Page:Popular Science Monthly Volume 80.djvu/339

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SCIENCE IN HIGHWAY CONSTRUCTION

more particularly the sulphur compounds, of which it is composed. The consistency of such material and its suitability in this regard for use as a road binder is further dependent on the relative proportions of liquid and solid bitumens of which it is composed. It is, of course, the province of the chemist to determine these characteristics for all bitumens proposed for use in road construction, and to interpret the result in the light of practical experience.

From the point of view of their solubility or insolubility in petroleum naphtha the liquid and solid bitumens in use in bituminous highway construction are composed of two components, one of which has been arbitrarily named as a class petrolenes, soluble in naphtha, and the other, asphaltenes, insoluble in naphtha. The one consists of the liquid and the other of the solid components. Whatever value a bitumen may have as a binding material for highway construction, is due to the presence and the character of the petrolenes. The solid material in itself has no binding power, but by its solution in or mixture with the petrolenes it gives to the latter their binding power, and also adds to their stability.

The value of a bitumen as a road binder will further depend upon the character of the petrolenes which it contains. If the petrolenes are of a sticky nature, the particular bitumen will be adhesive and ceraentitious, whereas if they are merely oily and not sticky, the material will lack in cementitious properties. The asphaltenes impart cohesiveness as distinguished from adhesiveness, and supply body or stability, as has been said, to the binding material. As an example it may be cited that the heavy residuum left on the distillation of paraffine petroleums in the preparation of burning and lubricating oils, consists of practically 100 per cent, petrolenes, but these petrolenes are oily and not sticky and adhesive. The same is true of any of the preparations from paraffine petroleums or petroleums containing a considerable amount of hydrocarbons of the paraffine series. It is not in itself a suitable binding material for highway construction as appears, not only for the reasons given, but by its behavior in actual use. The petroleums derived from the asphaltic oils, on the contrary, such as those of Trinidad, California and Mexico, are of a much more sticky character, and are not only in themselves, when reduced to a proper consistency, more suitable for a binding material, but are particularly desirable when used to soften solid and harder bitumens, known as asphalts, which possess great cohesiveness, but are wanting in cementing properties. The relative proportions of sticky petrolenes and cohesive asphaltenes is the most important element in bitumens which are used in the construction of asphalt pavements and bituminous highways. It has been found that the asphalt cement, that is to say, a solid asphalt combined with a suitable flux to provide the proper consistency for practical use, if it