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Methods
For Proper Concrete Curing
Note: The discussion below pertains only to the curing
of non-colored concrete for normal applications. For
colored, architectural or any exceptional types of
concrete, special curing procedures are required.
In such cases it is advisable to contact the material
manufacturer. For detailed information on recommended
practices for curing concrete, refer to "Standard
Practice for Curing Concrete" (ACI 308-81).
Why Curing Is So Important
Improper curing can easily cut the strength of
even the best concrete by 50%. Curing simply means
keeping the water in the concrete where it can Do
its job of chemically combining with the cement to
change the cement into a tough "glue" that will help
develop strong, durable concrete. Good curing means
keeping the concrete damp and at about 70°F (21°C)
until the concrete is strong enough to do its job.
Recommended practice calls for a minimum of seven
days curing for ambient temperatures about 40°F (4°C)
or the time necessary to attain 70% of the specified
compressive or flexural strength, whichever period
is less.
All concrete must be cured to attain maximum strength.
Correctly cured concrete is best from every standpoint:
It shrinks less, cracks less and dusts less. It is
stronger, more durable and has a more wear resistant
surface.
Start curing the concrete as soon as possible after
it has hardened. Early drying- especially in hot,
windy weather- must be prevented or the concrete will
not attain its full potential quality. Refer to ACI
Committee 305R, "Hot Weather Concreting".
Results of Proper Curing
More
Durable Concrete: Good concrete, properly cured, means
fewer pores and crevices where water can enter, freeze,
expand and crack the concrete. Air entrainment helps
make more durable concrete, but its use must also
be accompanied by proper curing.
More Wear-Resistant Concrete: Well-cured concrete
(28 Day curing period) will develop a surface twice
as wear resistant as a surface that us cured for only
three days. Proper curing prevents dusting, and means
less cracking, crazing and spalling of the concrete.
All in all, the better the curing, the better the
concrete.
Methods of Curing Membrane Curing Compounds Membrane
Curing Compounds: Are the preferred (most practical
and widely used) method of sealing moisture in the
concrete. Easy to spray, roll or brush on and low
in cost, only one application is generally needed.
This method is effective for slabs, vertical walls
or irregular surfaces. For flatwork, cover the dried
curing compound with scuff proof building paper or
polyethylene tarp to protect the surface from marring
by other trades until the curing is complete. If a
membrane curing compound is used, make sure it meets
the moisture retention requirements of ASTM C 309
at the coverage rate specified.
Water Spray: is a good curing method but only if the
concrete is kept continually damp. Allowing the concrete
surface to dry between sprinklings can cause crazing
of the surface and cracking of slabs.
Waterproof Curing Paper: holds moisture in the concrete
by preventing evaporation. First, water spray the
concrete surface, then cover with a non-staining,
waterproof paper. Overlap edges and seal with waterproof
tape. This method is acceptable for slabs but not
practical for walls, irregular surfaces or colored
floors.
Damp Burlap: is sometimes an effective method of curing.
However the burlap must be washed and free of any
foreign substances which might stain or harm the concrete.
Also wet burlap may be hard to handle for large areas.
Once the damp burlap has been spread over the for
large areas. Once the damp burlap has been spread
over the concrete surface, it must be kept continually
damp with water spray to replace the water lost through
evaporation. It should not be used on colored or metallic
floors.
Other Methods include plastic sheets which are completely
water tight, light in weight and easy to handle. The
give good protection during curing. However, care
must be taken to ensure that such sheets lie flat
against the surface, otherwise mottling of the surface
may result.
Damp earth, sand, straw and hay can also be recommended,
but their use should be limited to emergency situations.
Earth and sand are messy, hard to handle and to clean
up. Straw and hay dry out quickly can blow away and
can be a fire hazard. None of these methods should
be used on colored or metallic floors.
Cure Concrete Longer When Temperature is Below 70°F
(21°C) Concrete strength develops more slowly at lower
temperatures. Below 40°F (4°C) do not expect satisfactory
performance from the concrete unless special precautions
are taken. Closely follow job specifications on curing
time. In the absence of specifications, concrete should
be cured, protected from harmful temperatures and
not used until it has developed the required strength.
Heaters, if used, must be vented so combustion fumes
are exhausted outside the enclosure in order to avoid
carbonation of the fresh concrete. Refer to ACI Committee
306R, "Cold Weather Concreting".
Curing Compounds And Curing And Sealing Compounds
Difference
of CURING compounds and Curing And Sealing compounds
is the resin used. Curing Compounds use a resin that
breaks down in months, and curing and sealing compounds
use a resin that will resist sun light, abrasion,
most chemicals and will be on the surface of the concrete
for a while.
Effects
Of Water Mixed Into Concrete
If one gallon of water is added to a yard of properly
designed 3000 psi concrete mix, you: Increase
the slump about one inch.
Reduce the compressive strength
by as much as 200 psi.
Waste the effect of 1/4 bag of cement.
Increase the shrinkage potential
about 10%.
Increase the possibility of seepage
through concrete by up to 50%.
Decrease the freeze-thaw resistance
by 20%.
Decrease the resistance to attack
by de-icing salts.
Lower the quality in many other
ways.
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