How to Equip the CBR Testing Machine?

California Bearing Ratio is commonly known as the CBR testing machine, developed by the California Division of Highways in 1930, in order to justify the strength and resistance of subgrade soil and pavement designs. The civil engineers have subsequently observed and testified the authentication of the CBR testing machine. It is now a go-to option to be taken under place before any structure planning. It helps us in determining the thickness of various pavement layers through a penetration plunger.

The CBR test is enforced by computing the pressure required to get access to the soil with a plunger of the standard zone. The evaluated compression is then divided by the pressure required to achieve an equal penetration on a standard crushed rock component. In other words, the pressure up to penetration of 2.5 mm is measured and its ratio with respect to the bearing value of a standard crushed rock is taken into consideration. Moreover, it is observed that the rocks solid the surface, the bigger the CBR value. Generally, few areas of sand may have a CBR value of 10%, while a value of 2% indicates a sloppy surface and a value between 80-100% depicts a high-quality sub-base.

As per the guidance the procedure to conduct CBR testing is as follows:

Taking 3 samples each of about 7 kg must be compressed with a density range from 95% to 100%. Weigh of empty mold. Gradually, add water to the first sample (compressed it in five layers by giving 10 blows per layer). After compaction, eliminate the collar and level the surface. Take a sample of moisture content. Weight the molds with the addition of the compacted sample. Then, place the mold in the drenched tank for up to four days. In the meantime, take other samples and apply different blows and repeat the whole procedure. After four days, scale the swell reading and find %age swell. Take away the mold from the tank and drain the remaining water. Then place the sample under the penetration device and place some extra weight of 10lb. Apply the weight and note the penetration load values. Draw the graphs of penetration, penetration load and find out the value of at the required degree of compression.

Edward Powell