Hixtra Ltd 
Construction & Geotechnical Testing 

CBR, Laboratory 

CBR test conducted in the laboratory in either a normal or soaked condition 
The CBR test 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. It often forms part of site investigations and is commonly used to determine the thickness of materials needed for the proposed road construction. 
The laboratory 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. Typically, a value of 2% equates to clay, while some sands may have a CBR value of 10%. High quality sub-base will have a value over 30%. 
The Laboratory CBR test is carried out on soils with a maximum particle size of 20mm (please see CBR plate bearing tests for coarser material). The technique involves driving a small cylindrical plunger into a sample in the laboratory at a uniform rate. The sample can be soaked prior to test simulate the effects of groundwater on the CBR value. 
Soaked CBR Test 
Lab CBR specimens can be soaked prior to test to give a good indication of the effect of groundwater on the soil strength 
Effect of lime and cement 
We can test specimens at various percentages of lime and/or cement to measure the increase in CBR with increasing doses of additive to find the most cost effective option. 
High Capacity 
We have equipment and resources to test 30 lab CBR samples per day for your largest contract needs. 
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