Test method explained
California Bearing Ratio (CBR)
Aitken Rowe is NATA accredited for CBR testing at all three laboratories — Wagga Wagga, Griffith and Albury/Wodonga — to the Australian Standard and the corresponding state variants. CBR testing is what sets pavement thickness for roads and concrete slabs.
What a CBR test tells you
CBR testing informs pavement design for roads and concrete slabs. The result is a percentage: the strength of your material measured against a benchmark of well-graded crushed rock. The lower the number, the thicker the pavement has to be to carry the same traffic.
Typical CBR strengths
Scale 0–100%. California limestone was the original benchmark at 100%.
How the test is performed
- 01Remould and compactThe soil is compacted into a mould at a nominated density.
- 02CureStored in a controlled environment until moisture is evenly distributed throughout the specimen.
- 03SoakThe specimen is soaked until saturated — four days for coarse grained soils, ten for fine grained.
- 04PenetrateA metal plunger is driven in at 1.25mm per minute. The forces at 2.5mm and 5.0mm penetration are interpolated to give the CBR strength.
Why curing is the step that cannot be skipped
Curing stores the specimen in controlled conditions until moisture is evenly distributed. For low-permeability clays that can take more than seven days. Skipping it returns an erroneously high CBR strength — which under-designs the pavement and shows up later as early pavement failure.
This is why a cheap, fast CBR result is worth asking questions about.
Where the name comes from
The test is named for its Californian origins, where it was first performed on well-graded crushed California limestone — the force needed to penetrate that material was set at 100%, and every other material measured against it.
Its simplicity and consistency led the U.S. Army Corps of Engineers to adopt it during the Second World War, and it spread worldwide once the war ended.
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