The California Bearing Ratio (CBR) test rates the strength of subgrade, sub-base and base materials and is a primary input to flexible pavement thickness design.
What the Test Measures
CBR compares the load needed to push a standard plunger into the compacted soil against the load for the same penetration into a standard crushed-stone reference. It is expressed as a percentage; higher CBR means a stronger, stiffer layer.
Apparatus Required
- CBR loading frame with plunger and load/penetration measurement
- CBR moulds, surcharge weights and soaking tank
- Compaction rammer and density equipment
- Dial gauges or transducers for penetration
NL Scientific manufactures the ECO-SMARTZ Advance CBR / Marshall Loading Frame 50 kN and the full soil testing range, built to ASTM, EN, BS and AASHTO requirements with ISO/IEC 17025 accredited calibration.
Test Procedure
- Compact the soil into the CBR mould at the required density and moisture.
- Apply surcharge weights and, where specified, soak the specimen (commonly 4 days) recording swell.
- Place the mould in the loading frame and seat the plunger.
- Penetrate at 1.27 mm/min, recording load at standard penetrations.
- Read the loads at 2.5 mm and 5.0 mm penetration.
Calculation & Reporting
CBR (%) = (Test load ÷ Standard load) × 100 at 2.5 mm and 5.0 mm; report the higher value. Standard loads are 13.24 kN (2.5 mm) and 19.96 kN (5.0 mm).
Acceptance Criteria
Subgrade CBR often ranges from 2–10% for clays up to 30%+ for granular sub-base, with minimums set by the pavement design method. Soaked CBR is normally specified to represent worst-case moisture.
Frequently Asked Questions
Why is the specimen soaked?
Soaking simulates the weakest in-service moisture condition, giving a conservative design value and a measure of swell potential.
Can the same frame run Marshall tests?
Yes — a combined CBR/Marshall loading frame runs both at their required rates, which is efficient for combined soil and asphalt laboratories.
Need the right equipment for your laboratory? Contact NL Scientific for a quotation — Asia’s leading manufacturer of civil engineering and material testing equipment, shipping worldwide.
