when the form has been filled up to the level of the
block, as shown in figure 3-12.
DESIGN OF CONCRETE MIXTURES
From your previous studies, you know that the
basic ingredients used in the production of concrete
are cement (usually portland cement), water, and both
fine and coarse aggregates. You also know that certain
admixtures are used occasionally to meet special
requirements. The design of a concrete mixture
consists of determining the correct amount of each
ingredient needed to produce a concrete that has the
necessary consistency or workability in the freshly
mixed condition and that has desired strength and
durability characteristics in the hardened condition.
The characteristics of concrete should be
considered on a relative basis and in terms of degree
of quality required for a given construction project.
Figure 3-13 shows some of the properties of good
concrete, their interrelationships, and various
elements that control the properties. A study of this
figure points up the relative basis of the
characteristics. A single batch of concrete cannot
possess the maximum of strength, durability, and
economy. For example, entrained air makes handling
easier and is, therefore, conducive to economy;
entrained air promotes watertightness; but entrained
air makes concrete less dense and thereby reduces the
strength. The goal is to achieve an optimum balance
of all the elements. A thorough discussion of all the
factors involved in the production of good concrete is
beyond the scope of this manual. A wealth of
information is available to you in government and
commercial publications, especially the American
Concrete Institute (ACI) manuals.
The design of or the selection of a mix, the
necessity for a trial mix, the methods of controlling the
mix proportions, and the units of measure to be used
in the batching all depend on the nature and size of the
job and the extent to which requirements are set forth
in the specifications or on the plans.
An example of the simplest form of concrete
batching is the mixing of a very small amount of
concrete using the 1:2:4 carpenters mix. The relative
volumes of cement, sand, and gravel could be
measured in bucketfuls, or even in shovelful, and
with sufficient water added to give reasonable
consistency. A more refined procedure is to fabricate
a 1-cubic-foot wooden measuring box to give you
greater control over the proportions of the ingredients.
To mix approximately 1 cubic yard of 1:2:4 concrete,
you use the Rule of 42. Add the numbers of the mix
design together 1 + 2 + 4 = 7, then divide the rule (42)
by the mix design (7), which equals 6. This means it
Figure 3-12.Steel in place in a wall.