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Road construction tools and tests – what is CBR?

The Californian Bearing Ration (CBR) is a test that should be known by all roading foresters.

The CBR is a penetration test used to determine the strength of road subgrades. We are trying to get an idea of how well the road will last when vehicles start to move over it. A stronger subgrade will resist deflection under load and require a thinner pavement layer, with obvious cost savings. The test is carried out in a standard way to ensure that comparisons of different subgrade materials are possible. The subgrade soil during CBR testing should be at the same moisture content as when the road will be used.

The test is carried out with a plunger of known dimensions being forced into a subgrade sample either in the laboratory or on site. The load required to cause penetration of the plunger is measured at regular time intervals. The results are plotted on a graph. The higher the CBR reading, the stronger the subgrade. The lower the CBR, the weaker the subgrade. Usually CBR values of less than 3% are considered poor (a pavement is definitely required), and greater than 5% are good.

The CBR test is not 100% accurate, but it still provides a very good indication of the subgrade strength. An easier field test that can give an indication of the CBR is the Dynamic Cone Penetrometer (DCP) test, which will be discussed in the next issue.