Keep Your Concrete
Pliable with the Right Aggregate
By Gail Elber
Any
concrete mix is an attempt to balance cost, placeability and
setting time while maintaining the necessary qualities of
strength and durability. Ready-mix producers use different
ingredients from region to region depending on what's available
and what kind of weather the concrete will experience before
and after curing.
Coarse aggregate makes up more than
40 percent of a typical concrete mix. The type of aggregate
that is available varies in different areas. In the northern
parts of North America, round gravel occurs naturally, the
result of glacial action during the Ice Ages. Other aggregates
available are crushed rock, which is angular in form, and
recycled concrete.
So what is the best aggregate to use
for stamping applications? To answer this question requires
an understanding of aggregate's role in the mix.
 A
tale of two mixes
There's no such thing as "the best mix', for decorative
applications. "There's not necessarily one right way,
but there are upper and lower limits for each ingredient,
and if you work within them, you'll get better results,"
says Bruce Chattin, executive director of the Washington Aggregate
and Concrete Association. Examining two mixes from decorative
concrete contractors in different parts of North America shows
how great regional and personal differences can be.
Tom Ralston, of Tom Ralston Concrete
in Santa Cruz, California (www.tomralstonconcrete.com), starts
with this mix for one cubic yard of concrete:
Cement: 564 lb (6 sacks)
Sand: 1300-1500 lb
Aggregate: 1600-1800 lb
Water: 312 lb (39 gal)
Water reducer: 18 oz
Air-entraining admixture to produce 1 percent air (freezing
weather is rare in Santa Cruz, which is on the coast south
of San Francisco)
Ralston specifies that the mixture
be poured at a 3- to 4-ince slump. Rather than trying to predict
the weather, he orders the truckload to arrive 30 to 45 minutes
before the pour, then doses it on the spot with Fritz-Pak
admixtures (www.fritzpak.com) to accelerate or retard setting
as needed.
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