California Bearing Ratio (CBR) test was developed by
the California Division of Highway as a method of classifying and evaluating
soil-sub grade and base course materials for flexible pavements. CBR test, an
empirical test, has been used to determine the material properties for pavement
design. Empirical tests measure the strength of the material and are not a true
representation of the resilient modulus. It is a penetration test wherein a
standard piston, having an area of 3 in$^2$ (or 50 mm diameter), is used to
penetrate the soil at a standard rate of 1.25 mm/minute. The pressure up to a
penetration of 12.5 mm and it's ratio to the bearing value of a standard
crushed rock is termed as the CBR.
Figure : CBR test |
In most cases, CBR decreases as the penetration
increases. The ratio at 2.5 mm penetration is used as the CBR. In some case,
the ratio at 5 mm may be greater than that at 2.5 mm. If this occurs, the ratio
at 5 mm should be used. The CBR is a measure of resistance of a material to
penetration of standard plunger under controlled density and moisture
conditions. The test procedure should be strictly adhered if high degree of
reproducibility is desired. The CBR test may be conducted in re-moulded or
undisturbed specimen in the laboratory. The test is simple and has been extensively
investigated for field correlations of flexible pavement thickness requirement.
Test
Procedure
The laboratory CBR apparatus consists of a mould 150
mm diameter with a base plate and a collar, a loading frame and dial gauges for
measuring the penetration values and the expansion on soaking.
The specimen in the mould is soaked in water for
four days and the swelling and water absorption values are noted. The surcharge
weight is placed on the top of the specimen in the mould and the assembly is placed
under the plunger of the loading frame.
Load is applied on the sample by a standard plunger
with dia of 50 mm at the rate of 1.25 mm/min. A load penetration curve is
drawn. The load values on standard crushed stones are 1370 kg and 2055 kg at
2.5 mm and 5.0 mm penetrations respectively.
CBR value is expressed as a percentage of the actual
load causing the penetrations of 2.5 mm or 5.0 mm to the standard loads
mentioned above. Therefore,
Two values of CBR will be obtained. If the value of
2.5 mm is greater than that of 5.0 mm penetration, the former is adopted. If
the CBR value obtained from test at 5.0 mm penetration is higher than that at
2.5 mm, then the test is to be repeated for checking. If the check test again
gives similar results, then higher value obtained at 5.0 mm penetration is
reported as the CBR value. The average CBR value of three test specimens is
reported as the CBR value of the sample.
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