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California Bearing Ratio (CBR) Method for Pavement Design

Learn about the California Bearing Ratio (CBR) method, a widely accepted pavement design approach. Understand the laboratory testing process, specimen preparation, and load-penetration measurements. Discover how CBR values are utilized in determining pavement thickness. Explore CBR equipment and the design method. This method uses CBR values to calculate flexible pavement layers. Find out more through this comprehensive guide.

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California Bearing Ratio (CBR) Method for Pavement Design

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  1. CALIFORNIA BEARING RATIO (CBR) METHOD Typical Testing Machine

  2. 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.

  3. CBR LABORATORY TEST Specimen may be • Prepared • In-situ Mould • 150mm diameter • 127.3mm height • Separate base plate • A collar

  4. 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.

  5. CBR Equipment Soaking Samples Typical Testing Machine

  6. CBR Equipment CBR Test – Sample Preparation CBR Test – Load-Penetration Measurement

  7. Load a piston at a constant rate See what load it takes to make it penetrate a known amount California Bearing Ratio (CBR)

  8. CBR Equation

  9. Typical CBR Ranges

  10. California Bearing Ratio Vs Moisture content California Bearing Ratio Equipment

  11. 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.

  12. Flexible Pavement Design Using CBR (Road Note 31)

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