TM
5-822-14/AFJMAN
32-1019
Step 1. Spread the aggregate to uniform
clusters of fine aggregate saturated and coated
grade and cross section with motor graders.
with excessive amounts of asphalt can make a mix
Step 2. Thoroughly mix the aggregate by
difficult to spread and compact. This condition can
one or more passes of the mixer. When ready for
be corrected by windowing the mixture into a tight
the asphalt the moisture content of the aggregate
windrow and allowing it to cure for a few days.
should not exceed 3 percent, unless laboratory
After mixing and aeration have been completed,
tests indicate that a higher moisture content will
the windrow is moved to one side of the roadbed in
not be harmful when the asphalt is added.
readiness for subsequent spreading. If it is left for
Step 3. Add asphalt in increments of about
any length of time, periodic breaks in the windrow
0.50 gallons per square yard until the total re-
quired amount of asphalt is applied and mixed in.
the roadbed.
(5) Aeration. Before compaction, most of the
A total of 0.4 to 0.7 gallons per square yard per
diluents that have made the asphalt cold mix
inch of compacted thickness of the course is usu-
workable must be allowed to evaporate. In most
ally necessary. If the mixer is not equipped with
spraybars the asphalt usually is applied with an
cases, this occurs during mixing and spreading
and very little additional aeration is required, but
asphalt distributor.
Step 4. Make one or more passes of the
extra manipulation on the roadbed is needed occa-
sionally to help speed the process and dissipate the
mixer between applications of asphalt, as neces-
excess diluents. Until the mix is sufficiently aer-
sary to thoroughly mix it in.
Step 5. Maintain the surface true to grade
ated, it usually will not support rollers without
excessive pushing under the rolls. Generally, the
and cross-section by using a motor grader during
mixture is sufficiently aerated when it becomes
the mixing operations.
Step 6. Aerate the mixture by additional
tacky and appears to "crawl." Many factors affect
the rate and the required amount of aeration.
manipulation, if needed.
(c) Blade mixing. With blade mixing, the
Fine-g-rained and well-graded mixtures will re-
quire longer aeration than open-graded and coarse-
imported or in-place material is shaped into a
grained mixtures, all other things being equal.
measured windrow, either through a spreader box
Also, if an asphalt cold-mix base course is to be
or by running through a windrow shaper. The
surfaced within a short length of time, aeration
windrow is then flattened with the blade to about
before compaction should be more complete than if
the width of the distributor spraybar. The asphalt
the course is not to be surfaced for some time; the
is applied by successive passes of the asphalt
surface acts as a seal, greatly retarding the re-
distributor over the flattened windrow, each appli-
moval of diluents.
cation not exceeding 0.75 gallons per square yard.
(a) Emulsified asphalt mixes. Experience
After each pass of the distributor the mixture is
has shown that break-down rolling of emulsified
worked back and forth across the roadbed with the
asphalt mixes should begin immediately before, or
blade, sometimes added by auxiliary mixing equip-
at the same time as, the emulsion starts to break
ment. Prior to each succeeding application of
(indicated by a marked color change from brown to
asphalt, the mixture is reformed into a flattened
black). About this time, the moisture content of
windrow. The material in the windrow is subjected
the mixture is sufficient to act as a lubricant
to as many mixings, spreading, shapings, and
between the aggregate particles, but is reduced to
flattenings as are needed to disperse the asphalt
the point where it does not fill the void spaces,
thoroughly throughout the mixture, and to coat
thus allowing their reduction under compactive
effectively the aggregate particles. During mixing,
forces. Also, by this time, the mixture should be
the vertical angle of the mold board may require
able to support the roller without undue displace-
adjustment from time to time in order to achieve a
complete rolling action of the windrow as it is
ment.
(b) Cutback asphalt mixes. When using cut-
worked. As large a roll as possible should be
back asphalt, correct aeration will be achieved
carried ahead of the blade, since pressure from the
when volatile content is reduced to about 50
weight of the aggregate facilitates mixing. Addi-
percent of that contained in the original asphaltic
tionally, during mixing, care must be taken to see
that neither extra material be taken from the
material, and the moisture content does not exceed
2 percent by weight of the total mixture.
mixing table and incorporated into the windrow
(6) Spreading and compacting. With mixing
nor any of the windrow be lost over the edge of the
and aeration completed, spreading and compacting
mixing table or left on the mixing table without
the cold mix follows. Achieving a finished section
being treated. Sometimes, when cutback asphalt is
used, the formation of "oil balls," i.e., concentrated
and smooth riding surface conforming to the plans
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