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[1]).  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.


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 (Table 1).

Table 1: AASHO Road Test Weather

Average Mean Temperature (July) 24.5°C (76°F)
Average Mean Temperature (January) -2.8°C (27°F)
Annual Average Rainfall 837 mm (34 inches)
Average Depth of Frost (for fine-grained soil) 711 mm (28 inches)


  • 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[1]).


Figure 2: Axle weights and distributions used on various loops of the AASHO Road Test (redrawn from Highway Research Board, 1961[1]).

Performance Measurements

The following measurements of performance were collected:

HMA Pavement Materials

(Information for this section taken from Highway Research Board, 1962a and Highway Research Board, 1962b[2])

The primary materials used in the HMA 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.


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

Sieve Surface Course Gradation Limits Binder Course Gradation Limits
1 in. 100
3/4 in. 100 88-100
1/2 in. 86-100 55-86
3/8 in. 70-90 45-72
No. 4 45-70 31-50
No. 10 30-52 19-35
No. 20 22-40 12-26
No. 40 16-30 7-20
No. 80 9-19 4-12
No. 200 3-7 0-6

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

Sieve Specification Range Percent Passing Actual Mean Percent Passing
1-1/2 in. 010 100
1 in. 80-100 90
3/4 in. 70-90 81
1/2 in. 60-80 68
No. 4 40-60 48
No. 10 28-46 35
No. 40 16-33 20
No. 100 7-20 13.5
No. 200 3-12 10


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

Sieve Specification Range Percent Passing Actual Mean Percent Passing
1-1/2 in. 100 100
1 in. 95-100 100
3/4 in. 90-100 96
1/2 in. 80-100 90
No. 4 55-100 71
No. 10 40-80 52
No. 40 10-30 25
No. 200 5-9 6.5

Footnotes    (↵ returns to text)
  1. The AASHO Road Test: Report 1, History and Description of the Project.  Special Report 61A.  Highway Research Board, National Academy of Sciences.  Washington, D.C.
  2. The AASHO Road Test: Report 5, Pavement Research.  Special Report 61E.  Highway Research Board, National Academy of Sciences.  Washington, D.C.