Admixtures are sometimes used in concrete mixtures
to improve certain qualities, such as workability,
strength, durability, watertightness, and wear
They may also be added to reduce
segregation, reduce the heat of hydration, entrain air,
and accelerate or retard setting and hardening.
We should note that the same results can often be
obtained by changing the mix proportions or by
selecting other suitable materials without resorting to
the use of admixtures (except air-entraining admixtures
Whenever possible, comparison
should be made between these alternatives to determine
which is more economical or convenient.
admixture should be added according to current
specifications and under the direction of the crew leader.
Materials, such as hydrated lime and bentonite,
are used to improve workability. These materials
increase the fines in a concrete mix when an
aggregate is tested deficient in fines (that is, lacks
sufficient fine material).
The deliberate adding of millions of minute
disconnected air bubbles to cement paste, if evenly
diffused, changes the basic concrete mix and
increases durability, workability, and strength. The
acceptable amount of entrained air in a concrete mix,
by volume, is 3 to 7 percent. Air-entraining agents,
used with types I, II, or III cement, are derivatives of
natural wood resins, animal or vegetable fats, oils,
alkali salts of sulfated organic compounds, and
water-soluble soaps. Most air-entraining agents are in
liquid form for use in the mixing water.
The only accepted accelerator for general
concrete work is calcium chloride with not more than
2 percent by weight of the cement being used. This
accelerator is added as a solution to the mix water and
is used to speed up the strength gain. Although the
final strength is not affected, the strength gain for the
first 7 days is greatly affected. The strength gain for
the first 7 days can be as high as 1,000 pounds per
square inch (psi) over that of normal concrete mixes.
The accepted use for retarders is to reduce the rate
This permits the placement and
consolidation of concrete before initial set. Agents
normally used are fatty acids, sugar, and starches.
Portland cement is packed in cloth or paper sacks,
each weighing 94 pounds. A 94-pound sack of cement
amounts to about 1 cubic foot by loose volume.
Cement will retain its quality indefinitely if it
does not come in contact with moisture. If allowed to
absorb appreciable moisture in storage, however, it
sets more slowly and strength is reduced. Sacked
cement should be stored in warehouses or sheds made
as watertight and airtight as possible. All cracks in
roofs and walls should be closed, and there should be
no openings between walls and roof. The floor
should be above ground to protect the cement against
dampness. All doors and windows should be kept
Sacks should be stacked against each other to
prevent circulation of air between them, but they
should not be stacked against outside walls. If stacks
are to stand undisturbed for long intervals, they
should be covered with tarpaulins.
When shed or warehouse storage cannot be
provided, sacks that must be stored in the open should
be stacked on raised platforms and covered with
waterproof tarps. The tarps should extend beyond the
edges of the platform to deflect water away from the
platform and the cement.
Cement sacks stacked in storage for long periods
sometimes acquire a hardness called warehouse pack.
This can usually be loosened by rolling the sack
around. However, cement that has lumps or is not
free flowing should not be used.
CONCRETE MIX DESIGN
LEARNING OBJECTIVE: Upon completing
this section, you should be able to calculate
concrete mix designs.
Before proportioning a concrete mix, you need
information concerning the job, such as size and
shapes of structural members, required strength of the
concrete, and exposure conditions. The end use of the
concrete and conditions at time of placement are
additional factors to consider.
The ingredient proportions for the concrete on a
particular job are usually set forth in the speci-
fications under CONCRETEGeneral Require-
See table 6-3 for examples of normal