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Publication Number: FHWA-SA-97-028
Date: October 1997
The researchers at WesTrack, the Federal Highway Administration's (FHWA) hot- mix asphalt test track in Nevada, were not surprised when several sections of the track had to be replaced in June. In its first year of operation, WesTrack had taken a beating from more than 2.8 million 80kN equivalent single-axle loads (ESALs), which caused severe rutting and fatigue cracking in many sections. What operators didn't expect was that those new sections would begin showing signs of permanent deformation after only a few days of service.
An independent team of experts from State highway agencies and industry was formed and charged with determining the likely causes of the early rutting and recommending steps that could be taken to avoid similar problems with other coarse-graded mixes.
The team consisted of:
John D'Angelo and Chris Williams from FHWA provided technical support to the team.
The WesTrack project is primarily designed to further the development of performance-related specifications for hot-mix asphalt construction and to validate the Superpave performance prediction models.
In August, the team traveled to WesTrack for a firsthand look at the failed sections. After meeting with the WesTrack project team and reviewing all available data, they inspected the asphalt plant, the quarry, and the test track. They looked at the structural design, construction procedures, environment, traffic, and mix properties.
The sections placed in June had been designed to meet the Superpave requirements for binder and aggregate properties. The sections were constructed with a coarse Superpave aggregate gradation made from quarried stone; the binder was an unmodified PG64-22. The mixes had varying levels of asphalt content and air voids, in conformance with the experiment underway at WesTrack; thus, some of the sections met the Superpave requirements for mix volumetrics, while others did not.
After evaluating all the factors that could have contributed to the early rutting in the reconstructed test sections, the team concluded that the major cause was likely a combination of high asphalt content and low binder stiffness in a coarse-graded mix (see sidebar, page 1).
What does this mean for highway agencies and contractors using the Superpave system to design coarse-graded mixes? After all, most of the asphalt roads being built today are coarse-graded mixes. And more and more of those mixes are being designed to the Superpave specifications.
"The mix used at WesTrack was designed for 10 million ESALs, but the application rate is very concentrated. If traffic continued for 15 years there would be 75 million ESALs applied," says Gerry Huber from Heritage Research Group and a member of the evaluation team. "What happened at WesTrack is not representative of what we're seeing out on the highways across the country. Superpave pavements in service across the United States are performing as well or better than pavements designed with the Marshall hammer."
Larry Michael of the Maryland Department of Transportation agrees. "Maryland has built several projects with a similar gradation to the coarse-graded mix used at WesTrack, and the performance to date has been excellent," said Michael, who is also a member of the investigating team. He noted, however, that the binders used in Maryland's coarse-graded mixes are much stiffer than that used at WesTrack.
Michael says the premature rutting at WesTrack shows that the track is doing its job. "We need to remember that WesTrack is a test track," he says. "We expect to have failures there; it's all part of the experiment. What we learn at WesTrack will help us improve the Superpave system and the performance of asphalt pavements."
"Highway agencies across the country have placed more than 300 Superpave pavements on moderate and high-traffic highways, and they generally are performing well," says Gary Henderson, leader of FHWA's Superpave Technology Delivery Team. "The need for more fully documented performance data on the in-service Superpave pavements has been widely recognized, and we are pleased that the AASHTO Superpave Lead States team is addressing this issue." The team has developed a strategy to collect performance data from Superpave projects (see sidebar below).
For more information, or to obtain a copy of the team's report, contact John D'Angelo at FHWA (phone: 202-366-0121; fax: 202-366-7909; email: email@example.com) or Chris Williams at FHWA (phone: 703-285-1018; fax: 703-285-2767; email: firstname.lastname@example.org).
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