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 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.
CBR (%) = ( x / y ) x 100
where:
x = material resistance or the unit load on the piston (pressure)
for 2.54 mm (0.1″) or 5.08 mm (0.2″) of penetration
y = standard unit load (pressure) for well-graded crushed stone
= for 2.54 mm (0.1″) penetration = 6.9 MPa (1000 psi)
= for 5.08 mm (0.2″) penetration = 10.3 MPa (1500 psi)