| With this
in mind, Dave Pettigrew, principal of Diamond D Concrete (www.diamonddcompany.com)
in Capitola, California (near Santa Cruz), used to use pea
gravel for stamping applications, the size of aggregate he
still favors for concrete that is going to be pumped. But
now, for stamping, he specifies a 1/2-inch angular rock for
interior floors and a 3/4-inch angular rock for foundations,
which require more strength. He points out, "With smaller
rock, you have to use more control joints because it is not
as strong," something to consider when you're planning
a stamp job and want to position those control joints inconspicuously.
Aggregate
and water
Aggregate size also affects the amount of water needed in
the mix. Ralston explains, "Because smaller aggregate
particles have more space between them, more sand is added.
To make the concrete more workable, people want to add more
water, which increases the shrinkage potential." Instead
of adding water, he says, a better practice is to add water-reducing
admixture to improve workability without introducing these
problems. Slump should not exceed 4 inches.
Other
admixtures
Water reducers aren't the only admixture that is useful. Another
ingredient that can improve workability without compromising
strength is type F fly ash. Chattin suggests using it to replace
10-15 percent of cement in a mix. It will produce some of
the effects of adding more water -- increased setting time
and improved workability -- without compromising strength
as increased water does. Granulated stag cements, relatively
inexpensive in the steel-making regions of North America,
can have the same effect and, being more uniform in content,
are more predictable in their air-entrainment properties.
An important consideration for decorative applications is
that Type F fly ash makes concrete that's darker than concrete
made with plain portland cement, and granulated slag cement
makes it lighter in color.
Tom Ralston uses 15-20 percent fly ash
to replace cement, but he doesn't do it to get workability
benefits. In his town, many of his residential and commercial
customers have heard of the environmental benefits of fly
ash -- getting particulates out of the air while it reduces
the carbon dioxide production associated with cement manufacture
-- and are eager to specify it for that reason.
An ingredient to avoid if you are planning
to use color hardener is calcium chloride, which will interfere
with the action of the colorant. If you're going to experiment
with other admixtures, make sure to consult the manufacturers
about each chemical's interactions with other components of
the mix. "Crack reducing and air-entrainment admixtures
'fight,' and it weakens the mix, which becomes spongy,"
Ralston explains. "It won't set up properly. I've seen
it not ready to trowel after 24 hours."
Still, correct use of admixtures can
shave time off a job. "They may cost more money, but
you could be done an hour sooner or use fewer workers on the
job," Chattin points out. Just consult the manufacturers
and the ready-mix producer to make sure that you don't brew
up a batch of trouble in your mix.
An
important relationship
In fact, it's always a good idea
to keep your ready-mix provider informed about what you're
going to do with the concrete. Sit down with the ready-mix
rep every so often to talk about the mixes you need for different
kinds of work. "Having a relationship with a ready-mix
producer is important," says Chattin. You know the habits
of your crew and the patterns of your climate, and the ready-mix
rep knows the materials that are available locally and the
quirks of the admixtures on the market. Together you can put
together a mix tailored for your decorative applications.
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