DUCTILITY TEST OF BITUMINOUS MATERIAL

In flexible pavement construction where bitumen binders are used, it is of significant importance that the binders form ductile thin films around the aggregates. This serves as a satisfactory binder in improving physical interlocking of the aggregates. The binder material, which does not possess sufficient ductility, would crack and thus provide pervious pavement surface. It has been stated by some agencies that the penetration and ductility properties, go together; but depending upon the chemical composition and the type of crude source of the bitumen, sometimes it has been observed that the above statement is incorrect. It may hence be mentioned that the bitumen may satisfy the penetration value, but may fail to satisfy the ductility requirements. Bitumen paving engineer would however want that both test requirements be satisfied in field jobs. Penetration or ductility cannot in any case replace each other. The ductility is expressed as the distance in centimeters to which a standard briquette of bitumen can be stretched before the thread cracks. The test is conducted at 27+/-0.5⁰C and a rate of pull of 50+/-2.5 mm per minute. The test has been standardized by the ISI.

OBJECTS OF BITUMINOUS PAVING MIX DESIGN

According to Asphalt institute design manual, the over all objective for the design of the bituminous paving mixes, to determine an economical blend and grading of aggregates and a corresponding bitumen content which would yield a mix having the following characteristics.

• Mix should exhibit sufficient stability to satisfy the service requirements of the pavement and the traffic conditions, without undue displacements.

• It should have sufficient bitumen content to ensure a durable pavement by coating the aggregates and bonding them together and also by water-proofing the mix.

• Bitumen mix should have sufficient voids in the total compacted mix as to provide a reservoir space for a slight amount of additional compaction due to traffic to avoid flushing, bleeding and loss of stability.

• It should have sufficient flexibility even in the coldest season to prevent cracking due to repeated application of traffic loads.

• The mix should have sufficient workability while placing and compacting the mix. 

• The mix should be the most economical one that would produce a stable, durable and skid resistant pavement.

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.