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.