AASHO Road Test

The AASHO Road Test, a $27 million (1960 dollars) investment and the largest road experiment of its time, was conceived and sponsored by the American Association of State Highway Officials (AASHO) as a study of the performance of highway pavement structures of known thickness under moving loads of known magnitude and frequency (Highway Research Board, 1961).  The test studied both portland cement concrete and asphaltic concrete pavements, as well as certain types of short-span bridges.   

The information obtained from the AASHO Road Test was crucial in advancing knowledge of pavement structural design, pavement performance, load equivalencies, climate effects, and much more.  The basic performance information resulted in the performance equations and nomographs used in the AASHTO Guide.  This section provides some background information on the AASHO Road Test.  It should be helpful in understanding the experiment's strengths, weaknesses and limitations.

Background

This section provides some of the basic background for the AASHO Road Test and is taken primarily from Highway Research Board's Special Report 61A, The AASHO Road Test: History and Description of the Project (1961).

Location:  Ottawa, Illinois

The AASHO Road Test site (which eventually became part of I-80) at Ottawa, Illinois, was typical of northern climates (see Table 1).

Table 1: AASHO Road Test Weather

Time

• Construction: August 1956 - September 1958
• Test Traffic: October 1958 - November 1960
• Special Studies: Spring and early summer 1961

Test Facilities

• Six 2-lane test loops (see Figure 1)
• Loop 1 = not subject to traffic, used to test environmental effects
• Loops 2 through 6 = subject to traffic described in Figure 2

Figure 1: Loop 5 and 6 AASHO Road Test Layout (redrawn from Highway Research Board, 1961)

Figure 2: Axle Weights and Distributions Used on Various Loops of the AASHO Road Test
(redrawn from Highway Research Board, 1961)

 Performance Measurements

The following measurements of performance were collected:

• Roughness and visual distress
• Deflections, strains, etc.
• Pavement Serviceability Index (PSI)

Flexible Pavement Materials

(Information for this section taken from Highway Research Board, 1962a and Highway Research Board, 1962b)

The primary materials used in the flexible pavement sections will be briefly described below.  This kind of information is helpful in comparing a particular project’s set of  materials to those used at the Road Test.

HMA

The HMA mixes used at the Road Test used:

• Crushed limestone coarse aggregate

• Natural siliceous coarse sand

• Mineral filler which was limestone dust

• Penetration grade asphalt cement (85-100 pen)

The gradation requirements for the surface and binder courses are given in Table 2.

Table 2: AASHO Road Test HMA Gradation Requirements

For the HMA surfaced test sections (in Loops 3-6), the surface course was 1.5 inches thick.  The HMA mixes were designed by the Marshall method using 50 blows per face.   The typical field asphalt contents were about 5.4 and 4.4 percent by weight of total mix for the surface and binder courses, respectively.  The field air voids averaged 7.7 percent for the six test loops.

Base Course

The base course material was a crushed dolomitic limestone that conformed to the gradation range shown in Table 3.

Table 3: AASHO Road Test Base Course Gradation Requirements

It is significant to note that 10 percent passed the 0.075 mm (No. 200 sieve).  Thus, the base was likely a bit frost susceptible.  The CBR of the base material averaged 107.7 percent based on laboratory tests; however, the test results ranged from a low of 52 percent to a high of 160 percent (total of 11 tests).  The minimum specified CBR was 75 percent.  Typical in-place mean dry densities were about 2242 to 2275 kg/m³ (140 to 142 lb/ft³⁾, with mean moisture contents ranging from 5.6 to 6.1 percent.

Subbase Course

The subbase material was a sand-gravel mixture which conformed to the following gradation range shown in Table 4.

Table 4: AASHO Road Test Subbase Course Gradation Requirements

The CBR was specified not to exceed 60 percent.  Typical laboratory CBR values ranged from 28 to 51 percent.  The mean percent passing the 0.075 mm (No. 200) sieve was 6.5 percent (a lesser percentage than the base course).  Typical in-place mean dry densities ranged from 2227 to 2259 kg/m³ (139 to 141 lb/ft³), with moisture contents ranging from 6.1 to 6.8 percent.

Subgrade

One Road Test requirement was that all test pavements be built on a uniform embankment with the top 1 m (3 ft.) constructed of an A-6 soil.  Tests on this soil revealed:

Field CBR and density tests on the completed embankment just prior to paving were (5.12):

Flexible Pavement Designs

(Information for this section taken from Highway Research Board, 1962a and Highway Research Board, 1962b)

The structural design portion of the AASHO Road Test flexible pavement experiment was designed as a full factorial experiment with the design factors being:

•  surfacing thickness
• base thickness
• subbase thickness

To accomplish the many different factorial combinations, the test loops were divided up into small test sections.   Overall, there were 288 different flexible pavement test sections in the main experiments (332 counting replications).   Each flexible pavement section in the main experiment was 30 m (100 ft.) long with all sections being separated by a transition pavement of at least 5 m (15 ft.)  Table 5 summarizes the thicknesses for each loop.

Table 5: AASHO Road Test Structural Design Thickness Summary

1.0

5.0

3.0

Flexible Pavement Performance

Of the various axle loads applied to each of the test loops, a total of 1,114,000 load applications were made (not ESALs).  Of the 332 test sections, 264 sections (or 80 percent) had reached a PSI of 1.5 or less on or before the end of the load application period (from November 1958 to June 1960).  Tables 6 and 7 are used to overview the actual performance for Loops 4 and 6 (Loop 4 had an 18,000 lb single axle as one of its standard loads and Loop 6 had the heaviest axle loads used at the Road Test).  Data from these two loops are shown because they are likely of the greatest contemporary interest.  Table 6 shows that increasing AC thickness (from 3.0 to 5.0 in.) results in increased performance (number of loads until failure); however, not at as significant a rate as one might expect.  Similar results are observed for Loop 6. 

Each section, if it reached a PSI of 1.5 prior to completion of all load applications (recall the total was 1,114,000 axle repetitions) was overlaid with new AC so that the trucks applying the loads could safely continue.  For Loop 4, all of these overlays were applied during the months of March, April, and May.  For Loop 6, 75 percent of the overlays were applied during March, April, and May.  This suggests that freeze-thaw effects strongly influenced flexible pavement performance at the Road Test.

The depth of freeze varied a bit from year-to-year but averaged about 28 in. for the flexible sections and generally reached 30 in.  Such performance might provide additional insight into the required depth of pavement to resist freeze-thaw effects. 

Table 6: AASHO Road Test Loop 4 Results^2,3—Flexible Test Sections (after Highway Research Board, 1962b)

Table 7: AASHO Road Test Loop 6 Results—Flexible Test Sections (after Highway Research Board, 1962b)