CBR test

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Faculty of Civil and Environmental Engineering Department :

Department of Geotechnical and Transportation Engineering Title:

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 relationships.

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 applicablefor 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 ona standard sample of crushed stone. Thus,

stress unit Standard stress unit Test  CBR

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 valueof CBR at 5.0 mm penetration then this larger value should be adopted.

The CBR test is usually made on test specimens at optimum moisture content (OMC) for the crushedaggregate/soil as determined from modified compaction test.

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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-grade

20 – 50 Good Base of sub-base

>50 Excellent 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.

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CALIFORNIA BERING RATIO TEST 4.0 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. The mould is clamped to the base plate, the extension collar is attached and weighted. The

spacer disk is inserted into the mold and a coarse filter paper is placed on the top of the disk.

7. The aggregate /soil water mixture is compacted 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. The water content of the crushed aggregate /soil mixture is determined.

9. The extension collar is removed, and using on straight edge, the compacted crushed aggregate/soil even with the top of the mold surface is trimmed. The spacer disks is removed and the mold with sample is weighted.

10. The mold with crushed aggregate/soil is placed on the CBR machine and the surcharge is placed weight .seat the penetration piston, the dial gauges is set for load and penetration. 11. The loads is applied to the penetration piston at the rate of 1.27mm/min and the load is

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CALIFORNIA BERING RATIO TEST 5.0 CALCULATION

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. 100 stress unit Standard stress unit Test _  CBR

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



V A B  Water Content w Dry Density d w   1



Plot the CBR vs Dry density and determine the CBR at 95% of maximum dry density and repeat this value of CBR.

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CALIFORNIA BERING RATIO TEST Summary of Test Results

Sample

No. No. ofBlows



d (gm/cm3) CBR (%)

1 10 1.848 56.00

2 30 1.892 127.50

3 65 1.968 178.75

CBR at 0.95



d max : 100%

The graph Dry Density (g/cm3) versus CBR (%) has been plotted as shown in appendix.

REFERANCES

1. American Association of State Highway and Transportation Officals. AASHTO 1990.

2. ASTM D1556-1982

3. The Asphalt Institute. The Asphalt Handbook

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Department of Geotechnical and Transportation Engineering

CBR TEST – PENETRATION DATA

Penetration (mm)

Load

Sample 1 Sample 2 Sample 3

Div. Corrected Div. Corrected Div. Corrected

0.0 0.00 0.00 0.00 0.5 0.91 0.58 1.32 1.0 2.04 1.99 3.10 1.5 3.35 4.36 5.19 2.0 4.78 7.85 8.25 2.5 6.08 11.60 12.40 3.0 7.25 14.96 16.53 3.5 8.58 17.82 21.47 4.0 10.07 20.52 26.72 4.5 11.53 23.49 30.78 5.0 12.71 25.98 34.00 5.5 13.94 28.66 36.67 6.0 15.15 30.88 39.43 6.5 16.59 33.09 41.18 7.0 18.01 34.94 43.34

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DATA SHEET (CBR TEST)

Type of the Test : Soaked/Unsoaked

OMC Date : 05/02/2009 OMC : 11.3% d max : 1.86 gm/cm3

Sample 1 2 3

No. of Blows 10 30 65

Empty wt. of mould, W1 (kg) 16.700 16.800 16.650 Wt of mould + wet sample, W2 (kg) 21.150 21.350 21.260 Volume of sample, V (cm3₎ _2104.920 _2104.920 _2104.920 Wet density  = ( W2 - W1) / V (g/cm3₎ _2.114 _2.162 _2.190

Can no. 10 30 65

Wt.of empty can, A (g) 29.000 39.000 57.000 Wt. of can + wet sample, B (g) 49.600 59.800 77.700 Wt. of can + dry sample, C (g) 47.000 57.200 75.600 Water content, W% = [ (B - C) / (C - A)] * 100 14.400 14.300 11.300 Dry Density, d =  / ( 1 + W ) (g/cm3) 1.848 1.892 1.968 Graph Dry Density (g/cm3_{) versus Moisture Content (%) has been plotted in appendix.}

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DATA ANALYSIS

Sample 1 (10 blows)

Weight of Mould, W1 = 16700 g

Weight of Mould + wet sample, W2 = 21150 g

Volume, V = 2104.92 cm 3 Wet density,

_γ









3 1 2

/

114 .

2

92 .

2104

16700

21150

cm

g

V

W











Weight of can, A = 29.0 g

Weight of can + wet sample, B = 49.6 g Weight of can + dry sample, C = 47.0 g Water content, W

















% 4 . 14 100 0 . 29 0 . 47 0 . 47 6 . 49 100                      A C C B Dry Density,

γ

d









3 3 / 848 . 1 144 . 0 1 / 114 . 2 1 cm g cm g W      

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Sample 2 (30 blows)

Weight of Mould + wet sample, W2 = 21350 g

Volume, V = 2104.92 cm 3 Wet density,

_γ









3 2 1

/

162 .

2

92 .

2104

16800

21350

cm

g

V

W



























% 3 . 14 100 0 . 39 2 . 57 2 . 57 8 . 59 100                      A C C B Dry Density,

γ

d









3 3 / 892 . 1 143 . 0 1 / 162 . 2 1 cm g cm g W      

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Sample 3 (65 blows)

Weight of Mould + wet sample, W 2 = 21260 g Volume, V = 2104.92 cm 3 Wet density, γ









3 1 2

/

190 .

2

92 .

2104

16700

21150

cm

g

V

W



























% 3 . 11 100 0 . 57 6 . 75 6 . 75 7 . 77 100                      A C C B Dry Density, γd









3 3 / 968 . 1 113 . 0 1 / 190 . 2 1 cm g cm g W      

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DISCUSSION

The California Bearing Ratio is carried out to determine the CBR value of the given crushed aggregate or soil sample. The value of the experiment is compared with the standard value of CBR.

In the CBR test, the sample is separated into 3 types which consist of 10 blows, 30 blows and 65 blows. Every sample is compacted to 5 layers for each blow.

The compacted soil will be tested with the depth of penetration of the metal piston. The value of penetration is recorded into the table. The compaction that is carried out causes the soil to become more compact.

From the graph load (kN) versus penetration (mm) that is been plotted, the value of the corrected load at 2.5mm and 5.0mm is obtained. The CBR value for sample 1 is

56.0%, sample 2 is 127.5% and sample 3 is 178.75% According to the result, sample 3 is soil with good compaction that gives a higher value of CBR.

The graph of CBR versus Dry density has been plotted as appendices and we have determine the value of CBR at 0.95 dry density maximum is 100%. This shows that the sample is suitable (excellent) to be used as the base layer because the value is more than 50%.

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CONCLUSION

The objective of this experiment is to determine the California Bearing Ratio (CBR) value of the given crushed aggregate or soil sample.

From this experiment, the value of CBR is used for road pavement design. It includes the thickness of the pavement layer.

There are 4 graphs plotted. Pavement layer functions to support and separate load from vehicles. So, CBR is important in design to make sure the record is able to support.

We can conclude that our experiment is successful. It is because the aggregate value of CBR is more than 50%, which is excellent in general rating. It can be use as base layer. By using this aggregate for road construction, it is safe for vehicles because has a good strength.

REFERENCES

1. Traffic engineering and safety, BFC 2082 , Universiti Tun Hussein Onn Malaysia 2. Nicholars J. Garber; Lester A.Hoel; Traffic & Highway Engineering (3rd_Edition),

University of Virginia, 1995.

3. Paul H.wright & Radnor J.Paquette; Higway Engineering (5th_{Edition), Georgia} Institute of Technology, 1979.

4. Bent Tagesen; Highway Engineering & Traffic (1st_{Edition), Technical University of} Denmark.

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