California Bearing Ratio (CBR) test
History of CBR Test
The California Bearing Ratio (CBR) test was developed by the California State Highway Department. As a method for evaluating the strength of subgrade soil. Also, other pavement materials for the design and construction of flexible pavements. The CBR test results have been correlated with flexible pavement thickness requirements for highways and air fields. Being an empirical test method, CBR test results cannot be related accurately with any fundamental property of soil tested. The CBR method of test has also been standardised by the Bureau of Indian Standards (BIS).
Method of CBR Test
The CBR test denotes a measure of resistance to penetration of a soil or flexible pavement material, of standard plunger under controlled test conditions. The CBR test may be conducting in the laboratory generally, on re-moulded specimens. The test may also be conducting on undisturbed soil specimens. The laboratory test procedure should be strictly adhering if a high degree of reproducibility is desire. Procedure for field determination of CBR value of soil in place or in-situ has also been develop and standardised by different agencies including the BIS.
The basic principle in CBR test is by causing a cylindrical plunger of 50 mm diameter to penetrate into the specimen of soil or pavement component material at a rate of 1.25 mm per minute. The loads requiring for 2.5 mm and 5.0 mm penetration of the plunger into the soil are record. The CBR value of the material testing is express as a percentage of standard load value in a standard material. The Standard load values have been establishing based on a large number of tests on standard crushed stone aggregates. This is perform at the respective penetration levels of 2.5 and 5.0 mm. These standard load values given below may directly be using to compute the CBR value of the test material.
Determination of California Bearing Ratio (CBR) Test value in the laboratory
Apparatus
The laboratory CBR apparatus consists of :-
- Mould 150 mm diameter
- Base plate
- Collar
- A loading frame with the cylindrical plunger of 50 mm diameter and a dial gauges for measuring the expansion on soaking and the penetration values.
The specimen in the mould is compacting to a dry density corresponding to the minimum state of compaction likely to be achieve in practice. In the absence of information the specimens may be compacting to maximum dry density at the Optimum Moisture Content (OMC). IS heavy compaction as per IS: 2720 Part VIII preferring for high traffic roads like expressways and national and state highways: however IS light compaction as per IS: 2720 Part VII may adopted for low volume roads. The specimen is subject to four days soaking and the swelling and water absorption values are note.
The surcharge weight is place on the top of the specimen in the mould and the assembly is place under the plunger of the loading frame. The load values are note corresponding to penetration values of 0.0, 0.5, 1.0, 1.5.2.0, 2.5, 3.0, 4.0, 5.0, 7.5,10.0 and 12.5 mm. The load – penetration graph is plot as shown in below. Alternatively the load values may be converting to pressure values and plotted against the penetration values.
Two typical types of load –
penetration curves may be obtain as shown in Fig. The normal curve is with convexity upwards as for specimen no.1 and the loads corresponding to 2.5 and 5.0 mm penetration values are note. Sometimes a curve with initial upward concavity is obtaining, indicating the necessity of correction as can be seen for specimen no. 2 in the graph. In this case, the ‘corrected origin’ is establishing by drawing a tangent from the steepest point on the curve, to obtain the corrected origin. The load values corresponding to 2.5 and 5.0 mm penetration values from the corrected origin are note.
The CBR value is calculate using the relation –
The causes for the initial concavity of the load-penetration curve calling for the correction in origin are due to: (i) the top layer of the soaked soil is too soft or slushy after soaking in water (ii) top surface of the soil specimen is not even and (iii) the penetration plunger of the loading machine is not vertical; therefore the bottom surface of the plunger is not horizontal and is not fully in contact with the top surface of the specimen.
Normally the CBR value at 2.5 mm penetration is higher than that at 5.0 mm. And the higher value is reporting as the CBR value of the material. However if the CBR value obtaining at 5.0 mm penetration is higher than that obtaining at 2.5 mm. Then the test is to be repeat for checking. If the check test again gives similar results. The higher value obtaining at 5.0 mm penetration is report as the CBR value. The average CBR value of three test specimens is report as the CBR value of the material. To the first decimal place. If the variation in CBR value between the three specimens is more than the prescribing limits. Tests should repeated on additional three samples and the average CBR value of six specimens accepted.
Conclusion –
The CBR test is essentially an arbitrary strength test and hence cannot be using to evaluate the soil properties like cohesion or angle of internal friction or shearing resistance. Unless the test procedure is strictly follow, dependable results cannot be obtain. Presence of coarse grained particles would result in poor reproducibility of CBR test results. Material passing 20 mm sieve is only using in the test.
Field CBR test is carry out using in-situ penetration equipment. In-situ tests and not generally recommending for design purposes, as it is not possible to satisfactorily simulate the critical conditions of dry density and moisture content in the field.
Applications of California Bearing Ratio (CBR) test in flexible pavement design
Several agencies in different countries generally have standardised CBR test method. And have developed charts for design of flexible pavements for roads, however runways based on CBR values of subgrade soil materials. CBR test as well as CBR method of flexible pavement design are simple. Their performance studies of these pavements have extensively investigated and found to generally satisfactory. The Indian Roads Congress (IRC) has standardised guidelines for design of flexible pavements based on CBR test (IRC: 37 – 2001). This method being followed for the design of flexible pavements for all the categories of roads in India and also in other countries.
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