| In Cincinnati, Ohio,
Bob Ware, president of the Decorative Concrete Store (www.decorativeconcretestore.com),
favors a different mix appropriate to his freeze-thaw climate:
Cement: 564 lb
Sand: 1330 lb
No. 8 gravel (pea gravel): 770 lb
No. 57 gravel (a crushed stone that can be as large as 1 1/2
in.): 1130 lb
Water: 230 lb
Water reducer: 21 oz
Air-entraining admixture to produce 6 percent air content
Though Ware also pours at a 4-in slump,
his mix contains a lot of water reducer to make the water
content unusually low in comparison with Ralston's.
With these general mix proportions in
mind, let's look at how the size and shape of aggregate affect
the properties of the wet mix and the finished product.
Aggregate
and strength
First, the shape of the aggregate affects the strength of
the finished concrete. In general, angular crushed particles
give concrete greater strength because they interlock more
than rounded pea gravel particles do. Mixing sizes, as Ware
does, enhances that interlocking action.
"In our area, we have a lot of
round aggregate because of deposits of glacial till,"
Ware explains. "To get an angular gravel, we would have
to have it trucked in." When he does a job in the southern
part of his territory, south of where the glaciers stopped,
he takes advantage of the ready availability of crushed rock.
"Angular gravel is better,"
he says. “Round gavel is soft. You have to push it deeper
into concrete than angular gravel, or it will break and spall
the surface."
Size also influences strength: The bigger
the aggregate, the stronger the finished concrete will be,
since the chunks of rock don't crack as cement paste does.
Aggregate
and stamping
But if you're going to stamp the
concrete, big chunks of rock will interfere with the impression
of the stamps. Brickform Rafco (www.brickfixm.com) recommends
using aggregate no bigger than 3/8 inch with its stamps; other
manufacturers let you go as high as 3/4 inch. Ralston uses
3/4-inch angular hard aggregate for jobs that will be textured
with mats. If he plans to use cookie-cutter-type stamps, which
penetrate more deeply, Ralston switches to pea gravel or 1/2-inch
crushed stone. "The larger stone won’t allow the
stamp to make an impression," he says.
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