CBR Test: Procedure, Calculations & Use in Pavement Design (IRC 37)

The California Bearing Ratio (CBR) test is the single most important test for subgrade and sub-base material evaluation in road and pavement design. Developed by the California Division of Highways in the 1930s, it became the foundation of flexible pavement design methods used across India, the UK, and Commonwealth countries. This article covers the complete IS 2720 Part 16 procedure, calculations, and direct application in IRC 37 pavement design.

What is CBR?

CBR is the ratio of test load causing 2.5 mm or 5.0 mm penetration of a standard plunger into a soil specimen, to the standard load causing the same penetration in a standard crushed stone:

CBR (%) = (Test Load / Standard Load) × 100

Normally CBR at 2.5 mm > CBR at 5.0 mm. If CBR at 5 mm is higher, rerun the test. If the result repeats, report the 5 mm value as CBR.

Apparatus (IS 2720 Part 16:1979)

• CBR mould: 150 mm dia × 175 mm high, with collar and perforated base plate
• Spacer disc: 148 mm dia × 47.7 mm high
• Standard plunger: 50 mm dia (1963 mm²), 100 mm long
• Annular surcharge weights: 2.5 kg each (minimum 2 weights for soaked test)
• Loading machine: minimum 50 kN capacity, 1.25 mm/min penetration rate
• Soaking tank for 96-hour soaked test

Sample Preparation

1. Obtain representative soil sample and air-dry
2. Sieve through 20 mm IS sieve (replace oversize material with equivalent 4.75–20 mm material)
3. Determine OMC and MDD using Proctor compaction test (IS 2720 Part 7 or Part 8)
4. Add water to achieve OMC, mix thoroughly, allow to stand 1–2 hours in sealed bag
5. Compact in 3 layers using standard Proctor effort (IS 2720 Part 7) or heavy compaction (IS 2720 Part 8) as per design specification
6. Remove collar, trim off excess soil, weigh and measure for density

Soaked vs Unsoaked CBR

Note: IRC 37:2018 mandates the design CBR be based on soaked condition unless drainage is guaranteed. Most Indian pavements deteriorate primarily during monsoon season when water ingress is maximum.

Test Procedure

1. Place the mould (with compacted sample) in the soaking tank with surcharge weights in place
2. Add water — soil swells upward; measure swell with dial gauge on tripod (read at 0, 24, 48, 72, 96 hours)
3. After 96 hours, remove mould from water, drain 15 minutes, then test
4. Place mould in loading machine, apply 45 N seating load, zero dial gauge
5. Apply load at 1.25 mm/min; record load at 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0, 12.5 mm penetrations
6. Plot load-penetration curve; correct for initial concavity if needed (draw tangent from inflection point)

CBR Calculation

CBR at 2.5 mm = (Load at 2.5 mm from corrected curve / 13.44) × 100 %
CBR at 5.0 mm = (Load at 5.0 mm from corrected curve / 20.05) × 100 %
Report CBR = higher of corrected values (usually 2.5 mm governs)

Design CBR for Pavement

IRC 37:2018 recommends the design subgrade CBR as the 90th percentile (P90) value from a series of tests along the road alignment:

• Collect minimum 10 test values per subgrade type
• Rank in ascending order
• P90 = value below which 90% of results fall (i.e., only 10% are weaker)

CBR-Based Pavement Design: IRC 37:2018 Approach

IRC 37:2018 uses mechanistic-empirical approach with CBR as primary subgrade input:

Typical Pavement Thickness from IRC 37 Design Charts:

For NH/SH (Traffic = 10 msa, CBR = 4%, BC + DBM + WMM + GSB):

• Bituminous Concrete (BC): 40 mm
• Dense Bituminous Macadam (DBM): 85 mm
• Wet Mix Macadam (WMM): 250 mm
• Granular Sub-Base (GSB): 200 mm
• Total: 575 mm

Improving CBR of Poor Subgrade

Frequently Asked Questions