CBR Test Equipment: Complete Guide for Soil Subgrade Testing
Published October 2025 Β· 5 min read Β· Elipslab Technical Team
What Is the CBR Test?
The California Bearing Ratio (CBR) test measures the strength of subgrade soils and base course materials used in road and pavement construction. The CBR value expresses the ratio of the force required to penetrate a soil sample to the force required to penetrate a standard crushed stone material to the same depth.
CBR values are used directly in pavement design methods (including AASHTO and Asphalt Institute methods) to determine the required pavement thickness over a given subgrade. A weak subgrade (CBR 3β5%) requires much thicker pavement than a strong one (CBR 30%+).
The test is standardized under ASTM D1883 (laboratory CBR) and BS 1377-4 (UK/European equivalent). Field CBR can also be measured in-situ using a dynamic cone penetrometer or field CBR apparatus.
CBR Testing Equipment β What You Need
- CBR Testing Machine: A motorized loading frame with a capacity of at least 44.5 kN (10,000 lbf). The machine applies the piston at a constant penetration rate of 1.27 mm/min (0.05 in/min). Both benchtop and floor-standing models are available.
- Penetration Piston: A 49.65 cmΒ² (1987 mmΒ²) circular piston, 50 mm in diameter, used to penetrate the soil specimen.
- CBR Molds: Cylindrical steel molds, 152.4 mm (6 in) internal diameter Γ 177.8 mm (7 in) height, with base plate, collar extension, and spacer disk.
- Surcharge Weights: Annular weights (typically 4.54 kg each) placed on the sample surface during penetration to simulate overburden pressure.
- Compaction Equipment: Standard Proctor hammer (ASTM D698) or Modified Proctor hammer (ASTM D1557) for specimen preparation.
- Dial Gauges / LVDTs: One for penetration depth measurement, one for swelling measurement during soaking.
- Soaking Tank: For soaking specimens 96 hours (4 days) to simulate worst-case moisture conditions.
Laboratory CBR Test Procedure (ASTM D1883)
- Determine optimum moisture content (OMC) using Proctor compaction test (ASTM D698 or D1557). Prepare soil at OMC.
- Compact specimen in the CBR mold in three equal layers, 56 blows per layer (standard effort) or 25/10 blows for lighter efforts.
- Add surcharge weights representing the expected pavement weight.
- Soak specimen for 96 hours if testing soaked conditions. Record initial and final swelling readings.
- Set up the machine: Place the mold in the loading frame, seat the piston on the soil surface, and zero the gauges.
- Apply load at 1.27 mm/min. Record force at penetrations of 0.64, 1.27, 1.91, 2.54, 3.81, 5.08, 7.62, and 10.16 mm.
- Calculate CBR at 2.54 mm (0.1 in) penetration: CBR = (test load / standard load) Γ 100%. Standard loads: 13.34 kN at 2.54 mm, 20.01 kN at 5.08 mm.
Understanding CBR Results
| CBR Value | Subgrade Class | Typical Use |
|---|---|---|
| < 3% | Very poor | Unsuitable β require stabilization |
| 3β5% | Poor | Light traffic only, thick pavement |
| 5β10% | Fair | Standard roads with adequate design |
| 10β30% | Good | Most highway applications |
| > 30% | Excellent | Low pavement thickness required |
Selecting a CBR Testing Machine
When specifying CBR equipment for your laboratory, consider:
- Motorized vs hydraulic: Motorized machines with screw-driven frames maintain consistent penetration rate more accurately than hydraulic systems.
- Capacity: 50 kN is sufficient for most soils. Stabilized materials may require higher capacity.
- Digital readout: Load cells with digital displays eliminate manual force-ring reading errors.
- Multi-function: Many CBR frames can also perform unconfined compression tests and CBR on stabilized soils, increasing laboratory versatility.
CBR Testing Equipment from Elipslab
Elipslab supplies complete CBR testing setups including motorized CBR frames, mold sets, surcharge weights, and soaking tanks, compliant with ASTM D1883 and BS 1377. Manufactured in Ankara, Turkey β available worldwide.