CBR Test

Pavement
Material Lab
California Bearing Ratio Test
Priyansh Singh
Department of Civil Engineering
Indian Institute of Technology Delhi
shivakumarmathur@gmail.com
web.iitd.ac.in/~cez138070
Pavement Engg. Lab
12th January 2016

Pavement
Material Lab
California Bearing Ratio
12th January 2016 c Priyansh Singh
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Introduction
Flexible Pavement
It typically consist of asphalt concrete placed over granular
base or sub-base layers supported by the compacted soil,
referred to as the sub-grade.
Figure : Flexible Pavement

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Stress Distribution in Flexible Pavement
Load distribution characteristics depends upon
Materials
Layer Thickness

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CBR Test
The California Bearing Ratio devised by engineers of the
California Division of Highways in nine years period to 1938.
The California bearing ratio (CBR) is a penetration test for
evaluation of the mechanical strength of road subgrades
and base courses .
The test is performed by measuring the pressure required
to penetrate a soil sample with a plunger of standard area .
The measured pressure is then divided by the pressure re-
quired to achieve an equal penetration on a standard crushed
rock material .

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CBR Test Relevance
It is used for the evaluation of sub-grade strength of roads
and pavements.
The results obtained by these tests are used with the em-
pirical curves to determine the thickness of pavement and
its component layers.
CBR-value is used as an index of soil strength and bearing
capacity.
Indian Roads Congress (IRC) has standardized the guidelines
for the design of ﬂexible pavements based on CBR test
(IRC: 37-2001).
The CBR test can be conducted for both sub-grade soil and
Granular sub-base material.

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Relevant Standards
(a) IS 9669
(b) IS 2720 Part 16

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CBR Apparatus

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CBR Apparatus
Figure 1—California Bearing Ratio Apparatus
TS-1aT193-3AASHTO
©2010bytheAmericanAssociationofStateHighwayandTransportationOfficials.
Allrightsreserved.Duplicationisaviolationofapplicablelaw.

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CBR Mould IS t 9669 - 1980
seatings at the ends for positioning the collar and the base plate (see
Fig. 1 ).
4.2 Collar - The collar shall be made from same material as that of
mould. Two similar ears as in the case of the mould shall. be cast integral
with the body or welded. It shall have suitable seatings at the lower end
for sitting flush with the mould ( see Fig. 2 ).
(c) Top View
All dimensions in millimetres.
FIG. 1 MOULD
5
(d) Fornt View
Figure : CBR Mould (Dimensions in mm)

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CBR Plunger
FIG.6
ADJUSTABLE STEM AND PERFORATED PLATES
z
2
B
1
I-5:
L-502.1 m-l
L
FIG.~ PENETRATION PLUNGER
9

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Optimum Moisture Content

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Sample Preparation
Undisturbed specimen
Samples are obtained from the ﬁle soil by cutting mould.
Remoulded Specimen
Prepare the remoulded specimen at Proctor’s maximum dry
density or any other density at which C.B.R> is required.
Maintain the specimen at optimum moisture content or the
ﬁeld moisture as required.
The material used should pass 20 mm I.S. sieve but it should
be retained on 4.75 mm I.S. sieve.
Prepare the specimen either by dynamic compaction or by
static compaction.

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Dynamic Compaction
Take about 4.5 to 5.5 kg of soil and mix thoroughly with the
required water.
Fix the extension collar and the base plate to the mould.
Insert the spacer disc over the base.
Place the ﬁlter paper on the top of the spacer disc.
Compact the mix soil in the mould using either light com-
paction or heavy compaction. For light compaction, com-
pact the soil in 3 equal layers, each layer being given 55
blows by the 2.6 kg rammer. For heavy compaction com-
pact the soil in 5 layers, 56 blows to each layer by the 4.89
kg rammer.
Remove the collar and trim off soil.

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Dynamic Compaction (cont.)
Turn the mould upside down and remove the base plate and
the displacer disc.
Weigh the mould with compacted soil and determine the
bulk density and dry density.
Put ﬁlter paper on the top of the compacted soil (collar side)
and clamp the perforated base plate on to it.

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Static Compaction
Calculate the weight of the wet soil at the required wa-
ter content to give the desired density when occupying the
standard specimen volume in the mould from the expres-
sion.
W = γd × (1 + w) × Vm (1)

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Soaking
Place the swell plate with adjustable stem on the soil sample
in the mold and apply sufﬁcient annular weights to produce
an intensity of loading equal to the mass of the subbase and
base courses and surfacing above the tested material.
Place the tripod with dial indicator on top of the mold and
make an initial dial reading.
Immerse the mold in water to allow free access of water to
top and bottom of the specimen. During soaking, maintain
the water level in the mold and the soaking tank approxim-
ately 25 mm (1 in.) above the top of the specimen. Soak
the specimen 96 hours (4 days).

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Soaking (cont.)
At the end of 96 hours, make a ﬁnal dial reading on the
soaked specimens and calculate the swell as a percentage
of the initial sample length:
Percent swell =
Change in length in mm during soaking
Orignal sample length
×100
(2)

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Penetration Test
Place the mould assembly with the surcharge weights on
the penetration test machine.
Seat the penetration piston at the center of the specimen
with the smallest possible load, but in no case in excess
of 4 kg so that full contact of the piston on the sample is
established.
Set the stress and strain dial gauge to read zero. Apply the
load on the piston so that the penetration rate is about 1.25
mm/min.
Record the load readings at penetrations of 0.5, 1.0, 1.5,
2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10 and 12.5 mm.
Note the maximum load and corresponding penetration if it
occurs for a penetration less than 12.5 mm.

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Penetration Test (cont.)
Detach the mould from the loading equipment. Take about
20 to 50 g of soil from the top 3 cm layer and determine the
moisture content.
• Note the maximum load and corresponding penetration if it occurs for a penetration less
than 12.5 mm.
• Detach the mould from the loading equipment. Take about 20 to 50 g of soil from the top
3 cm layer and determine the moisture content.
The fallowing figure 1 shown bellow represents the CBR test apparatus.

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Observation
Optimum Moisture Content (%) =
Dry Density (g/cc) =
Weight of empty mould =
Weight of Mould + Compacted Specimen =
Volume of specimen =
PR Constant =

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Observation (cont.)
PR Constant =
Table 1 Observed Readings of load with respect to the penetration:
Penetration
(mm)
PR Reading Load on
Plunger
Corrected Load
(kg)
Standard
Load (kg)
CBR%
0
0.5
1
1.5
2
2.5
3
4
5
7.5
10
12.5
PRESENTATION OF RESULTS:
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

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Penetration Result
A load penetration curve of load against penetration is then
plotted with load in ordinate axis and penetration in ab-
scissa axis
the loads corresponding to 2.5 and 5.0mm penetration val-
ues 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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Penetration Result (cont.)
Penetration, mm
ResistancetoPenetration,kPa
Figure 2—Correction of Stress-Strain Curves

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Results
Calculate the CBR value using following formula.
CBR% =
Corrected Load Value
Standard Load
× 100 (3)
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.
Formula Used:
‫ܴܤܥ‬ % =
ܶ݁‫ݐܽ݀ܽ݋݈ݐݏ‬ 2.5݉݉‫ݎ݋‬ 5.0݉݉‫݊݋݅ݐܽݎݐ݁݊݁݌‬
ܵ‫ݐݎ݋݂݀ܽ݋݈݀ݎܽ݀݊ܽݐ‬ℎ݁‫݊݋݅ݐܽݎݐ݁݊݁݌݁݉ܽݏ‬
∗ 100
Standard load values on for specified penetration values as per IS: 2720(Part-XVI)-1987:
PENETRATION(MM) TOTAL STANDARD
LOAD (Kgf)
UNIT STANDARD LOAD
(kg/cm2)
2.5 1370 70
5.0 2055 105
RESULT:

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Report Format

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Report Format

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Report Format
Figure : Page margins
Font : Times new roman
Paragraph : Justiﬁed
Title 14pt.
Section 12 bold.
Subsection 12 bold
italic.
Matter 12 pt & 1.5 line
spacing.
Staple at left corner only.

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Report Submission
Soft Copy Submission
Submit soft copy of report within 48 hours of lab.
Submit copy to shivakumarmathur@gmail.com and carbon
copy (cc) to akswamy.iitd@gmail.com.
The Submitted mail should have subject as <Course
Code><Group NO.><Experiment Name>
Hard Copy Submission
Submit hard copy before Next Experiment1.
1
If next lab date is holiday submit it before that

CALIFORNIA BEARING RATIO TEST
AIM OF THE STUDY: The California Bearing Ratio test is penetration test meant for the
evaluation of sub-grade strength of roads and pavements. The results obtained by these tests are
used with the empirical curves to determine the thickness of pavement and its component layers.
This is the most widely used method for the design of flexible pavement. The objective of this
test is to determine the California bearing ratio by conducting a load penetration test in the
laboratory.
APPARATUS REQUIRED:
1. Moulds with Base plate, Collar and wing nut: Cylindrical mould with inside diameter
150 mm and height 175 mm, provided with a detachable extension collar 50 mm height
and a detachable perforated base plate 10 mm thick. (Confirming to IS : 9669 – 1980).
2. Spacer Disc: 148 mm in diameter and 47.7 mm in height along with handle(Confirming
to IS: 9669-1980)
3. Compaction Rammer: For dynamic compaction having light rammer of 2.6kg or
4.89kg for heavy rammer. For static compaction, compression machine (conforming to
IS: 9198-1979) can also be used.
4. Surcharge Weights: One annular metal weight and several slotted weights weighing 2.5
kg each, 147 mm in diameter, with a central hole 53 mm in diameter.
5. Dial Gauges: Two dial gauges reading to 0’01 mm.
6. Penetration Plunger: Metal penetration piston 50 mm diameter and minimum of 100
mm in length (Conforming to IS: 9669-1980).
7. Loading Machine: With a capacity of at least 5 000 kg and equipped with a movable
head or base which enables the plunger to penetrate into the specimen at a deformation
rate of 1’25 mm/min- The machine shall be equipped with a load machine device that can
read to suitable accuracy.
8. Sieves: 47’5 mm IS Sieve and 19 mm IS Sieve [IS : 460 ( Part 1 ) - 1985: I].
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9. Miscellaneous Apparatus: Other general apparatus, such as a mixing bowl, straightedge,
scales, soaking tank or pan, drying oven, filter paper, dishes and calibrated measuring jar.
RELAVANT STANDARDS:
IS: 9669-1980: Indian Standard Specifications for CBR moulds and its accessories.
IS-2720 (Part-XVI)-1987: Indian Standard Specification laboratory determination of
CBR.
TEST DESCRIPTION:
The California Bearing Ratio Test (CBR Test) is a penetration test developed by
California State Highway Department (U.S.A.) in 1950 for evaluating the bearing capacity of sub
grade soil for design of flexible pavement. The test results have been correlated with flexible
pavement thickness requirements for highways and air fields.
California bearing ratio is the ratio of force per unit area required to penetrate into a soil
mass with a circular plunger of 50mm diameter at the rate of 1.25mm /min. It is a penetration test
where a standard piston, having an area of 50 mm diameter, is used to penetrate the soil at a
standard rate of 1.25 mm/minute. The pressure up to a penetration of 12.5 mm and it's ratio to the
bearing value of a standard crushed rock is termed as the CBR. In most cases, CBR decreases as
the penetration increases. The ratio at 2.5 mm penetration is used as the CBR. In some case, the
ratio at 5 mm may be greater than that at 2.5 mm. If this occurs, the ratio at 5 mm should be
used.
The CBR is a measure of resistance of a material to penetration of standard plunger under
controlled density and moisture conditions. The test procedure should be strictly adhered if high
degree of reproducibility is desired. The CBR test may be conducted in remoulded or
undisturbed specimen in the laboratory. The test is simple and has been extensively investigated
for field correlations of flexible pavement thickness requirement.
RELEVANCE AND IMPORTANCE:
The California bearing ratio test is a penetration test, used for the evaluation of sub-grade
strength of roads and pavements. The results obtained by these tests are used with the empirical
curves to determine the thickness of pavement and its component layers. This is the most widely
used method for the design of flexible pavement. CBR-value is used as an index of soil strength
and bearing capacity. This value is broadly used and applied in design of the base and the sub-
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base material for pavement. The Indian Roads Congress (IRC) has standardised the guidelines
for the design of flexible pavements based on CBR test (IRC: 37-2001). The CBR test can be
conducted for both sub-grade soil and Granular sub-base material.
TEST PROCEDURE:
The test may be performed on undisturbed specimen and on remoulded specimen, which may be
compacted either statistically or dynamically.
PREPARATION OF SPECIMEN
Undisturbed specimen: Attach the cutting edge to the mould and push it gently into the ground.
Remove the soil from the outside of the mould which is pushed in. When the mould is full of
soil, remove it from weighing the soil with the mould or by any field method near the spot.
Remoulded Specimen: Prepare the remoulded specimen at Proctor9s maximum dry density or
any other density at which C.B.R> is required. Maintain the specimen at optimum moisture
content or the field moisture as required. The material used should pass 20 mm I.S. sieve but it
should be retained on 4.75 mm I.S. sieve. Prepare the specimen either by dynamic compaction or
by static compaction.
Dynamic Compaction:
• Take about 4.5 to 5.5 kg of soil and mix thoroughly with the required water.
• Fix the extension collar and the base plate to the mould. Insert the spacer disc over the
base.
• Place the filter paper on the top of the spacer disc.
• Compact the mix soil in the mould using either light compaction or heavy compaction.
For light compaction, compact the soil in 3 equal layers, each layer being given 55 blows
by the 2.6 kg rammer. For heavy compaction compact the soil in 5 layers, 56 blows to
each layer by the 4.89 kg rammer.
• Remove the collar and trim off soil.
• Turn the mould upside down and remove the base plate and the displacer disc.
• Weigh the mould with compacted soil and determine the bulk density and dry density.
• Put filter paper on the top of the compacted soil (collar side) and clamp the perforated
base plate on to it.
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Static compaction
• Calculate the weight of the wet soil at the required water content to give the desired
density when occupying the standard specimen volume in the mould from the expression.
W =desired dry density * (1+w) V
Where, W = Weight of the wet soil
w = desired water content
V = volume of the specimen in the mould = 2250 cm3 (as per the mould available in
laboratory)
• Take the weight W (calculated as above) of the mix soil and place it in the mould.
• Place a filter paper and the displacer disc on the top of soil.
• Keep the mould assembly in static loading frame and compact by pressing the displacer
disc till the level of disc reaches the top of the mould.
• Keep the load for some time and then release the load. Remove the displacer disc.
The test may be conducted for both soaked as well as un-soaked conditions.
• If the sample is to be soaked, in both the cases of compaction, put a filter paper on the top
of the soil and place the adjustable stem and perforated plate on the top of filter paper.
• Put annular weights to produce a surcharge equal to weight of base material and
pavement expected in actual construction. Each 2.5 kg weight is equivalent to 7 cm
construction. A minimum of two weights should be put.
• Immerse the mould assembly and weights in a tank of water and soak it for 96 hours.
Remove the mould from tank.
PROCEDURE FOR PENETRATION TEST:
• Place the mould assembly with the surcharge weights on the penetration test machine.
• Seat the penetration piston at the center of the specimen with the smallest possible load,
but in no case in excess of 4 kg so that full contact of the piston on the sample is
established.
• Set the stress and strain dial gauge to read zero. Apply the load on the piston so that the
penetration rate is about 1.25 mm/min.
• Record the load readings at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10 and
12.5 mm.
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• Note the maximum load and corresponding penetration if it occurs for a penetration less
than 12.5 mm.
• Detach the mould from the loading equipment. Take about 20 to 50 g of soil from the top
3 cm layer and determine the moisture content.
The fallowing figure 1 shown bellow represents the CBR test apparatus.
(Figure 1 Representing CBR test Apparatus)
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OBSERVATION AND CALCULATION:
Optimum Moisture Content (%) =
Dry Density (g/cc) =
Weight of empty mould =
Weight of Mould + Compacted Specimen =
Volume of specimen =
PR Constant =
Table 1 Observed Readings of load with respect to the penetration:
Penetration
(mm)
PR Reading Load on
Plunger
Corrected Load
(kg)
Standard
Load (kg)
CBR%
0
0.5
1
1.5
2
2.5
3
4
5
7.5
10
12.5
PRESENTATION OF RESULTS:
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
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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.
Formula Used:
‫ܴܤܥ‬ % =
ܶ݁‫ݐܽ݀ܽ݋݈ݐݏ‬ 2.5݉݉‫ݎ݋‬ 5.0݉݉‫݊݋݅ݐܽݎݐ݁݊݁݌‬
ܵ‫ݐݎ݋݂݀ܽ݋݈݀ݎܽ݀݊ܽݐ‬ℎ݁‫݊݋݅ݐܽݎݐ݁݊݁݌݁݉ܽݏ‬
∗ 100
Standard load values on for specified penetration values as per IS: 2720(Part-XVI)-1987:
PENETRATION(MM) TOTAL STANDARD
LOAD (Kgf)
UNIT STANDARD LOAD
(kg/cm2)
2.5 1370 70
5.0 2055 105
RESULT:
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References
P. Chakorborty and A. Das. Principles of Transportation
Engineering. PHI Learning (formerly Prentice Hall of
India), 2003.
SK Khanna and CEG Justo. Highway engineering. Nem
Chand & Bros, 1991.
Pavement Interactive. California Bearing Ratio. [Online;
accessed 12-January-2015]. 2007. URL: http:
//www.pavementinteractive.org/article/california-
bearing-ratio/.
Wikipedia. California bearing ratio — Wikipedia, The Free
Encyclopedia. [Online; accessed 12-January-2015].
2014. URL: http://en.wikipedia.org/w/index.php?
title=California_bearing_ratio&oldid=635397353.

Indian Institute of
Technology Delhi
Pavement Material Lab
c Priyansh Singh
Available at: web.iitd.ac.in/~cez138070
Questions ?
Thanks !

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