CBR Test

California Bearing ratio test is used to measure the bearing capacity of the soil. These values are used in the pavement design.

1. Pavement
Material Lab California Bearing Ratio Test
Priyansh Singh
Department of Civil Engineering
Indian Institute of Technology Delhi
@[email protected]
mweb.iitd.ac.in/~cez138070
Pavement Engg. Lab
12th January 2016

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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 :

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Stress Distribution in Flexible Pavement
Load distribution characteristics depends upon
IMaterials
ILayer Thickness

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CBR Test
IThe California Bearing Ratio devised by engineers of the
California Division of Highways in nine years period to 1938.
IThe California bearing ratio (CBR) is a penetration test for
evaluation of the mechanical strength of road subgrades
and base courses .
IThe test is performed by measuring the pressure required
to penetrate a soil sample with a plunger of standard area .
IThe 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
IIt is used for the evaluation of sub-grade strength of roads
and pavements.
IThe results obtained by these tests are used with the em-
pirical curves to determine the thickness of pavement and
its component layers.
ICBR-value is used as an index of soil strength and bearing
capacity.
IIndian Roads Congress (IRC) has standardized the guidelines
for the design of exible pavements based on CBR test
(IRC: 37-2001).
IThe 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 ApparatusFigure 1 —California Bearing Ratio Apparatus
TS-1a T 193-3 AASHTO
© 2010 by the American Association of State Highway and Transportation Officials.
All rights reserved. Duplication is a violation of applicable law.

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CBR MouldIS 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 ).
All dimensions in millimetres.
FIG. 1 MOULD
5
(c) Top ViewIS 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 ).
All dimensions in millimetres.
FIG. 1 MOULD
5 (d) Fornt View
Figure :

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CBR PlungerIS : 9669 - 1980
FIG. 6
HOLES EQUALLY
All dimensions in millimetres.
ADJUSTABLE STEM AND PERFORATED PLATES
z
2
B
1
I-
5:
L-50 2.1 m-l
L
All dimensions in millimetres.
FIG.~ PENETRATION PLUNGER
9

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Optimum Moisture Content

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

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Dynamic Compaction
ITake about 4.5 to 5.5 kg of soil and mix thoroughly with the
required water.
IFix the extension collar and the base plate to the mould.
Insert the spacer disc over the base.
IPlace the lter paper on the top of the spacer disc.
ICompact 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.
IRemove the collar and trim off soil.

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Dynamic Compaction
ITurn the mould upside down and remove the base plate and
the displacer disc.
IWeigh the mould with compacted soil and determine the
bulk density and dry density.
IPut lter 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
ICalculate 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=d(1+w)Vm (1)

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Soaking
IPlace the swell plate with adjustable stem on the soil sample
in the mold and apply sufcient annular weights to produce
an intensity of loading equal to the mass of the subbase and
base courses and surfacing above the tested material.
IPlace the tripod with dial indicator on top of the mold and
make an initial dial reading.
IImmerse 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
IAt the end of 96 hours, make a nal 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
IPlace the mould assembly with the surcharge weights on
the penetration test machine.
ISeat 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.
ISet 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.
IRecord 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.
INote the maximum load and corresponding penetration if it
occurs for a penetration less than 12.5 mm.

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Penetration Test
IDetach 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. 
 

(Figure 1 Representing CBR test Apparatus) 

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Observation
IOptimum Moisture Content (%) =
IDry Density (g/cc) =
IWeight of empty mould =
IWeight of Mould + Compacted Specimen =
IVolume of specimen =
IPR Constant =

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ObservationOBSERVATION 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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Penetration Result
IA load penetration curve of load against penetration is then
plotted with load in ordinate axis and penetration in ab-
scissa axis
Ithe loads corresponding to 2.5 and 5.0mm penetration val-
ues are noted.
ISometimes 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.
IA tangent is drawn at a point of greatest slope.
IThe point where this tangent meets penetration axis is the
corrected zero reading of penetration.

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Penetration ResultTS-1a T 193-9 AASHTO
Penetration, mm
Resistance to Penetration, kPa
Figure 2—Correction of Stress-Strain Curves
10.2 California Bearing Ratio—The corrected load values shall be determined for each specimen at
2.54 and 5.08 mm (0.10 and 0.20 in.) penetration. California Bearing Ratio values are obtained in
percent by dividing the corrected load values at 2.54 and 5.08 mm (0.10 and 0.20 in.) by the
standard loads of 6.9 and 10.3 MPa (1000 and 1500 psi), respectively, and multiplying these ratios
by 100.
Corrected load value
CBR = 100
Standard load

(2)
10.2.1
The CBR is generally selected at 2.54 mm (0.10 in.) penetration. If the ratio at 5.08 mm (0.20 in.)
penetration is greater, the test shall be rerun. If the check test gives a similar result, the ratio at
5.08 mm (0.20 in.) penetration shall be used.
10.3 Design CBR for One Water Content Only —Using the data obtained from the three specimens, plot
the CBR-Dry Density as Molded relation as shown in Figure 3. The design CBR may then be
determined at the desired percentage of the maximum dry density, normally the minimum
percentage compaction permitted by the agency’s co mpaction specifications.
© 2010 by the American Association of State Highway and Transportation Officials.
All rights reserved. Duplication is a violation of applicable law.

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Results
ICalculate 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: 
CALIF O
RN BEGTSTBIMHUDDGYIUHPDDQN:NBYTB1G:
.BT:STYSEGTSoGYBuN TDNQN:NBYTB1G:
∗ 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 :
Font: Times new roman
Paragraph: Justied
ITitle 14pt.
ISection 12 bold.
ISubsection 12 bold
italic.
IMatter 12 pt & 1.5 line
spacing.
Staple at left corner only.

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Report Submission
mSoft Copy Submission
Submit soft copy of report within 48 hours of lab.
Submit copy [email protected]
copy (cc) [email protected].
The Submitted mail should have subject as
Code><Group NO.><Experiment Name>
vHard Copy Submission
Submit hard copy before Next Experiment
1
.
1
If next lab date is holiday submit it before that

29. 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].  Pavement
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30. 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: 
C IS: 9669-1980: Indian Standard Specifications for CBR moulds and its accessories. 
C 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- Pavement
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31. 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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32. 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.  Pavement
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33. • 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)  Pavement
Material Lab California Bearing Ratio
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34. 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  Pavement
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35. 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: 
CALIF O
RN BEGTSTBIMHUDDGYIUHPDDQN:NBYTB1G:
.BT:STYSEGTSoGYBuN TDNQN:NBYTB1G:
∗ 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: 
 
 
 
 
 
 
 
 
 
    Pavement
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36. Pavement
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References
P. Chakorborty and A. Das.Principles of Transportation
Engineering.
India), 2003.
SK Khanna and CEG Justo.Highway engineering.
Chand & Bros, 1991.
Pavement Interactive.California Bearing Ratio.
accessed 12-January-2015]. 2007.URL:http:
//www.pavementinteractive.org/article/california-
bearing-ratio/.
Wikipedia.California bearing ratio — Wikipedia, The Free
Encyclopedia.
2014.URL:http://en.wikipedia.org/w/index.php?
title=California_bearing_ratio&oldid=635397353.

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