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Concrete is the most versatile building material
in the world. Concrete that has been properly placed, cured and protected
will last longer than any other building material.
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It is critical to concrete’s long-term durability that it
be kept as moist as possible for the first seven days after placement.
In addition, it must be allowed sufficient drying time before being
subjected to freezing temperatures or to de-icing salts. Curing concrete
properly requires the correct control of moisture and temperature
and without proper curing, concrete may only achieve 50% of its potential
design strength. The logic behind the practice of a seven-day curing
period rests on two facts:
- Cement, the "active" ingredient in concrete requires
constant moisture to gain strength.
- If the concrete is kept moist for the seven-day period, it will
not only gain strength, but will also shrink less and produce fewer
Curing can be assisted in a number of ways:
- Spray-on liquid curing agents and sealers.
- Water ponding or spraying a mist over the concrete.
- Coverings such as wet burlap, polyethylene, insulating blankets,
- Leave forms in place.
A liquid curing agent is the most effective and convenient method.
The curing agent should be applied as soon as finishing is complete.
Curing agents form a membrane on the surface of the concrete in order
to retain moisture. Once the curing agent has been applied, no further
working of the concrete can be done. Use a spray or roller to apply
According to CSA A23.1-94, curing is a mandatory part of concrete
construction and requires moisture to maintain the concrete at 100%
relative humidity and a minimum temperature of 10°C for a period
of at least 3 days. This period is extended to a minimum of 7 days
for the durability requirements of exposure classes F1, C1, C2, S1,
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COLD WEATHER CURING
Concrete can be placed safely throughout the colder months if precautions
are taken. During colder weather, the mix should be adapted to the
ambient temperature by heating the concrete, adding accelerators,
and providing protection. From November 1 through March 15, Rempel
Bros. heats all concrete to meet the requirements of CSA and to maintain
reasonable setting times and strength gain. Rempel's Polarcon™ is
an accelerating system, which decreases setting times during cold
Concrete generates heat during hydration, the chemical process by
which cement reacts with water to form a hard stable paste. Hydration
is affected by initial concrete temperature, ambient air temperature,
the dimensions of the concrete and mix design. The temperature of
the concrete during and after placement is critical since concrete
sets more slowly as the temperature drops. Finishing and form removal
may be delayed in cool weather and strength development can be impaired
if appropriate precautions are not taken.
Do not place concrete on frozen ground, on snow, or in freezing
weather. If concrete freezes while in a saturated condition, surface
problems like scaling, spalling or cracking can arise due to the
expansion and contraction of frozen water inside the concrete. During
freezing weather, water curing of concrete should be terminated 12
hours before the end of the protection period. Do not use a curing
agent if there is any chance that the concrete will freeze during
the curing period.
- Protect concrete from freezing temperatures for 3 to 7 days after
- Leave forms in place as long as possible. Corners and edges are
most vulnerable (cover and heat if necessary).
- Protect flatwork by covering and heating, or using insulated
blankets, or covering with plastic and straw.
This information has been provided to assist you with your cold
weather construction. For a complete review of cold weather concrete
practices, please refer to CSA A23.1 – 94 Section 21.
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After the concrete is fully cured, the application of a concrete
sealer is recommended. The advantages of sealing concrete include:
- Provides protection against de-icing salts
- Resists mildew and surface staining
- Improves appearance
- Makes surfaces easier to clean
- Inhibits efflorescence (a whitish powder on the surface)
Due to the porous finish of exposed aggregate concrete, a surface
sealer is highly recommended. Sealers should not be used on coloured
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Superplasticizers are admixtures that increase slump to make concrete
easier to place. Superplasticizers disperse particles of cement throughout
a concrete mix. The effect is the same as adding water, but without
the detrimental side effects. In addition, they reduce water in the
mixture to gain higher, earlier strengths.
As a contractor, superplasticizers can help you to build more economically
and to produce higher quality concrete at the same time, since moderately
flowing concrete can be placed more easily providing good consolidation
around rebar and tight forms with minimal vibration. Combine superplasticizers
with low initial slump concrete to produce a workable mix with higher
Use superplasticized concrete for floors, suspended slabs, toppings,
walls, columns, and anywhere that higher slump concrete is required.
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