CBR
The California Bearing Ratio test is a simple strength test that compares the bearing capacity of a material to that of a well-graded crushed stone.
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Bearing capacity is the capacity of soil to support the loads applied to the ground (pavement / traffic / cranes / buildings etc.)
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The test was developed nearly 100 years ago, around 1930, by the California Division of Highways. Basically, they performed a heap of tests on a local crushed limestone source, this result became the standard that all CBRs are compared against (i.e. CBR 100).
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When performing a CBR test, you’re comparing your material against the California limestone.
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CBRs are penetrated at 1.0mm per minute, a load-penetration curve is plotted and the loads corresponding to penetrations of 2.5mm and 5.0mm are determined.
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California limestone requires a force of 13.2kN to penetrate 2.5mm and 19.8kN to penetrate 5.0mm
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To determine your CBR, divide the kN force at 2.5mm penetration by 13.2 and the kN force at 5.0mm penetration by 19.8. The highest percentage result is the CBR value.
Example:
In the example above, the blue line is representative of the California limestone (CBR 100)
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The sample for the green line took 7900N of force to penetrate 5.0mm
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7.9kN / 19.8kN = CBR 40%
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Generally, the harder the material, the higher the CBR value. A CBR value of 2% is usually found for clay, high-quality sub-base will have CBR values between 80% and 100%, and some sands may have values around 10%
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