a chassis equipped with wheels for road towing. Sizes
range from 3 1/2 to 10 cubic feet and can be powered
by an electric or a gasoline motor. Mixing takes place
either by rotation of the drum or by rotation of the blades
inside the drum. Tilt the drum to discharge plaster into
a wheelbarrow or other receptacle.
When using a plaster mixer, add the water first, then
add about half the sand. Next, add the cement and any
admixture desired. Last, add the rest of the sand. Mix
until the batch is uniform and has the proper
consistency—3 to 4 minutes is usually sufficient. Note
that excessive agitation of mortar speeds up the setting
time. Most mixers operate at top capacity when the
mortar is about 2 inches, at most, above the blades.
When the mixer is charged higher than this, proper
mixing fails to take place. Instead of blending the
materials, the mixer simply folds the material over and
over, resulting in excessively dry mix on top and too wet
mix underneath-a bad mix. Eliminate this situation by
not overloading the machine.
Handling Materials
Personnel handling cement or lime bags should
wear recommended personnel protective gear. Always
practice personal cleanliness. Never wear clothing that
is hard and stiff with cement. Such clothing irritates the
skin and may cause serious infection. Any susceptibility
of skin to cement and lime bums should be immediately
reported to your supervisor.
Don’t pile bags of cement or lime more than 10 bags
high on a pallet except when stored in bins or enclosures
built for such purposes. Place the bags around the
outside of the pallet with the tops of the bags facing the
center. To prevent piled bags from falling outward,
crosspile the first five tiers of bags, each way from any
comer, and make a setback starting with the sixth tier.
If you have to pile above the 10th tier, make another
setback. The back tier, when not resting against a wall
of sufficient strength to withstand the pressure, should
be set back one bag every five tiers, the same as the end
tiers.
During unpiling, the entire top of the pile should be
kept level and the setbacks maintained for every five
tiers.
Lime and cement must be stored in a dry place to
help prevent the lime from crumbling and the cement
from hydrating before it is used.
PLASTER APPLICATION TOOLS
AND TECHNIQUES
LEARNING OBJECTIVE: Upon completing
this section, you should be able to state the uses
of plastering tools, and describe the techniques
of plastering.
A plaster layer must have uniform thickness to attain
complete structural integrity. Also, a plane plaster
surface must be flat enough to appear flat to the eye and
receive surface-applied materials, such as casings and
other trim, without the appearance of noticeable spaces.
Specified flatness tolerance is usually 1/8 inch in 10 feet.
TOOLS
Plastering requires the use of a number of tools,
some specialized, including trowels, hawk, float,
straight and feather edges, darby, scarifier, and
plastering machines.
Trowels
Steel trowels are used to apply, spread, and smooth
plaster. The shape and size of the trowel blade are
determined by the purpose for which the tool is used and
the manner of using it.
The four common types of plastering trowels are
shown in figure 7-12. The rectangular trowel, with a
blade approximately 4 1/2 inches wide by 11 inches
long, serves as the principle conveyor and manipulator
Figure 7-12.—Plasterting trowels.
7-12
