, Whitaker said. “I think our higher average daily traffic [ADT] roads—those with more than 2,000 vehicles per day—will be candidates for thin-lift overlays, while our lowest ADT roads—those with fewer than 400 vehicles per day—will be candidates for chip seals,” he explained. “Exactly where the line is between the two treatments is something we are currently trying to determine.”

Polk County has been placing thin-lift overlays during the last two paving seasons and in 2011 placed successful thin-lift overlays that included both reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS). One of Polk County’s thin-lift overlays earned first place in the rural road category from the Asphalt Pavement Association of Oregon at its annual paving awards presentation.

Trying it out

In the spring of 2012, the Oregon Department of Transportation will place a thin-lift overlay on a 2-mile, two-lane stretch of I-5 in the far southern portion of the state. The treatment will serve as a preventive-maintenance measure, but also provide a means to test alternate treatments to what normally would be addressed with a 2-in. mill and fill (or with a chip seal on lower-volume roads). 

“We’re looking for an alternative to the 2-in. mill-and-fills we’ve been doing on open-graded courses,” said ODOT Pavement Specialist Jeff Shambaugh. The thin-lift approach requires less material and can be completed in less time, he added. 

Shambaugh said ODOT also is hoping to find an alternative to chip seals for noninterstate roads. “Chip seals deteriorate on interstates after two to three years, so that will be the bar for evaluation of this thin-lift overlay,” he explained. “We actually anticipate the overlay to last as many as five to eight years.”

Shambaugh noted that there may be other applications for thin-lift overlays within ODOT’s jurisdiction if this test section performs as well as expected. Aside from being a chip-seal alternative, thin-lift overlays could be applied to roads with ruts deeper than 0.5 in. and to open-graded wearing courses where deeper grinds and inlays have been required in the past. AT

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