The CBR test is one of the most commonly used methods to evaluate the strength of a sub grade soil, sub base, and base course material for design of thickness for highways and airfield pavement.
The California Bearing Ratio (CBR) test is a simple strength test that compares the bearing capacity of a material with that of a well-graded crushed stone (thus, a high quality crushed stone material should have a CBR @ 100%). The basic CBR test involves applying load to a small penetration piston at a rate of 1.3 mm (0.05″) per minute and recording the total load at penetrations ranging from 0.64 mm (0.025 in.) up to 7.62 mm (0.300 in.). Figure 1 is a sketch of a typical CBR sample. Figure 1. CBR Sample. 1. Definition of CBR: It is the ratio of force per unit area required to penetrate a soil mass with standard circular piston at the rate of 1.25 mm/min. to that required for the corresponding penetration of a standard material. 6. The CBR test is one of the most commonly used methods to evaluate the strength of a sub grade soil, sub base, and base course material for design of thickness for highways and airfield pavement. The california bearing ratio test is penetration test meant for the evaluation of subgrade strength of roads and pavements. The results obtained by these tests are used with the empirical curves to determine the thickness of pavement and its component layers. It is the ratio of force per unit area required to penetrate a soil mass with standard circular piston at the rate of 1.25 mm/min. to that required for the corresponding penetration of a standard material. The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength of road subgrades and basecourses. It was developed by the California Department of Transportation before World War II. The CBR can also be used for measuring the load-bearing capacity of unimproved airstrips or for soils under paved airstrips. The harder the surface, the higher the CBR rating. A CBR of 3 equates to tilled farmland, a CBR of 4.75 equates to turf or moist clay, while moist sand may have a CBR of 10. The California Bearing Ratio (CBR) Test is used to estimate the bearing value and the mechanical strength of highway subbases and subgrades. It requires a reaction load, usually a Land Rover or similar, to provide the force to the plunger and CBR press. The CBR test is performed by measuring the pressure required to penetrate a soil sample with a plunger of standard area. The measured pressure is then divided by the pressure required to achieve an equal penetration on a standard crushed rock material. The harder the surface, the higher the CBR value. The CBR test is fully described in BS1377: Part 9: 1990: Soils for civil engineering purposes. It was first developed by the California State Highway Department and is used in the field as an arbitrary strength test which is considered to stress soils and replicate wheel loads. The CBR test is performed by measuring the pressure required to penetrate a soil sample with a plunger of standard area. The measured pressure is then divided by the pressure required to achieve an equal penetration on a standard crushed rock material. The harder the surface, the higher the CBR value. NEED AND SCOPE. The california bearing ratio test is penetration test meant for the evaluation of subgrade strength of roads and pavements. The results obtained by these tests are used with the empirical curves to determine the thickness of pavement and its component layers.  CBR Testing Apparatus. The California Bearing Ratio (CBR) test is a penetration test developed by the California State Highway Department and is used to determine the mechanical strength of a soil. The test can be carried out on site or in the laboratory. The CBR test is carried out using a cylindrical plunger which penetrates the soil at a given rate. The relationship between the force and penetration can then be used to calculate the CBR value of the soil being tested. The California Bearing Ratio test, described in ASTM D 1883, is a penetration test commonly used to evaluate the potential strength of subgrade, subbase and base course material. The CBR value is the ratio of this stress at different penetration levels to the bearing value of a standard crushed rock. In most cases, CBR decreases as the penetration increases, so the ratio at 0.1-inch penetration is used as the recorded CBR value.
http://www.pavementinteractive.org/article/California-Bearing-Ratio/ http://www.aboutcivil.org/california-bearing-ratio-test.html http://www.aboutcivil.org/california-bearing-ratio-test.html https://en.wikipedia.org/wiki/California_bearing_ratio http://www.southerntesting.co.uk/services/ground-investigation/cbr-test http://www.southerntesting.co.uk/services/ground-investigation/cbr-test http://home.iitk.ac.in/~madhav/expt14.html http://www.engineersdaily.com/2014/04/what-is-california-bearing-ratiocbr-test.html
what is a cbr test