Proportioning and Mixing Concrete

A CONCRETE MIX is designed to produce concrete
that can be easily placed at the lowest cost.
The concrete must be workable and cohesive when
plastic, then set and harden to give strong and
durable concrete.
The mix design must consider the environment that
the concrete will be in; ie exposure to sea water,
trucks, cars, forklifts, foot traffic or extremes of hot
and cold.
PROPORTIONING Concrete is a mixture of
Cement, Water, Coarse and Fine Aggregates and
Admixtures.
The proportions of each material in the mixture affects the properties of the final hardened
concrete. These proportions are best measured by weight. Measurement by volume is not
as accurate, but is suitable for minor projects.
CEMENT CONTENT As the cement content
increases, so does strength and durability.
Therefore to increase the strength, increase
the cement content of a mix.
WATER CONTENT Adding MORE WATER to
a mix gives a WEAKER hardened concrete.
Always use as little water as possible, only
enough to make the mix workable.
WATER TO CEMENT RATIO As the Water to
Cement ratio INCREASES, the strength and
durability of hardened concrete DECREASES.
To increase the strength and durability of
concrete, decrease the Water-Cement ratio.
See CHAPTER 2 Concrete Properties



AGGREGATES Too much fine aggregate gives a sticky mix.


Too much coarse aggregate gives a harsh or boney mix.
MIXING Concrete must be mixed so the Cement, Water,
Aggregates and Admixtures blend into an even mix.
Concrete is normally mixed by MACHINE.
Machine mixing can be done on-site or be a Pre-Mixed
concrete company. Pre-Mixed concrete is batched
(proportioned) at the plant to the job requirements.
Truck Mixing The materials are normally added to the
trucks at batching plants and mixed for required time and
speed at the plant. The trucks drum continues to rotate to
agitate the concrete as it is delivered to the site.
Site Mixing When site mixing begin by loading a MEASURED AMOUNT of coarse
aggregate into the mixer drum. Add the sand before the cement, both in measured amounts.
NEVER USE A SHOVEL AS A MEASURE AS VOLUMES CAN VARY WIDELY.
Mix materials together until there is no visible sand in the mix.
Add enough water to get a workable mix.
Be careful not to overload the mixer. Too much
concrete in the mixer means each batch takes longer
to be properly mixed, which causes costly delays
in the long run or it will not mix at all.
Always check how much the mixer holds so you know
how much concrete can be produced at once.
Avoid delays between batches to get maximum output.
Small quantities of concrete may be mixed by hand with
a shovel. Mixing should be done on a clean board, or plate,
or in a wheelbarrow. Mix the materials together until they are even.
Then dish the material and add water. Use only enough water to get
an even, workable mix. Finish mixing.