TM
5-822-14/AFJMAN
32-1019
windrows prior to mixing and spreading. If wind-
(4) Mixing.
rows are to be used, the roadway must be cleared
(a) Travel plant mixing. Travel-plant mixing
of all vegetation to a width sufficient to accommo-
offers the advantage of closer control of the mixing
date both windrow and traffic while the mixture
operation than is possible with blade mixing. With
cures. Because the thickness of the new pavement
the windrow-type travel plant, the machine moves
is directly proportional to the amount of aggregate
along the windrow, picking up the aggregate,
in the windrow(s), accurate control and measure-
mixing it with asphalt in the pugmill, and deposit-
ment of the volume of the windrowed material is
ing the mixture in a windrow, ready for aerating
necessary. Usually, there is not enough loose mate-
or spreading. For this type of plant, the asphalt
rial on the road surface to use in the road mix. In
application rate must be matched accurately with
this case, it is best to blade the loose material onto
the width and thickness of the course, forward
the shoulder rather than perform the several
speed of the mixer, and the density of the in-place
operations that are necessary to blend it with the
aggregate. As the thickness is specified, the den-
material brought in from other sources. Some-
sity is fixed, and the asphalt application rate is
times, however, incorporating the existing mate-
set; the variable is the forward speed. If the
rial on the roadbed into the mixture is considered
aggregate windrow is so large that all of the
practical, if it is uniform and enough is available.
asphalt cannot be incorporated in one mixing pass,
When this is done, the loose aggregate must first
it should be split into two or more windrows and
be bladed into a windrow and measured. Next, it
the proper amount of asphalt added to each wind-
must be made to meet grading specifications by
row as it is mixed. Sometimes, further mixing of
adding other aggregates as necessary. Finally, the
the windrowed material may be necessary after
windrow is built up to the required volume with
the addition of the asphalt. Unless the travel
implanted material that meets the specifications.
mixer can be used as a multiple pass mixer, this
If two or more materials are to be combined on the
addition mixing usually is done with a motor
road to be surfaced, each should be placed in its
grader. This ensures that all of the windrowed
own windrow. These windrows are then mixed
material is incorporated into the mix. It also
together thoroughly before asphalt is added.
aerates the mixture for the removal of diluents.
(2) Determining asphalt application rate. Be-
The number of passes with the motor grader
fore mixing operations begin, the correct asphalt
required for this purpose varies with different job
application rate and forward speed of the spray
conditions. After the mixing and aeration proce-
bar equipped mixer or asphalt distributor must be
dure is completed, the windrow should be moved to
determined for the quantity of aggregate in the
one side of the area to be surfaced in preparation
windrow. Also, when using emulsified asphalt, it
for spreading. The hopper-type travel plant oper-
is frequently necessary to moisten the aggregate
ates by mixing, in its pugmill, the proper amount
before applying the asphalt and the water applica-
tion rate and forward speed of the water distribu-
of asphalt with aggregate that is deposited by haul
tor must be determined.
trucks, directly into the plant's hopper; then it
(3) Control of asphalt. Asphalt is added to the
spreads the mixture. Except when using open-
aggregate from an asphalt distributor or by a
graded mixtures, care must be taken to ensure
travel mixer. Whichever method is used, close
sufficient evaporation of diluents from the mix
control of quantity and viscosity is required to
prior to compaction.
ensure a proper mixture. Maintaining the correct
(b) Rotary mixing. As with windrow travel
viscosity is critical because the asphaltic material
plants, rotary mixers equipped with built-in spray-
must be fluid enough to move easily through the
ing systems require that the asphalt application
spray nozzles and to coat adequately the aggregate
rates be matched accurately with the width and
particles: Cutback asphalts, and occasionally emul-
thickness of the course, forward speed of the mix-
sified asphalts, even though already fluid, require
er, and the density of the in-place aggregate. How-
some heating in order to bring them to a viscosity
ever, when utilizing a rotary mixer not equipped
suitable for spraying. If the proper grade of as-
with spraybars, an asphalt distributor, operating
phalt has been used, and the mixing is done
ahead of the mixer, applies asphalt to the aggre-
correctly, the cutback or emulsified asphalt will
gate. Incremental applications of asphalt and
remain fluid until the completion of mixing. As
passes of the mixer are usually necessary to
the actual temperature of the mixture is controlled
achieve the specified mixture. Most rotary mixers
by that of the aggregate, care must be taken to see
are now equipped with a spray system. When
that mixing is not attempted at aggregate temper-
using this type of mixer the following steps are
atures below 50 degrees F.
recommended:
4-23
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