CBR TEST
- 1. CALIFORNIA BEARING RATIO TEST By :- ABHIK BORTHAKUR NITESH KHOKER ANJALI MEENA NAGENDRA MALAPATI 1
- 2. CALIFORNIA BEARING RATIO (CBR) :- The ratio of the force per unit area required to penetrate a soil mass with standard penetration plunger at a uniform rate of 1.25 mm/min., to the corresponding penetration load of the standard material or crushed stone is called CBR. CBR (%) = Test load x 100 Penetration load (standard material) 2
- 3. SIGNIFICANCE :- It is generally used to classify & evaluate the soil subgrade & base coarse materials for the flexible pavement. This test is empirical & the results cannot be accurately related with any fundamental properties of the material. 3
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- 5. PROCEDURE :- Take 4.5-5 kg of soil & mix it thoroughly with required quantity of water (OMC) or field moisture content. The spacer disc is placed at the bottom of the mould over the base plate & a coarse filter paper is placed over the spacer disc. The moist soil is to be compacted over this in the mould by adopting either IS light or heavy compaction. 5
- 6. After compacting the last layer, the collar is removed & the excess of soil above the top of the mould is evenly trimmed off. The clamps are removed & the mould with the compacted soil is lifted, leaving below the base plate & spacer disc is removed. A filter paper is placed on the base plate, the mould with compacted soil is inverted & placed over the base plate& clamps are tightened. Weight of 2.5-5 kg are placed over the soil in the mould. Then the whole mould is placed in the water tank to allow soaking for 4 days. 6
- 7. A swelling measuring device are placed on the top edge of the mould & the spindle of the dial gauge is placed touching the top of the sample. Initial dial gauge reading are recorded & the test sample is kept undisturbed in the water tank. After 4 days of soaking, the mould is clamped over the base plate & the same surcharge weights are placed on the specimen as the test could be conducted. The complete assembly is placed under the loading machine. 7
- 8. Penetration plunger is brought in contact with the top surface of the soil by applying 4 kg of load. The dial gauge of proving ring & the penetration dial gauge is set on 0. The load is applied through penetration rate of 1.25 mm/min. The load readings are taken at 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0, 12.5 mm of penetration. 8
- 9. The max. load value & the corresponding penetration value are recorded. A graph is plotted by penetration (mm) on x-axis & load (kN) on y-axis & the value of load at penetration 2.5 & 5.5 mm are found from the graph. 9
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- 11. Finally, the CBR value is calculated from the formulae. As per IRC recommendation the min. value of CBR required for a subgrade should be 8%. 11
- 12. OBSERVATION TABLE: Following are the observed values of loading in kg against penetration values in mm on a sample Sr.no. Penetration Load(kg) Sr.no . Penetrati on Load(kg) 1. 0.0 7. 3.0 2. 0.5 8. 4.0 3. 1.0 9. 5.0 4. 1.5 10. 7.5 5. 2.0 11. 10.0 6. 2.5 12. 12.5
- 13. OBSERVATIONS Area of Plunger, 𝐴 = 𝜋𝑟2 = 3.14x2.5²(d=5cm) =19.625 cm² Pressure at 2.5 mm Penetration = Pressure at 5.0 mm penetration = Standard pressure for 2.5 mm penetration= Standard pressure for 5.0 mm penetration= C.B.R. At 2.5 mm = C.B.R. At 5.0 mm =
- 14. Volume of mould : 2244 cm³ Rate of loading : 1.25 mm/ min. CBR = Load carried by specimen x100 Load carried by std. crushed stone specimen
- 15. Volume of mould : 2244 cm³ Rate of loading : 1.25 mm/ min. CBR = Load carried by specimen x100 Load carried by std. crushed stone specimen
- 16. PENETRATION RESULT ◮ A load penetration curve of load against penetration is then plotted with load in ordinate axis and penetration in abscissa axis ◮ the loads corresponding to 2.5 and 5.0mm penetration values are noted. ◮ Sometimes a curve with initial upward concave may also be obtained due to surface CBR% irregularities and in this case a correction is to be done. ◮ A tangent is drawn at a point of greatest slope. ◮ The point where this tangent meets penetration axis is the corrected zero reading of penetration.
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