the overwhelming boom in decorative concrete, it seems more and
more newcomers are getting into the world of stamped concrete.
These newcomers - as
well as some seasoned professionals - are not aware there are products
available to make their job easier and increase productivity. Admixtures
are an ingredient in concrete used to modify the properties of concrete
in its freshly mixed, setting or hardened states. Admixtures can
help you fine-tune your concrete mix and enhance performance, both
in the plastic and hardened states.
When used in the right proportions,
pozzolans and other cement replacements can have positive effects
on concrete by improving finishability, reducing permeability, reducing
efflorescence and minimizing bleeding of color. Pozzolans can also
have an effect on the setting time of the concrete. Granulated blast-furnace
slag, for example, can slow setting and extend the working time
of the concrete. Based on my experience, however, this retardation
effect occurs only at cooler temperatures. When the air temperature
exceeds 80 to 85 degrees F, I haven't seen any retardation with
Keep in mind that concrete is influenced
by temperature: The warmer the weather, the faster concrete sets.
To slow the set, try using a retarding admixture, which can be predosed
at the batch plant or added at the jobsite. A retarder can buy you
additional stamping time, especially when temperatures heat up.
Conversely, if you are faced with
cool conditions, you can use an accelerating admixture to speed
setting time and minimize bleeding and segregation. Avoid products
that contain calcium chloride or added chloride ions, which can
result in discoloration of colored concrete.
To improve the durability of concrete
without sacrificing workability, consider using a water reducer.
The most commonly used water reducers in the field of stamped concrete
generally reduce the water requirement by 7 to 10 percent which
can greatly improve concrete strength at all ages and reduce permeability
Another admixture that contributes
to durability, and one that is essential for concrete exposed to
freezing and thawing conditions, is an air-entraining agent. An
air-entraining admixture can be added to fresh concrete to cause
the development of a system of microscopic air bubbles. This helps
to improve the freeze-thaw resistance of hardened concrete. If using
a color hardener with a mix design that contains an air-entraining
admixture, check with the manufacturer of the color hardener for
their recommendation of the maximum air content since this could
greatly affect how the hardener works.
On most decorative stamped concrete
projects, especially larger jobs, time is of the essence. But what
happens if unexpected delays occur? Or what if the concrete that
arrives at the jobsite is already too stiff to place and finish
properly or the air content is too low?
Prepackaged admixtures that you can
dose on the jobsite will be an important part of your tool kit.
They give you greater control
over the concrete you receive and the amount of time available for
decorative stamping. Powdered admixtures, conveniently packaged
in water-soluble bags, can simply be tossed into a batch of concrete
whenever needed. The premeasured quick-fix powders provide first
aid for a number of common problems.
For example, on a recent project that
was installed during the heat of summer, we ordered our concrete
mix with a small dose of retarder batched at the ready mix plant.
We had already predetermined the pour area and broken it up into
roughly three equal sections. We poured the first section of the
pool deck without altering the concrete. For the next third we added
toss-in retarder packs per the suggested dosage rate recommended
by the manufacturer. On the last portion we once again added additional
retarder packs. This common technique is referred to as step retardation.
Had we not used this method, there is no way we would have effectively
been able to stamp this complex design.
In addition to retarders, prepackaged
admixtures include air entrainers, superplasticizers (or water reducers),
and finishing aids that enhance surface finishing characteristics
and make stamping and the addition of color hardeners easier.
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 * December
2004 * www.concreteconceptsmag.comONCRETE