California bearing ratio

The California Bearing Ratio (CBR) is a measure of the strength of the subgrade of a road or other paved area, and of the materials used in its construction.

The ratio is measured using a standardized penetration test first developed by the California Division of Highways for highway engineering. Empirical tests measure the strength of the material and are not a true representation of the resilient modulus.

Definition

The CBR is the ratio of the bearing load that penetrates a material to a specific depth compared with the load giving the same penetration into crushed stone. The test measures neither Stiffness

Modulus nor Shear Strength directly, but gives a combined measure of both.

Penetration is measured by applying the bearing load on the sample using a standard plunger of diameter 50 mm at the rate of 1.25 mm/min. The CBR is expressed as a percentage of the actual load causing the penetrations of 2.5 mm or 5.0 mm to the standard loads on crushed stone. A load penetration curve is drawn. The load values on standard crushed stones are 1,370 kgf (13.44 kN) and 2,055 kgf (20.15 kN) at 2.5 mm and 5.0 mm penetrations respectively.

The CBR can be mathematically expressed as:

C

B

R

=

p

p

s

⋅

100

%

{\displaystyle CBR={\frac {p}{p_{s}}}\cdot 100\%}

Test procedure

The CBR test is a penetration test in which a standard piston, with a diameter of 50 mm (1.969 in), is used to penetrate the soil at a standard rate of 1.25 mm/minute.