California Bearing Ratio (CBR)

The California Bearing Ratio (CBR) test was developed by the California state highway department and is widely used to determine the resistance strength of sub-grade and sub-base materials. The test is basically a simple penetration test using a load frame and a standard compacted CBR test mold of the material to be tested. It 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. The results of the test are then compared and evaluated to known standards already established for the material being tested. Application standards are ASTM D1883; AASHTO T193; BS 1377 Part 4.

• Load cell: 10 kN and 50 kN
• LVDT: 5 mm X 0.001 mm

• CBR value at soaked condition for base, subbase, and subgrade
• CBR value at un-soaked condition for base, subbase, and subgrade

• Evaluation of subgrade strength of roads and pavements
• CBR value is used with empirical curves to determine the thickness of flexible pavement layers