Standard Method of Test for
The California Bearing Ratio
AASHTO Designation: T 193-99 (2003)
This test method covers the determination of the California Bearing Ratio (CBR) of pavement
subgrade, subbase, and base/course materials from laboratory compacted specimens. The test
method is primarily intended for but not limited to, evaluating the strength of cohesive materials
having maximum particle sizes less than 19 mm (3/4 in.).
When materials having maximum particle sizes greater than 19 mm (3/4 in.) are to be tested, this
test method provides for modifying the gradation of the material so that the material used for tests
all passes the 19.0-mm (3/4-in.) sieve while the total gravel 4.75-mm (No. 4) to 75-mm (3-in.)
fraction remains the same. While traditionally this method of specimen preparation has been used
to avoid the error inherent in testing materials containing large particles in the CBR test apparatus,
the modified material may have significantly different strength properties than the original
material. However, a large experience base has developed using this test method for materials for
which the gradation has been modified, and satisfactory design methods are in use based on the
results of tests using this procedure.
Past practice has shown that CBR results for those materials having substantial percentages of
particles retained on the 4.75-mm (No. 4) sieve are more variable than for finer materials.
Consequently, more trials may be required for these materials to establish a reliable CBR.
This test method provides for the determination of the CBR of a material at optimum water
content or a range of water content from a specified compaction test and a specified dry unit mass.
The dry unit mass is usually given as a percentage of maximum dry unit mass from the
compaction tests of T 99 or T 180.
The agency requesting the test shall specify the water content or range of water content and the
dry unit mass for which the CBR is desired.
Unless specified otherwise by the requesting agency, or unless it has been shown to have no effect
on test results for the material being tested, all specimens shall be soaked prior to penetration.
The values stated in SI units are to be regarded as the standard.
M 92, Wire-Cloth Sieves for Testing Purposes
M 145, Classification of Soils and Soil-Aggregate Mixtures for Highway Construction
T 2, Sampling of Aggregates
T 87, Dry Preparation of Disturbed Soil and Soil Aggregate Samples for Test
T 88, Particle Size Analysis of Soils
T 89, Determining the Liquid Limit of Soils
T 90, Determining the Plastic Limit and Plasticity Index of Soils
T 99, Moisture-Density Relations of Soils Using a 2.5-kg (5.5-lb) Rammer and a 305-mm
T 180, Moisture-Density Relations of Soils Using a 4.54-kg (10-lb) Rammer and a 457-mm
T 265, Laboratory Determination of Moisture Content of Soils
SIGNIFICANCE AND USE
This test method is used to evaluate the potential strength of subgrade, subbase, and base course
material, including recycled materials for use in road and airfield pavements. The CBR value
obtained in this test forms an integral part of several flexible pavement design methods.
For applications where the effect of compaction water content on CBR is small, such as
cohesionless, coarse-grained materials, or where an allowance is made for the effect of differing
compaction water contents in the design procedure, the CBR may be determined at the optimum
water content of a specified compaction effort. The dry unit mass specified is normally the
minimum percent compaction allowed by using the agency’s field compaction specification.
For applications where the effect of compaction water content on CBR is unknown or where it is
desired to account for its effect, the CBR is determined for a range of water content, usually the
range of water content permitted for field compaction by using the agency’s field compaction
The criteria for test specimen preparation of self-cementing (and other) materials, which gain
strength with time, must be based on a geotechnical engineering evaluation. As directed by the
engineer, self-cementing materials shall be properly cured until bearing ratios representing longterm service conditions can be measured.
Molds—The molds shall be cylindrical in shape, made of metal, with an internal diameter of
152.40 ± 0.66 mm (6.0 ± 0.026 in.) and a height of 177.80 ± 0.46 mm (7.0 ± 0.018 in.) provided
with an extension collar approximately 50 mm (2.0 in.) in height and a perforated base plate that
can be fitted to either end of the mold. (See Figure 1.) It is desirable to have at least three molds
for each soil to be tested.
Figure 1—California Bearing Ratio Apparatus