Table 8-2.-Penetration Grade and AP Number of Asphalt Cement
Table 8-3.-AsphaIt Cutback Composition (Expressed in Percent of Total Volume)
from 40 (hard) to 300 (soft) (table 8-2). The number is
derived from a penetration test that is the distance that
a standard needle penetrates the asphalt cement under a
standard loading weight, in a given time, under known
temperature conditions.
All asphalt cements are solid or semisolid at room
temperature (77°F) and must be converted to a fluid for
mixing with aggregate or for spraying. Asphalt cement
must be heated to a temperature ranging from 250°F to
350F, depending upon the grade of the asphalt cement.
The various penetration grades of asphalt cement
are suitable for different uses, such as plant mixes,
penetration macadam, and surface treatment. Soft
penetration grades of asphalt cement are preferred for
use in cold climates, medium grades in moderate
climates, and had grades in warm climates.
Asphalt Cutbacks
The special equipment needed to heat asphalt
cements is not always available. Since asphalt must be
in a fluid condition to spray or to mix with an aggregate,
the solid asphalt cement would not be suitable. Asphalt
cement (AC) can be made fluid by adding solvents
called Cutterstock or Flux Oil. Cutterstock maybe any
one of the more volatile petroleum distillate products.
The resulting combination is called Asphalt Cutback.
Exposure to air causes the petroleum distillate to
evaporate and leave the asphalt cement to perform its
function.
The rate of evaporation determines the type of
asphalt cutback that is in the mixture. Gasoline or
naphtha (highly volatile) will produce a rapid-cure
cutback (RC) with a curing time of 4 to 8 hours; kerosene
(medium volatility) will produce a medium-curing
cutback (MC) with a curing time of 12 to 24 hours; and
a fuel oil (low volatility) will produce a slow-curing
cutback (SC) with a curing time of 48 to 60 hours, Table
8-3 shows the percentage of components by grade for
the three types of asphalt cutbacks.
8-20
