The California Bering Ratio (CBR) test is a penetration test developed by the California Division of Highways widely used method for designing the flexible pavements and the thickness of the pavement elements is determined by the CBR value.
It is the ratio of the force per unit area required to penetrate the soil mass with a circular plunger of 50 mm diameter at the rate of 1.25 mm/ minute required for the respective penetration into a standard material.
The ratio is usually set for penetration of 2.5 mm and 5 mm.
Significance of CBR Test:
- CBR test is used to analyze existing pavements, layer by layer with respect to their strength and load carrying capacity.
2. It also helps identify the causes of load failure of pavements.
3. CBR values are usually calculated for penetration of 2.5 mm and 5 mm.
- The CBR value at 2.5 mm penetration will be higher than the 5 mm penetration taken as the CBR value for design purposes.
- If the CBR value is greater than 2.5 mm corresponding to a 5 mm penetration, the test is repeated.
Procedure for CBR Test:
1. Representative sample preparation:
This procedure involves following steps as follows:
1. Sieve the soil obtained from the farm using 19 mm IS sieve, soil passing through 19 mm IS sieve is used for sample preparation.
2. Weight the portion retained on 19 mm IS strainer and discard it.
3. Take soil equal amount soil passing through the 19 mm IS strainer, but retained on 4.75 mm IS strainer.
4. Add it to the soil passing through the 19 mm IS sieve, so that allowance can be made for materials larger than 19 mm.
5. This gives a representative soil sample, you should have about 5 kg of this sample of cbr test.
2. Preparation of specimen:
Sample prepared at the field density or maximum dry density for preparing a sample in MDD, it is necessary to know the MDD and the corresponding OMC of the soil sample.
This procedure is to prepare a remolded sample by dynamic compaction using the compaction method:
1. Take the accurately calculated mass of soil from the representative sample, required to prepare the sample in MDD and add the calculated amount of water to bring the sample to the OMC, mixing it well.
If an appreciable amount of clay is found in the sample, it is better to keep mixed with water for 18 to 24 hours.
This will ensure uniform distribution of water throughout the soil mass in cbr test.
2. Remix the soil before compacting and make three equal parts.
3. Place the spacer disc on the perforated base plate and place a filter paper on the spacer disc.
4 Weigh the empty mould and apply a thin layer of oil inside the mold, it will make it easier to remove the soil from the mould.
5. Clamp the mold with the collar on the base plate.
6. Fill one part of soil mixed together and compact it evenly using a rammer giving 56 blows, distribute the blows evenly on the cross-section.
Similarly, compaction to the two more layers, giving 56 blow to each layer use a mechanical rammer (if available).
7. Remove the extension collar and carefully trim the level of compact soil along the top of the mold, compacted soil should not spread more than 5 mm from the top of the mould.
8. Patch any holes that have developed while trimming the top surface of the soil in cbr test.
9. Remove the mold and weigh it with the sample, this will help to calculate the bulk and dry density of the sample.
10. Also, remove the spacer disc and place a filter paper on the perforated base plate.
Turn the mold upside down and fasten it in contact with the filter paper on the base plate with the soil surface.
3. Penetration test:
This procedure involves following steps as follows:
1. Keep the mould on loading machine and place the annular surcharge weight on top of the soil.
2. By manually operating the loading machine, bring the piston to the soil surface and the piston should be seated under a load of 4 kg which establishes complete contact between the specimen surface and the piston.
3. Now place additional surcharge load over the annular weight.
4. Adjust the plunger of the gauge, measuring the penetration of the piston on a hard surface preferably on the mold wall, set it to zero.
5. Set the other dial gauge measuring the load also to zero.
6. Adjust the loading machine to give a penetration rate of 1.25 mm / min, then start it for the cbr test.
7. Record the load dial gauge reading at penetration of 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5 and 10.0 mm.
8. Turn the machine off when the penetration reaches 12.5 mm, if the maximum load occurs before penetration of 12.5 mm, record the load and penetration.
9. Raise the piston by manual operation and remove the mould.
10. Repeat the penetration test as a check test on the back side of the sample.
11. Collect three samples of about 50 grams, each in moisture cans, from the top 30 mm layer of the specimen and determine the mean moisture content.
Precautions during CBR Test:
1. Care must be taken to take a representative sample for cbr test.
2. The annular surcharge weight should be placed on the specimen in such a way that the plunger does not touch the edges of the central hole of the weight.
3. Before testing, check that the penetration rate is 1.25 mm / min.
Also read: Slump Test & Rock quality designation
The CBR test is performed by measuring the pressure required to penetrate the soil sample with a plunger of standard area.
Typically, a value of 2% is equivalent to clay, while some sand may have a CBR value of 10% and a high-quality sub-base will have a value between 80–100% (maximum).