Table 13-4.-Penetration Grades and AP Numbers of Asphalt Cement
bituminous cement are to hold the aggregate particles
together and to seal the surface, which then resists the
penetration of water.
Bituminous cements are available in several forms
suitable for different procedures of mixing or
application under wide variations in temperature. Some
asphalts and tars are solid or semisolid at room
temperature. Other grades are relatively viscous (thick)
liquids at room temperatures, and all become liquid at
higher temperatures. Mixing bitumens with petroleum
solvents or water produces cutbacks or emulsions that
become liquid at atmospheric temperatures. Such liquid
asphalts and tars are used for cold mixes or applied as
sprays in building pavements.
ASPHALTS.— Asphalts may be natural or manu-
factured and they maybe solid, semisolid or liquid in
consistency. Natural asphalts occur in lakes (as lake
asphalt), pits, or rock structures (as rock asphalts).
Asphalt cement is one of the by-products from the
refining of crude petroleum.
Generally, the military engineer depends upon the
manufactured asphalts that are obtained when crude
petroleum is refined for the purpose of separating the
various fractions (fig. 13-26). The crude oil vapors are
separated into gasoline, kerosene, and fuel oils, and the
residue is asphalt cement and lubricating oils. The
longer the process and the higher the temperatures, the
harder the residue becomes because of the increased loss
in volatiles.
Asphalt cement is commercially available in
different standard ranges of consistency (grades). The
ranges for the penetration grade are based on
measurements by the penetration test in which the
relative hardness of asphalt cement is determined by the
distance that a standard needle, under a standard
loading, will penetrate a sample in a given time under
known temperature conditions. The asphalt petroleum
(AP) number is a number from 00 to 7 that is assigned
to these penetration ranges. Table 13-4 lists the ranges
presently recognized along with relative consistencies
corresponding to those ranges.
Asphalt cement is also graded on the basis of
viscosity, using special testing equipment (not in the
Naval Construction Force Table of Allowance) to
measure the time that a given amount of liquid asphalt
material will flow through a tube of standard dimensions
under rigidly controlled temperature and pressure
conditions. Multiplying that measured time by a
calibration factor for the equipment gives a numerical
designation called kinematic viscosity, measured in
stokes (square centimeters per second) or centistokes
(stokes 100).
The viscosity grades of asphalt cement are available
in two series. One series includes grades AC-2.5, AC-5,
AC-10, AC-20, and AC-40. The other series includes
grades AR-1000, AR-2000, AR4000, AR-8000, and
AR-16000. Normally, but not always, the lower
viscosity-graded asphalts correlate with the softer
asphalts having higher penetration values, and the
higher viscosity-graded asphalts correlate with the
lower penetration grades.
Since all asphalt cements are solid or semisolid at
room temperature (77°F), they must be converted in one
of three ways to a fluid state before they can be pumped
or sprayed through pipes or nozzles and be mixed with
aggregate. One way to liquify an asphalt cement is to
heat it. Then, when it cools, it becomes a semisolid
cementing material. The other methods to liquify
asphalt cement are dissolution (producing cutback
asphalt) and emulsification (producing emulsified
asphalt).
CUTBACKS.– When asphalt cement is dissolved
in volatile petroleum solvents (called cutterstock or
flux oils), the resulting liquid is known as cutback
asphalt. The idea behind cutback asphalt is that upon
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