Page 77 - Masonry and Concrete

Concrete Construction Techniques
76 CHAPTER THREE

QUICK>>>TIP concrete surfaces because it can cause the
Always use clear poly- color to become splotchy. Keep the con-
ethylene for moist-curing concrete. Black crete moist for seven days.
polyethylene will absorb too much heat
After concrete slabs have cured for 24
in hot weather, and white will reflect too
hours, and concrete walls and footings for
much heat in cold weather.
three days, remove the forms, but do not
pry or hammer against the concrete itself.
The concrete will continue to cure slowly for another month until it
reaches full strength, but slabs are safe to use for foot traffic after the
first day and for light rubber-tired vehicles after the first week. Heavy
traffic areas should be protected with plywood. Foundation walls and
footings should cure for at least two weeks before substantial framing
loads are added.

3.8.1 Cold Weather Concreting
Cold weather can have damaging effects on freshly placed concrete.
Both setting time and rate of strength gain are slower in cold weather,
and if the concrete freezes during the first few days of curing, it will
suffer reduced strength and weather resistance, and increased moisture
permeability. When it is necessary to work in cold weather, certain pre-
cautions must be taken to assure the quality of the finished concrete.
Cold weather is defined as a period when the mean daily tempera-
ture drops below 40°F for more than three consecutive days. On com-
mercial projects, heated enclosures are often provided to protect
concrete and masonry work during cold weather. Although this is not
usually done on residential work because of the expense, the follow-
ing protective measures can and should be taken.

■ For slabs and other flatwork such as driveways, sidewalks, and
patios, reduce the amount of mixing water so that the concrete
has a slump of 4 in. or less. This will minimize bleeding of mix
water to the surface and decrease the time until initial set.
■ Use air-entrained cement or an air-entraining admixture even if
the concrete will not be exposed to freeze-thaw cycling in service.
■ Use either an extra bag of cement per cubic yard of concrete, a
high-early-strength cement (Type III), or a nonchloride set accel-
erator to develop strength faster.

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