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Published On: Fri, Sep 15th, 2017

Procedure of CBR Testing

CBR is the abbreviation of California Bearing Ratio field in-place tests and was first conceptualised by the California State Highway Department. The CBR analysis is crucial and is used to determine the mechanical strength of the ground and the depth of materials required for the planned road construction.

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The CBR is the ratio of the force applied to a piston to penetrate a mass of soil at the rate of 1.25 mm/min to the force needed to make a corresponding penetration in a standard material. These tests are conducted on compacted, natural, water soaked or un-soaked soils and the results obtained are compared with those of standard test to know the strength of the sub grade material.

The CBR experiment is commonly used to estimate the stability of a sub grade earth, base course, and sub base material to design the thickness of airfield pavement and highways. The results of these tests combined with the empirical curves can be used to ascertain the depth of pavement and its layers. This method is widely used for designing flexible pavement.

How a CBR Test is done
The first thing is to measure the pressure needed to penetrate a sample with a piston of a standard area. A level ground way in for a 4WD vehicle is necessary. If the field is not level, manual scraping can be done to make sure that the condition of the site meets the test requirements. The pressure required to make the penetration is divided by the pressure required to make the same penetration of a standard material. If the surface is hard, the CBR value becomes high. Normally, clay will have a value of 2%, while some variety of sand will have a CBR value of 10%. High-quality sub-base should have a value ranging from 80-100%. For more information regarding how a CBR test is carried out, click here.

Site testing is done on material with a particle size of up to 20mm. For material that is greater than 20mm, Plate Bearing Test is done instead. The tests are also done at surface level or depths ranging from 500-1000mm at intervals of 20-30m along the planned construction centre line. A minimum of three tests should be done out at each site. On a standard site with surfaces are already prepared, 8-10 tests can be done in a day, by one operator, and interim results can be availed on-site.

Equipment and tool used
• A cylindrical mould with an inner diameter of 150 mm and a height 175 mm, which comes with a removable extension collar of 50 mm height and a removable perforated base plate of a 10mm thickness. 
• A loading machine with a minimum capacity of 5000 kg which is equipped with a variable head or base that can travel at a consistent rate of 1.25 mm/min.
• Metallic penetration piston of 50 mm diameter and a minimum of 100 mm length. 
• Two dial gauges of 0.01 mm reading.
• 4.75 mm and 20 mm sieves.
• Assorted apparatus such as a scale soaking pan or tank, mixing bowls filter papers, drying oven, and containers. 

How CBR is calculated
CBR experiments are conducted in the lab or on-site. CBR Testing can be done on either dry or wet sample. In the lab, the material being examined is prepared at Proctor’s maximum dry density or the required density condition. A piston of a standard surface area is then injected into the earth at a steady penetration rate. The force needed to maintain this rate of penetration is recorded on a form.

The formula used to calculate is:
CBR= (F/Fs) x100%
Where F is the force needed to penetrate the material, and Fs is the force that can be used to penetrate a standard sample. According to the CBR principle, the hardness of the sample is directly proportional to the CBR rating.

The pros and cons of the CBR experiment

Pros
• Little experience and training is required to do the test.
• This method is more suitable for airfield pavements compared to any other methods.
• The test is conducted using unsophisticated and movable equipment.
• The tests can be carried out both in the field and in a lab.
• Representative materials of future moisture conditions can be utilised for this test.

Cons
• The lab and field tests are not identical. Findings from the field and in the lab can have a slight variance.
• Considering that most of the tests are arbitrary, the conditions have to be precise for the results to be considered as valid.

Author: Mahendra

About the Author

- Outside contributors to the Dispatch are always welcome to offer their unique voices, contradictory opinions or presentation of information not included on the site.

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