CALIFORNIA BEARING RATIO (CBR) TEST:-

 

CALIFORNIA BEARING RATIO (CBR) TEST:-This is a penetration test developed by the California Division of Highways, as a method for evaluating the stability of soil subgrade and other flexible pavement materials.

The laboratory CBR apparatus consist of a mould 150mm diameter with a base plate and a collar, a loading frame with cylindrical plunger of 50mm diameter and dial gauges for measuring the expansion on soaking and the penetration values. Briefly the penetration test consists of causing a cylindrical plunger of 50mm diameter to penetrate a pavement component material at 1.25mm/minute. The load values to cause 2.5mm and 5.0mm penetration are recorded. The loads are expressed as percentage of standard load values at respective deformation levels to obtain CBR value. The standard loads values are obtained from the average of a large number of tests on crushed stones are 1370 and 2055 kg(70 and 105kg/cm²) respectively at 2.5 and 5.0mm penetration.The load values are noted corresponding to penetration values of 0.0, 0.5, 1.0, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0, and 12.5mm. The load penetration graph is plotted. Alternatively the load values are converted to pressure values and plotted against the penetration values.

BITUMINOUS MATERIALS:-Bituminous binders used in pavement construction works include both bitumen and tar. Bituminous material used in highway construction may be divided as:

Bitumen: - Bitumen is a petroleum product obtained by the distillation of petroleum crude. The paving bitumen available in India is classified into two categories. (i) Paving bitumen from Assam petroleum denoted as A-type and designated as grades A-35, A-90 etc.

(ii) Paving bitumen from other sources denoted as S-type and designated as grades S-35, S-90 etc. there are different forms in which native asphalts are available. Native asphalts are those which occur in a pure or nearly pure state in nature. Native asphalts which are associated with a large proportion of mineral matter are called rock asphalt.

Properties: - The bitumen should possess the following desirable properties.

(i)                The viscosity of the bitumen at the time of mixing and compaction should be adequate.

(ii)              The bituminous material should not be highly temperature susceptible. During the hottest weather of the region the bituminous mix should not become too soft or unstable.

(iii)            In presence of water the bitumen should not strip off from the aggregate.

Tar: - Road tar is obtained by the destructive distillation of coal or wood. Both bitumen and tar have similar appearance, black in colour though they have different characteristics. Both materials can be used in pavements works. There are five grades of road tars, viz, RT-1, RT-2, RT-3, RT-4 and RT-5, based on their viscosity and other properties. RT-1 has lowest viscosity and is used for surface painting. RT-2 is recommended for standard surface painting under normal Indian climatic conditions. RT-3 may be used for surface painting, renewal coats and premixing chips for top course and light carpets. RT-4 is used for premixing tar macadam in base course. RT-5 is used for grouting purposes which has the highest viscosity.