If
you are in the fortunate circumstance of getting in on a project
early, you can have a positive influence over the concrete you will
be staining.
Concrete mix
Generally speaking, the more cement
in the mix, the stronger the reaction you will get from chemical
stains, producing more intense colors. Therefore, use straight cement
mixes, such as a five- or six-sack mix or higher, when possible.
But be consistent: Staining an area of five-sack mix concrete that
adjoins or butts up to an area made with a six-sack mix could cause
color variations, despite the use of the same stain color.
Pozzolans and some cement replacements
(such as fly ash and granulated blast-furnace slag) can have positive
effects on concrete by improving finishability, reducing permeability,
reducing efflorescence and minimizing color bleeding. But they also
can lessen the reaction between the chemical stain and the concrete
surface because removing a percentage of the cement reduces the
amount of calcium hydroxide (a reaction product of hydrated lime).
Metallic salts in chemical stains react with the calcium hydroxide
to produce color.
Concrete admixtures
Most water reducers or air-entraining
admixtures will not have a dramatic effect on the final stain appearance.
Calcium-chloride-based accelerating admixtures, however, will have
a visual impact, typically resulting in dark, splotchy areas. If
it is necessary when pouring concrete in cooler weather to accelerate
the setting time, consider using a nonchloride accelerator or hot
water.
Aggregates used in concrete mix
While most contractors cannot influence
the type of aggregate used in ready-mixed concrete, be aware that
some lime-based aggregates, especially if dose to the surface, can
actually absorb stain and darken the color of the cement paste above
the underlying aggregates. Also, certain types of aggregates found
in the United States are not as absorbent and will not readily accept
the stain.
It is good practice to routinely test
the absorbency of the concrete substrate by wetting the surface
with water prior to chemical stain application. In many cases, this
simple test can help you determine if absorbent aggregates are too
close to the surface. If you begin chemical staining before making
this determination, it is generally too late. If you find that the
surface is not a presentable enough canvas for your staining application,
you may need to skim or overlay the concrete and then stain it.
Concrete finishing methods
The optimum finish, and my personal
preference because I like the way the stain looks on it, is a hard-troweled
surface. Although open finishes achieved with minimal troweling
(such as a broom or float finish) tend to take more stain and produce
more intense colors than hard-troweled surfaces, they are less dense,
wear faster and lose color sooner. On slabs that have been power
troweled to the point of the surface
being "blackened," the surface may need to be opened up
by sanding, diamond grinding, or in extreme cases, a diluted acid
wash.
Use the same finish from pour to pour
if possible. Different textural finishes will give you different
final stain effects. Also, finish the concrete with hand tools around
floor outlets, plumbing risers, electrical sleeves and other obstructions
so these areas are consistent in finish with the rest of the slab.
Concrete curing
Spray-applied liquid curing compounds
should not be used because they produce residual buildup, which
impedes stain penetration. Instead, use unwrinkled, nonstaining
curing paper. If water curing has been specified, understand that
this procedure could trap moisture in the slab. A high moisture
content can affect the performance of sealer and topcoats.
Remember, there are many different
factors that affect how a chemical stain takes to a concrete surface.
Don't cut corners to try and save time. Do the necessary upfront
work so you can minimize any unforeseen surprises on your stain
projects.
Bob
Harris is the founder and president of the Decorative Concrete Institute
in Douglasville, Gal, which provides hands-on training seminars
in architectural concrete. He has pesonally placed or supervised
the placement of more than 3 million sq. ft. of decorative concrete
and has authored two best-selling books, Bob Harris' Guide
to Stained Concrete Interior Floors and Bob Harris' Guide
to Stamped Concrete.
CONCRETE
CONCEPTS * February 2005 * www.concreteconceptsmag.com
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Decorative Concrete Institute 