FOR A STRONG PRODUCT,
KEEP FRESH CONCRETE MOIST AND WARM FOR AS LONG AS POSSIBLE
BY DAVID CARNS
projects call for concrete in some form, whether as footings,
walls, or flatwork. As a frequently used material, concrete
is also a common source of problems. Foulups can occur at
any stage, from batching, mixing, and transporting to placement,
finishing, or even curing, the final step. Concrete that has
been handled correctly all along can still be ruined if it's
not properly cured.
Warm and Wet: Keys to Curing
Curing means taking steps to keep the concrete under the right
temperature and moisture conditions during the first few days
of hardening after placement. Proper curing is vital because
the concrete will eventually be much harder and stronger if
it is cured correctly.
hardening of concrete is not a drying process, but rather
the result of a chemical reaction between the finely ground
portland cement particles and the water in the mix. This reaction
is known as hydration. Like most chemical reactions, hydration
is greatly influenced by temperature. The basic idea behind
proper curing is to allow this reaction to continue as long
as practical by maintaining a suitable curing temperature,
usually 50°F to 90°F, and by keeping the concrete
wet. If the temperature of the concrete drops below 50°F,
hydration begins to slow, and if the water in the mix freezes,
the concrete will be ruined. Also, if too much water escapes
from the concrete, hydration will stop altogether. The longer
favorable conditions are maintained, the longer the concrete
will cure, resulting in a better product.
of Good Curing
Although it may require some extra effort on the average residential
job site, maintaining proper curing conditions will result
in superior concrete. Properly cured concrete has several
- It is stronger in compression (see
"Moist-Curing for Maximum
Strength," next page).
- It resists abrasion much better.
This is important for concrete floors and pavements. o It
is more durable; in particular, it is better able to withstand
repeated freezing and thawing over the years.
- It is less permeable. This makes
big difference in concrete walls designed to keep out moisture,
such as basements.
A note of caution. Keep in mind
that proper curing will not compensate for poor quality concrete.
Concrete that is poured too wet will be weak regardless of
how it is cured. Not all of the water that is added to the
concrete mix is necessary for the hydration process. In fact,
the amount of water required to completely hydrate the cement
is only about one half to two thirds of what is usually added
at the batch plant or on the job site. The rest is added strictly
to make the mix more workable. Adding too much water, however,
might save work during placement, but it will also result
in very weak, porous concrete, even with proper curing.
How and When to Cure
Hydration begins as soon as the cement particles come in contact
with the mix water. Curing begins immediately after placement,
or in the case of flatwork, immediately after the final finishing
is done. You should move quickly to create suitable curing
conditions. Even when the temperature is within the 50°F
to 90°F range, you need to prevent evap- oration from
the concrete by sealing it or keeping it wet. If hot, dry,
or windy weather is likely to cause excessive evaporation,
or if cold temperatures threaten to freeze the concrete, the
need to control temperature and moisture becomes even greater.
For milder weather, here are some techniques that work well
on residential job sites:
Slabs and flatwork. You can
keep a slab wet with a continuous water spray, such as a lawn
sprinkler. This provides the moisture necessary for the slab
surface to properly cure, and cools the slab to slow evaporation
during hot weather. However, you have to keep an eye on things,
which isn't always practical. I remember one time when a homeowner
came home and helpfully turned off the sprinkler, thinking
that the contractor had forgotten about it. The slab dried
out and curing stopped.
You can dam the edges of a slab with
dirt or plastic and pond the surface with water. This works
fine in theory, but it has the same drawback as the water
spray: You have to keep watch.
You can also cover a slab with polyethylene
sheets. Use clear poly, 4 mil or heavier. Do not use black
poly, especially in hot weather. The idea is to keep the moisture
contained in the concrete from evaporating. If the plastic
has even small holes or tears, or if a tight seal is not maintained
around the edge of the slab, moisture will escape and curing
will stop. Tape the plastic to any protruding rebar or plumbing,
lap and weight any seams, and weight all the edges. This method
is practical and inexpensive, and works well when the poly
is completely sealed.
There is one drawback: Where the plastic makes contact with
the concrete, it can discolor the slab surface. If appearance
is important, consider one of the other methods. Curing compound.
You can apply a curing compound. These
commercially available compounds provide a thin sealing film
that accomplishes the same thing as a plastic sheet, holding
moisture in the concrete. These compounds may be either clear
or pigmented; with the pigmented product, you can tell where
you've sprayed. They are relatively inexpensive - a gallon
sells for about $8 and covers 200 to 300 hundred square feet.
A garden sprayer works well for applying the compounds. Usually
two coats are recommended, the second at right angles to the
first, to ensure complete coverage. Keep in mind that all
surfaces should be coated, including slab edges.
Although curing compounds will eventually
wear off, some will prevent adhesion of paint, carpet, vinyl
cements, or future concrete. Consult your supplier if this
is a consideration. Curing compounds are widely used
on both residential and commercial jobs,
with good reason. They are easy to apply, inexpensive and
effective, and they require little or no attention once applied.
Best of all, you can use the slab as soon as it has cured
enough that it won't be easily damaged.
Formed concrete. The best way
to cure formed concrete walls is to leave the forms in place
as long as practical - a minimum of three days is best. The
formwork holds in moisture and, in cold weather, heat. In
hot weather, keep the forms hosed down to control temperature
and reduce moisture evaporation from the concrete. Any exposed
concrete, such as the top of a concrete wall, should be covered
with plastic or sprayed with curing compound. If the forms
are stripped earlier than three days, the concrete should
be either covered with plastic for several days or sprayed
with curing compound. There are some obvious practical considerations
here. The idea is to allow the concrete to cure for a reasonable
length of time, yet not delay the rest of the job.
How Long to Cure? The answer
to this question is the longer the better. Concrete will continue
to hydrate and gain strength almost indefinitely as long as
moisture is present and a suitable temperature is maintained.
Try drilling or jackhammering 30-year-old concrete and you'll
see what I mean. If concrete is cured in cool temperatures
(32°F to 50°F), strength gain will be slow but the
concrete will eventually reach a high strength as long as
moisture is continuously present. However, concrete should
not be allowed to get hotter than 90°F or to dry out during
the curing period. If the concrete dries out too early in
the hydration process (within the first three days), long-term
strength will be compromised even if moist conditions return.
If your intention is to produce the highest quality concrete,
keep freshly poured concrete moist at 50°F to 90°F
for seven days for Types I or II portland cement ("regular"
concrete) or for three days if Type III cement (highearly,
or rapid-hardening) or accelerators are used.
As a practical matter, though, most
concrete subs strip their wall forms after a few days, and
few builders take the trouble to continue the curing process
beyond that point. This is usually not a problem since, after
curing for a few days behind the forms, 8- inch-thick concrete
walls are plenty strong to carry residential loads (though
you have to be careful about backfilling too soon). For slabs,
though, with their increased surface area, you should pay
more attention to curing, especially since conspicuous surface
defects may be unacceptable.
Hot Weather Curing
The biggest problem in hot
weather is excessive moisture loss from the concrete surface
(see "Bad Weather Curing," facing page). This can
be serious for thin concrete members, such as slabs, that
have a large surface exposed to the weather. Surface evaporation
is affected by a combination of four factors: concrete temperature,
air temperature, relative humidity, and wind speed. Placing
concrete in hot weather is complicated by the fact that the
hydration reaction generates heat, adding to the problem of
Crazing. If you pour a slab
on a hot, windy day without proper precautions, several defects
can result. One defect, known as crazing, causes fine surface
cracks in a chicken wire pattern that are especially visible
if the slab is dampened. Crazing results from too much surface
evaporation during the initial curing period. It is a cosmetic
problem, not a structural problem. Applying a curing compound
or sealing the slab with plastic helps to prevent crazing;
this should be done as quickly as possible after the slab
Plastic shrinkage cracks. A
more serious problem, plastic shrinkage cracks, can happen
when moisture evaporates from the slab while the concrete
is still "plastic" - that is, wet and workable.
This typically happens on a windy day, leaving short, parallel
cracks (6 inches to 3 feet long) at right angles to the wind.
Plastic shrinkage cracks can allow water to enter an outdoor
slab, where it may freeze, causing further deterioration.
Under extremely hot, dry, and windy
conditions, you may not be able to prevent crazing and plastic
shrinkage cracks; it's best to pour early in the day or wait
until conditions improve. You can also use set retarders to
slow hydration, and some batch plants may offer ice as a replacement
for part of the mix water.
Lower strength. Furthermore,
excessive evaporation will lower the final strength of the
slab, since hydration may not resume once moisture is again
present. A slab that is not sealed with plastic or a curing
compound must be kept continuously wet throughout the initial
curing period if it is to achieve full strength.
Cold Weather Curing
Placing concrete in cold
weather gives rise to two concerns: Strength gain is slowed
because hydration slows, and the water in exposed concrete
may freeze. Job specs often require concrete to attain a specified
strength before use or form removal; in that case, slow strength
gain may slow the job's progress. On the other hand, if moisture
in curing concrete is allowed to freeze, the result can be
weak, poor quality concrete or spalling (flaking) of the slab
surface. The more freezing cycles the wet concrete endures,
the worse the problem. Freezing becomes less of a concern
after the first few days, tion process continues to reduce
the moisture in the concrete.
Heat of hydration. Often the
"heat of hydration" will help speed coldweather
curing. Insulating blankets, straw-covered plastic, insulated
forms, and windbreaks will all help retain this internal heat,
speeding strength gain and preventing freezing. Accelerators
or Type III cement can also be added to the mix at the batch
plant to speed curing. This won't cause more heat to be released
overall, but it does result in more heat being released during
the first few days when maintaining concrete temperature is
most critical. you use an accelerator, try to avoid the calcium-chloride
type; the chloride tends to corrode reinforcing steel.
Enclosures. Heat can also be
added to the concrete by building temporary enclosures of
polyethylene or canvas and using a kerosene or propane "salamander"
heater. Combustion heaters should be vented outside the enclosure,
however, because carbon dioxide in the exhaust will react
with and soften the concrete surface. Also, in colder regions,
ready-mix suppliers are often set up to heat either the mix
water or the aggregate, or both, as is the case here in central
Common sense. Protecting concrete
in cold weather becomes a function of the air temperature
and common sense. If, for example, you pour a slab when it's
40°F outside and weather forecast calls for temperatures
to drop to the mid-twenties night, covering the slab with
an insulating blanket for the first night should keep the
concrete warm enough until the next day, when blanket can
be removed as it warms up. If it's already below freezing
when you pour, and the forecast calls lower temperatures to
come, keep concrete covered or heated for least several days.
The major concern is to prevent the moisture in the concrete
from freezing during the initial curing period.
Although concrete gains strength more
slowly at lower temperatures, the strength of concrete cured
at will eventually reach that of concrete cured at 70°F.
Even if the concrete temperature drops to the mid teens, the
hydration process will continue a very slow rate.
David Carns, P.E., teaches engineering at Central Washington
University, Ellensburg, Wash.