Frequently Asked Questions
Cement & Concrete
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Q: What precautions do I have to take during hot-weather
concreting?
I am planning to have a large volume of concrete placed as part
of an industrial development project that will be built during the
summer of this year. I have been told that the quality of concrete
can be affected not only by cold weather but also hot weather conditions.
What are the effects of hot weather on concrete, and what precautions
should be taken to assure that the concrete will provide good durable
service?
A: It is true that hot weather conditions above
approximately 25°C (77°F) can adversely impact the quality of concrete.
The precautions that should be taken to assure a quality end product
will vary depending on the actual conditions during concrete placement
and the specific application for which the concrete will be used.
In general if the temperature at the time of concrete placement
will exceed 25°C (77°F) a plan should be developed to negate the
effects of high temperatures. The precautions may include some or
all of the following:
1) Moisten subgrade, steel reinforcement,
and form work prior to concrete placement.
2) Erect temporary wind breaks to limit wind velocities and sunshades
to reduce concrete surface temperatures.
3) Cool aggregates and mixing water added to the concrete mixture
to reduce its initial temperature.
4) Use a concrete consistency that allows rapid placement and consolidation.
5) Protect the concrete surface during placement with plastic sheeting
or evaporation retarders to maintain the
initial moisture in the concrete mixture.
6) Provide sufficient labor to minimize the time required to place
and finish the concrete, as hot weather conditions substantially
shorted the times to initial and final set.
7) Consider fogging the area above the concrete placement to raise
the relative humidity and satisfy moisture demand of the ambient
air.
8) Provide appropriate curing methods as soon as possible after
the concrete finishing processes have been completed.
9) In extreme conditions consider adjusting the time of concrete
placement to take advantage of cooler temperatures, such as early
morning or night time placement.
With proper planning and execution concrete can be successfully
placed and finished to produce high quality durable concrete at
temperatures of 35°C (95°F) or more.
Additional information can be found in:
Portland Cement Association, Hot
Weather Concreting, IS014.05 , reprinted from Chapter 13 of Design and Control of Concrete
Mixtures, EB001.14, Portland Cement Association, 2002, 12 pages.
ACI Committee 305, Hot Weather Concreting, ACI 305-99, American
Concrete Institute, Farmington Hills, Michigan, 1999, 20 pages
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