What is CBR Testing?
The California Bearing Ratio (known as CBR) is a penetration assessment of mechanical strength and bearing value. This test was initially conceptualized by the Department of Transport in California before the World War 2.
CBR now applies around the globe in the construction of roads, parking lots, and pavements. This test can also be used to ascertain the load-bearing ability of paved airstrips and underdeveloped airstrips. CBR tests are also done on highways and base courses under new carriageway construction. This test is usually done in a laboratory (in-situ).
How to calculate CBR
CBR is calculated by finding the ratio of the force used to penetrate a sample, to that needed to make the same penetration into a universal sample. The global sample used is ground California limestone rock which has a CBR value of 100.
CBR experiments are done in the laboratory or the field. CBR test is also conducted on both on dry and wet samples. In the laboratory, the sample is presented at Proctor's maximum dry density or any other density conditions required.
A nozzle of a unique area is then pressed into the soil at a constant penetration rate. The strength needed to keep this rate of penetration is then recorded.
The formula used is:
CBR= (F/Fs) x100%
Where F is the force used to penetrate the sample, and Fs is the force needed to penetrate the standard sample. The CBR principle dictates that the tougher the sample, the higher the CBR.
How to do CBR site testing
The basic site test is done by ascertaining the force needed to push a standard plunger into the soil. The pressure recorded is then divided by the normal pressure requires penetrating ground California limestone.
Level ground access for a 4WD vehicle is required. In case the field is not level, it is manually scrapped to ensure the conditions meet the test requirements.
A cylindrical nozzle with a surface area of 1935mm2 is then pushed into the ground at a fixed rate of 1.25 mm/min. A 4WD vehicle is used to give the reaction force to inject the CBR press. The force applied to the nozzle is then measured and recorded in intervals.
The values recorded are then used obtained CBR by dividing the nozzle force when pressed to 2.5 and 5.0 mm by force given at the same levels in the standard ground California limestone.
On a good site, a single operator can do 6-8 tests. Provisional results are also made available on site. After the site tests, tabulated data can be provided as soon as the following day.
Pros and Cons of the CBR test
• Little training and experience is needed in order to conduct the test
• The CBR method is more suited for airfield pavements than any other methods
• The test is done using uncomplicated and transportable equipment
• The tests can be done either in the field or at the laboratory
• Representative samples of future water conditions can be used for this test
• The field and lab tests are not the same. Results from the field and in the laboratory can differ slightly.
• Since most of the procedures are random, the standards have to be exact for the findings to be regarded as valid