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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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|Publication Number: Date: Fall 1996|
Issue No: Vol. 60 No. 2
Date: Fall 1996
Test tracks, in which large numbers of pavement test sections can be loaded to failure by trucks in controlled experiments, are key contributors to improving pavement design.
The AASHO (American Association of State Highway Officials) Road Test, for example, was a group of six test track loops built and trafficked in the late 1950s; the results of that test series remain the foundation for many of today's standard theories of highway pavement design. The Penn State Test Track, built in the early 1970s, has contributed significantly to pavement design and rehabilitation in Pennsylvania and surrounding states. The closed-loop portion of the Minnesota Research Project (Mn/Road), currently in operation, is expected to further improve pavement structural designs in Minnesota and other northern states.
The most recent addition to the stock of test track sites is WesTrack, a 2.9-kilometer (km) oval loop in western Nevada. Construction of the first pavement test sections on WesTrack was completed in October 1995, and truck loading was initiated in March 1996. The construction and the two-year truck loading of the track are being funded by the Federal Highway Administration (FHWA) as part of a significant study of hot-mix asphalt (HMA) paving materials and construction.
The current WesTrack program has two objectives:
When the study is completed early in 1999, the highway community can expect to have improved tools for specifying high-quality HMA construction and for predicting the short- and long-term performance of HMA pavements from the results of quality-control tests during construction.
WesTrack is located in a high desert area on the grounds of the Nevada Automotive Test Center (NATC), some 60 km southeast of Reno. NATC is the prime contractor for the FHWA study, but the project is better described as being carried out by the seven organizational members of the WesTrack Team. The site has advantages for test track construction and operation especially its limited precipitation, which averages 100 millimeters (mm) per year, and mild winters.
The 2.9-km oval track consists of two tangent (straight-line) pieces and the superelevated curves connecting them. Each tangent contains 13 test sections, each of which is 70 meters (m) long. There are no test sections along the curves.
The track is two lanes wide. The 3.7-m-wide inside lane is the "trial lane," and the 3.7-m-wide outside lane is the "test lane." Because each test section had distinct mixture designs and/or construction requirements, placing a section in the trial lane first gave the researchers and the construction contractor the opportunity to "practice" before placing the section in the test lane. The trial lane also provided a path for tests of the autonomous truck controlling systems, which will be described later in this article.
The experiment was designed to examine those materials and construction properties that are most likely to affect pavement performance and to cover the range of variabilities in properties common to typical paving jobs. The variables being examined are asphalt content, compaction, and aggregate gradation. The pavement structural cross section is uniform throughout the 26 test sections -- 150 mm of HMA placed over 300 mm of aggregate base and 450 mm of compacted fill subgrade.
Each of the 70-m-long test sections consists of three zones. The first 25 m (in the direction in which the trucks are moving) is a transition zone, which allows for variability during the construction of the section and for damping of any truck dynamic motion generated by damaged pavement in the previous test section. The next 40 m is the test area in which all nondestructive pavement performance measurements are made, and the last 5 m is the area in which destructive sampling and test measurements are made periodically.
Nevada Automotive Test Center
Nichols Consulting Engineers
Granite Construction Co.
Harding Lawson Alpha
University of Nevada at Reno
Oregon State University
University of California at Berkeley
The plan calls for the application of 10 million equivalent single-axle loads (ESAL) of 80 kilonewtons (kN) to the track sections over a two-year period during the study. Other constraints limit single-axle loads to 89 kN and allow a single tandem axle per vehicle with a load not to exceed 178 kN. The WesTrack team elected to apply the loads with four identical trucks -- triple-trailer combinations. The trucks are loaded with tied-down steel plates. A fully loaded truck weighs 676 kN and applies approximately 10.3 ESAL per pass to the test sections.
Achieving the 10-million ESAL application target will require the vehicles to operate at 64 km per hour for 20 hours each day, seven days a week (with a two-day shutdown every two weeks for pavement performance measurements) for most of the two-year test period. Maintaining operation of the trucks at this level for two years presented particular concerns for driver safety, especially because the closed traffic loop will provide little visual stimulation. The study required the WesTrack team to seriously examine autonomous vehicle operation, i.e., computer controlled and without human drivers.
At the time of this writing, driverless vehicle development and operation have been strikingly successful. The NATC-led team settled on a wire-in-the-road approach, with Global Positioning System (GPS) satellite vehicle location as a backup. Cables were installed around the track at the top of the base layer during construction. The front bumper of each truck is equipped with a guidance antenna array that picks up guide tones emitted by the wires under the pavement. A vehicle control computer mounted in the sleeper area of the truck's cab activates steering, braking, and accelerating actuators to constantly adjust the vehicle's path and speed. The vehicle path is repeatable to within Ã±13 mm transversely on repeated trips around the track. Because the path is so repeatable, it has been necessary to artificially induce transverse wander into the vehicles to simulate the variability produced by human drivers.
Each truck has a second computer to collect and analyze data from a number of critical components, e.g., tire temperature and oil pressure. If any critical measurement is not within preset limits, the vehicle-monitoring computer signals the parent system control computer to shut down all of the vehicles.
In addition to the wire-in-the-road primary control, the system control computer constantly examines the location of each truck as reported by GPS. If any individual truck stops or even slows down relative to the others, the parent system control computer observes that two trucks are not maintaining the proper separation and automatically shuts down all of the vehicles. Numerous other redundancies have been incorporated within the vehicles and throughout the control system to ensure the operation is as safe as possible.
The aggressive loading schedule and the use of the driverless vehicle technology required new tractors, automatic transmissions, and state-of-the-art electronic controls on the engines and transmissions. The WesTrack team brought in truck and truck component manufacturers -- Navistar, Detroit Diesel, Twin Disc, and Goodyear -- as partners in the driverless vehicle development. The manufacturers provided accelerated delivery, reduced costs, and state-of-the-art technology in exchange for performance data during the planned two-year track loading. (Each of the four trucks is expected to cover more than 800,000 km during the loading period.)
As in any test track or other accelerated pavement testing project, achieving the study's objectives depends on obtaining extensive and high-quality data throughout the study. These efforts at WesTrack will be described in more detail in future publications. Below is a list of some of the testing that has been or will be accomplished:
Throughout the WesTrack project, one of the secondary goals has been to take full advantage of the data being developed by cooperating with other research programs. For example, material samples have been shipped to researchers at Georgia Tech, the Colorado Department of Transportation, FHWA's Turner-Fairbank Highway Research Center, and Koch Materials for use in new laboratory rut-testing equipment. Results of these tests can then be compared with the field rutting data obtained on the WesTrack pavements. Track data and information from strain gauges installed under a section of the track ramp are being made available to a new National Cooperative Highway Research Program project aimed at determining the relative pavement damage caused by super-single and singled-out dual truck tires. And significant quantities of binder, subgrade, aggregate, and HMA are being stored in the FHWA-supported Materials Reference Library in Reno for future testing by other researchers.
The performance data from WesTrack are expected to add significantly to the development of performance-related specifications for HMA construction and to provide an early verification of the SuperpaveTM performance-prediction models and mixture analysis system. Both of these accomplishments should, in turn, lead to improvements in the quality of HMA construction.
Terry M. Mitchell is a research materials engineer in the Pavement Performance Division at the Turner-Fairbank Highway Research Center in McLean, Va. He joined FHWA in 1971. He received bachelor's degrees in aeronautical engineering and mathematics and a doctorate in nuclear engineering from the University of Michigan.