Murray Boulevard is a major arterial best known for being the access to Bowerman Drive, the main entrance to Nike World Headquarters where thousands of employees work on a sprawling campus that would make many prestigious colleges jealous. To asphalt pavement engineers Murray Boulevard is important not because of its famous tenant, but because of the sections of thin asphalt pavement.
The first section (about 2,700 feet) of 1-inch thick asphalt pavement was paved in 2001 after Washington County noticed cracking in the existing surface. Washington County extended the 1-inch section another 2,200 feet in 2002. The mix was a Level 3 (100 gyration) 3/8-inch NMAS mix, with PG 64-22 asphalt binder, and no recycled asphalt pavement. Although many of the project documents no longer exist, the remaining records show that the contract price was $40 per ton.
As of 2017, Washington County sealed cracks and performed some minor maintenance, but has performed no significant resurfacing on the initial thin lift pavement sections.
In 2007 Washington County needed a preservation treatment to extend the life of a much larger section of Murray Boulevard at least 10 years. Funding was too low for a major rehabilitation, but something had to be done because fatigue cracking would soon accelerate and result in a serious failure.
A chip seal was unsuitable because of high traffic volume, and other seals did not have the needed life. The County’s engineers decided to overlay the existing surface with thin lift paving like what was done in 2001 and 2002. With help from the local asphalt pavement association and similar to the pavement in 2001, they specified a 1-inch section of Level 3 (100 gyration) 3/8-inch NMAS mix, with PG 64-22 asphalt binder and no recycled asphalt pavement, for over 2 miles of Murray Boulevard (approximately 788,000 square feet), which required about 4,800 tons of asphalt mix. Recognizing that there was no way to accurately measure the density of a 1-inch lift, the County did not specify a specific density requirement and required compaction “as directed.”
Baker Rock Resources was awarded the project with a bid price of $46.70 per ton of placed asphalt pavement, which was applied to an estimated quantity of 4,797 tons, for a total contract price of about $224,000. Baker Rock Resources developed a mix design with the following:
Baker Rock tested the mix for rutting potential with the local pavement association’s Asphalt Pavement Analyzer. Oregon’s typical maximum allowed rutting for samples tested with the Asphalt Pavement Analyzer is 6 millimeters, and all of the samples tested well under the limit at less than 3 millimeters.
Because of the significant daytime traffic, Baker Rock completed the paving at night, averaging just over 1,000 tons per night. Paving 1-inch with lower night temperatures necessitated tarping the trucks and running the breakdown roller close to the paver. Baker Rock targeted four roller passes for compaction.
During production, because Baker Rock was concerned about getting compaction with cooler nighttime temperatures and thin paving, it added additional asphalt binder. Although additional asphalt binder could increase the risk of rutting, the rut tests showed that the mix was sufficiently stable. The first night Baker Rock produced mix at 6.57 percent asphalt binder, and increased that percentage to around 7 percent for the remaining nights of paving. The actual quantity of asphalt pavement placed was 4,730 tons, for a total contract price of $220,892, which calculates to about $2.63 per cubic foot.
All of the sections performed exceptionally through 2016. However, the 2016-2017 winter was one of the worst on record for Washington County (it was the coldest winter in the Pacific Northwest since 1992-93, with 4 ice storms and 4 snow events), which caused the surface to show signs of failure with some cracking and areas of delamination. Washington County plans to spot repair the failures over the 2017 paving season (something that was not previously needed), and repave Murray Boulevard in 2018.
The 2001 and 2002 sections provided over 15 years of service with little maintenance, and the
2007 section provided 10 years of maintenance-free service. With respect to the 2007 section, it is likely that the fantastic durability was due to the additional asphalt binder Baker Rock added to the mix. Without knowing it, Baker Rock essentially did what the local asphalt pavement association presently recommends for pavement preservation – designing for durability with additional asphalt binder if rut testing shows adequate stability.
Thinlays provided Washington County exactly what it needed – an affordable and dependable preservation treatment that greatly extended the life of a critical urban arterial. Had the County used a chip or other seal, significantly more maintenance would have been required (possibly repeated seals), and the underlying pavement would not have had the structural benefit of an additional inch of asphalt pavement.
After Murray Boulevard was paved, a consultant recommended microsurfacing to preserve the County’s arterials. After a couple of seasons of inconsistent performance and delays in opening microsurfaced sections to traffic, the County switched back to thin asphalt pavement treatments. Baker Rock Resources paved another 1-inch Thinlay on SW 170th Street (another major arterial in Washington County) in 2015, and it looks and rides great. None of the Thinlays have failed prematurely and the recently paved Thinlays are expected to last at least as long as the Murray Boulevard sections.
Because of the performance on Murray Boulevard, Washington County considers Thinlays for the preservation of all of its urban arterials.