Civil Engineering Reference
In-Depth Information
kneeboards. These operations should be delayed until the
concrete has hardened sufficiently so that water and fine
material are not brought to the surface. Too long a delay,
of course, will result in a surface that is too hard to float
and trowel. The tendency, however, is to float and trowel
the surface too soon. Premature floating and troweling can
cause scaling, crazing, or dusting and a surface with
reduced wear resistance.
Spreading dry cement on a wet surface to take up
excess water is a bad practice and can cause crazing.
Such wet spots should be avoided, if possible, by adjust-
ments in aggregate gradation, mix proportions, and con-
sistency. When wet spots do occur, finishing operations
should be delayed until the water either evaporates or is
removed with a rubber floor squeegee or by dragging a
soft rubber garden hose. If a squeegee or hose is used,
care must be taken so that excess cement paste is not
removed with the water.
The first troweling may produce the desired surface free
of defects. However, surface smoothness, density, and wear
resistance can all be improved by additional trowelings.
There should be a lapse of time between successive trowel-
ings to permit the concrete to become harder. As the surface
stiffens, each successive troweling should be made with
smaller trowels, using progressively more tilt and pressure
on the trowel blade. The final pass should make a ringing
sound as the trowel moves over the hardening surface.
A power trowel is similar to a power float, except that
the machine is fitted with smaller, individual steel trowel
blades that are adjustable for tilt and pressure on the con-
crete surface. When the first troweling is done by machine,
at least one additional troweling by hand should be done
to remove small irregularities. If necessary, tooled edges
and joints should be rerun after troweling to maintain uni-
formity and true lines.
Exterior concrete should not be troweled for several
reasons: (1) because it can lead to a loss of entrained air
caused by overworking the surface, and (2) troweled sur-
faces can be slippery when wet. Floating and brooming
should be sufficient for outdoor concrete.
Fig. 11-20. Brooming provides a slip-resistant surface
mainly used on exterior concrete. (69943)
Brooming
Brooming should be performed before the concrete has
thoroughly hardened, but it should be sufficiently hard to
retain the scoring impression to produce a slip-resistant
surface (Fig. 11-20). Rough scoring can be achieved with a
rake, a steel-wire broom, or a stiff, coarse, fiber broom;
such coarse-textured brooming usually follows floating. If
a finer texture is desired, the concrete should be floated to
a smooth surface and then brushed with a soft-bristled
broom. Interior concrete could also be troweled before
brooming. Best results are obtained with brooms that are
specially made for texturing concrete. Slabs are usually
broomed transversely to the main direction of traffic.
Highway pavements are textured by “tining” the sur-
face with stiff wires; this improves traction and reduces
hydroplaning (Fig. 11-21).
Fig. 11-21. (top) This machine is tining the surface of fresh
concrete. (bottom) Tining of pavements improves tire trac-
tion and reduces hydroplaning. (69944, 69945)
Curing and Protection
All newly placed and finished concrete slabs should be
cured and protected from drying, from extreme changes in
temperature, and from damage by subsequent construc-
tion and traffic.
