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CHARACTERISTICS AND USES OF BITUMENS

Selection of a particular bituminous material depends upon the type of pavement, climatic conditions, seasonal factors, and availability of equipment. In general, soft penetration grades of asphalt cement are preferred for use in cold climates, medium grades in moderate climates, and hard grades in warm climates. Heavier grades of asphalt cutbacks and tars are normally used in warm weather and lighter grades in cold weather. Tables 13-6 and 13-7 list the bituminous materials, sources, curing, temperatures, and grades associated with bituminous operations.

Figure 13-29.-Field identification of unknown bituminous materials.

Advantages and disadvantages of the bituminous materials used in construction are discussed. Asphalt cement cutbacks are flammable. Also, all asphalts can be dissolved by petroleum products that may be spilled on them, such as during refueling at an airfield. Tars, on the other hand, are not affected by petroleum derivatives since they are products of coal. Tars, when used as a prime for base courses, also seem to possess better penetration qualities than asphalts. Tars are very temperature-susceptible, having a wide range in viscosity with normal ambient temperature changes. Tar can become so soft during warmer weather that the pavement will rut under traffic. In colder weather it can become so brittle that the pavement will crack. Asphalt emulsions are not flammable and are liquid at normal temperatures. Since they are mixed with water, they can be used with a damper aggregate than required for the cutbacks. Additional water may be added to the emulsion up to proportions of 1:3 for use in slurry seal coats. Because emulsions contain water, they have certain disadvantages. During freezing weather the emulsions can freeze, and the components can separate. Also, emulsions are difficult to store for extended periods because they tend to "break" even in unopened drums. When emulsions are shipped, the water in the mixture takes up valuable space, which could be used to transport hard-to-obtain materials.

FIELD IDENTIFICATION OF BITUMENS

A fairly accurate identification of bituminous materials is necessary to make decisions regarding types and methods of construction, types and quantities of equipment, and applicable safety regulations. Field tests must be performed to identify a bituminous material as asphalt cement, asphalt cutback asphalt emulsion, road tar, or road-tar cutback and to field-identify the grade. Field identification of bitumens is summarized in figure 13-29.







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