In its plastic state, concrete will freeze if its temperature falls
below about 25°F [-4°C]. If plastic concrete freezes, its potential
strength can be reduced by more than 50% and its durability
will be adversely affected. Concrete should be protected
from freezing until it attains a minimum compressive
strength of 500 psi [3.5 MPa], which is about two days after
placement for most concrete maintained at 50°F [10°C].
Low concrete temperature has a major effect on the rate of
cement hydration, which results in slower setting and rate
of strength gain. A good rule of thumb is that a drop in concrete
temperature by 20°F [10°C] will approximately double
the setting time. The slower rate of setting and strength gain
should be accounted for when scheduling construction operations,
such as form removal.
Concrete in contact with water and exposed to cycles of
freezing and thawing, even if only during construction,
should be air-entrained. Newly placed concrete is saturated
with water and should be protected from cycles of freezing
and thawing until it has attained a compressive strength of
at least 3500 psi [24.0 MPa].
Cement hydration is a chemical reaction that generates heat.
Newly placed concrete should be adequately insulated to retain
this heat and thereby maintain favorable curing temperatures.
Large temperature differences between the surface and
the interior of the concrete mass should be prevented as cracking
may result when this difference exceeds about 35°F [20°C].
Insulation or protective measures should be gradually removed
to avoid thermal shock.
HOW to Place Concrete In Cold Weather?
Recommended concrete temperatures at the time of
placement are shown below. The ready mixed concrete producer
can control concrete temperature by heating the mixing
water and/or the aggregates and furnish concrete in accordance
with the guidelines in ASTM C 94.
Cold weather concrete temperature should not exceed these
recommended temperatures by more than 20°F [10°C]. Concrete
at a higher temperature requires more mixing water, has a
higher rate of slump loss, and is more susceptible to cracking.
Placing concrete in cold weather provides the opportunity for
better quality, as cooler initial concrete temperature will
typically result in higher ultimate strength
Slower setting time and strength gain of concrete during cold
weather typically delays finishing operations and form removal.
Chemical admixtures and other modifications to the
concrete mixture can accelerate the rate of setting and strength
gain. Accelerating chemical admixtures, conforming to
ASTM C 494—Types C (accelerating) and E (water-reducing
and accelerating), are commonly used in the winter time.
Calcium chloride is a common and effective accelerating
admixture, but should not exceed a maximum dosage of 2%
by weight of cement. Non-chloride, non-corrosive accelerators
should be used for prestressed concrete or when corrosion
of steel reinforcement or metal in contact with concrete
is a concern. Accelerating admixtures do not prevent concrete
from freezing and their use does not preclude the requirements
for concrete temperature and appropriate curing
and protection from freezing.
Accelerating the rate of set and strength gain can also be accomplished
by increasing the amount of portland cement or
by using a Type III cement (high early strength). The relative
percentage of fly ash or ground slag in the cementitious material
component may be reduced in cold weather but this may
not be possible if the mixture has been specifically designed
for durability. The appropriate decision should afford an economically
viable solution with the least impact on the ultimate
Concrete should be placed at the lowest practical slump as this
reduces bleeding and setting time. Adding 1 to 2 gallons of
water per cubic yard [5 to 10 L/m3] will delay set time by ½ to
2 hours. Retarded set times will prolong the duration of bleeding.
Do not start finishing operations while the concrete continues
to bleed as this will result in a weak surface
Adequate preparations should be made prior to concrete
placement. Snow, ice and frost should be removed and the
temperature of surfaces and metallic embedments in contact
with concrete should be above freezing. This might require
insulating or heating subgrades and contact surfaces
prior to placement.
Materials and equipment should be in place to protect concrete,
both during and after placement, from early age freezing
and to retain the heat generated by cement hydration.
Insulated blankets and tarps, as well as straw covered with
plastic sheets, are commonly used measures. Enclosures and
insulated forms may be needed for additional protection
depending on ambient conditions. Corners and edges are
most susceptible to heat loss and need particular attention.
Fossil-fueled heaters in enclosed spaces should be vented
for safety reasons and to prevent carbonation of newly placed
concrete surfaces, which causes dusting.
The concrete surface should not be allowed to dry out while it
is plastic as this causes plastic shrinkage cracks. Subsequently,
concrete should be adequately cured. Water curing is not recommended
when freezing temperatures are imminent. Use
membrane-forming curing compounds or impervious paper
and plastic sheets for concrete slabs.
Forming materials, except for metals, serve to maintain and
evenly distribute heat, thereby providing adequate protection
in moderately cold weather. With extremely cold temperatures,
insulating blankets or insulated forms should be used,
especially for thin sections. Forms should not be
stripped for 1 to 7 days depending on the setting characteristics,
ambient conditions and anticipated loading on the structure.
Field-cured cylinders or nondestructive methods should
be used to estimate in-place concrete strength prior to stripping
forms or applying loads. Field-cured cylinders should not
be used for quality assurance.
Special care should be taken with concrete test specimens
used for acceptance of concrete. Cylinders should be stored
in insulated boxes, which may need temperature controls, to
insure that they are cured at 60°F to 80°F [16°C to 27°C]
for the first 24 to 48 hours. A minimum/maximum thermometer
should be placed in the curing box to maintain a temperature