Procedure for CBR Test, results and uses.

 CBR Stands for California Bearing Ratio, introduced by the California Division of Highways to determine the stability of soil sub-grade. In this post, we will learn about the procedure for the CBR test. CBR test is used for the design of flexible pavement. The thickness of pavement and its component layers is determined by using the results of this test.

What is CBR Value?

The ratio (in %) of force per unit area required to penetrate a soil sample with a cylindrical plunger of 50 mm diameter at the rate of 1.25 mm/min to that required for corresponding penetration in a standard material (crushed aggregates) is called CBR value.

The ratio is usually determined for penetration of 2.5 and 5 mm. If the ratio at 5 mm is consistently higher than that at 2.5 mm, the ratio at 5 mm is used.

 How CBR Test is performed?

The test can be conducted on either undisturbed samples or remolded samples and can either be conducted in the laboratory or in the field. Laboratory testing consists of CBR testing equipment/apparatus having a mould of 150 mm diameter with a collar, stay rod, and base plate.

The dial gauge is attached with this to measure penetration of the sample or expansion after soaking. The loading frame (having a capacity of at least 5000 Kg) consists of a cylindrical plunger of 50 mm diameter and arrangement for measurement of the load.

CBR value is determined by penetrating this 50 mm cylindrical plunger into the soil sample at the rate of 1.25 mm/minute and load at 2.5 mm penetration and 5.0 mm penetration is noted by proving ring.

CBR value is expressed as a percentage of standard load value which is obtained after numerous tests on coarse aggregates.

What is this standard value? the load which has been obtained from the test on the crushed stone is defined as having a California Bearing Ratio of 100 percent. This load is the Standard value.

Standard values are 1370 (70 kg/cm2) kg for 2.5 mm penetration and 2055 (105 kg/cm2) kg for 5.0 mm penetration.

CBR testing machine demonstrating the procedure for cbr test
CBR Testing Machine

The Specimen is subjected to four days of soaking and swelling and water absorption values are noted. Surcharge weight is placed on the mould and the plunger is kept at surface level. Then the load is increased at the rate of 1.25 mm/ minute and load is measured at penetration 0, to 12.5 mm, and then load vs. Penetration curve is plotted. 

How to Prepare Test Specimen for CBR Test?

1. Undisturbed Specimen

First, the surface from where the sample is to be taken is cleaned and grass, unwanted material etc. is removed. A cutting edge is then fitted into the mould and then mould is inserted into the ground. Keep inserting the mould until it gets filled with the soil sample.

Then soil around the mould is dug and mould is taken out, top and bottom of the mould is then leveled.

2. Remolded Specimen

For remolded specimen soil shall pass through the 19 mm sieve and shall be compacted at either the field density or maximum dry density or dry density at which CBR is desired. There are two ways for compaction.

(i) Statically compacted

In that case, we have to estimate the mass of soil required at certain moisture content to give desired density. Then the soil is mixed with calculated water and then placed in the mould. A filter paper is placed between the disc and the soil and then compaction is obtained by pressing the displacer disc.

(ii) Dynamically Compacted

In this case, approximately 4.5 kg or more soil (if fine-grained) and 5.5kg or more (if granular) are taken and then thoroughly mixed with the water and compacted to the desired density.

Procedure for CBR Test

(i) Test for swelling

  • A filter paper is placed on the soil sample and then an adjustable stem and the perforated plate are placed on the soil sample.
  • Surcharge (around 2.5 kg) to imitate the soil condition at the site (weight of the pavement) is placed on the mould.
  • Dial Gauge for measurement of the swelling is attached and then the whole assembly is immersed in water for 96 hours to ensure proper soaking of the material. Measurement of swelling is then taken every day.
  • After the end of the soaking period, the final reading of the gauge is noted and mould is then taken out of the water and allowed to drain for 15 minutes. Surcharge weight, perforated plate, and filter paper is then removed and the weight of the mould is noted.

(ii) Penetration test

  • Mould is first placed into the testing machine and then surcharge weight is placed. The Surcharge weight should be sufficient to imitate the weight of base material and pavement at the site.
  • The surcharge is sufficiently pressed against the soil sample to prevent upheaval of the soil into the hole of surcharge weight.
  • Load and deformation gauges are then set to zero.
  • Reading of the load shall be taken at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, 10.0 and 12.5 mm. Care should be taken to note the maximum load that is occurring even before penetration of 12.5 mm.
  • The Plunger is then lifted and mould is taken out. Around 20 to 50 g of the soil sample is taken out of the mould to determine the water content.
  • The same procedure is then repeated for 2 other samples.

CBR Test Results

Expansion ratio

Expansion Ratio = ( df-di)/h
df =final dial gauge reading in mm,
di =initial dial gauge reading in mm, and
h=initial height of the specimen in mm.

This is used to identify the potential expansiveness of the soil.

Load Penetration Curve.

Load vs Penetration curve is plotted. This curve is usually convex upwards sometimes the initial portion of the curve may be convex downwards due to surface irregularities.

In that case, a correction is applied by drawing a tangent to the point of the greatest slope so that zero penetration is taken as the point where the tangent cuts the axis of penetration.

CBR Value (%) = Load sustained by the sample at 2.5 or 5mm penetration / Load sustained by Standard sample at 2.5 or 5mm penetration.

Load vs penetration curve in CBR Testing

CBR value at 2.5 mm penetration is usually higher than CBR at 5.0 mm penetration but sometimes, CBR value at 5.0 mm penetration appears more. In that case, the sample is tested again and if the same results appear CBR at 5.0 mm is noted at the CBR of the sample. 

The average value of 3 sample tests is noted as CBR of the sample. If the difference between the CBR Value is more than the prescribed limit then additional three tests are done and the average of 6 samples is taken as the CBR value of the sample.

Use of CBR Test

To determine the strength of Soil sub-grade, sub-base, and base course. Through this thickness of these layers is determined for flexible pavement design.

Procedure for CBR Testing

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