Page 20 - Masonry and Concrete

Understanding Concrete
19
UNDERSTANDING CONCRETE

1 / 5 FORM TO FORM

3 / 4 REBAR TO FORM
1 / 3 SLAB
DEPTH

FIGURE 2-7
Maximum aggregate size relationship to concrete forms and reinforcing steel.

ity and make slab surfaces easier to finish. Masonry sand is gener-
ally not appropriate for concrete because it contains only the
smaller particle sizes and can cause the mix to be sticky rather than
plastic.
Sand should be free of contaminants that can be harmful to con-
crete such as silt, clay, and organic materials such as leaves and roots.
The cleanness of coarse aggregate can usually be judged by visual
inspection, but sand contamination is a little more difficult to detect.
There are sophisticated laboratory tests which can determine the exact
type and amount of contaminants in concrete aggregates, but there is
also a simple field test. Put 2 inches of sand in a quart jar, add water
until the jar is about three-fourths full, shake it for one minute, then let
it stand for an hour. If more than /8 in. of sediment settles on top of the
1
sand, it should be washed by drenching with a garden hose the day
before it will be used (Figure 2-9).
Gravel and crushed stone are sold by the ton or half ton, and can be
purchased from an aggregate company or a ready-mix producer. Order
1
3
1
a clean, graded mix ranging from /4 in. to either /4-, 1-, or 1- /2-in.
diameter as appropriate for the project. Small quantities of sand can
usually be purchased at building supply yards. Larger quantities of
sand are sold by the ton or half ton by aggregate suppliers and ready-
mix producers. Order clean, natural, concrete sand.

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