Crude asphalts as they occur in nature usually con, tain considerable water, which needs to be removed before the material can be used. In order to remove this water and any vegetable impurities which the crude material may contain, the asphalt is refined by heating sufficiently to vaporize the water and melt the bitumen. This is accomplished either by the use of a large kettle heated directly by fire, or by passing steam through pipes inside the tank containing the asphalt. During the heating the material is agitated by a current of air or steam. When the water has been driven off, and the material is thoroughly melted, the liquid asphalt is drawn off and is known as refined asphalt. Some asphalts occur in a practically anhy drous condition and do not need refining.
Refined asphalt is brittle at ordinary temperatures and possesses little cementitious value. To bring it to a proper consistency it is heated to a temperature of about 300° F. and mixed with heavy bituminous oil, which serves as flux. The product is then known as asphalt cement.
Fluxes. The material commonly used to soften asphalt in preparing paving cement is the oil residuum resulting from the distillation of petroleum. In the distillation of the petroleum the lighter oils are driven off, the petrolenes being removed. The material re maining is composed of bitumens similar in character to those of the asphalts, and when carefully prepared, almost entirely malthenes (soluble in 88° naphtha solution). These residuums differ considerably in character according to the nature of the petroleum from which they are prepared and the care used in the preparation. Those prepared from the California asphaltic petroleums differ from the residuums of Eastern petroleums in containing less saturated hydro carbons, and in having no paraffines, the presence of which characterize most of the other residuum oils.
Natural malthas have sometimes been used as fluxes for asphaltic cement, but in most instances difficulty has been met in their use due to the fact that they contain considerable of the lighter oils, petrolenes, which are volatilized at the temperature required for mixing, thus leaving the maltha too hard to act satisfactorily as a flux. The same difficulty is met in
use of carelessly prepared petroleum residuum, which may lack uniformity of composition, and contain both volatile oils and hard asphaltenes.
The amount of fluxing materials necessary to give proper consistency to asphalt cement depends upon the density of the oil as well as the nature of the asphalt. With dense and heavy oils a larger amount of oil must be used to reach the same consistency than with light oils, and the stability of the mixture is greater when maintained in heated condition for a long time, on account of the less volatile nature of the oils. For Trinidad and Bermudez asphalts the amount of resid uum oil required may vary from about 15 per cent to 25 per cent of the weight of the asphalt. For harder asphalts a larger quantity is required, and where the amount of malthenes in the bitumens is low, a high percentage of dense oil may be required in order to give a proper relation between the mal thenes and asphaltenes in the resulting asphaltic cement. • Preparation of Cement. In preparing asphalt cement the asphalt is first melted and raised to a temperature of about 300° F. The flux is then added at a tempera ture of 150° to 20o° F. The mass is then agitated with jets of steam or air. The agitation is continued from 4 to 8 hours, or until the mass comes to a uni form and homogeneous condition. The refined asphalt cement is then drawn off.
Careful and expert manipulation is necessary to secure a uniform product of proper consistency. Continued agitation with air causes hardening of some of the bitumens and volatilizes some of the lighter oils. The consistency is commonly tested by chewing in the mouth a small piece of the asphalt cement which has been cooled in water. For accurate deter minations, the penetration is determined, at standard temperature, by a penetration machine (see Art. 58).