Laborate Star

The CBR (California Bearing Ratio) Testing Machine is a laboratory apparatus used in geotechnical and highway engineering to determine the bearing capacity and strength of subgrade soil, sub-base, and base course materials used in road and pavement construction.

Purpose

It measures the resistance of soil to penetration under controlled conditions, which helps engineers decide:

• Pavement thickness

• Suitability of soil for road construction

Main Components

1. Loading Frame

   • Rigid frame that applies vertical load

   • Can be manual or motorized

2. Penetration Plunger

   • Standard diameter: 50 mm

   • Penetrates the soil sample at a constant rate

3. Load Measuring Device

   • Proving ring or load cell

   • Measures applied load accurately

4. Dial Gauges

   • Measure penetration depth and deformation

5. CBR Mould

   • Cylindrical mould where soil is compacted

   • Includes base plate and collar

6. Soaking Tank (optional)

   • Used for soaked CBR tests (usually 4 days)

Working Principle

• Soil is compacted in the mould at a specified density.

• The plunger penetrates the soil at 1.25 mm per minute.

• Load readings are taken at standard penetrations (2.5 mm and 5 mm).

• CBR value is calculated as a percentage of standard load.

Applications

• Road and highway pavement design

• Airport runway construction

• Evaluation of subgrade soil strength

Advantages

• Simple and standardized test

• Widely accepted in pavement design

• Gives reliable comparison of soil strength