Figure 8-9.-Gradation control unit.
transferred to a storage silo. This material is often used
in the hot mix.
When the material removed from the dust collector
can be recombined satisfactorily with the aggregates in
the mix, some or all of it may be returned to the plant.
The amount returned depends upon the combined
grading of the finished mix. When the collected dust is
unsatisfactory or is prohibited by the mix specifications,
it is removed from the bottom of the collector and
wasted.
Hot Screens
After the aggregates have been heated and dried,
they are carried by a hot elevator to the gradation unit,
In the gradation unit, the hot aggregate passes over a
Figure 8-10.-Screen wear.
series of screens. The function of the screens is to
separate the hot aggregate into the specified sizes
accurately and deposit those sizes in hot bins.
The gradation control unit (fig. 8-9) or screening
unit includes a set of several different-sized vibrating
screens. The top screen is a scalping screen that rejects
and carries off oversized aggregates. This is followed by
one or two intermediate-sized screens, decreasing in
size from the top to bottom. The very bottom screen is
normally a sand screen. The effective screening area
must be large enough to handle the maximum amount
of feed delivered to separate the hot aggregates properly;
therefore, the capacity of the screens should be checked
against the capacity of the dryer and the capacity of the
pugmill.
When too much material is fed to the screens or the
screen openings are plugged, many particles that should
pass through ride over the screen and drop into a bin
designated for larger sized aggregate. When screens are
worn (fig. 8-10) or torn, resulting in enlarged openings
and holes, oversized material will go into bins intended
for smaller sized aggregate. Fine aggregate misdirected
into bins intended for larger aggregate is know as
“carry-over.”
Carry-over can cause a lack of uniformity in the
aggregate gradation and in the mixture. Additionally,
excessive carry-over adds to the amount of fine
aggregate in the total mix, thus increasing the surface
area to be coated with asphalt. Excessive carry-over, or
its fluctuations, can be detected by a sieve analyses made
from the contents of the individual hot bins and must be
corrected immediately. Corrective measures include the
cleaning of screens, the regulation of the quantity of
material coming from the cold feed, or a combination of
both. Some carry-over is permitted in normal screening;
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