Introduction - Design of Reinforced Concrete - page 10

closely spaced air bubbles are incorporated into the concrete. When concrete
freezes, water moves into the air bubbles, relieving the pressure in the concrete.
When the concrete thaws, the water can move out of the bubbles, with the result
that there is less cracking than if air-entrainment had not been used.

2. The addition of accelerating admixtures such as calcium chloride to concrete will
accelerate its early-strength development. The results of such additions (particularly
useful in cold climates) are reduced times required for curing and protection of the
concrete and the earlier removal of forms. (Section 3.6.3 of the ACI Code states that
because of corrosion problems, calcium chloride may not be added to concretes with
embedded aluminum, concretes cast against stay-in-place galvanized steel forms, or
prestressed concretes.) Other accelerating admixtures that may be used include vari-
ous soluble salts as well as some other organic compounds.

3. Retarding admixtures are used to slow the setting of the concrete and to retard
temperature increases. They consist of various acids or sugars or sugar deriva-
tives. Some concrete truck drivers keep sacks of sugar on hand to throw into the
concrete in case they get caught in traffic jams or are otherwise delayed. Retarding
admixtures are particularly useful for large pours where significant temperature
increases may occur. They also prolong the plasticity of the concrete, enabling
better blending or bonding together of successive pours.

4. Superplasticizers are admixtures made from organic sulfonates. Their use enables
engineers to reduce the water content in concretes substantially while at the same
time increasing their slumps. Although superplasticizers can also be used to keep
constant water-cement ratios while using less cement, they are more commonly
used to produce workable concretes with considerably higher strengths while
using the same amount of cement.

5. Usually, waterproofing materials are applied to hardened concrete surfaces, but
they may be added to concrete mixes. These admixtures generally consist of some
type of soap or petroleum products, as perhaps asphalt emulsions. They may help
retard the penetration of water into porous concretes but probably don't help
dense, well-cured concretes very much.

1.11 PROPERTIES OF REINFORCED CONCRETE

A thorough knowledge of the properties of concrete is necessary for the student before he
or she begins to design reinforced concrete structures. An introduction to several of these
properties is presented in this section.

Compressive Strength
The compressive strength of concrete ( ) is determined by testing to failure 28-day-old
6-in. by 12-in. concrete cylinders at a specified rate of loading. For the 28-day period the
cylinders are usually kept under water or in a room with constant temperature and 100%
humidity. Although concretes are available with 28-day ultimate strengths from 2500 psi
up to as high as 10,000 to 20,000 psi, most of the concretes used fall into the 3000- to

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