Figure 8-12.-Asphalt return line.
samples cannot be obtained even when the sampling
device is used correctly.
Asphalt Heating and Circulation
Provisions should be made for the circulation of the
asphalt through the feeding and storage system. All
storage tanks, transfer lines, and pumps should have
heating coils and/or jackets to maintain the asphalt at the
required temperature.
Return lines discharging into the storage tanks
should be submerged below the asphalt level in the tank
to prevent oxidation of the asphalt. When the pump is
reversed, two or three vertical slots should be cut in the
return line within the tank to break the vacuum in the
lines. The slots should be cut above the high level mark
of the stored asphalt (fig. 8-12).
To assure temperature control of the asphalt, you
should place an armored thermometer or a pyrometer
with a recorder in the asphalt feed line at a location near
the discharge valve at the mixer unit. Also, the asphalt
storage tank should be equipped with a recording
thermometer, having a minimum time range of 24 hours.
An approved valve or spigot should be installed in
the tank or in the circulating system to provide a means
for sampling the asphalt. Sufficient material must be
drawn and wasted before the sample is taken to ensure
the material obtained is truly representative of the
storage supply.
When the temperature of the asphalt is maintained
by circulating heating oil, the level of the hot oil in the
reservoir of the heating unit should be inspected
frequently. If the hot-oil level falls, check for leakage of
the hot oil into the stored asphalt.
Temperature of Mixture
Both asphalt and aggregate must be heated before
they are combined in the pugmill. The asphalt is heated
to make it fluid enough to coat the aggregate particles.
The aggregate is heated to make it dry and hot enough
to keep the asphalt in a fluid state while it is coating the
particles.
Asphalt is a thermoplastic material that decreases in
viscosity with increasing temperature; however, the
relationship between temperature and viscosity may not
be the same for different sources or types and grades of
asphalt material.
The temperature of the aggregate controls the
temperature of the mixture, and a mixing temperature
normally is specified based on factors relating to
placement and compacting conditions. Another
consideration is the temperature required to dry the
aggregate sufficiently to obtain a satisfactory mix.
Mixing should be accomplished at the lowest
temperature that provides complete coating of the
aggregate particles and a mixture of satisfactory
workability. Table 8-1 provides a guide for suggested
asphalt temperatures ranges.
Mineral Filler
Mineral filler is a fine material (dust) that passes
through the No. 200 sieve during a sieve analysis.
Mineral filler is normally part of the asphalt mix design,
used to fill in the voids of the aggregates. Mineral fillers
commonly used are the following: portland cement,
pulverized limestone (limestone dust), silva, and
hydrated lime.
High production plants often have a separate
feeding system for introducing mineral filler into the
asphalt mix. Part of this system is a storage silo that
maintains several days supply of mineral filler. A
receiving hopper, screw conveyer, and dust elevator are
used to charge the storage silo, and a vane feeder meters
the filler introduced into the mix. The ultimate choice of
this system is usually dependent on the availability of
bulk filler and their price in relation to bagged fines.
In plant operations where the volume of filler
required does not justify a bulk silo, a bag feeding
system is used. This system consists of a ground-
mounted feeder, dust-tight elevator, surge hopper, vane
feeder or screw conveyer, and an overflow chute.
8-10
