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2012-08-21T13:24:50Z

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The '''California bearing ratio''' ('''CBR''') is a penetration test for evaluation of the [[mechanical strength]] of [[road]] [[subgrade]]s and basecourses. It was developed by the [[California Department of Transportation]] before World War II.

The 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 CBR test is described in [[ASTM]] Standards D1883-05 (for laboratory-prepared samples) and D4429 (for soils in place in field), and [[AASHTO]] T193. The CBR test is fully described in BS 1377 : Soils for civil engineering purposes : Part 4, Compaction related tests.

The CBR rating was developed for measuring the [[load-bearing]] capacity of soils used for building roads. 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. High quality crushed rock has a CBR over 80. The standard material for this test is crushed California [[limestone]] which has a value of 100.

<math>CBR=\frac {p}{p_s} \cdot 100 \quad </math>

{|
|<math>CBR \quad</math> || = CBR [%]
|-
|<math>p \quad</math> || = measured pressure for site soils [N/mm²]
|-
|<math>p_s \quad</math> ||= pressure to achieve equal penetration on standard soil [N/mm²]
|}

==External links==
*[http://www.highwaysmaintenance.com/cbrtext.htm CBR]

{{Geotechnical engineering}}

[[Category:Road transport]]
[[Category:Pavements]]hdsj sdjfs dfks

[[Category:Soil tests]]

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en>Maschen: tag for expert attension