Mechanical Troweling Machine
The mechanical troweling machine (figure 7-50)
is used to good advantage on flat slabs with a stiff
consistency. Mechanical trowels come with a set of
float blades that slip over the steel blades. With these
blades, you can float a slab with the mechanical
trowels. The concrete must be set enough to support
the weight of the machine and the operator. Machine
finishing is faster than hand finishing. However, it
cannot be used with all types of construction. Refer
to the manufacturers manual for operation and
maintenance of the machine you are using.
A nonskid surface can be produced by brooming
the concrete before it has thoroughly hardened.
Brooming is carried out after the floating operation.
For some floors and sidewalks where scoring is not
desirable, a similar finish can be produced with a
hairbrush after the surface has been troweled once.
Where rough scoring is required, a stiff broom made
of steel wire or coarse fiber should be used.
Brooming should be done so that the direction of the
scoring is at right angles to the direction of the traffic.
When grinding of a concrete floor is specified, it
should be started after the surface has hardened
sufficiently to prevent dislodgement of aggregate
particles and should be continued until the coarse
aggregate is exposed. The machines used should be
of an approved type with stones that cut freely and
rapidly. The floor is kept wet during the grinding
process, and the cuttings are removed by squeegeeing
and flushing with water.
After the surface is ground, air holes, pits, and
other blemishes are filled with a thin grout composed
of one part No. 80-grain carborundum grit and one
part portland cement. This grout is spread over the
floor and worked into the pits with a straightedge.
Next, the grout is rubbed into the floor with the
grinding machine. When the filings have hardened
for 17 days, the floor receives a final grinding to
remove the film and to give the finish a polish. All
surplus material is then removed by washing
When properly constructed of
good-quality materials, ground floors are dustless,
dense, easily cleaned, and attractive in appearance.
Figure 7-50.-Mechanical troweling machine.
A sack-rubbed finish is sometimes necessary
when the appearance of formed concrete falls
considerably below expectations. This treatment is
performed after all required patching and correction
of major imperfections have been completed. The
surfaces are thoroughly wetted, and sack rubbing is
The mortar used consists of one part cement; two
parts, by volume, of sand passing a No. 16 screen; and
enough water so that the consistency of the mortar
will be that of thick cream. It may be necessary to
blend the cement with white cement to obtain a color
matching that of the surrounding concrete surface.
The mortar is rubbed thoroughly over the area with
clean burlap or a sponge rubber float, so that it fills all
pits. While the mortar in the pits is still plastic, the
surface should be rubbed over with a dry mix of the
This removes all excess plastic
material and places enough dry material in the pits to
stiffen and solidify the mortar. The filings will then
be flush with the surface. No material should remain
on the surface above the pits. Curing of the surface is
A rubbed finish is required when a uniform and
attractive surface must be obtained. A surface of
satisfactory appearance can be obtained without