Colored Concrete Cautions
When you hear the word "concrete," you probably picture
structural elements like footings, foundations, and slabs. And
whatever you picture, the material is probably smooth and gray.
Yet there are lots of ways to transform this common structural
material into an impressive decorative element.
Most decorative techniques involve changing the color and
texture of the concrete surface. These methods allow concrete
to be used for nontraditional applications like countertops but
are most frequently used for flatwork, which is the focus of
The most common treatments make concrete look like more
expensive materials such as brick, flagstone, and slate.
Skilled finishers can produce stone or tile-like surfaces that
are hard to tell from the real thing. But techniques aren't
limited to copying other materials. Color and texture can also
be used to produce abstract or painting-like effects that are
not possible with other materials.
In researching this article, I talked to a number of suppliers
and spent time visiting job sites with Tom Ralston of Tom
Ralston Concrete in Santa Cruz, Calif. With the exception of
stenciling, his company has done just about every kind of
decorative concrete there is.
There are a number of ways to color concrete. One of the older
methods is to color the surface by exposing attractively
colored aggregate. Other methods involve dying or staining the
cement that's in the concrete.
After scoring a damp exterior slab to
suggest stone pavers, workers sprinkle on color hardener to
change the tint (see photo at top of article). Latex molds
taken from actual stone are pounded into place to provide the
Integral color. The easiest way to color
concrete is to have the supplier add color pigment at the batch
plant. You can also add pigment to the mix after the truck
arrives on site. Either way, the concrete is colored before
it's poured, so placing and finishing aren't much different
than usual. Concrete prices vary widely from area to area, but
depending on who you talk to, using integral color may add 10%
to 50% to the cost of the material. The labor to finish is
about the same as for a standard mix.
Pigments are usually powdered and come in a wide range of
hues. You can get reds, greens, yellows, blues, browns, tans,
grays, and black. The color will go all the way through the
crete, so chips and dings will not be noticeable.
Adding integral color is a simple way to color concrete, but
there are ways to mess up. The most important thing is to be
consistent, especially for jobs that require more than one
load. Each batch should contain the same materials and the same
amount of pigment and arrive at the recommended slump --
typically 4 or 5 inches. Each batch should be mixed, placed,
finished, and cured exactly the same way. Change anything and
you can end up with areas that don't match.
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.
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
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
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.
Color hardener comes in a wide array of colors. There's more
design flexibility than with integral color because the
finisher controls the process. He or she decides how much color
to apply and is able to create highlights by using more than
It takes between 60 and 120 pounds of color hardener to color
100 square feet of concrete. Lighter colors typically require
more. Most color hardener costs between 35¢ and $1.00
per square foot. But blues and greens are more, costing between
$1.20 and $6.50 per square foot. As with integral color, you
should avoid using chloride-based accelerators with this
Cured concrete can be colored by treating it with acid stain.
The stain, which is made from water, acid, and inorganic salts,
can be applied with a roller, brush, or garden sprayer. It's
applied to the surface but does not form a coating like paint.
Instead, it soaks in and reacts with the free lime that's in
the concrete. Free lime is not evenly distributed, so the
treatment produces an attractive mottled effect.
You can use a single color, multiple colors, or mix colors on
the surface. Geometric patterns can be created by using a
diamond blade to score the slab prior to staining. The kerfs
can be grouted later on to create a surface that looks like it
was tiled. Free-form patterns can be created by masking off
portions of the slab and selectively staining the
Manufacturers recommend allowing new concrete to cure for two
to four weeks before staining. The slab should be carefully
cleaned before treating because surface contamination by oil or
drywall dust can mask the surface and keep the stain from
soaking in evenly. Check with the stain manufacturer before
using any cleaning solution on the slab. And whatever you do,
don't acid-etch it first because the acid will react with and
"use up" the free lime that's in the slab. The stain won't take
if there's nothing in the concrete for it to react with.
It's possible to acid-stain old concrete, but the results are
unpredictable because there's no telling what has gotten onto
or into the slab in the years since it was poured. In general,
colors are likely to be more intense on new material than on
Acid stain can be used indoors or out. But it works better on
those smooth, hard finishes that are more commonly used
indoors. Stain is typically applied in two coats and should be
sealed after it dries. A gallon of stain costs about $50 and
will color between 100 and 200 square feet. Acid stain is
available in many hues of blue, green, black, tan, and red.
That said, the range of color is much narrower than what's
available with other coloring methods. Acid stain can produce
deep colors, but they are usually somewhat muted.