WesTrack, FHWA's hot-mix asphalt test track in Nevada, is less than 1 year old, but already the day-in, day-out heavy truck traffic is beginning to take its toll. Five of the 26 test sections had such significant permanent deformations (ruts 25 to 35 mm [1 to 1.4 in] deep or more) that they had to be rehabilitated in November.

Surprised? Don't be. The WesTrack project was designed to evaluate how variations in construction quality—namely asphalt content, aggregate gradation, and compaction—affect pavement performance. Many of the test sections were intentionally built with less-than-optimum construction properties. The rutting was expected—and confirms the importance of proper materials and construction practices.

The other 21 test sections at WesTrack also show some rutting, but to a lesser degree. This rutting is also not unexpected in light of the very concentrated, heavy loading the track receives and the high temperatures during the Nevada summer. In the first 5 months of loading, more than 1 million equivalent single-axle loads (ESALs) were applied to the pavement—roughly equal to the loads many Interstate highways carry in a year or more.

What Does It Mean?

Although data are still being collected and analyzed, the team has made several initial observations:

• The five worst-performing sections all have high amounts of asphalt, ranging from 0.7 to 1.2 percent above the optimum design values for their aggregate gradations. The high asphalt content was intentionally designed into the asphalt mixes for these test sections, to mimic the asphalt contents occasionally encountered in the field. Severe rutting was thus expected on these test sections. The impact of high asphalt content on performance is clear.

• As with other accelerated pavement tests, care must be taken in relating the test results at WesTrack to "real world" conditions. The WesTrack tractor-trailer trucks are controlled by computer, and during the initial months of operation only a limited amount of wander was programmed into their paths. As a result, the trucks tracked a narrower path than conventionally driven trucks would have. The amount of rutting caused by 1 million ESALs over a 5-month period at WesTrack is likely to be much higher than would be expected from 1 million ESALs applied to an Interstate highway over a period of a year or more. And once the initial ruts were formed, the WesTrack truck trailers tended to stay in the ruts regardless of the path of the tractor.

• Liquid level gauges capable of detecting deformations as small as 2.5 mm (0.1 in) in the aggregate base and visual examinations of trenches cut in each of the five failed test sections give no indication of any significant deformation in the base layer to date.

A second objective of the WesTrack project is verification of the Superpave performance prediction models and the Superpave mix analysis system. Only 4 of the 26 WesTrack sections were designed to be true Superpave pavements—that is, with optimum asphalt contents, air voids, and aggregate gradations. The other 22 sections have significant variations in one or more of these three properties (representing variations often encountered in the field) and would not be expected to perform nearly as well as the optimum sections. The laboratory and field performance data do indicate inadequate coarse aggregate angularity may be contributing to the early rutting in even the optimum sections.

For more information, contact Terry Mitchell at FHWA (telephone: 703-285-2434; fax: 703-285-2767; email: terry.mitchell@fhwa.dot.gov).

Back to December 1996 - List of Articles

   This page was created with Netscape Navigator Gold