California Bearing Ratio (CBR) — Lab Manual

Objective & standard

Strength of subgrade and sub-base for pavement design.

Standard: ASTM D1883 / AASHTO T193

Apparatus

• CBR mould 2124 cm³ with spacer disc
• Rammer, surcharge weights 4.5 kg
• Loading machine with 50 mm plunger, dial gauges
• Soaking tank, swell gauge

Procedure

1. Compact the soil at OMC (3 moulds at 10, 25 and 56 blows for a soaked CBR curve).
2. Put surcharge weights, soak 96 hours and record swell.
3. Drain 15 min, place under the plunger with the surcharge.
4. Load at 1.27 mm/min and record load at 0.5 mm intervals up to 12.5 mm.
5. Correct the curve for concave start if needed.

Calculations

• CBR (2.5 mm) = load ÷ 13.24 kN × 100
• CBR (5.0 mm) = load ÷ 19.96 kN × 100 — use the higher (repeat if 5 mm is higher)

Typical values: Clay subgrade 2–5 %, silty sand 8–20 %, granular sub-base ≥ 30 %, base ≥ 80 %.

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Data sheet (print this page)

Project: ____________________   Sample: __________   Date: __________   Tested by: __________

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