Hot and Cold Weather Concreting

In extremes of heat and cold concrete must be:
HANDLED
PLACED
COMPACTED
FINISHED and
CURED carefully.
Extremes of heat and cold mainly cause problems of cracking. When conditions of heat
and cold are expected some of the following precautions will improve the quality of your
final concrete.
IN HOT WEATHER
Workability In hot and/or windy weather a concrete mix may stiffen rapidly and not be
workable.
A 'set retarding' admixture may be added to the concrete during mixing to give a longer
working time.
See CHAPTER 2 Concrete Properties
In hot and/or windy weather, if concrete stiffens quickly, a cold unbonded joint may form
between concrete already in place and the new concrete.
If there is a chance of this happening you may
need to make a construction joint.
See CHAPTER 11 Joints in Concrete
To Stop Concrete Drying out and Cracking
Use one or more of the following:
Use SHADE to keep all materials out of direct sun
and keep the aggregates MOIST.
DAMPEN subgrade and formwork,
but don’t leave excess water lying around.



Put up SHADES and WINDBREAKS or try
to place in the cooler parts of the day.
Avoid DELAYS once the job begins by
planning ahead.
Use a 'Set-Retarding' ADMIXTURE.
SPRAY concrete with 'aliphatic Alcohol' after
the initial finishing, which reduces
EVAPORATION and CRACKING.
CURE concrete carefully.
See CHAPTER 10 Curing Concrete
Keep it COOL. In extreme conditions iced water, or other methods, may be used in the mix
to keep it cool. Do not apply cold water to a hot concrete surface as thermal cracking may
result due to the sudden cooling.



IN COLD WEATHER Frozen or very cold water will also slow down the setting time which
can cause costly delays.
In extremely cold weather water turns to ice, EXPANDS and can CRACK hardened
concrete.
To Stop Water Freezing and Cracking Concrete Use one or more of the following:
Keep all MATERIALS warm.
Use WARM WATER in the mix.
COVER the formwork and subgrade, to keep them free
of frost and ice.



Use a 'Set-Accelerating' ADMIXTURE.
Try to keep concrete as much above 10°C
as possible for the first few days.
NEVER lay concrete on frozen ground.
CURE concrete carefully to keep it warm. The best method will be one that KEEPS HEAT
in the concrete. The curing methods should not cool the concrete. An insulating layer may
be needed.
In cold climates with frequent freeze/thaw conditions the concrete may need an
Air-Entraining Admixture for long term durability.