California Bearing Ratio (CBR)

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Definition - What does California Bearing Ratio (CBR) mean?

The California bearing ratio (CBR) test is a penetration test used to evaluate the potential strength of subgrade, subbase, and base course material including recycled materials. The results are used for the design of roads and airfield pavements.

The test was first developed by the California State Highway Department. The test works by testing the pressure required to penetrate a soil sample with a plunger of standard area. The CBR value increases with an increase in the hardness of the surface.

ASTM D1883-16 describes the standard test method for testing CBR of laboratory compacted soils.

Trenchlesspedia explains California Bearing Ratio (CBR)

The California bearing ratio (or CBR) test can be carried out on soils having a maximum particle size of 20mm. For soils greater than this size, the plate bearing test is used.

The CBR test requires a loading machine, a cylindrical mold, a detachable extension collar, a spacer disc, weights, and a compaction hammer. The mold with the test specimen is placed on the lower plate of the testing machine.

The penetration piston is positioned at the center of the specimen with the smallest possible load, but not exceeding 4 kg. The piston is placed such that it is in full contact with the specimen.

The load and deformation gauges are set to zero and the load is applied on the piston so that the penetration rate is about 1.25 mm/min. The load readings are recorded at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0. 7.5, 10, and 12.5 mm.

Using these readings, the load versus penetration graph is plotted. The moisture content of the sample is determined by taking about 20 to 50 gm of soil from the top 30 mm layer and testing it.

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