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Using Admixtures
in Decor Work
How-to tips from the pros that save time and money
BY CINDY
GRAWL
Admixtures
are an important part of concrete mix design. They are those
materials other than cement, water, and aggregate, and they
are put into the mix just before or during the mixture to
help it meet the design and finish needs of the installation.
Says Gabriel Ojeda, president of Fritz-Pak, Dallas, admixtures
can be used in two ways: to improve concrete's ultimate durability
and strength and to modify its behavior to make it easier
to work with.
They can reduce the cost of concrete
work, modify the qualities of the hardened concrete, and assure
the quality of the concrete during the mix, transport, pour
and set, while they help to modify setting times in response
to weather or workability needs and make the concrete stronger
and more durable.
According to the PortlandCement Association,
Skokie, Illinois, the effectiveness of an admixture depends
on the type and amount of cement used, water content, mixi ng
time, slump, and the temperatures of concrete and air: Admixtures
include the following broad categories:
1. air entrainment chemicals that trap
air in the mix, allowing it to withstand freeze-thaw cycles
by giving water a place to go;
2. water reducers to increase the slump
and enhance the concrete's placeability without adding water
and thus reducing the strength and permeability of the concrete;
and
3. accelerators and reducers to help
modify set times and thus allow more time to do design work
or keep the job on track with time constraints.
4. other admixtures, says the PCA, can
be used to color the concrete, inhibit corrosion, reduce shrinkage,
enhance workability, damp proof and bond the concrete.
Air
entrainment admixtures
According to Master Builders, a manufacturer of admixtures
based in Cleveland, Ohio, concrete containing these admixtures,
meant to safeguard concrete in exterior installations from
the rigors of Northern climates, is more workable. However,
adding too much air entrainment can reduce the compressive
strength of the concrete. The bubbles in airentrained concrete,
says Ojeda, make it stronger and easier to trowel, as well
as reducing bleed to produce a better finish.
Bob Harris of L.M. Scofield, Los Angeles,
agrees. "You can't do the finish work in excessive bell
water, so air entrainment lets you get to it sooner and assures
better quality," he says.
However, according to Kevin Copeland,
manager of technical services for the Southwest Region at
Boral Material Technologies, San Antonio, Texas because air
entrainment slows the rate of bleeding, it can cause delamination
if the surface seats before the bleed out, so beware.
Air-entrainment additives can be used
to save contractors money if they have a load that will be
used for both interior and exterior installation. They can
do the interior pour first and then add the air entrainment
chemicals that will help the concrete stand up to weather
for the exterior pour.
Water-reducers
These admixtures come in three strengths - normal range, mid
range and high range, also known as superplasticizers that
improve workability and decrease the need for water in the
mix by varying amounts, allowing for a stronger concrete without
increasing the amount of cement, helping to save on costs.
Adding water reducers, says Rick Seymour, technical services
manager at Lafarge, Denver, Colorado, can help contractors
get better detailing - he's seen the engraving on a coin reproduced
- and also save labor costs, as a four-man crew can do the
work of six.
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