TM
5-822-14/AFJMAN
32-1019
regulates the quantity of cement placed on the
After the cement is added, a second plow attach-
prepared soil. To obtain a uniform cement spread,
ment a little farther up on the main feeder belt
the spreader should be operated at a constant,
closes the furrow and covers the cement. A cover
slow speed and with a constant level of cement in
on the main feeder belt will also minimize cement
the hopper. A true line at the pavement edge
loss due to wind. One of three types of cement
should be maintained with a string line. The
meters-belt, screw, or vane-can be used to pro-
mechanical spreader must have adequate traction
portion the cement on a volumetric basis. Each
to produce a uniform cement spread. Traction can
requires a 450- to 750-pound capacity surge tank
be aided by wetting and rolling the soil material
or hopper between the cement silo and the cement
before spreading the cement. When operating in
feeder. This tank maintains a constant head of
loose sands or gravel, slippage can be overcome by
cement for the feeder, thus providing a more
the use of cleats on the spreader wheels or by
uniform cement discharge. Compressed air of 2- to
other modifications; sometimes, the spreader is
4-pounds per square inch pressure should be used
mounted on a tractor or high lift. The mechanical
to prevent arching of cement in the silo and the
cement spreader can also be attached directly
surge tank. Portable vibrators attached to the
behind a bulk cement truck. Cement is then
surge tank can be used instead of air jets. A
moved pneumatically from the truck through an
positive system should be included to stop the
air separator cyclone that dissipates the air pres-
plant automatically if the cement flow suddenly
sure, and falls into the hopper of the spreader.
stops. The correct proportion of cement, soil mate-
Forward speed must be slow and even. Sometimes
rial, and water entering the mixing chamber must
a motor grader or loader pulls the truck to main-
be determined by calibrating the plant before
tain this slow, even forward speed. Pipe cement
mixing and placing operations begin.
(b) Mixing and application of water. Proce-
spreaders attached to cement transport trucks
have been used in some areas with variable
dures for applying water and mixing depend on
results. Improvements in this type of equipment
the type of mixing machine used. A thorough
are being made.
mixture of pulverized soil material, cement, and
2. Bagged-cement spread, mixed-in-place
water must be obtained. Uniformity of the mix is
construction. When bags of cement are used on
easily checked by digging trenches or a series of
small jobs, a simple but exact method for properly
holes at regular intervals for the full depth of
placing the bags is necessary. The bags should be
treatment and inspecting the color of the exposed
spaced at approximately equal transverse and
soil-cement mixture. Uniform color and texture
longitudinal intervals that will ensure the proper
from top to bottom indicate a satisfactory mix; a
percentage of cement. Positions can be spotted by
streaked appearance indicates insufficient mixing.
flags or markers fastened to chains at proper inter-
Proper width and depth of mixing are also impor-
vals to mark the transverse and longitudinal rows.
tant. Following are methods of applying water and
When the bags are opened, the cement should be
mixing for the different types of mixing machines.
1. Windrow-type traveling mixing ma-
dumped so that it forms fairly uniform transverse
chine. Windrow-type traveling mixing machines
windrows across the area being processed. A spike-
tooth harrow, a nail drag, or a length of chain-link
will pulverize friable soil materials. Other soils,
fence can be used to spread the cement evenly.
however, may need preliminary pulverizing to
The drag should make at least two round trips
meet specification requirements. This is usually
over the area to spread the cement uniformly.
done before the soil is placed in windrows for
3. Cement application, central-mixing-
processing. The prepared soil material is bladed
plant construction. When a continuous-flow central
into windrows and a proportion pulled along to
mixing plant is used, the cement is usually me-
make them uniform in cross section. When borrow
tered onto the soil aggregate and the two materi-
materials are used, a windrow spreader can be
als are carried to the pugmill mixer on the main
used to proportion the material. Nonuniform wind-
rows cause variations in cement content, moisture
feeder belt. Variations in moisture and in grada-
tion of the soil aggregate will result in variations
content, and pavement thickness. The number and
size of windrows needed depend on the width and
in the amount of material being fed onto the
feeder belt. A high bulkhead placed in front of the
depth of treatment and on the capacity of the
mixing machine. Cement is spread on top of the
soil hopper will help to obtain a more uniform flow
partially flattened or slightly trenched, prepared
through the soil material feeder. The chance of
windrow. The mixing machine then picks up the
loss of cement due to wind can be minimized by
soil material and cement and dry-mixes them with
the use of a small plow attachment that will form
a furrow for the cement in the soil aggregate.
the first few paddles in the mixing drum. At that
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