Cbr test

Cbr test

1. VISHWAKARMA GOVT. ENGINEERING COLLEGE,CHANDKHEDA Prepared By :- Gamit Kaushal A.

2. CALIFORNIA BERING RATIO (CBR) TEST This is a penetration test developed by the california division of highway. This is most widely used method for the design of flexible pavement. The thickness of the different element comprising a pavement is determine by CBR value.

3. Apparatus Cylindrical mould of inside dia. 150 mm & height 175 mm with a detachable collar & a base plate, Spacer disc of 148 mm in Dia. & 47.7 mm height, Surcharge weight of mass 2.5 kg, Penetration plunger of 50 mm dia. & height 100 mm, Loading m/c of capacity of 500 kg, Compaction rammers, Dial gauges, Filter paper, Mixing tools, Measuring cylinder,

4. TEST PROCEDURE Test specimen can be prepared either by static compaction or by dynamic compaction. In static compaction soil is compacted in loading m/c, while in dynamic compaction soil is compacted by specified rammers. Dynamically compacted specimen can be prepared by light compaction or by heavy compaction. Dynamic Compaction Light compaction Heavy Compaction Rammer Weight 2.6 kg 4.89 kg No. of layer 3 5 Free Fall 31 cm 45 cm Blows per layer 56 56

5. Fix the collar to the top of the mould & the base plate to its bottom. Insert the spacer disc over the base with the central hole of disc at lower side & put disc of course filter paper on the top of spacer disc. Take about 5 kg of soil passing 20 mm sieve but retained on 4.75 mm sieve. Add predetermined quantity of water such that w/c of soil is either equal to OMC or FMC & mix throughly. Compact the mixed soil in the mould using either the light or heavy compaction. After compacting remove the collar ,base plate ,& filter paper. Note down weight of soil along with mould.

6. Then place the mould assembly on the penetration test m/c & fix the penetration plunger. Allow the plunger to penetrate at rate of 1.25 mm / min. Record the load reading at penetrations of 0, 0.5, 1, 1.5, 2, 4, 5, 7.5, 10 & 12.5 mm. Draw graph of penetration v/s load. CBR Value = Test load corresponding to 2.5mm or 5 mm * 100% Standard load for 2.5 or 5 mm penetration Standard Load Value Penetration ( mm ) Unit Load ( kg / cm^2 ) Total Load ( kg ) 2.5 70 1350 5.0 105 2055

7. The curve is mainly convex upward although the initial portion of the curve may be concave upwards. A correction is then applied by drawing a tangent to the curve at the point of greatest slope. The corrected origin will be point where the tangent meets the abscissa.

8. Interpretation & Recording CBR value are usually calculated for penetration of 2.5 mm & 5.0 mm Generally the CBR value of 2.5 mm will be greater then that at 5.0 mm & in such Cash the formar (2.5 mm CBR value ) should be taken as design CBR. If CBR for 5.0 mm exceeds that for 2.5 mm, the test shall be repeated. CBR value. Subgrade strength ​ 3% & less. Poor ​ 3% - 5% Normal ​ 5% - 15% Good

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