10-08-2012, 03:00 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. If temperatures are favorable, hydration is relatively
rapid the first few days after concrete is placed; however, it
is important for water to be retained in the concrete during
this period, that is, for evaporation to be prevented or substantially
reduced.
With proper curing, concrete becomes stronger, more
impermeable, and more resistant to stress, abrasion, and
freezing and thawing. The improvement is rapid at early
ages but continues more slowly thereafter for an indefinite
period. Fig. 12-2 shows the strength gain of concrete with
age for different moist curing periods and Fig. 12-3 shows
the relative strength gain of concrete cured at different
temperatures.
CURING METHODS AND MATERIALS
Concrete can be kept moist (and in some cases at a favorable
temperature) by three curing methods:
1. Methods that maintain the presence of mixing water
in the concrete during the early hardening period.
These include ponding or immersion, spraying or
fogging, and saturated wet coverings. These methods
afford some cooling through evaporation, which is
beneficial in hot weather.
2. Methods that reduce the loss of mixing water from
the surface of the concrete. This can be done by covering
the concrete with impervious paper or plastic
sheets, or by applying membrane-forming curing
compounds.
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.