Page 26 - Masonry and Concrete

Understanding Concrete
25
UNDERSTANDING CONCRETE

ment. The slump recommended to assure
proper water-cement ratio for residential
concrete is 3 to 5 inches. Slump tests can SLUMP
also be used to ensure consistent mixes
from batch to batch.
As a general guideline for ordering
ready-mix concrete, Figure 2-12 shows
recommended mix requirements for vari-
ous exposure conditions. The weathering
regions indicated on the map are intended CONE FILLED CONE REMOVED
IN THREE AND SLUMP
only as a guide. Particularly in mountain-
LAYERS MEASURED
ous regions, local conditions can change
FIGURE 2-11
within a very short distance and may be
Concrete slump test (from Concrete Construction Pub-
more or less severe than indicated by the
lications, Basics of Concrete, Concrete Construction
region classification. Severe exposures are Publications, Inc., Addison, Illinois).
those in which deicing salts are used
because of significant snowfall combined with extended periods in
which natural thawing does not occur. If you are in doubt about which
classification applies, always use the more severe exposure. Actual
concrete ingredient proportions can be measured either by volume or
by weight, as described in Chapter 3.

2.4 Formwork and Reinforcement

Formwork is used to shape the fluid concrete mixture and hold it in
place while it cures. It must be strong enough to withstand the pres-
sure of the wet mix, which can exert a considerable force until it
begins to harden and hold its own shape. Reinforcement is used to add
tensile strength to the concrete and to help resist shrinkage cracking.

2.4.1 Formwork Materials
Lumber and plywood are used to build forms or molds to contain the
concrete mix and shape it. Usually, 2 4s, 2 6s or 2 8s are used for
the actual form or mold, and 1 2s, 1 4s, or 2 4s for stakes and
1
braces to hold it in place. Metal landscape edging or /4-in. plywood or
hardboard can be used to form curved slab edges. Plywood used to form
curves will bend more easily if it is cut in strips perpendicular to the face
grain rather than along the grain. Form boards should be free of holes,

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