How is a CBR Test Performed?

How is a CBR Test Performed?

The California Bearing Ratio (CBR) is a model that was devised by the California Division of Highways engineers in 1938. It is a test for evaluating the mechanical strength of the base courses and subgrades. The performance of the test is done by getting the measure of the required pressure for soil penetration in a sample of soil using a plunger of a predetermined area that is standard, basically a standard piston that is spherical is used at a 1.25 mm/min. The division is then done between the pressure that has been measured and the pressure that is needed, to realize a penetration that is equal on a rock material which is at standard proportion.

Apparatus for CBR Testing.

Compacted or natural soils are used for the performance of the test. These soils could either be in a soaked state or in conditions that are un-soaked. Whatever results that come out of this, are then put into comparison with standard test curves to determine the strength of the soil of the subgrade. Some of the apparatus into application for the process are: mould; spacer disc; steel cutting collar; surcharge weight; dial gauges; penetration plunger; IS Sieves; miscellaneous apparatus; and the loading machine.

The process of testing the CBR

Normally, three specimens that weigh approximately 7kg must be compressed to achieve 95% to 100% densities of compactness, by utilizing 10, 30 and 65 blows. The empty mould is then weighed. Water is then added to the primary specimen which is then compacted into five layers by the application of 10 blows for each layer. Once this has been achieved on the first specimen, the collar is then removed and the surface leveled so that the process of determining the content of moisture takes effect.

The compacted specimen is then added to the weight of the mould. It is from this that the mould is placed into a soaking tank for a duration of four days. The other samples are then taken and a repeat of the same process takes effect after the application of blows that are different to them. The swell readings are taken to determine the percentage swell after four days. This then followed by the removal of the mould for water to drain. The piston is placed beneath the penetration piston and a 10lb surcharge load placed on it.

The value of penetration load is then taken scrutinized, after which graphs are formulated between the penetration (in) and the load penetration (in) to arrive at the CBR value. Finally, a graph is drawn between the percentage CBR and the Dry Density. This will get the needed compaction degree. CBR is instrumental in the testing of the soil compactness. For this to be realized, the right apparatus are needed and the due process followed.