The California Bearing Ratio (CBR) test is a penetration test used to evaluate the subgrade strength of roads and pavements. The pressure up to a penetration of 2.5 mm is measured and its ratio to the bearing value of a standard crushed rock is termed as the CBR.

The harder the surface, the higher the CBR value. Typically, a value of 2% equates to clay, while some sands may have a CBR value of 10%. High quality sub-base will have a value of between 80-100% (maximum). The CBR test is carried out on soils with a maximum particle size of 20mm.

What is CBR Test in Civil

What is CBR Test?

CBR Test Full Name is California Bearing Ratio (CBR)

In the year 1930, this test was developed by the California Division of Highways around 1930 and was consequently accepted by several states, nations, the U.S. federal agencies, and worldwide.

As a result, in the US, most agency and commercial geotechnical laboratories are set to perform CBR tests. The CBR(California bearing ratio) test is one kind of strength testing system that allows to compares of the bearing capacity of a standard graded cursed stone (mainly it should have its maximum of 100% of CBR value). The CBR(California Bearing Ratio) test is a determination of the resistance of a material to penetration of standard plunger in controlled density and moisture conditions.

It is mainly projected for, examine the strength of cohesive materials having the largest particle sizes lower than 19 mm (0.75 inches). The intent of this test is the evaluation of the CBR (California Bearing Ratio) value of the subgrade soil.

The evaluation of the bearing strength of subgrade soil and bases is to fulfill the criteria of the road and pavement construction by using this testing method. This is the main objective of the CBR test.

California Bearing Ration test is the proportion articulated infraction of force per unit area necessary for penetrating a soil mass with a standard circular plunger of 25 mm radius at the rate of 1.25 mm/min to that required for related penetration in a standard material.

The proportion is generally found out for penetration of 2.5 and 5 mm.

If the ratio at 5 mm is consistently upper than that at 2.5 mm, the ratio at 5 mm is used. This method covers the laboratory technique for finding out the C.B.R. of compacted soil specimens in a wet state.

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How to CBR Test Complete?

Apparatus Required for CBR Test

Loading Machine for CBR Test :

California Bearing Ratio Test apparatus includes a loading machine with a minimum capacity of 5000 kg and set with a mobile head or base, which enables Plunger of 50 mm diameter, and 1.25 mm per minute is the rate of penetration to penetrate the specimen.

Mould in this test with Stay Rod, Base Plate, and Wing Nut Cylindrical mold:

Inside diameter of 150mm and height of 175mm with a separable perforated base plate of 10mm thickness and 235mm diameter.
It has a net capacity of 2250 ml, meeting the requirements of IS-9669:1980.
There is one detachable extension collar with 60 mm height, Spacer Disc with 148 mm diameter, and height of 47.7 mm along with the handle.
Furthermore, several slotted weights weighing 2.5 kg each weight and single annular metal weight and, 147 mm in diameter, with a hole in the center of 53 mm in diameter.
In this test, the height of the drop is 450 mm for compaction Rammer, and its weight is 4.89 kg.

Step up Apparatus for CBR Test

Lubricating oil is applied in the internal surface of the mould to make it frictionless removals of soil.
Start compacting the mixed soil in the mould by helping bit heavy compaction. i.e.By using 4.89 kg rammer, compact the soil in 5 layers with 55 blows to each segment.
Afterward, remove the extension collar and level and trim the compacted soil carefully at the topmost level of mould or leveling of the top surface.
In case if there are any voids developed on the top surface of the compacted soil after removal of the coarse material, all voids refilled by using smaller particles of soil material.
The next step is to measure the weight of the compacted soil specimen and mass of the mould, after removing the perforated base plate, Spacer disc, and filter paper.
Put this filter paper on the base plate and reverse the mould and remove the compacted soil. Hence this soil came in contact with the filter paper.
Set a filter paper above the specimen, on the other hand, place a perforated plate on top of the compacted soil specimen in the mould.
Lay annular weights to generate a surcharge equivalent to the weight of base material and pavement, to the nearby 2.5 kg.
Now, submerge the mould assembly and weights into a tank of water and saturate it up to 96 hours. Set the stand for expansion
measuring device on the edge of the mould, record preliminary dial gauge reading.
The second step is to Note down the readings every day adjacent to the time of reading. Make sure that the water level shall remain constant to maintained in the tank right through the period.
Final step or at the end of the soaking period, parallel note down the last reading of the dial gauge and get the mould out from the water tank.
Take out the extra water collected in the mould and allocate the specimen to draw off for 15 minutes. At last, Remove the perforated plate and the filter paper from the top layer, evaluate the soaked soil sample and record the weight.

The Procedure Apparatus for CBR Test

Firstly, set the mould assembly with a test specimen on the lower plate of the penetration testing machine.
For avoiding disruption of soil into the hole of the extra weights, 2.5 kg annular pressure shall be sited. Before setting out surcharge weight placed on the soil surface, first to set the penetration plunger.
The penetration piston is set at the middle of the specimen with the nominal possible load; however, in some cases, over 4 kg, thus full contact of the piston on the pattern, is established.
Set the weight on it, so deformation gauges are to be read zero, by applying the load on the piston hence the penetration rate is nearly 1.25 mm/min.note down all load readings at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, 10 and 12.5 mm.
Pull up the plunger and separate the mould from the loading equipment.
Afterward, Take out 20 to 50 g of soil from the top 30 mm layer and find out the moisture content.

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The Calculation Required During the Experiment:

Let start with the initial portion of the curve is concave to upwards directions.

To correct by sketching a tangent on the curve at the point of the maximum slope and move the origin. After that, Find out and note down the accurate load reading corresponding to each penetration.

Formula of calculation

C.B.R. = (PT/PS) X 100

where,

PS = Standard load for the same penetration taken from the table above, and PT = Corrected test load corresponding to the selected penetration from the load penetration curve.

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Penetration(mm)	Standard Load(kg)	Unit Standard Load(kg/cm2)
2.5	1370	70
5	2055	105
7.5	2630	134
10	3180	162
12.5	3600	183

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Standard Load Values for CBR Test

C.B.R. value at 2.5 mm penetration = 3.
C.B.R. value at 5.0 mm penetration = 4.

The C.B.R. results are usually calculated for penetration of 2.5 mm and 5 mm. In general, the C.B.R. value at 2.5 mm will be bigger than at 5 mm, and in such a case/the previous shall be taken as C.B.R. for the design point.

In case C.B.R. values on 5 mm cross that of 2.5 mm, the test should be done again. If matching results pursue, the C.B.R.value corresponding to 5 mm penetration should be in use for design.

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What Is a CBR?

The California Bearing Ratio (CBR) test is a penetration test used to evaluate the subgrade strength of roads and pavements. The pressure up to penetration of 2.5 mm is measured and its ratio to the bearing value of a standard crushed rock is termed the CBR.

What Is a CBR Test Used For?

CBR Testing (California Bearing Ratio) is primarily undertaken to provide data for road pavement design. It was first developed by the California State Highway Department. It is a penetration test which is used to evaluate the subgrade strength primarily of roads, pavements, and foundations.

How Is CBR Value Calculated?

The C.B.R. values are usually calculated for penetration of 2.5 mm and 5 mm. Generally, the C.B.R. value at 2.5 mm will be greater than at 5 mm and in such a case/the former shall be taken as C.B.R. for design purposes.

California Bearing Ratio Values

The CBR value is the stress at the 0.100in (2.54mm) penetration depth. Occasionally, the ratio at the 0.200in (5.08mm) penetration depth is higher than the 0.100in depth, and the test must be rerun. If the check test shows similar results, the stress from the 0.200in penetration depth is the reported CBR.

CBR Value

The harder the surface, the higher the CBR value. Typically, a value of 2% equates to clay, while some sands may have a CBR value of 10%. A high-quality sub-base will have a value of between 80-100% (maximum). The CBR test is carried out on soils with a maximum particle size of 20mm.

What Is a Good Cbr Test Result?

A clayey soil generally has a low CBR value (less than 8). Sands are more granular and drain better and will generally have CBR values between 15 and 35. Gravel will have the best CBR values, generally 25 and up. That is why it is suggested to save the best material to cap the subgrade.

CBR Test Results

The CBR value is the stress at the 0.100in (2.54mm) penetration depth. Occasionally, the ratio at the 0.200in (5.08mm) penetration depth is higher than the 0.100in-depth, and the test must be rerun. If the check test shows similar results, the stress from the 0.200in penetration depth is the reported CBR.

CBR Test Results Interpretation

In Situ CBR Test Results

CBR test results can be gathered either in-situ or within the accredited soil laboratories using samples of the subgrade natural soils to ensure that thickness of the capping layer and/or subbase materials are correct and the underlying subgrade can carry the required loads.

CBR Results Interpretation

CBR Test Procedure

The CBR 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 required to achieve an equal penetration on a standard crushed rock material. The harder the surface, the higher the CBR value.

California Bearing Ratio Test Procedure

The test consists of causing a cylindrical plunger of 50mm diameter to penetrate a pavement component material at 1.25mm/minute. The loads for 2.5mm and 5mm are recorded. This load is expressed as a percentage of the standard load value at a respective deformation level to obtain the CBR value.

California Bearing Test Procedure

The laboratory CBR test consists essentially of preparing a sample of soil in a cylindrical steel mold and then forcing a cylindrical steel plunger, of a nominal diameter of 50 mm, into the sample at a controlled rate, whilst measuring the force required to penetrate the sample.

CBR Formula

The crude birth rate (CBR) is equal to the number of live births (b) in a year divided by the total midyear population (p), with the ratio multiplied by 1,000 to arrive at the number of births per 1,000 people.

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