concrete will not be as watertight as a cement-only
concrete, nor will it have as much initial strength.
Additional tests may have to be made to determine when
to remove the forms. Its final strength, however, will be
as great as a cement-only concrete.
Densifiers
Dense concrete is required in some types of
construction, such as in prestressed structures. This
density is achieved when cement particles are separated
evenly throughout the mix or at least prevented from
attaching to each other (flocculating). A detergent
admixture will disperse the particles individually and
will create a more uniform paste. These admixtures also
reduce the formation of a cement gel that expands at the
early stages of hydration and pushes the particles apart,
thus increasing the volume. Prevention of this expansion
results in a denser paste.
Waterproofing Agents
Watertightness can be controlled to a great extent by
lowering the water-cement ratio. This may not always
be practical, and sometimes even with a low
water-cement ratio, capillaries still form through the
concrete. Densifying or using an accelerator like
calcium chloride improves the watertightness.
Air-Entraining Agents
The greatest improvement in watertightness and
resistance to deterioration under freezing and thawing
is obtained by incorporating 4 to 6 percent, by volume,
of entrained air into the mix. Workability of fresh
concrete is enhanced by such entrained air. Air-
entrained cement contains the necessary admixture.
Soaps, butylstearate, some of the fine pozzolans, and
several proprietary compounds are available for use as
air-entraining admixtures with ordinary cements. These
agents minimize the formation of capillaries and plug
the tiny holes with a water-repellant or sealing material.
They provide small, uniformly spaced, discrete air voids
that prevent the buildup of damaging pressures from the
expansion of freezing water into ice.
CURING
Concrete does not develop its full strength until the
chemical process of curing (or hydration) is complete.
Curing takes place over an extended period—the most
critical portion of which is from the day of placement
through the 10th day. The extent and rate of curing
depends upon the presence of moisture and the correct
temperature within the mix.
Temperature
The ideal temperatures for concrete work are
between 55°F and 70°F. Above this, rapid evaporation
of moisture creates a problem. At lower temperatures,
the curing or setting is delayed. Temperatures below
32°F stop the hydration process. Since the chemical
action gives off some heat, some method must be used
to keep the heat within the structure during times of low
temperatures. Cold weather construction may even
require heating the ingredients, or mix, and covering the
emplaced concrete or providing a heated enclosure. In
hot weather, extra care is required to prevent a high
temperature rise and too rapid drying of the fresh
concrete. Moistening the aggregate with cool water will
lower the generated temperature. The water is kept cool
as possible by the application of reflective white or
aluminum paint to water supply lines and storage tanks.
On massive construction, such as dams and heavy
retaining walls, the mixing water is often cooled
artificially or ice is substituted for part of the water. This
ice must be melted by the time the concrete is fully
mixed and ready to leave the mixer. Cement replace-
ment materials (such as pozzolans of diatomaceous
earth, pumicites, or fly ash) may be used to depress
concrete temperature by reduction of the heat of
hydration in a structure; however, pozzolans vary
widely and may have adverse effects on strength, air
content, and durability, if used in excessive amounts.
Moisture
Concrete curing depends upon chemical action in
the presence of water. Any loss of moisture during the
process by seepage or evaporation prevents complete
hydration and development of optimum strength and
watertightness. Saturating the subgrade on which the
concrete will be placed will delay, if not prevent,
seepage. Wood forms should be thoroughly wetted if
they have not been treated otherwise. Covering the
concrete without marring the surface as soon as possible
after finishing is one method used to reduce evaporation.
This covering may be some material, such as burlap,
straw, or plastic film, or it may be a chemical curing
compound that is sprayed over the finished surface.
After the initial set is attained, water can be applied
directly to the surface to keep the hydration process in
action. This water application can be part of the
temperature control. The increase on concrete
compressive strength with age is shown by curves in
13-28
