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.

HISTORICAL BACKGROUND OF BITUMEN

Bituminous road materials have used for well over a century in the construction and maintenance of roads in the United Kingdom. Up to about 1900 most of rural roads were constructed of layers of broken stone, following the general scheme of Telford or macadam, and were classified as water bound macadam. Even as early as the1830’s, however, experiments were being made to replace the wet fine aggregate use as  the cementing mortar in water-bound macadam by a more effective adhesive, namely the coal tar then becoming available as a by-product from the carbonization of coal in gas works. This tar could be used either to make tar macadam or as a ‘Surface dressing’ on the water-bound macadam roads. In surface dressing the tar was sprayed over the road surface as uniformly as possible and covered with a layer of sound, and in later work with stone chipping or gravel.

Surface dressing is still the principal method of maintaining road surface and over half of all the roads in this country have a surface dressing as the running surface coated macadam forms part of the structure of most roads in Great Britain, and is used as the running surface of a substantial of them. Some ten million tons of coated macadam are now used annually for road construction and maintenance here.

Natural rock asphalt was first used for surfacing city streets in the middle of the nineteenth century and its use increased greatly later in the century. In America, in about 1870, experiments were made with ‘synthetic’ asphalt’s as substitutes for the natural material; fine mineral matter was mixed with fluxed lake asphalt and later with the bitumen made available by the development of the petroleum industry. These ‘synthetic’ mixtures, develop to a great extent by Clifford Richardson in the 1890’s, are the basic of the modern rolled-asphalt surfacing which has long held the pre-eminent position as a heavy-duty surfacing for heavily trafficked trunk roads and urban streets.

One of the principal developments in the last twenty years has been the widespread mechanization of all forms of bituminous road surfacing. Mixing plants have been automated; premixed materials are largely laid by a traveling machine, and in surface dressing both the binder and the chipping are now applied mechanically. Much more rigid control of the quality of the material and the technique of application is therefore essential for the maintenance of the high standard of road construction that is traditional in the United Kingdom.

INTRODUCTION OF BITUMINOUS MATERIAL

Bituminous mixes are most commonly used all over the world in pavement construction. They were being used as a mortar and water proofing agent; as early as 3800 B.C. Early bitumen was of natural origin, found in pools and lakes. Many of these pools and lakes exist even today. The bitumen lake on the Trinidad island and the Bermudez deposit in Venezuela, are the largest known sources of bitumen supply. Most of the roads are constructed in Bangladesh by bituminous materials.

In different parts of the world, bitumen is also found in porous rocks such as sandstone and limestone. These bitumen-impregnated rocks have been of limited commercial value, because of the range of bitumen content. Gilsonite is another form of natural bitumen rock and is related to rock asphalt. It is a form of asphalt or bitumen occurring in rock crevices or veins.

Many theories have been put forward to explain the formation of petroleum and bitumen. But it has now been accepted that these materials were produced by decomposition of Dead Sea organisms such as Alge, molluscs, radiolaria, and probaly fish also. The organic matter changed chemically to^, hydrocarbons, which constitute crude petroleum. Due to distortion in earth's crust, crude petroleum deposits got exposed to evaporation. On exposure, lighter oils and gases were driven off, leaving behind a residue called natural asphalt which may be in the form of lake asphalt or rock asphalt. Crude petroleum is obtained by tapping the under ground reservoir by drilling.

There has been some confusion in regard to the terms `Asphalt' and `Bitumen’. In U.S.A. the term `asphalt' is used to refer to both the products manufactured from crude petroleum in the refinery and also to the natural or artificial mixture in which it is associated with inert mineral matter. But else-where in the world including India, the refinery product is termed bitumen and the mixture of bitumen and inert mineral matter as `asphalt'.

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.