exposure to air the solvents evaporate and the asphalt cement is left to perform its function.

The classification of the cutback is based on the rate of evaporation of the distillate that is in the mixture. Gasoline or naphtha (highly volatile) will produce a rapid-curing cutback (RC); kerosene (medium volatility), a medium-curing cutback (MC); and a fuel oil (low volatility), a slow-curing cutback (SC). At times, reference will be found to road oils, which are one of the SC grades of liquid asphalt, or in effect, a heavy petroleum oil. Table 13-5 shows the percentage of components by grade for the three types of asphalt cutbacks.

As more cutterstock is mixed with a given amount of asphalt cement, a thinner liquid results. In practice, different amounts of cutterstock are added to a given amount of asphalt cement to obtain various viscosities, or grades, of cutbacks. The number assigned to each grade corresponds to the lower limit of kinematic viscosity measured in stokes or centistokes. The upper limit of each grade is equal to twice the lower limit or grade number. Thus a number 70 cutback refers to a viscosity range of 70 to 140 centistokes. The other grades and their limits are 250 (250-500), 800 (800-1600), and 3000 (3000-6000); in addition, the MC has a 30 grade. Figure 13-28 shows the scale of viscosity grades. The higher the viscosity, the thicker the liquid.

Asphaltic penetrative soil binder is a special cutback asphalt composed of low penetration grade asphalt and a solvent blend of kerosene and naphtha. It is similar in character to a standard low viscosity, medium-curing

Table 13-6.-Characteristics of Bituminous Materials

cutback asphalt, but differs in many specific properties. It is used as a soil binder and dust palliative.

EMULSIONS. Emulsification is the third process in which asphalt is liquified. In the emulsification process, hot asphalt cement is mechanically separated into minute globules and is dispersed in water that is treated with a small quantity of an emulsifying agent, such as soap, colloidal clay, or one of numerous other organic agents. The rate at which the asphalt globules separate from the water is called the breaking or setting time. This rate is generally dependent upon the emulsifier used and the proportion of water to asphalt. Based in the breaking time, emulsions are described as rapid setting (RS), medium setting (MS), and slow numbers (fig. 13-28).

Emulsions are also grouped according to their ability to mix with damp aggregate. The RS emulsion breaks so fast that it cannot be mixed; therefore, it is called a nonmixing emulsion. The MS and SS emulsions break slowly enough to permit good mixing; that is, each particle of the aggregate is uniformly coated. Emulsions may be satisfactorily used as a tack coat for bituminous pavements.

ROAD TARS. Tars are products of the distillation of coal. No natural sources of tar exist. Coal tar is a general term applied to all varieties of tar obtained from coal. It is produced by one of several methods, depending on the desired end product. When

Table 13-7.Typical Uses of Bituminous Materials

This tar may vary widely in specific gravities, viscosities, and other physical and chemical properties. Road tars are manufactured in 12 grades of viscosity (figs. 13-27 and 13-28). There are also some special grades for use in rubberized-tar binders. Grades 1 through 7 are liquid at room temperature, and grades 8 through 12 are semisolid or solid. The difference occurs because of different amounts of the liquid coal distillates in the tar; the more distillate, the more liquid (or less solid) the tar. The road-tar cutbacks (RTCBs) are the products of cutting back the heavier or harder grades Tar, which is insoluble in petroleum distillates, is sometimes mixed with oil-resistant, unvulcanized rubber to form a rubberized-tar binder material.