LimsPulse
Construction materials testing

California Bearing Ratio (CBR) Test Apparatus

A load frame that pushes a 50 mm plunger into a compacted soil specimen at 1.25 mm/min and compares the load needed with that for a standard crushed stone. The ratio (CBR) is used to design road and pavement thickness.

  • Road and highway construction
  • Geotechnical testing
  • Airports and railways
  • Construction materials testing
  • Academic labs
Illustration of California Bearing Ratio (CBR) Test Apparatus
WORKING PRINCIPLE

How CBR apparatus works

The resistance of a soil to penetration by a plunger under controlled conditions is compared with that of a standard crushed stone. CBR (%) = test load ÷ standard load × 100 at the same penetration, usually 2.5 mm; if the 5.0 mm value is higher, the test is repeated and the higher value used if confirmed.

FLOW

The path of a sample

  1. 01

    Prepare specimen

    Soil is compacted in the CBR mould at the target dry density and moisture (static or dynamic compaction) over a spacer disc.

  2. 02

    Soak (if required)

    Surcharge weights are placed on the specimen and it is soaked for typically 96 hours, with a dial gauge on a tripod measuring swell.

  3. 03

    Set up

    The mould is placed on the frame with surcharge weights; the plunger is seated with a small load (about 4 kg) and gauges zeroed.

  4. 04

    Penetrate

    The plunger is driven at 1.25 mm/min; load is read at set penetrations (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10 and 12.5 mm).

  5. 05

    Plot and correct

    Load is plotted against penetration; if the curve is concave upward at the start, the zero is corrected.

  6. 06

    Calculate CBR

    Loads at 2.5 and 5.0 mm are divided by the standard loads and expressed as percentages.

COMPONENTS

Main parts and what they do

1

Load frame

Motorised screw jack or hydraulic frame that raises the mould against the plunger at a constant rate.

2

Proving ring or load cell

Measures the penetration load; rings of e.g. 10, 25 or 50 kN capacity are common.

3

Penetration plunger

Hardened steel cylinder about 50 mm in diameter (area about 19.6 cm²).

4

CBR mould with collar and base

Typically 150 mm internal diameter and 175 mm high, with a perforated base for soaking.

5

Spacer disc

Disc placed under the soil during compaction so the specimen height is about 127 mm.

6

Surcharge weights

Annular and slotted weights, typically 2.5 kg each.

7

Penetration dial gauge

Measures plunger penetration, typically 0.01 mm divisions.

8

Swell tripod and dial gauge

Measures specimen swell during soaking.

9

Soaking tank

Holds the moulds submerged during the soak period.

KEY CONCEPTS

Terms every analyst should know

Standard load

Load carried by standard crushed stone: typically 1370 kg at 2.5 mm and 2055 kg at 5.0 mm (IS); ASTM uses about 6.9 MPa and 10.3 MPa on the plunger area.

Rate of penetration

1.25 mm/min (IS) or 1.27 mm/min (0.05 in/min, ASTM). Load readings depend on this rate.

Surcharge

Annular weights, typically 2.5 kg each, placed on the specimen to represent the weight of the pavement above.

Swell

Increase in specimen height during soaking, expressed as a percentage of the initial height.

Proving ring

Elastic steel ring whose deflection, read on a dial gauge, is proportional to load; its constant comes from a calibration certificate.

In LimsPulse · Calibration Management

Keep every CBR apparatus in control: usage logs, calibration and maintenance schedules, and automatic blocking of overdue instruments.

See how