overall appearance of the concrete.

overall appearance of the concrete.
Aggregate exposure methods that fracture,
grind, or polish the aggregate also produce
a different appearance than techniques
that merely expose the aggregate through
washing and scrubbing.
Calcium chloride, commonly used to
accelerate concrete curing, can produce
discoloration and should not be used with
concrete colors (see Photo 9). Some pigment
producers offer "color-conditioned"
admixtures, which are pigments preblended
with plasticizers, water reducers, or
other concrete admixtures. The same
result, plus greater economy and control
of the mix design, can be achieved by
specifying the admixtures separately from
the color additives or by using a performance
specification that allows the concrete
producer to determine the optimum
combination of admixtures and colors.
Concrete producers routinely stock and
use admixtures that have a proven track
record with their cement and other raw
materials.
Mixes
Variations in the dosage rate of color additives
affect concrete's color (see Photo 10).
The pigment dosage rate is based on the
weight of all cementitious materials in a
mix, including portland cement, fly ash,
and lime.
Increasing the dosage rate produces
more intense colors. Color saturation
generally occurs at dosage rates beyond
6 percent; thus, adding pigment will not
intensify the concrete color. Color addition
rates above 10 percent should not be used.
For uniform color, the same color
dosage rate must be used throughout the
project. In recent years, it has become easier
for concrete producers to add the correct
amount of concrete color to a batch.
In the past, powdered pigments were dirty
to work with and had to be manually
weighed and poured into a concrete
mixer. Color additives are now available
in a variety of forms. Concentrated powder,
liquid, granules, disintegrating packages,
and automated dispensing systems
are all designed to make coloring clean,
simple, and accurate (see Photo 11).
Adding water to a concrete mix produces
a paler color (see Photo 12). It is
generally best to use the lowest water/
cement ratio that will provide a workable
mix. For cast-in-place construction, a 127
mm (5 in.) slump generally is acceptable.
If additional slump is required, a waterreducer
or plasticizing admixture should
be included in the mix instead of more
water.
Adding water has other negative
effects on concrete. Too much water contributes
to dusting, cracking, and reduced
durability of the concrete surface. Adding
3.8 L (1 gal) of water per 0.76 m3 (1 yd3
)
of a properly designed mix creates an
additional 25 mm (1 in.) of slump, reduces
compressive strength approximately 1380
kPa (200 psi), increases shrinkage by 10
percent, and increases the potential for
efflorescence.
December 1998 The Construction Specifier
11. Disintegrating bags of color additives can
be tossed into a mixer without opening
or pouring.
12. different water/cement ratios make plain
(top) and col
2. different water/cement ratios make plain
(top) and colored concrete (bottom) appear pale
or faded.