CALIFORNIA BEARING RATIO CBR METHOD Typical Testing Machine

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 loaddeformation 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 • • 150 mm diameter 127. 3 mm 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 50 mm diameter and penetration is measured. • Rate of penetration is maintained at 1. 25 mm/minute. • Loads are recorded for 2. 5 and 5. 0 mm. • 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 Equipment CBR Test – Sample Preparation CBR Test – Load-Penetration Measurement

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 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 Typical CBR Ranges Clean gravels Gravels with fines Clean sands Sands with fines Silts and clays USC Soil Type CBR Range GW 40 – 80 GP 30 – 60 GM 20 – 60 GC 20 – 40 SW 20 – 40 SP 10 – 40 SM 10 – 40 SC 5 – 20 ML 15 or less CL 15 or less OL 5 or less MH 10 or less CH (LL>50%) 15 or less OH 5 or less

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)