THE AASHO ROAD TEST
THE AASHO ROAD TEST
One of the most significant pavement research projects conducted in the past 50 years was the AASHO Road Test, which was constructed in the North Central region. Conducted between 1958 and 1960 on the future alignment of I-80 near Ottawa, Illinois ( Figure 10 ), the project not only evaluated the performance and behavior of pavement structures under a variety of axle loadings, but also investigated the performance of highway bridge structures under known loading conditions (58, 59, 60, 61, 62, 63, 64).
The pavement construction, pavement monitoring, and data analysis activities conducted at the Road Test established many standards and protocols and helped define design, construction, and evaluation practices. Even now, nearly five decades after the completion of the Road Test, the AASHO (and now AASHTO) design procedures continue to serve as the cornerstone for both PCC and HMA pavement design in the United States, as well as in other countries. Additionally, data from the Road Test are often used by researchers in a variety of ways unforeseen by the original AASHO Road Test developers (65).
Six pavement loops comprised the AASHO Road Test; five of them were exposed to traffic loadings ( Figure 11 ). Each traffic loop consisted of two straight tangent sections and a superelevated turnaround connecting them at each end. Tangent lengths were 6,800 ft for Loops
3 through 6, 4,400 ft for Loop 2, and 2,000 ft in Loop 1. Two traffic lanes were constructed throughout the large loops, and each lane in the traffic loops were subjected to a specific truck of known axle type and axle load (65).
FIGURE 10 Location of AASHO Road Test.
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FIGURE 11 Layout of test loops at AASHO Road Test.
The northern tangent section and the eastern turnaround of each loop consisted of HMA test sections, whereas the southern tangent section and the western turnaround of each loop consisted of PCC test sections. A series of short test sections were constructed within each tangent (representing a factorial set of pavement thicknesses), separated by a short transition. Each test section was separated into two identical pavement sections by the centerline of the pavement (as specific truck axle type/axle load combinations operated in each lane). In total, there were 836 test sections constructed in all loops at the AASHO Road Test: 468 HMA pavement test sections and 368 PCC pavement test sections.
The performance of the pavement sections were closely monitored over the 2-year period. In general, measurements were made on those variables that had been demonstrated in previous research to be related to pavement performance. Routine performance measures included
• HMA pavements: – Longitudinal profile, – Roughness, – Cracking, – Patching, and
– Rut depths and • PCC pavements: – Longitudinal profile, – Cracking, – Patching, – Spalling, and – Joint/crack faulting.
The roughness and profile testing were conducted at 2-week intervals, whereas the other pavement condition data were collected as part of weekly surveys. Other measurements, such as
82 Pavement Lessons Learned from the AASHO Road Test and Performance of the Interstate Highway System
surface deflections, surface strains, vertical subgrade pressures, and pavement temperature distributions, were obtained at various times during the test period.
Because one of the principal objectives of the Road Test was to determine relationships between pavement performance and design variables, it became necessary to develop a rational method of determining and expressing as a single number the performance of each pavement section at periodic intervals (62). To address this need, Bill Carey (chief engineer for research) and Paul Irick (chief, data processing and analysis) developed the concept of “pavement serviceability,” which is founded on the principle that the primary function of a pavement is to serve the traveling public. As such, users of a pavement facility can provide their subjective opinions as to how well the pavement is meeting their needs, and it is assumed that the serviceability of a given highway may be expressed as the mean evaluation of all highway users (66). Serviceability was expressed on a scale of 0 to 5, with 0 representing a pavement that was impassible and 5 representing a perfectly smooth riding pavement.
Since it was impractical to convene a panel of raters regularly to provide their opinion of road serviceability on the more than 800 pavement sections at the Road Test, an alternate means of assessing pavement serviceability based on objective pavement condition measurements (roughness and distress) taken on each test section, was developed. With these objective measures and models developed from a panel review of pavements in Illinois, Minnesota, and Indiana, a PSI was computed for each section that served as an estimate of the mean panel serviceability rating. Figure
12 shows typical serviceability histories for selected pavement sections included in the Road Test.
Equations were developed to define the serviceability trends predicted from the number, type, and weight of axle loads relative to either the thickness index (now called the structural number) for HMA pavements or the slab thickness for PCC pavements. Figure 13 presents the graphical relationship that was developed for HMA pavements.
FIGURE 12 Typical serviceability histories for selected pavement test sections (62).
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FIGURE 13 HMA pavement “thickness index” as a function of axle loadings for PSI of 2.5 (60).
As mentioned earlier, the results of the AASHO Road Test were used in the development of the interim pavement design procedures that were released to the states in 1962 for a 1-year trial usage. The design procedures were widely accepted by many highway agencies and were used to design many of the pavements constructed during the Interstate era. A portion of the original AASHO Road Test sections (Loop 1, the untrafficked loop) still remains today in the right-of-way of I-80 as a visible reminder of the significant contributions of the Road Test (see Figure 14 ).