The test is specifically used to

determine the mechanical strength as well as the potential strength of road

subgrades and basecourses materials including the recycled material generally

used for road and airfield pavements. CBR value is a percentage comparison with

the standard crushed rock from California and thus this test is a comparison

test.

The test

is standardized by American Society Of Testing Materials (ASTM) as D1883-05

(this standard is used for laboratory prepared samples or re-molded samples)

and D4429 (for on field soils); and by American Association of State Highway

and Transportation officials (AASHTO) T193; and by British Standard as BS 1377;

and by International Standard (IS) as 2720 (Part XVI).

Main components of Pavement design

The

results obtained by CBR test are used with the empirical curves to determine

the thickness of different layers of flexible pavement like subgrade, subbase,

base courses. This is the most widely used method for the design of flexible

pavement.

CBR value

is used to quantify the response of the pavement foundation and subgrade to

loading. CBR does not provide any data regarding the properties of the soil

except as to compare its resistance to penetration to the base crushed rock’s

resistance to penetration. Although CBR test is empirical and has some

limitations but it is still used around the world due to its low equipment

requirement, ease of performance prediction and history of use.

The two

empirical methods for pavement design used now days are CBR method and Group

Index Method. In CBR method beside many other steps one step is to determined

the CBR value. This test allows the Engineer to design the Capping Layer and

the sub-base layer by determining the strength of the underlying soil.

Flowchart for pavement design using CBR method

Scope and Objective of the CBR Test

The

Objective of the California Bearing Ratio test is to determine the CBR value

for a soil under consideration as a pavement foundation.

CBR is

generally used for cohesive soils where effect of water content on CBR is more

and generally CBR is determined for a range of water content but it can also be

performed for cohesionless soils or coarse grained materials where the effect

of water content on CBR is small and thus CBR is performed at Optimum Moisture

Content (OMC).

According

to ASTM; CBR is primarily intended for evaluating the strength of materials

having maximum particle sizes less than ¾ inches or 19 mm, but this restriction

is not strict one; if the sample has a material larger than 19 mm; equal amount

of such material be replaced by material of size smaller than 19 mm but from

the same representative sample.

If the

sample to be tested contains much fraction of particles of sizes 3 in (Sieve

No. 4), the CBR results will fluctuate and thus more trials are required to

establish a reliable CBR; thus generally the sample to be tested consists of

particles smaller than 19 mm but larger than 3 in.

CBR test

can be performed for both in soaked conditions or un-soaked conditions; but

mostly soaked conditions is preferred so as to evaluate the material strength

in worst conditions.

Description of Apparatus

Apparatus

required to perform the CBR test is as follows;

Molds: Cylindrical

mold with an internal diameter of 6 in. and a height of 7 in. with an extension

collar of 2 in. height and a perforated base plate.

mold with an internal diameter of 6 in. and a height of 7 in. with an extension collar of 2 in. height and a perforated base plate. Spacer Disk: A circular disk of metal 5 –

15/16 in. diameter and 2.416 in. height.

15/16 in. diameter and 2.416 in. height. Rammer: A rammer of mass 4.54 kg (10 lbs)

Apparatus for measuring expansions. This

consists of a swell plate with adjustable stem and a tripod support for a dial

indicator.

consists of a swell plate with adjustable stem and a tripod support for a dial indicator. Surcharge weights: Several slotted or split

metal plates of 149.2 mm; diameter and 5 lb weight.

metal plates of 149.2 mm; diameter and 5 lb weight. Penetration Piston: A metal Piston of circular

cross – section having diameter of 1.954 in. Area = 3in2 and

not less than 4 inches in length.

cross – section having diameter of 1.954 in. Area = 3in2 and not less than 4 inches in length. Loading Device: A compression type apparatus

capable of applying a uniformly increasing load up to 10,000 lb at a rate of

1.3 mm/min.Soaking Tank: A tank suitable for maintaining

the water level of 1 in, above the top of specimen.

capable of applying a uniformly increasing load up to 10,000 lb at a rate of 1.3 mm/min.Soaking Tank: A tank suitable for maintaining the water level of 1 in, above the top of specimen. Drying Oven: Oven Capable of maintaining a

temperature of 110 + 5 0C for drying samples.

temperature of 110 5 0C for drying samples. Moisture content Containers

Miscellaneous: Tools such as mixing pans,

spoons, straightedge, filter paper, balances etc.

Test procedure

Approximately 18 kg soil pass of 19mm sieve and

retain of sieve no. 4 is taken. Moisture and dry density curve is obtained

using the standard AASHTO T 99 or T 180. Optimum Moisture Content (OPC) is obtained

from the graph between moisture content and dry density Prepare the sample by adding optimum moisture

content and then compact the soil in five layers by applying 10,30 and 65 blows

respectively in three CBR molds using 10 lb rammer having 18 in. height of

fall. The compacted densities of the three specimens range from 95 percent to

100 % of the maximum dry density already

determined by the T 180 compaction test. 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 sub-base and base courses

and surfacing above the tested material, but not less than 4.54 kg (10 lbs) .

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. Place the sample in water for

96 hours (4 days) Make a dial reading on soaked specimen and

calculate swell as a percentage of initial sample height. Remove the sample from tank and allow to drain

for 15 minutes. Penetration Test: Place the mold on the

loading frame and adjust its potion until the piston is centered on the

specimen. Seat the penetration piston with a 44 N (10

lb) load, and set both the load dial and the strain dial to zero. This initial

load is considered as the zero load when determining the stress-penetration

relationship. Place the surcharge weights on the specimens

equal to that used during soaking. Apply load at a rate of 1.3 mm / min and

record the load for penetration of 0.025 in, 0.05 in, 0.075 in, 0.10 in and so

on up to 0.5 inches. Stress strain curve: Plot curves between load

and penetration for each specimen. Apply the corrections to the curves if

required. Take the readings of load for 0.1 in and 0.2 in. penetration and find

CBR for both penetrations. The greater values is the required CBR for that

specimen. Also find the dry density for each specimen. CBR = Test load value, divided by, the

standard load, multiplied by 100. Design

CBR: it is calculated by plotting a graph between CBR values and dry densities

of all the three specimens and then calculating the design CBR against value of

85 % maximum dry density.

Test results, table ,graphs and calculations

Step 1:

Getting the relationship and graph between moisture content and dry density

Moisture Density Relationship

Dry

Density (lb/cft)

Sample No. Moisture Content (%) Dry Density (lb/cft) 1 2 3 4 5

Step 2: Finding the density of the three

samples each of 10 blows, 30 blows and 60 blows

Step 3: Load v/s Penetration Graph

Load Penetration Curve

For sample made with 10 blows

Sr.No Load (lbs) Penetration (mm) 1 0.5 mm 2 1.0 mm 3 1.5 mm 4 2.0 mm 5 2.5 mm 6 3.0 mm 7 3.5 mm 8 4.0 mm 9 4.5 mm 10 5.0 mm 11 5.5 mm 12 6.0 mm 13 6.5 mm 14 7.0 mm 15 7.5 mm 16 8.0 mm 17 8.5 mm 18 9.0 mm 19 9.5 mm 20 10 mm

For sample made with 30 blows

Sr.No Load (lbs) Penetration (mm) 1 0.5 mm 2 1.0 mm 3 1.5 mm 4 2.0 mm 5 2.5 mm 6 3.0 mm 7 3.5 mm 8 4.0 mm 9 4.5 mm 10 5.0 mm 11 5.5 mm 12 6.0 mm 13 6.5 mm 14 7.0 mm 15 7.5 mm 16 8.0 mm 17 8.5 mm 18 9.0 mm 19 9.5 mm 20 10 mm

For sample made with 60 blows

Sr.No Load (lbs) Penetration (mm) 1 0.5 mm 2 1.0 mm 3 1.5 mm 4 2.0 mm 5 2.5 mm 6 3.0 mm 7 3.5 mm 8 4.0 mm 9 4.5 mm 10 5.0 mm 11 5.5 mm 12 6.0 mm 13 6.5 mm 14 7.0 mm 15 7.5 mm 16 8.0 mm 17 8.5 mm 18 9.0 mm 19 9.5 mm 20 10 mm

Summary of CBR Test and conclusion

Sample No. Compaction Effort Dry Density (lb/cft) CBR value (%) 1. 10 113 14.5 2. 30 121 28 3. 65 125 34.5

Maximum

Dry density _____________________ (lb./cft)

95% of

maximum dry density________________(lb/cft)

Resultant

CBR or Designed CBR __________________ (lb/cft)

CBR – Density Relation

Limitations

The

laboratory and field compaction methods are not identical, however, comparative

tests indicate that reasonable correlation of results can be obtained from

field compact materials and samples compacted under similar conditions in the

laboratory.

Because added strength to highly stabilized surfaces such as

asphaltic concrete is neglected, the assumption of a completely saturated

subgrade condition sometimes results in a too conservative factor of safety.

Because many of the procedures are of an arbitrary nature, you

must run the test to exact standards in order for the design tables to be

valid.

Conclusion:

California

bearing ratio is a widely used method to design the flexible pavements, beside

all the limitations it is easy to perform and still does not need any big

instruments etc. The value of the CBR test is then compared with the following

table to get the quality of material from which the required thicknesses and

other parameters are decided.

CBR VALUE SUBGRADE STRENGTH COMMENTS 3%

and less Poor ”

Capping is required 3%

– 5% Normal Widely

encountered CBR range capping considered according to road category 5%

– 15% Good “Capping”

normally unnecessary except on very heavily trafficked roads.

CaliforniaBearing Ratio (CBR) test is a compressive nature penetration test. It wasoriginally developed by Caltrans i.e. California Department of Transportationafter World War II i.e. in the late 1930s.