Introduction of Cold Weather
Concreting
Concrete placed during cold weather will develop sufficient
strength and durability to satisfy intended service requirements only if it is
properly produced, placed and protected.
ACI 306 “Cold Weather Concreting” defines cold weather
concreting as a period when for more than three consecutive days, the following
conditions exist:
•The average daily air temperature is less than 5°C (40°F)
and,
•The air temperature is not greater than 10°C (50°F) for
more than one-half of any 24 hour period.
Even though not defined as cold weather, protection during
Spring and Fall is required during the first 24 hours to avoid freezing.
What Happens When Concrete Freezes?
•Pore water in concrete starts to freeze around -1°C (30°F)
• As some water freezes the ion concentration in the
unfrozen water goes up, further depressing the freezing point.
• At around -3 to -4°C (25 to 27°F), enough of the pore
water will freeze so that hydration will completely stop, and depending on the
extent of hydration, and thus the strength of the concrete, the forces
generated by the expansion of ice (ice occupies ~9% more volume than water) may
be detrimental to the long term integrity of the concrete.
Objectives of Cold Weather Concreting
The objectives of cold weather concreting are to:
•Prevent damage to concrete due to freezing at early ages
• Assure that concrete develops the required strength for
the safe removal of forms
• Maintain curing conditions that foster normal strength
development without using excessive heat
• Limit rapid temperature changes in the concrete to prevent
thermal cracking
•Provide protection consistent with the intended
serviceability of the structure
For every 10°C (18°F) reduction in concrete temperature, the
times of setting of the concrete double, thus increasing the amount of time
that the concrete is vulnerable to damage due to freezing. It should be noted
that warm concrete placed on cold sub-grade will lose heat and its temperature
will drop. It is important to understand that having the concrete reach the
specified 28-day strength is irrelevant if the structure is damaged by
inadequate curing and protection.
Concrete that is protected from freezing until it has
attained a compressive strength of at least 3.45 Mpa (500 psi) will not be
damaged by exposure to a single freezing cycle. Concrete that is protected and
properly cured will mature to its potential strength despite subsequent
exposure to cold weather.
Except in heated, protective enclosures, little or no
external supply of moisture is required for curing during cold weather.
