California Bearing Ratio (CBR) Test


A penetration test is performed to determine the California bearing ratio (CBR) value for base, subbase, and subgrade materials upon which pavement design thicknesses are based.

Laboratory Test Procedure (ASTM D1883)

Aseries of specimens are compacted in 6-in.-diameter molds to bracket either the standard or modified optimum moisture. Asurcharge weight is placed on the soil surface and the specimens immersed in a water tank for 4 days to permit saturation while swelling is measured. Amold is placed in the testing apparatus (Figure 1) and a plunger 1.91 in. in diameter is forced to a penetration of 0.1 to 0.2 in. (penetrations to 0.5 in. are used) while the penetration resistance is recorded.

FIGURE 1 – Laboratory CBR loading device (electric motor is often used to drive jack at a rate of 0.05 in./min).

The CBR is determined as the ratio of the test resistance at 0.1 in. penetration to the standard resistance of crushed stone at the same penetration (1000 psi or 70.3 tsf) (Asphalt Institute, 1969).

Field Test Procedure (ASTM D4429-93)

The plunger is jacked against the reaction of a truck weight. The test should not be performed during the dry season because resistances will be higher than those to be relied upon for pavement support. Field values are usually lower than laboratory values, probably because of the effect of confinement in the lab.

Soil Rating System

A soil rating system for subgrade, subbase, or base for use in the design of light traffic pavements in terms of CBR values is given in Figure 2. Typical CBR values for compacted materials are given in Table 1.

FIGURE 2 – Approximate correlation of soil ratings based on CBR values for use in design of light-traffic pavements.
Table 1

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