Civil Engineering Reference
In-Depth Information
Finishing Formed Surfaces
with grout must be completed in one day, since grout
remaining on the surface overnight is difficult to remove.
If possible, work should be done in the shade and
preferably during cool, damp weather. During hot or
dry weather, the concrete can be kept moist with a fine
fog spray.
The completed surface should be moist-cured by
keeping the area wet for 36 hours following the clean
down. When completely dry, the surface should have a
uniform color and texture.
Many off-the-form concrete surfaces require little or no
additional treatment when they are carefully constructed
with the proper forming materials. These surfaces are
divided into two general classes: smooth and textured or
patterned. Smooth surfaces are produced with plastic-
coated forms, steel forms, fiberglass-reinforced plastic
forms, formica forms, or tempered-hardboard forms. Tex-
tured or patterned surfaces are achieved with form liners,
rough-sawn lumber, special grades and textures of ply-
wood, or by fracturing the projections of a striated surface.
Rough-form finishes
require patching of all tieholes
and defects, unless tieholes are left open for architectural
effect. Otherwise, these surfaces need no further work
since texture and finish are imparted by the forms.
For a
smooth off-the-form finish
, it is important to
arrange the smooth-facing forming material and tierods in
a symmetrical pattern. Studs and wales that are capable of
preventing excessive deflections must support smooth-
finish forms that are somewhat lightweight.
A
smooth, rubbed finish
is produced on a newly hard-
ened concrete surface no later than the day following form
removal. The forms are removed and necessary patching
completed as soon as possible. The surface is then wetted
and rubbed with a carborundum brick or other abrasive
until a satisfactory uniform color and texture are produced.
A
sand-floated finish
can also be produced on newly
hardened concrete surfaces. No later than 5 to 6 hours fol-
lowing form removal, the surface should be thoroughly
wetted and rubbed with a wood float in a circular motion,
working fine sand into the surface until the resulting
finish is even and uniform in texture and color.
A
grout cleandown (sack-rubbed finish)
can be used
to impart a uniform color and appearance to a smooth sur-
face. After defects have been repaired, the surface should
be saturated thoroughly with water and kept wet at least
one hour before finishing operations begin. Next a grout
of 1 part cement, 1
1
⁄
2
to 2 parts of fine sand passing a
600 µm (No. 30) sieve, and sufficient water for a thick,
creamy consistency should be prepared. It should be
preshrunk by mixing at least one hour before use and then
remixed without the addition of water and applied uni-
formly by brush, plasterer's trowel, or rubber float to com-
pletely fill all air bubbles and holes.
The surface should be vigorously floated with a wood,
sponge rubber, or cork float immediately after applying the
grout to fill any small air holes (bugholes) that are left; any
remaining excess grout should be scraped off with a
sponge-rubber float. If the float pulls grout from the holes,
a sawing motion of the tool should correct the difficulty;
any grout remaining on the surface should be allowed to
stand undisturbed until it loses some of its plasticity but not
its damp appearance. Then the surface should be rubbed
with clean, dry burlap to remove all excess grout. All air
holes should remain filled, but no visible film of grout
should remain after the rubbing. Any section being cleaned
SPECIAL SURFACE FINISHES
Patterns and Textures
A variety of patterns and textures can be used to produce
decorative finishes. Patterns can be formed with divider
strips or by scoring or stamping the surface just before the
concrete hardens. Textures can be produced with little
effort and expense with floats, trowels, and brooms; more
elaborate textures can be achieved with special techniques
(Fig. 11-35). See
Kosmatka (1991)
.
Exposed-Aggregate Concrete
An exposed-aggregate finish provides a rugged, attractive
surface in a wide range of textures and colors. Select aggre-
gates are carefully chosen to avoid deleterious substances;
they are usually of uniform size such as 9.5 to 12.5 mm (
3
⁄
8
to
1
⁄
2
in.) or larger. They should be washed thoroughly
before use to assure satisfactory bond. Flat or elongated
aggregate particles should not be used since they are easily
dislodged when the aggregate is exposed. Caution should
be exercised when using crushed stone; it not only has a
greater tendency to stack during the seeding operation
(requiring more labor), but it also may be undesirable in
some applications (pool decks, for example).
The aggregate should be evenly distributed or seeded
in one layer onto the concrete surface immediately after
the slab has been bullfloated or darbied. The particles
must be completely embedded in the concrete. This can be
done by lightly tapping with a wooden hand float, a
darby, or the broad side of a piece of lumber; then, when
the concrete can support a finisher on kneeboards, the sur-
face should be hand-floated with a magnesium float or
darby until the mortar completely surrounds and slightly
covers all the aggregate particles.
Methods of exposing the aggregate usually include
washing and brushing, using retarders, and scrubbing.
When the concrete has hardened sufficiently, simultane-
ously brushing and flushing with water should expose
the aggregate. In washing and brushing, the surface
layer of mortar should be carefully washed away with a
light spray of water and brushed until the desired expo-
sure is achieved.
