Table 13-5.-Asphalt Cutback Composition (Expressed in Percent of Total Volume)
Figure 13-28.-Viscosity grades at room temperature.
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
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