CALIFORNIA BEARING RATIO (CBR) METHOD

Typical Testing Machine

Introduction

• The California Bearing Ratio devised by engineers of the California Division of Highways in nine years period to 1938.

• Most universally accepted pavement design methods.

• A standard penetration-type load-deformation test is carried out, and using the values obtained from the test from an empirical design chart, the pavement thickness are calculated.

CBR LABORATORY TEST

Specimen may be• Prepared• In-situ

Mould • 150mm diameter• 127.3mm height• Separate base plate• A collar

• The specimen is soaked and the expansion is measured.

• Load is applied by the loading frame through a cylinder plunger of 50mm diameter and penetration is measured.

• Rate of penetration is maintained at 1.25mm/minute.

• Loads are recorded for 2.5 and 5.0mm.• Load is expressed as a percentage of the

standard load at the respective deformation level, and is known as the CBR value.

CBR Equipment

Typical Testing Machine Soaking Samples

CBR Test – Load-Penetration Measurement

CBR Test – Sample Preparation

CBR Equipment

California Bearing Ratio (CBR)

• Load a piston at a constant rate

• See what load it takes to make it penetrate a known amount

CBR Equation

yxCBR 100

x = material resistance or the unit load on the piston (pressure) for 2.5 mm or 5 mm of penetration

y = standard unit load (pressure) for well graded crushed stone. For 2.5 mm of penetration = 1000 psi For 5.0 mm of penetration = 1500 psi

General Soil Type USC Soil Type CBR Range

Clean gravelsGW 40 – 80GP 30 – 60

Gravels with finesGM 20 – 60GC 20 – 40

Clean sandsSW 20 – 40SP 10 – 40

Sands with finesSM 10 – 40SC 5 – 20

Silts and clays

ML 15 or lessCL 15 or lessOL 5 or lessMH 10 or less

CH (LL>50%) 15 or lessOH 5 or less

Typi

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California Bearing Ratio Equipment

California Bearing Ratio Vs Moisture content

CBR DESIGN METHOD

• In this method, the CBR values are used to determine the total thickness of the flexible pavement and its various layers.

• Following are the graphs used in the design of the pavement thickness.

Flexible Pavement Design Using CBR (Road Note 31)