03-08-2012, 02:27 PM
Curing Concrete
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Curing is the maintenance of a satisfactory moisture content
and temperature in concrete for a period of time immediately
following placing and finishing so that the desired
properties may develop (Fig. 12-1). The need for adequate
curing of concrete cannot be overemphasized. Curing has a
strong influence on the properties of hardened concrete;
proper curing will increase durability, strength, watertightness,
abrasion resistance, volume stability, and resistance to
freezing and thawing and deicers. Exposed slab surfaces
are especially sensitive to curing as strength development
and freeze-thaw resistance of the top surface of a slab can
be reduced significantly when curing is defective.
When portland cement is mixed with water, a chemical
reaction called hydration takes place. The extent to which
this reaction is completed influences the strength and durability
of the concrete. Freshly mixed concrete normally contains
more water than is required for hydration of the cement;
however, excessive loss of water by evaporation can
delay or prevent adequate hydration. The surface is particularly
susceptible to insufficient hydration because it dries
first.
Ponding and Immersion
On flat surfaces, such as pavements and floors, concrete
can be cured by ponding. Earth or sand dikes around the
perimeter of the concrete surface can retain a pond of
water. Ponding is an ideal method for preventing loss of
moisture from the concrete; it is also effective for maintaining
a uniform temperature in the concrete. The curing
water should not be more than about 11°C (20°F) cooler
than the concrete to prevent thermal stresses that could
result in cracking. Since ponding requires considerable
labor and supervision, the method is generally used only
for small jobs.
The most thorough method of curing with water consists
of total immersion of the finished concrete element.
This method is commonly used in the laboratory for
curing concrete test specimens. Where appearance of the
concrete is important, the water used for curing by ponding
or immersion must be free of substances that will stain
or discolor the concrete. The material used for dikes may
also discolor the concrete.
Fogging and Sprinkling
Fogging (Fig. 12-4) and sprinkling with water are excellent
methods of curing when the ambient temperature is well
above freezing and the humidity is low. A fine fog mist is
frequently applied through a system of nozzles or
sprayers to raise the relative humidity of the air over flatwork,
thus slowing evaporation from the surface. Fogging
is applied to minimize plastic shrinkage cracking until finishing
operations are complete. Once the concrete has set
sufficiently to prevent water erosion, ordinary lawn sprinklers
are effective if good coverage is provided and water
runoff is of no concern. Soaker hoses are useful on surfaces
that are vertical or nearly so.
Plastic Sheets
Plastic sheet materials, such as polyethylene film, can be
used to cure concrete (Fig. 12-7). Polyethylene film is a
lightweight, effective moisture retarder and is easily
applied to complex as well as simple shapes. Its application
is the same as described for impervious paper.
Curing with polyethylene film (or impervious paper)
can cause patchy discoloration, especially if the concrete
contains calcium chloride and has been finished by hardsteel
troweling. This discoloration is more pronounced
when the film becomes wrinkled, but it is difficult and
time consuming on a large project to place sheet materials
without wrinkles. Flooding the surface under the covering
may prevent discoloration, but other means of curing
should be used when uniform color is important.
Membrane-Forming Curing Compounds
Liquid membrane-forming compounds consisting of
waxes, resins, chlorinated rubber, and other materials can
be used to retard or reduce evaporation of moisture from
concrete. They are the most practical and most widely
used method for curing not only freshly placed concrete
but also for extending curing of concrete after removal of
forms or after initial moist curing. However, the most
effective methods of curing concrete are wet coverings or
water spraying that keeps the concrete continually damp.
Curing compounds should be able to maintain the relative
humidity of the concrete surface above 80% for seven days
to sustain cement hydration.