UFC 3-250-03
15 May 2001
(e) Screening. The aggregate exits the dryer and is carried, along with the returned dust,
up the hot elevator, over the screening deck, and into the hot bins. Screen sizes are selected such that
the oversize material will be rejected and the remaining aggregates are separated into various sizes.
Ideally, the screen sizes should be selected so that the amount of material going into each hot bin is
proportional to the relative volume of that hot bin. For example, suppose that hot bin No. 1 has a volume
of 3 cubic meters (4 cubic yards), hot bin No. 2 has a volume of 1.5 cubic meters (2 cubic yards), and
hot bin No. 3 has a volume of 1.5 cubic meters (2 cubic yards). Screens should be selected so that
50 percent of the material will go into bin No. 1, 25 percent into bin No. 2, and 25 percent into bin No. 3.
(f) Percentage of each hot bin. The percentage of each hot bin to be used in the mixture
should be determined. Samples of each hot bin should be taken and the gradation for each sample
determined. The percentage of each bin should be selected so that the gradation of the combined
materials from the hot bins is equal to the JMF.
(g) Mixing aggregate and asphalt. After the cold feed and hot bins are properly set, the
combined aggregate from the hot bins is mixed with the proper amount of asphalt. The mixing time,
generally 5 seconds for dry mixing and 25 to 40 seconds for wet mixing, should be selected so that all
aggregate particles are coated. The plant should now be set to produce a uniform asphalt concrete
mixture having proper aggregate gradation, asphalt content, and temperature. The batch plant weighs
the various nominal size aggregates and asphalt to produce a batch of material that is then mixed for a
specified period of time.
(2) Drum mixer.
(a) General. The asphalt plant that has become popular throughout the paving industry is
the drum mixer (figure 2-2). The drum mixer is less expensive than the batch plant and generally
produces material at a higher production rate. When a drum mixer is used, the gradation must be
closely controlled at the cold feed bins because no additional screening of the mixture occurs. The drum
mixer is frequently used in the production of recycled hot-mix asphalt as well as conventional hot-mix
asphalt.
(b) Cold feed bin calibration. The cold feed bins are set up much the same way as for the
batch plant, but the drum mixers should have a weight sensor on the aggregate feed belt that weighs a
given length of the loaded belt. The asphalt pump adds binder based on the belt measured weight of
aggregate. Thus, to calibrate the cold feeds, each aggregate can be fed onto the belt at various gate
openings or individual belt speeds, weighed, and the feed rate computed. These steps should be
followed for each of the aggregates to be added to the mixture, and a calibration curve should be
developed.
(c) Dryer. For the drum mixer the burner for the dryer is usually located on the high side
of the drum. The aggregate enters the dryer just below the burner and helps to shield the asphalt binder
from direct contact with the flame. The asphalt cement is added to the dryer at approximately the
midpoint to two-thirds the length to prevent close contact with the flame, which could cause over-heating
and damage the asphalt binder. A double barrel drum mixer has the burner on the low end of the drum
and is more efficient than a conventional drum mixer. Information concerning the addition of a
recyclable asphalt pavement to a drum mixer is available in paragraph 7, paragraph 4 "Recycled Hot-Mix
Asphalt."
(3) Asphalt mixture storage silo. Asphalt storage silos are used to store hot-mix asphalt
mixture before loading onto trucks. Thus, plants can run continuously even when there is a temporary
shortage of trucks. Material can be stored in silos for short periods of time, but if stored too long, the
material may cool excessively or may oxidize excessively causing the bituminous binder to become hard
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