Home >> Textbook-on-roads-and-pavements-1908 >> Abrasion Test to Street Railway Track >> Bitumen


hydrocarbons, material, soluble, bitumens, classes, asphalt and asphaltenes


The bitumen which forms the essential part of asphalt consists of a mixture of hydrocarbons of various compositions and in varying proportions. It is not, therefore, a compound of definite composition, and little can be inferred, from its ultimate analysis as to the character of the material. These hydrocar bons are divided into classes according to certain of their physical and chemical properties, and the char acter of a bitumen is judged by determining the per centages of each class of hydrocarbons that it contains. This division into classes is purely arbitrary, and the details of the methods of analysis must be carefully specified and made uniform in order that the results of tests may be comparable for different bitumens.

Bitumen is separated from the other constituents of the asphalt by -its solubility in cold carbon bisul phide, any organic matter not soluble being considered foreign and not classed as bitumen. Sometimes chloro form or turpentine is used as a solvent for this purpose, giving usually a somewhat higher percentage of bitu men in the material.

The constituents of bitumen have commonly been divided into two classes, the first of which, called petrolene, is the oily and cementitious material; the other, called asphaltene, is the hard material lacking in cementing properties. These were separated by the solubility of the petrolene in naphtha. Mr. Clifford Richardson* has, however, proposed a more extended classification, which gives a better definition of the character of the material, and has also outlined in detail the methods he uses in their determination.

Petrolenes. Mr. Richardson limits the term "petro lenes" to those hydrocarbons which are volatilized at 325° F. in 7 hours. The petrolenes, thus defined, are found to but small extent in the asphalts used in paving, varying from about i per cent to 6 per cent of the total bitumen.

Malthenes. The name "malthenes" is applied to bitu mens soluble in naphtha solution (of density 88 Baume) on account of their resemblance to the malthas. These hydrocarbons comprise usually 6o per cent to 71 per cent of the total bitumen in asphalts commonly used for pavements. They are the heavy oils which tend to give plasticity to the asphalt, and lower its melting point.

Asphaltenes. Asphaltenes are defined as those hydro carbons not soluble in naphtha solution but soluble in cold carbon tetrachloride. These form a hard brittle

substance, which will not melt without decomposition, when heated in the absence of malthenes. The bitumens of the asphalts in common use are composed almost entirely of the malthenes and asphaltenes, the relative proportions between the two determining the consistency of the material and its availability for, and method of use.

Carbenes. Hydrocarbons soluble in carbon bisul phide but insoluble in cold carbon tetrachloride are classed as carbenes. These bitumens have hardened through exposure or other causes to a greater extent than the asphaltenes and thus become less soluble in the oils. They enter to a very limited extent into the materials used for asphalt pavements, but sometimes occur in bitumens of similar character and serve to distinguish between these classes of materials.

Saturated Hydrocarbons. The distinction between saturated and unsaturated hydrocarbons is drawn by observing the action of sulphuric acid upon the hydro carbons soluble in 88 degrees solution of naphtha; the saturated hydrocarbons being unacted upon while the unsaturated ones are removed from the solution by the acid.

Separation of Bitumens. As already stated, each of these classes consists of complicated mixtures of hydro carbons of varying compositions and properties, and the line between them is arbitrary and indefinite. The tests used, therefore, to separate them must be exactly defined and made with precision in order to give comparable results, and, while they may be employed with confidence to control the use of an asphalt of known properties, they cannot be conclusive as to the value of an untried material.

These bitumens being composed of a mixture of a number of hydrocarbons of different melting points, do not melt at any particular temperature, but gradu ally change in consistency with change of temperature, softening and finally flowing as the heat increases. It is common to determine arbitrarily certain temper atures which are known as the softening and flowing points of the material. The consistency or softness of the bitumen is commonly determined by the depth of penetration, in millimeters, of a needle at standard temperature (usually 78° F.).