Civil Engineering Reference
In-Depth Information
3. Methods that accelerate strength gain by supplying
heat and additional moisture to the concrete. This is
usually accomplished with live steam, heating coils,
or electrically heated forms or pads.
The method or combination of methods chosen
depends on factors such as availability of curing materials,
size, shape, and age of concrete, production facilities (in
place or in a plant), esthetic appearance, and economics.
As a result, curing often involves a series of procedures
used at a particular time as the concrete ages. For example,
fog spraying or plastic covered wet burlap can precede
application of a curing compound. The timing of each
procedure depends on the degree of hardening of the con-
crete needed to prevent the particular procedure from
damaging the concrete surface (
ACI 308 1997
).
Ponding and Immersion
Fig. 12-4. Fogging minimizes moisture loss during and after
placing and finishing of concrete. (69974)
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 main-
taining 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 con-
sists 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 pond-
ing or immersion must be free of substances that will stain
or discolor the concrete. The material used for dikes may
also discolor the concrete.
water by using burlap or similar materials; otherwise
alternate cycles of wetting and drying can cause surface
crazing or cracking.
Wet Coverings
Fabric coverings saturated with water, such as burlap,
cotton mats, rugs, or other moisture-retaining fabrics, are
commonly used for curing (Fig. 12-5). Treated burlaps that
reflect light and are resistant to rot and fire are available.
The requirements for burlap are described in the
Specifica-
tion for Burlap Cloths Made from Jute or Kenaf
(AASHTO M
182), and those for white burlap-polyethylene sheeting are
described in ASTM C 171 (AASHTO M 171).
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 flat-
work, thus slowing evaporation from the surface. Fogging
is applied to minimize plastic shrinkage cracking until fin-
ishing operations are complete. Once the concrete has set
sufficiently to prevent water erosion, ordinary lawn sprin-
klers 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.
The cost of sprinkling may be a disadvantage. The
method requires an ample water supply and careful
supervision. If sprinkling is done at intervals, the concrete
must be prevented from drying between applications of
Fig. 12-5. Lawn sprinklers saturating burlap with water keep
the concrete continuously moist. Intermittent sprinkling is
acceptable if no drying of the concrete surface occurs.
(50177)
