California bearing ratio test (CBR) | Theory and Scope | Apparatus | Procedure

To determine the California Bearing Ratio (CBR) of the subgrade soil by conducting a  load penetration test in the laboratory.

 

Theory and Scope

California Bearing Ratio (CBR) test originally developed by the California Division of Highways (U.S.A.) is one of the most commonly used methods to evaluate the strength of subgrade soil for the design of pavement thickness. CBR value as defined by IS: 2720 (Part XVI)-1979 is the ratio of the force per unit area required to penetrate a soil mass with a circular plunger of 50 mm diameter at the rate of 1.25 mm/minute, to that required for corresponding penetration of a standard material.

 

The standard load is that load that has been obtained from tests on a crushed stone whose CBR value is taken to be 100 per cent. The ratio is usually determined for penetrations of 2.5 mm and 5.0 mm. The results of this empirical test cannot be related accurately with the fundamental properties of the material but are useful in the design of flexible pavements.

 

Apparatus

Apparatus conforming to the essential requirements of IS: 2720 (Part XVI) with the mould as per IS: 9669.  The field CBR apparatus meets the requirements of IS: 2720 (Part XXXI).

 

A metallic cylinder mould; Loading machine; Perforated swell plate; Proving ring of 10 kN capacity; Two dial gauges reading to 0.01 mm; 4.75 mm and 20 mm IS sieves; Steel cutting collar; Penetration plunger;  Metal rammers; Mixing bowl, Spacer disc; Surcharge weights; Straight edge; Scales; Soaking tank; Drying oven; Filter paper; Dishes and Calibrated measuring jar.

 

Description of apparatus as per IS: 9669-1980

Mould consists of a gunmetal or steel cylinder of 150 mm internal diameter and 175 mm height; provided with a detachable metal extension collar of 50 mm in height. It also has a detachable perforated base plate of 10 mm thickness. The perforations in the base plate do not exceed 1.5 mm in diameter.

 

Loading machine with a capacity of at least 50 kN and equipped with a movable head or base that can travel vertically at a uniform rate of 1.25 mm/minute. Complete with load indicating device. There are two types  of test frames:

 

1.   Hand-operated load frame of capacity 50 KN.

2.   Motorised press or load frame of capacity 50 KN.

 

In place of dial gauge and proving ring assembly, electronic instrumentation is also available. Typical CBR  test loading machines are illustrated in Figs. below

A spacer disc is a metal disc of 148 mm in diameter and 47.7 mm in height. The spacer disc has a groove on one side so that a handle can be screwed to facilitate its lifting.

Metal rammer of 2.6 kg weight with a drop of 310 mm (or) 4.89 kg weight with a drop of 450 mm.

Steel cutting collar of 60 mm total height which can fit flush with the mould; Penetration plunger having a  diameter of 50 mm and at least 100 mm length.

Surcharge weights One annular metal weight and several slotted weights 2.5 kg each and 147 mm in diameter with a 53 mm diameter central hole.

The penetration plunger has a diameter of 50 mm and at least 100 mm in length.

 

Procedure

The method can be used for the determination of CBR of undisturbed and remoulded/compacted soil specimens in soaked as well as in unsoaked states. In the case of a remoulded soil test, the specimens may be compacted either statically or dynamically.

 

Part 1: Preparation of test specimen

Prepare two specimens of the required type as follows:

Step 1: Undisturbed or natural soil specimen Attach the steel cutting edge to the mould and push it gently into the ground till the mould is full of soil. Remove the soil from the sides and bottom. Trim the excessive soil from top and bottom; weigh the soil with the mould and determine its density.

 

Step 2: Remoulded or compacted specimen Prepare the remoulded specimen at Proctor’s maximum dry density or any other density at which CBR is required. Maintain the specimen at optimum moisture content or the field moisture as required. The material taken for remoulded specimen should pass the 20 mm IS  sieve and be retained on the 4.75 mm IS sieve. In case the soil contains larger than 20 mm size particles, it should be replaced by an equal amount of material passing a 20 mm sieve but retained in a 4.75 mm sieve.  The remoulded samples are compacted either statically or dynamically.

 

A) Statically compacted specimen

1. Calculate the weight of the soil at the required water content for the desired density after  compaction such that it fills the mould (excluding collar);

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 the laboratory)

 

2. Fix the extension collar to the mould and clamp it to the base plate. Take the weight W  (calculated as above) of the soil mixed thoroughly with water and place it in the mould.

 

3. Fill the mould with soil, gently pressing it with your hands so that it does not spill out of the mould.

 

4. Place a coarse filter paper over the levelled soil surface and then insert the spacer disc.

 

5. Place the mould assembly on the pedestal of the static loading machine and compact the soil by pressing the displacer disc till the disc is flush with the top of the mould. Keep the load for some time and then release the load and remove the displaced disc.

 

B) Dynamically compacted specimen

1. Take about 4.5 to 5.5 kg of granular soils sieved through a 20 mm IS sieve in a mixing pan and add water to the soil in the quantity such that the moisture content of the specimen is either equal to field moisture content or OMC (Optimum Moisture Content) as desired.  Mix together the soil and water uniformly.

 

2. Clamp the mould along with the extension collar to the base plate. Insert the spacer and discover the base with the hole on the bottom side. Place a coarse filter paper on the top of the spacer disc.

 

3. Compact the soil-water mix in the mould using either light compaction or heavy compaction. For light compaction, compact the soil into three equal layers, each layer being given 55 blows with the rammer weighing 2.6 kg dropping through 310 mm. For heavy compaction compact the soil in five layers, 56 blows to each layer by the 4.89 kg rammer dropping through 450 mm. After each layer compacted surface should be scratched before adding soil for the subsequent layer.

 

4. Remove the extension collar and trim off the excess soil by a straight edge.

 

5. Turn the mould upside down and remove the base plate, the displacer disc and the filter paper.

 

6. Weigh the mould with compacted soil and determine the bulk and dry densities.

 

7. Invert the mould and place a coarse filter paper on the top of the compacted soil (collar side) and clamp the perforated base plate onto it so that the soil is in contact with the filter paper in the base.

The test may be conducted for both soaked as well as unsoaked conditions.

 

Part 2: For the CBR test on the specimen soaked in both cases of compaction, soak the prepared specimen as described in Step 1 as follows; in the case of the unsoaked test move to Step 3.

 

Step 1: Weigh the sample excluding the base plate and spacer disc; put a filter paper on the top of the soil and the perforated plate on the top of the filter paper.

 

Step 2: Place annular weights to produce a surcharge equal to the weight of base material and pavement expected in actual construction; the surcharge weight may vary from 2.5 to 5 kg. Immerse the mould assembly and weights in a water tank for 4 days. Each 2.5 kg weight is equivalent to 70 mm construction. A minimum of two weights should be placed.

 

Step 3: Remove the mould from the tank and drain off the water from the sample hold it in a vertical position for about 15 minutes.

 

Step 4: Weigh the sample again to calculate the percentage of water absorbed.

 

Part 3: Test the specimen following the normal procedure

 

Step 1: Place the mould assembly containing the specimen along with the base plate and surcharge weight of 2.5 kg on the top surface of the soil on the penetration test machine.

 

Step 2: Install the proving ring assembly and penetration plunger onto the loading machine. Seat the plunger  at the centre of the specimen with the smallest possible load, but in no case in excess of 4 kg so that  full contact of the plunger on the sample is established as shown in Fig. below

 

Step 3: Place the remainder surcharge weight (slotted weight) so that the total surcharge weight equals 5 kg. This should be treated as a zero load position.

 

Step 4: Mount the dial gauge with the tip of its stem resting on the collar to measure the penetration; set the dial gauges to read zero.

 

Step 5: Apply load so that penetration rate is 1.25 mm/minute. Record the load at penetrations of 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, 10.0 and 12.5 mm. In case the load starts decreasing before 12.5 mm  penetration, record the maximum load and the corresponding penetration value.

 

Step 6: Detach the mould from the loading assembly and take about 20 to 50 g of soil from the top 30mm layer and determine the moisture content.

 

At least three specimens should be tested on each type of sample. The maximum permissible variation should be within the limits given below.

CBR, percent	Maximum permitted  variation, percent
< 10	3
10−30	5
30−60	10
> 60	Not Significant
CBR, per cent	The maximum permitted  variation, per cent

 

Part 4: Computation of test results

Step 1: Plot the load penetration curve with the load as ordinate and penetration as abscissa. Generally, the initial portion of the curve is concave upwards due to surface irregularities. In such a case apply a correction. Draw tangent at the point of the greatest slope. The point where this tangent meets the abscissa  is the corrected zero reading of penetration as illustrated in Fig. below

Typical load penetration curve

 

Step 2: From the load penetration curve, determine the load value corresponding to the penetration value at which the CBR is desired.

 

Step 3: The CBR value is computed from

where _PT = corrected test pressure corresponding to the chosen penetration from the load penetration curve in MPa.

Ps = pressure to achieve equal penetration on standard soil in MPa

In most cases, CBR decreases as the penetration increases. The CBR values are usually calculated for penetrations of 2.5 mm and 5 mm, and the greater of the two is adopted for design purposes.  Generally, the CBR value at 2.5 mm is greater than that at 5 mm. However, if CBR for 5 mm penetration exceeds that for 2.5 mm, then the test is to be repeated for checking. If the check test gives similar results, the CBR value corresponding to 5 mm penetration is taken as the design value.

 

The standard loads adopted for different penetrations for the standard material with a CBR value of 100 per cent are given in the table below.

 

Standard load for different penetration values

Penetration,  mm	Unit standard load,  kgf/cm2	Total standard load,  kgf
2.5	70	1350
5.0	105	2055
7.5	134	2630
10.0	162	3180
12.5	183	3600

 

Observations and Calculations 

1. Details of the sample

Sample details	Location
Type of sample	Undisturbed/Remoulded
Compaction of specimen	Static/Dynamic
Type of compaction	Light / Heavy
Condition of soaking	Soaked / Un-soaked
Period of soaking	96 hours
Surcharge weight,
Dry unit weight
Weight of material coarser than 20  mm replaced
The water content of the compacted sample	per cent
Sample details	Location

 

 

CBR at 2.5 mm penetration = _________

CBR at 5 mm penetration = __________

CBR of sample (design value) =__________

The average CBR value of three test specimens is reported as the CBR value of the sample.

 

Precautions

1. The holes of the base plate of the mould should not be blocked.

2. The surcharge weight should be aligned with the plunger so that the plunger penetrates freely into the soil sample.

 

Discussion

California Bearing Ratio is an empirical value that can be used in the design of flexible pavements. Tests are carried out on natural or compacted soils in water soaked or unsoaked conditions and the results so obtained are compared with the curves of the standard test to have an idea of the soil strength of the subgrade soil.

 

The CBR is a measure of the resistance of a material to penetration of a standard plunger under controlled density and moisture conditions. The test procedure should strictly adhere if a high degree of reproducibility is desired.

 

The harder the surface, the higher will be the CBR value. A CBR of 3 may represent tilled farmland, a  CBR of 4.75 the turf or moist clay, while moist sand may have a CBR of 10. High-quality crushed rock has a  CBR of over 80. The standard material for this test is crushed California limestone which has a value of 100.

 

Design curves developed by Road Research Laboratory, UK have been adopted by Indian Road Congress  IRC: 37. Depending upon the estimated traffic volume the thickness of the base course and sub-base course can be determined from their respective CBR values. The range of CBR values for various types of soils is listed  in the Table below

Range of CBR values for various types of soils

 

What is the CBR test used for?

California bearing ratio test (CBR) is defined as the ratio force per unit area of penetration test for the amount of the subgrade strength of roads and sidewalks. The results obtained from these tests are used along with empirical curves to determine the constituent layers and thickness of the pavement.

NATIONAL STANDARDS

IS:460 (Parts 1 and 2)-1985; Specification for Test Sieves.

IS:2386 (Part 1)-1963; Methods of Test for Aggregates for Concrete: Part-I Particle Size and Shape  (with Amendment No. 2).

IS:2386 (Part 3)-1963; Specific Gravity, Density, Voids, Absorption and Bulking.

IS:2386 (Part 4)-1966: Methods of Test for Aggregates for Concrete: Part-IV- Mechanical Properties (Crushing Value, Aggregate Impact Value, Abrasion, Polished Stone, Crushing Strength) (with  Amendment No. 3).

IS:2386 (Part 50) − 1963; Soundness.

2430−1996 (1st Revision): Methods for Sampling of Aggregates for Concrete.

IS:2720 (Part-XVI)-l 979; Methods of Test for Soils (Lab determination of CBR).

IS 5640−1970: Method of Test for Determining Aggregate Impact Value of Soft Coarse Aggregate.

IS:6241- 1971; Method of Test for Determining Stripping Value of Road Aggregates.

IS:6579-1981 (1st Revision); Specifications for Coarse Aggregate for Water Bound Macadam

(Amendment No.1)

 

REFERENCES

Gambhir, M. L., Concrete Technology, 4th Edition, McGraw-Hill Education (India), 2009.

Gambhir, M. L. and Neha Jamwal, Building Materials: Products, Properties and Systems, McGraw-  Hill Education (India), 2011.

Millard, R.S. (1993), Road Building in the Tropics: Transport Research Laboratory State-of-the-Art  Review 9, HMSO, London.