CALIFORNIA BERING RATIO TEST 1.0 INTRODUCTION
The California Bearing Ratio (CBR) was developed by California division of highways as a method of classifying and evaluating soil-sub-grade and base course materials for flexible pavements. The CBR test is currently used in pavement design for both roads and airfield pavement. In some methods CBR is used directly and in some others it is converted to Resilient Modulus MR using the following relation ships.
MR = 1500 x CBR (ibs/in2) MR = 10340 x CBR (Kpa)
The laboratory CBR test measures the shearing resistance of a crushed aggregate/soil under controlled moisture and density conditions. The test yields bearing ratio number that is applicable for the state of crushed aggregate/soil as tested.
The CBR is obtained as the ratio of the unit stress required of effect a certain depth of penetration of the piston (1935 mm) into a compacted specimen of crushed aggregate/soil at some water content and density to the standard unit stress required to obtain the same depth of penetration on a standard sample of crushed stone. Thus.
CBR = ** 100
The CBR is usually base on the load ratio for the penetration of 2-5mm. If the CBR value at the penetration of 5.0 mm is larger, the test should be repeated. If a second test yields a larger value of CBR at 5.0 mm penetration then this larger value should be adopted.
The CBR test are usually made on test specimens at optimum moisture content (OMC) for the crushed aggregate/soil as determined from modified compaction test.
Test unit stress Standard unit stress
CBR is used to rate the performances of soils used as bases and sub grade. The following table gives typical rating :
CBR General Rating Uses
0.3 Very poor Sub-grade
3-7 Poor to fair Sub-grade
7-20 Fair Sub-base
20-50 Good Base of sub-base
>50 Excellence Base
2.0 OBJECTIVE
To determine the CBR value of the given crushed aggregate/soil sample.
3.0 APPARATUS
1. CBR equipment consisting of 152.4 mm diameter and 178 mm height, An extension collar of a diameter 51 mm, spacer disk of 150.8mm diameter and 61.4 mm height.
2. Mechanical compaction rammer 50.8 mm die, 2.49 kg and capable of free fall of 305 mm. 3. Surcharge weight to simulate the effect of overlaying pavement weight.
4. CBR machine: A compression machine, which can operate at a constant rate of 1.3mm/min. A metal piston of 1935mm2 is attached to it.
4. PROCEDURE
1. CBR equipment consisting of 152.4 mm diameter and 178 mm height, An extension collar of diameter 51 mm, spacer disk of 150.8 mm diameter and 61.4 mm height.
2. Mechanical compaction rammer 50.8 mm die, 2.49 kg and capable of free fall of 305 mm. 3. Surcharge weight to simulate the effect of overlying pavement weight.
4. CBR machine: A compression machine, which can operate at a constant rate of 1.3 mm/min. A metal piston of 1935mm2 is attached to it.
5. The representative crushed aggregate/soil sample is sieved through 20 mm sieve. About 5 kg of crushed aggregate/soil is taken and mixed with optimum moisture content (OMC).
6. Clamp the mould to the base plate, attach the extension collar and weight. Insert the spacer disk into the mold and place a coarse filter paper on the top of the disk.
7. Compact the aggregate /soil water mixture into the world in 3 equal layers to give a height of 127 mm compact each layer in the 10 blows , 30 blows and 65 blows for each sample. 8. Determine the water content of the crushed aggregate /soil mixture.
9. Remove the extension collar, and using on straight edge, trim the compacted crushed aggregate/soil even with the top of the mold surface. Remove the spacer disk and weight the mold with sample.
10. Place the mold with crushed aggregate/soil on the CBR machine and place the surcharge weight .seat the penetration piston, set the dial gauges for load and penetration.
11. Apply the loads to the penetration piston at the rate of 1.27mm/min and record the load at 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, and 10.0mm penetration respectively.
5. CALCULATIONS
CBR : Plot the load deformation curve for each specimen. In some cases the initial penetration takes place without a proportional increase in the resistance to penetration and the curve may be concave upward. To obtain the true stress-strain relationships, correct the curve having concave upward shape near the origin by adjusting the location of the origin by extending the straight the portion of the stress strain curve down ward until it intersects with x-axis.
Determine the corrected load values at 2.5mm and 5.0 mm and determine the CBR by the following relationship.
CBR = * 100
Standard load at 2.5mm is taken 13.2kN and at 5.0mm it is on 20kN Dry Density:
Weight of the empty mold = A gm Weight of the mold + soil = B gm Volume of soil sample = V Weight density γ =
Water Content w
Dry Density γd =
Plot the CBR vs Dry density and determine the CBR at 95% of maximum dry density and repeat this value of CBR.
Test unit stress Standard unit stress
B - A V
λ 1 + w
Summary of Test Results Sample
No. No. of Blows γd (gm/cm3) CBR (%)
1 10 2 30 3 65 CBR at 0.95 γdmax : ……… Checked by ; ……….. Date :……….. 6. REFERANCES
1. American Association of State Highway and Transportation Officals. AASHTO T-193-1990. 2. ASTM D1556-1982
3. The Asphalt Institute. The Asphalt Handbook
7. DISCUSSION
DATA SHEET (CBR TEST) Type of the Test : Soaked/Unsoaked
OMC Date : ……….
Sample 1 2 3
No. of Blows
Empty wt. of mould, W1
Wt of mould + wet sample, W2
Volume of sample, V
Wet density γ = ( W2 - W1) / V
Can no.
Wt.of empty can, A
Wt. of can + wet sample, B
Wt. of can + dry sample, C
Water content, W% = [ (B - C) / (C - A)] * 100
Dry Density, γd = γ / ( 1 + W )
CBR TEST – PENETRATION DATA
Penetration Load
(mm) Sample 1 Sample 2 Sample 3
Div. Corrected Div. Corrected Div. Corrected
0.0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 8.0 9.0 10.0
