Civil Engineering Reference
In-Depth Information
Colored Finishes
should be dampened as soon as possible without dis-
turbing the paint. Damp curing of conventional portland
cement paint is essential. On open-textured surfaces, such
as concrete masonry, the paint should be applied with
stiff-bristle brushes (scrub brushes). Paint should be
worked well into the surface. For concrete with a smooth
or sandy finish, whitewash or Dutch-type calcimine
brushes are best.
The latex materials used in latex-modified portland
cement paints retard evaporation, thereby retaining the nec-
essary water for hydration of the portland cement. When
using latex-modified paints, moist curing is not required.
Most latex paints are resistant to alkali and can be
applied to new concrete after 10 days of good drying
weather. The preferred method of application is by long-
fiber, tapered nylon brushes 100 to 150 mm (4 to 6 in.) wide;
however, roller or spray methods can also be used. The
paints may be applied to damp, but not wet surfaces. If the
surface is moderately porous, or if extremely dry condi-
tions prevail, prewetting the surface is advisable.
Clear coatings are frequently used on concrete sur-
faces to (1) prevent soiling or discoloration of the concrete
by air pollution, (2) to facilitate cleaning the surface if it
does become dirty, (3) to brighten the color of the aggre-
gates, and (4) to render the surface water-repellent and
thus prevent color change due to rain and water absorp-
tion. The better coatings often consist of methyl methacry-
late forms of acrylic resin, as indicated by a laboratory
evaluation of commercial clear coatings (
Litvin 1968
). The
methyl methacrylate coatings should have a higher vis-
cosity and solids content when used on smooth concrete,
since the original appearance of smooth concrete is more
difficult to maintain than the original appearance of
exposed-aggregate concrete.
Other materials, such as silane and siloxane pene-
trating sealers, are commonly used as water repellents for
many exterior concrete applications.
Colored concrete finishes for decorative effects in both
interior and exterior applications can be achieved by four
different methods: (1) the one-course or integral method,
(2) the two-course method, (3) the dry-shake method, and
(4) stains and paints (discussed below).
Color pigments added to the concrete in the mixer to
produce a uniform color is the basis for the one-course
method. Both natural and synthetic pigments are satisfac-
tory if they are: (1) insoluble in water, (2) free from soluble
salts and acids, (3) fast to sunlight, (4) fast to alkali and
weak acids, (5) limited to small amounts of calcium sul-
phate, and (6) ground fine enough so that 90% passes a
45 micron screen. Use only the minimum amount necessary
to produce the desired color and not more than 10% by
weight of the cement.
In the two-course method, a base slab is placed and
left with a rough texture to bond better to a colored top-
ping layer. As soon as the base slab can support a cement
mason's weight, the topping course can be placed. If the
base slab has hardened, prepare a bonding grout for the
base slab prior to placing the topping mix. The topping
mix is normally 13 mm (
1
⁄
2
in.) to 25 mm (1 in.) thick, with
a ratio of cement to sand of 1:3 or 1:4. The mix is floated
and troweled in the prescribed manner. The two-course
method is more commonly used because it is more eco-
nomical than the one-course method.
In the dry-shake method, a prepackaged dry-color
material is cast onto the surface of a concrete slab. The dry-
shake material is applied after the concrete has been
screeded and darbied or bullfloated, excess moisture has
evaporated from the surface, and preliminary floating has
been done. Two-thirds of the dry material is shaken evenly
by hand over the surface and thoroughly floated into the
surface in a manner that evenly distributes the material.
Immediately, the rest of the material is cast onto the surface
and floated as before. The surface can then be troweled at
the same time as a typical slab. For exterior surfaces that
will be exposed to freezing and thawing, little or no trow-
eling followed by brooming with a soft bristle concrete
broom is usually sufficient.
PRECAUTIONS
Protect Your Head and Eyes.
Construction equipment and
tools represent constant potential hazards to busy construc-
tion personnel. That's why hard hats are required on con-
struction projects. It is therefore recommended that some
sort of head protection, such as a hard hat or safety hat, be
worn when working any construction job, large or small.
Proper eye protection is essential when working with
cement or concrete. Eyes are particularly vulnerable to
blowing dust, splattering concrete, and other foreign objects.
On some jobs it may be advisable to wear full-cover gog-
gles or safety glasses with side shields. Actions that cause
dust to become airborne should be avoided. Local or gen-
eral ventilation can control exposures below applicable
exposure limits; respirators may be used in poorly venti-
lated areas, where exposure limits are exceeded, or when
dust causes discomfort or irritation.
Stains, Paints, and Clear Coatings
Many types of stains, paints and clear coatings can be
applied to concrete surfaces. Among the principal paints
used are portland cement base, latex-modified portland
cement, and latex (acrylic and polyvinyl acetate) paints
(
PCA 1992
). However, stains and paints are used only
when it is necessary to color existing concrete. It is difficult
to obtain a uniform color with dyes or stains; therefore, the
manufacturer's directions should be closely followed.
Portland cement based paints can be used on either
interior or exterior exposures. The surface of the concrete
should be damp at the time of application and each coat
