| Lee Levig, of Levig Concrete
is San Francisco, agrees that mid-range water reducers can
give more "life" to the concrete - more detail time
- while making concrete easier and less labor-intensive to
place. They also make surfaces wetter, he says, so color hardeners
can have a better effect.
The high-range water reducers, or superplasticizers,
can produce highly flowable concrete while maintaining a specified
water to concrete ratio, according to Master Builders, but
their effect only lasts a short time. Fritz-Paks' Ojeda adds
that they do give the concrete more strength, by reducing
the water to cement ratio, but that as it becomes denser,
it may become harder to add color hardeners, as they need
to absorb the water that just isn't there. Another drawback,
says Levig, is that too much of a plastic effect can result
in shrinkage.
According to Boral's Copeland, superplasticizers
can be used to create self-compacting concrete, but that can
cause aggregate to sink down and segregate Out. To overcome
this, he recommends use of viscosity modifying admixtures
that will keep aggregates suspended throughout a mixture.
Superplasticizers can also lead to shrinkage
if overused, and many contractors use fibers to assure that
the concrete will stay put. However, warns Harris, polypropylene
fibers are to be used only to overcome surface cracking; they
will not give the hearing strength of rebar.
Accelerators
These admixtures make the concrete set faster and are often
used during cold weather to overcome the slowdown in the hydration
of the concrete that occurs when temperatures fall. They assure
that concrete reaches its full strength before it is subjected
to freezing. If fly ash or slag is used in the mix, an accelerator
can help regulate the set of the concrete. Using accelerators
can also help finish work begin earlier and keep an installation
on schedule - and on budget, says Master Builders.
Tom Ralston, president of Ralston Concrete,
Santa Cruz, California, agrees. "Use an accelerator to
reduce wait time. If your crew is standing around waiting
for a slab to go off on a cold day," he says, "it
costs a lot of money. Weigh that against the cost of using
an accelerator."
Ojeda adds that in addition to saving
labor costs, accelerators are great to reduce cure times,
so concrete can cure before a freeze is due. He also notes
that they achieve faster strength, so driveways and runways
are available for use by customers sooner.
One word of warning: Accelerators
can contain calcium chloride, but Fritz-Pak's Ojeda warns
decor contractors to stay away from the calcium chloride ones.
They can cause corrosion problems if exposed to water, react
negatively with integral colon, and promote efflorescence,
a drawback to aesthetics, he says.
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