CONSISTENCY OF BITUMINOUS MATERIAL

Consistency carries a great importance in the selection and use of bituminous materials. Role of consistency is two fold.

1. Effect of temperature on consistency and  
2. Consideration of consistency at some specified temperatures. This helps in grading various materials. 

To understand the concept of consistency and temperature: relationship, consider two semi-solid bitumen. One of them is said blown bitumen and the other regular paving grade straight run bitumen. Both bitumen at 25°C have the same penetration. If each of these bitumen are heated to 46°C arid tested for penetration it will be found that the consistency of straight run bitumen is much less than that of blown bitumen. When both are further heated the difference in consistency becomes even more pronounced. Straight run bitumen will become a liquid where as the-other is still in semi plastic state. At about 121°C, bitumen will become thin liquid where as blown bitumen will not acquire the same fluidity. At nearly 177°C both become equally thin fluids. Now if both the bitumen is allowed to cool to 0°C, paving bitumen will become much harder than blown bitumen. Thus it can be said that the consistency of the paving bitumen is affected more by temperature changes than that in blown bitumen. This property of bitumen is called temperature susceptibility. This property of bitumen differs for the bitumen obtained from different crude oils, but this change is minor, com­pared with that exist between blown and paving bitumen

BITUMEN-RUBBER

An experimental pavement bound with a bitumen-rubber mixture was laid in Holland in 1929. The first use of this binder in the United States was made in 1947 when a section was laid Akren, Ohio. The pavement was conventional, except that finely divided rubber amounting to 5 – 7.5 % of the bitumen by weight was included since that time experimental roads have been laid by (among others) the state highway departments of Virginia, Ohio, Texas, Massachusetts, California, Colorado, and Utah the cities of New York and Baltimore and the great Britain. In addition, bituminous binders modified with rubber have used for seal coats.

Certain advantages are attributed to rubber additives. For example, skid tests in Virginia, reported in 1950, showed very little improvement in coefficients of friction on newly laid pavements but considerable advantage after six months early test by the Bureau of public roads indicated both favorable and unfavorable results. Rubber added in powdered form brought unfavorable consequence, when preblended with the asphalt, it improved the stability of some but not all laboratory specimens. Recently, greater elasticity reduces temperature susceptibility and brittleness and longer life in the pavement have been claimed. In sum however the conclusions of a 1954 analysis by the Bureau of public roads still appears to be valid. It stands than an appraisal of the real economic value of the addition of rubber to asphalt must wait on further observation of the behavior of experimental pavements under the influence of age, weather and traffic.