TM
5-822-14/AFJMAN
32-1019
and mixes them in place. Water, supplied by a
tank truck, is usually applied to the mixture by a
spray bar mounted in the mixing chamber, or it
can be applied ahead of the mixer by water
pressure distributors. The soil material and ce-
ment must be sufficiently blended when water
contacts the mixture to prevent the formation of
cement balls. The number of mixing passes de-
pends on the type of soil material and its moisture
content and on the forward speed of the mixer. See
figure 4-14 for construction sequences.
(c) Central mixing plant. Central mixing
plants are often used for projects involving borrow
materials. The basic principles of thorough mixing,
adequate cement content, proper moisture content,
and adequate compaction apply. Friable granular
borrow materials are generally used because of
their low cement requirements and ease in han-
dling and mixing. Pugmill-type mixers, either
continuous flow or batch, or rotary-drum mixers
are used for this work. Generally the twin-shaft
continuous-flow pugmill is used on highway
projects. Facilities for efficiently storing, handling,
2 LANES
3 LANES
and proportioning materials must be provided at
the plant. Quantities of soil material, cement, and
water can be proportioned by volume for weight.
Figure 4-13. Plan for processing with
Mixing is continued until a uniform mixture of
multiple-transverse-shaft traveling mixing machine.
soil material, cement, and water is obtained. To
construction saves time during actual processing
reduce evaporation losses during hot, windy condi-
operations because most of the required water will
tions and to protect against sudden showers, haul
already have been added to the soil material. In
trucks should be equipped with protective covers.
very granular materials, prewetting prevents ce-
To prevent excessive haul time, not more than 60
ment from sifting to the bottom of the mix by
minutes should elapse between the start of moist-
causing it to adhere more readily to the sand and
mixing and the start of compaction. Haul time is
gravel particles. Mixing the soil material and
usually limited to 30 minutes. The mixed soil-
cement is easier if the moisture content of the raw
cement should be placed on the subgrade without
material is two or three percentage points below
segregation in a quantity that will produce a
optimum. However, very sandy materials can be
compacted base of uniform density conforming to
mixed even if the moisture content is one or two
the specified grade and cross section. The mixture
percentage points above optimum. Moisture should
should be spread to full roadway width either by
be applied uniformly during prewetting. By mix-
one full-width spreader or by two or more spread-
ing it into the soil material, evaporation losses are
ers operating in staggered positions across the
reduced. Because of the hazard of night rains,
roadway. Less preferable is the use of one piece of
some prefer to do the prewetting in the early
spreading equipment operating one lane at a time
morning. After scarifying and prewetting, the
in two or more lanes. No lane should be spread so
loose, moist soil material is shaped to crown and
far ahead of the adjoining lane that a time lapse of
grade. Cement is spread by a mechanical cement
more than 30 minutes occurs between the time of
spreader or from bags. Occasionally, the prewet
placing material in adjoining lanes at any loca-
soil material becomes compacted by cement-
tion. The subgrade should be damp when the
spreading equipment. In such cases, mixing can be
soil-cement is placed. Bituminous pavers have
hastened by loosening the material again after
been used for spreading soil-cement although mod-
cement is spread, usually with the scarifier on a
ification may be necessary to increase volume
motor grader. The scarifier teeth should be set so
capacity before they can be used. Compaction
that the cement will flow between them and not be
equipment should follow immediately behind the
carried forward or displaced by the scarifier frame.
spreader. When compacting the first lane, a nar-
The mixer picks up the soil material and cement
row compacted ridge should be left adjacent to the
4-7
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