Page 60 - Masonry and Concrete

Concrete Construction Techniques
59
CONCRETE CONSTRUCTION TECHNIQUES

buy a packaged concrete mix containing cement, sand, and gravel. The
ingredients are already in the correct proportions, and all you have to
do is add water to make fresh concrete. Packaged mixes are very con-
venient but are economical only for very small quantities of concrete.
For batches requiring more than a few bags of packaged mix but less
than the one cubic yard minimum which ready-mix suppliers usually
require, it is more economical to mix concrete on site using portland
cement and bulk aggregates.
Hand mixing concrete is very simple. Usually a large and sturdy
wheelbarrow is the best container to use because you can mix in it,
transport the concrete, and pour it into the forms. Clean and rinse the
wheelbarrow before adding your materials. Place the correct propor-
tion of each dry ingredient, and mix them together in the wheelbarrow
with a mason’s hoe. Make a depression in the middle, pour part of the
water in, and mix it with the dry ingredients. Add the rest of the water,
and mix all the ingredients thoroughly again. Hand mixing is not vig-
orous enough to produce proper air entrainment, so hand mixing
should not be used for concrete which requires air-entrained cement
or air-entraining admixtures.

3.5.2 Machine Mixing

Machine mixing is faster and a little less backbreaking than hand mix-
1
ing. You can rent a small concrete mixer with a capacity ranging from 2
to 6 cubic feet. The size of the concrete batch is usually only about 60%
of the total volume of the mixer. This allows room for proper mixing and
rotation without spilling. Never load a mixer beyond its maximum
batch size. For volume proportions, use a bucket or shovel to measure

LAP MINIMUM 30
BAR DIAMETER

FIGURE 3-19
Reinforcing bar lap splices (from Adams, J. T., The Complete Concrete, Masonry and
Brick Handbook, Van Nostrand Reinhold, New York).

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