| 
After scoring a damp
exterior slab to suggest stone pavers, workers sprinkle
on color hardener to change the tint (opposite page).
Latex molds taken from actual stone are pounded into
place to provide the texture. |
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Pigment should
be measured by weight and added in proportion to the amount
of cement that's in the mix. Intense colors are more expensive
than subtle ones because it takes more pigment to get them.
It might take 3 pounds of pigment to color a yard of light
tan concrete and as much as 24 pounds to produce a deep brick
red.
Watch out for cold weather. it's
common practice to use calcium chloride to speed setting in
cold weather. But you shouldn't add it to concrete that's
getting integral color because chloride-based accelerators
can discolor the slab or cause efflorescence. If you need
to pour in cold weather, ask your supplier to use hot water
or to increase the cement content of the mix. Other options
include using a more expensive nonchloride accelerator or
concrete containing fast-setting type III cement.
Hot days. Dark colors
absorb heat, so a dark mix may set faster than you expect
when it's sunny and dry. That can make the concrete hard to
finish and may lead to thermal cracking. If you can't avoid
pouring dark colors on hot sunny days, you should tent the
slab or use extra curing compound to keep the concrete from
drying out.
Using
Color Hardener
Another way to color concrete is to add pigment after it's
placed. This is done by heavily dusting the surface with a
dry-shake color hardener after the crete is floated and the
bleed water has disappeared. The moisture in the slab activates
the hardener, which is incorporated into the surface by floating,
troweling, or stamping with textured mats. The color typically
goes about 1/8 inch into the slab.
The main ingredients in color hardener
are pigment and Portland cement. The pigment provides color,
and the Portland enriches the surface and makes it harder
than the concrete below. You could pour material that's 3,000
psi and end up with a surface that's 6,000 psi. The extra
hard surface helps the slab wear better and makes it more
resistant to freeze-thaw cycles.
Skilled labor needed. Using
color hardener takes more skill and labor than using integrally
colored concrete. Color hardener is applied in stages. The
first two thirds are broadcast on the slab and worked in by
trowel or float. The final third is worked into areas that
didn't get enough color the first time. The slab is then ready
for finishing.
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