TM
5-822-14/AFJMAN
32-1019
is the objective of these final two construction
steps. The mixture should always be spread to a
uniform thickness (whether in a single pass or in
several thinner layers) so that no thin spots exist
in the final mat. Mixtures that do not require
aeration may be spread to the required thickness
immediately after mixing and then compacted
with pneumatic-tired vibratory or steel-tired roll-
ers. Mixtures that require aeration, however, are
generally deposited upon the roadbed in windrows
and then are spread from these windrows. The
windrow may be placed along the centerline of the
road, or along one side if the mixture is to be
Figure 4-52. Spreading and compacting train.
spread by blade. Because there is a tendency to
leave a hump in the road when blade spreading
(a) Stationary plants. Generally, stationary
from a center-line windrow, it is considered better
plant mixing is accomplished at a location away
practice to place the windrow to the side for
from the road site, frequently at the aggregate
spreading. Blade spreading should be accomplished
source. A stationary plant consists of a mixer and
in successive layers, with no layer thinner than
equipment for heating the asphalt (if necessary)
about 1.5 times the diameter of the maximum
and for feeding the asphalt, aggregate, and addi-
particle size. As each layer is spread, compaction
tives (if needed) to the mixer. It is similar in many
should follow almost immediately with a pneu-
respects to the hot-mix plant, except that it has no
matic-tired roller. Because the tires of the motor
dryer or screens other than a scalping screen. Like
grader compact the freshly spread mix, their tracks
the bigger hot-mix plant, a stationary cold-mix
will appear as ridges in the finished mat unless
plant may be either a batch or continuous type,
there is adequate rolling between the spreading of
although the latter is most prevalently used for
each successive layer. The roller should follow
cold-mix construction (figs 4-53 and 4-54). Any
directly behind the motor grader in order to
type of plant that can produce an asphalt mixture
eliminate these ridge marks (fig 4-52). If, at any
conforming to the specifications can be used. But,
time during compaction, the asphalt mixture ex-
as a minimum, it should be equipped with temper-
hibits undue rutting or shoving, rolling should be
ature and metering devices to control accurately
stopped. Compaction should not be attempted until
the asphalt material being applied to the aggre-
there is a reduction diluent content, occurring
gate and controlled feeders for proportioning ag-
either naturally or by mechanical aeration. After
gregates and additives. Although not always a
plant component, a storage silo allows a more
one course is thoroughly compacted and cured,
continuous mixing operation, resulting in better
other courses may be placed on it. This operation
mix uniformity.
should be repeated as many times as necessary to
(b) Haul trucks. Several types of haul trucks
bring the road to proper grade and crown. For a
may be used for cold mix produced in stationary
smooth riding surface the motor grader should be
plants; the type selected depends on the spreading
used to trim and level as the rollers complete
equipment. The traditional raised-bed end-dump
compaction of the upper layer. After the mat has
truck can be used with windrowers or pavers with
been shaped to its final required cross section, it
hoppers. Bottom dumps produce windrows and are
must then be finish rolled, preferably with a
not used with pavers with hoppers unless a low-lift
steel-tired roller, until all roller marks are elimi-
loader is used to transfer the mix to the hopper.
nated. Sometimes, a completed course may have to
Horizontal discharge trucks deposit the mix di-
be opened temporarily to traffic. In this event, to
rectly into the paver's hopper without raising the
prevent tire pickup, it may be advisable to seal the
bed. These trucks may also be used with windrow
surface by applying a dilution of slow-setting
spreader boxes. A sufficient number of haul trucks
emulsified asphalt and potable water (in equal
with smooth, clean beds should be available to
parts) at a rate of approximately 0.10 gallons per
ensure uniform operation of the mixing plant and
square yard. This should be allowed to cure until
paver.
no pickup occurs. For immediate passage of traffic,
(2) Spreading equipment.
sanding may be desirable to avoid pickup.
(a) Paver. If climatic conditions and aggre-
c. Equipment for Plant Mixing.
gate gradation permit evaporation of moisture or
(1) Mixing equipment.
4-25
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