The experimental test road facility, referred to as WesTrack, was constructed at the Nevada Automotive Test Center (NATC) near Fallon, Nevada (USA), under the Federal Highway Administration (FHWA) project “Accelerated Field Test of Performance-Related Specifications for Hot-Mix Asphalt Construction”. The experiment had two primary objectives. The first was to continue development of performance-related specifications for hot-mix asphalt (HMA) construction by evaluating the impact of deviations in materials and construction properties from design values on pavement performance in a full-scale, accelerated field test. The second was to provide some early field verification of the Superpave® mix design procedures. Because the WesTrack site typically experiences less than 100 mm of precipitation per year and no frost penetration, it was well suited for evaluating the direct effects of deviations of materials and construction properties on performance.

WesTrack was constructed as a 2.9-km oval loop incorporating twenty-six 70 m long experimental sections on the two tangents. The pavement cross sections consisted of various asphalt concrete mixes placed on a design thickness of 300 mm of aggregate base, with a thick layer of “engineered fill” below, sometimes referred to as the subgrade in this paper. The design thickness of the HMA layer (referred to as asphalt concrete [AC] in this paper) in all sections was 150 mm, placed in two 75 mm lifts.

Construction was completed in October 1995; trafficking was carried out between March 1996 and February 1999. During this period, four triple-trailer combinations composed of a tandem axle, Class 8 tractor and a lead semi-trailer followed by two single-axle trailers, operated on the track at a speed of 64 km/h, providing 10.3 equivalent single-axle load (ESAL) applications per vehicle pass. The use of autonomous (driver-less) vehicle technology provided an exceptional level of operational safety and permitted loading to occur up to 22 hours per day, 7 days per week.

The experimental variables were in the asphalt concrete mixes, and included 3 levels of asphalt content, three levels of air void content, and three aggregate gradations (Coarse, Fine and Fine Plus). The main performance variables were rut depth and percentage of the wheelpath area with fatigue cracking. Approximately 4.95 million ESALs were applied during the trafficking period. Several original sections failed early in the experiment; they were replaced with a mix design that duplicated the Coarse gradation mix experiment in the original construction, but changed the aggregate source. The replacement sections were constructed in June 1997 after the application of approximately 2.85 million ESALs. Only the 26 original test sections are considered in this paper. The experiment yielded clearly differentiated levels of permanent deformation and fatigue cracking among the experimental sections.

All of the initial 26 test sections used the same aggregate source and binder in the asphalt concrete. The symbols used in this paper for the three different gradations are: Fine (F), Coarse (C) and Fine Plus (P). The Fine mixes had a Superpave aggregate gradation that passed above the “Restricted Zone” in the Superpave mix design system. The Fine Plus mixes had a gradation that was slightly finer than the Fine gradation. The Coarse mixes had a gradation that passed below the Restricted Zone. For each mix type there were sections with high (H), medium (M) and low (L) asphalt content with target values of 4.7, 5.4 and 6.1 %, respectively for Fine and Fine Plus mixes and 5.0, 5.7 and 6.4 % for the Coarse mix, and with high (H), medium (M) and low (L) air voids content with target values of 4, 8 and 12 %, respectively.

In the naming system used for each section in this paper, “FML”, for instance, indicates a section with a Fine (F) mix with a Medium (M) AC content and a Low (L) air voids content (a 1 or 2 following the mix name would indicate whether the section was the first or the second of replicate sections for those cells that had replicates).
Measurements taken during the WesTrack experiment and used in this study included Falling Weight Deflectometer (FWD) deflections, pavement temperatures at several depths in the asphalt concrete, and pavement distress condition surveys following the LTPP protocol.