Team Evaluates Rutting Problems at WesTrack

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:

• Ray Brown, National Center for Asphalt Technology
• Erv Dukatz, Vulcan Materials Co.
• Gerry Huber, Heritage Research Group
• Larry Michael, Maryland Department of Transportation
• Jim Scherocman, consulting engineer
• Ron Sines, New York State Department of Transportation

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: john.d’angelo@fhwa.dot.gov) or Chris Williams at FHWA (phone: 703-285-1018; fax: 703-285-2767; email: ronald.c.williams@fhwa.dot.gov).

Preliminary WesTrack Report Released

The recent premature rutting of Superpave test sections at WesTrack (see September Focus, p. 8) was due to a combination of materials factors, according to a preliminary report by an independent team of investigators.

"The consensus of the team is that the major cause of early rutting of the reconstructed test sections is a combination of a coarse-graded mixture with high optimum asphalt content (5.7 percent) and low binder stiffness (PG 64-22)," the team’s preliminary report said. The team made several recommendations on how to avoid problems with similar mixes in the future. Two key recommendations for coarse-graded mixes are to consider increasing the binder stiffness when designing mixes for high traffic loads (design ESALs greater than 3 million) and to adjust the mix to reduce voids in the mineral aggregate (VMA) for mixes with a VMA more than 2 percent above minimum.

In response to the investigating team’s request, FHWA is conducting additional testing and evaluation of the materials used at WesTrack to validate the team’s findings. The team will use the results of the testing and evaluation to develop its final recommendations. Those recommendations, and the results of the materials testing, are scheduled to be released December 1.

"WesTrack is a research facility used to advance knowledge of pavement performance," said John D’Angelo in FHWA’s Office of Technology Applications. "Some of the mixes used at WesTrack, including the one that recently failed, were placed to evaluate the extremes of the Superpave system, and there were indications from performance testing that they might not perform well."

Truck Accident at WesTrack

Testing at WesTrack suffered a setback in September when one of the four driverless trucks failed to stop during a routine shutdown. The truck struck the truck that had stopped ahead of it, destroying the truck’s tractor and severely damaging the stopped truck’s three trailers. According to track operators, the accident was caused by an unlikely combination of failures in the computer system and in the truck’s brake system.

The accident is the first major incident at WesTrack. Since June 1996, the trucks have operated at least 15 hours a day and have logged more than 900,000 vehicle-km (600,000 vehicle-mi).

The three remaining trucks began operating again on October 10. The loss of one truck will reduce monthly traffic loading at WesTrack from 320,000 ESALs to about 250,000 ESALs. However, the total ESALs applied to the track will be more than enough for the purposes of the experiment, says Terry Mitchell, FHWA’s project manager for WesTrack. "We will get all the information we hoped for," he says.

Superpave Lead States Team To Collect Project Data

The Superpave Lead States team recently developed a strategy to collect performance data from Superpave projects and disseminate that information to highway agencies and industry.

The team is now working out some of the key details of the plan, including how to select the projects to collect data on and how to determine what data will be most useful. The team hopes to begin collecting and disseminating data later this year, with the help of FHWA and the Superpave regional centers.

According to Paul Mack of New York State DOT, head of the Superpave Lead States team, some of the most important data to be collected will be on how projects were constructed. "Highway agencies and industry want to know what Superpave mixes are supposed to look like and how they are supposed to behave," he says. "We will collect information on what worked, as well as on what didn’t work. This information will give States and contractors that don’t have extensive experience with the Superpave system more confidence when building Superpave projects."

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