Reportedly, it costs substantially more to pour a foundation because of additives and other precautions used to keep the concrete from freezing.
This page lists extensive guidelines and recommendations from the American Concrete Institute.
- Never pour concrete on frozen ground, snow, or ice.
- Concrete in cold weather is recommended to have low slump, and minimal water to cement ratio, to reduce bleeding and decreases setting time.
- Use insulation blankets or heated enclosures to maintain concrete temperatures above 50° degrees Fahrenheit for three to seven days. Maintain the concrete temperature above 40° degrees Fahrenheit for at least four more days after the use of the insulation blankets or heated enclosures.
- Fresh concrete frozen during the first 24 hours can lose 50% of its
potential 28 day strength.
- Keep concrete warm, over 5 degrees Celsius, for the first 48 hours, where concrete strength development is critical. When concrete is being placed below 5 degrees, but is not below freezing point, concrete will take longer to develop the required strength. Note that removing formwork when concrete is too cold or hasn’t reached desired strength, could damage concrete strength and surfaces and concrete might collapse.
- Concrete in cold weather is recommended to have low slump, and minimal water to cement ratio, to reduce bleeding and decreases setting time.
This page also offers a number of other useful considerations, effects, and construction guidelines during cold weather including:
- Chlorine-based additives can have a corrosive effect on metal re-bar
and shouldn't be used.
This thread contains a lot of good information about pouring foundations in cold winter weather.
Other notable points from Glenn Good:
It is recommended to keep concrete above 40 degrees. With the proper
insulation covering all areas exposed to the cold it will maintain
this temperature in most cases. Keep in mind that this insulation must
extend below the frost line as well. Rigid polystyrene foam insulation
board can be used below the grade and left in place. The warmer the
weather the less insulation required.
You may also want to try to keep track of is the amount of water in
the concrete and the time it is poured. Concrete should be placed
within 1 hour after it leaves the plant. The "slump" test is used to
determine the amount of liquid in the concrete. Concrete should not be
poured over a 4" slump. The lower the slump the less water in the mix
and the stronger the concrete will be. Excess amounts of water cause
the concrete to shrink more as it cures and as a result stress cracks
will begin to form and weaken it.
Another item I should mention is the addition of calcium to the mix.
This is often used in cold weather pours to accelerate the curing time
and prevent freezing. The one major draw back with using calcium (and
many other accelerators) is they have a tendency to deteriorate or
oxidize (rust) the reinforcing steel that is used in the concrete.
Chances are you will have steel rebar in your foundation and I would
advise you do not permit the use of accelerators in the concrete mix.
This will mean they will HAVE to use insulation to protect the
concrete from freezing but you will get a stronger product that will
last longer.
This site says:
Temperature extremes make it difficult to properly cure concrete. On
hot days, newly placed concrete losses too much water through
evaporation. If the temperature drops too close to freezing, hydration
slows to nearly a standstill. Under these conditions, concrete ceases
to gain strength and other desirable properties. In general, the
temperature of new concrete should not be allowed to fall below 50°F
(10°C) during the curing period.
Cold weather concreting is a common and necessary practice; every cold
weather application must be considered carefully to accommodate its
unique requirements. The current American Concrete Institute
definition of cold-weather concreting, as stated in ACI 306 is, “a
period when for more than 3 successive days the average daily air
temperature drops below 5°C (40°F) and stays below 10°C (50°F) for
more than one-half of any 24 hour period.”
Rule number ONE is that ALL concrete must be protected from freezing
until it has reached a minimum strength of 3.5MPa (500psi), which
typically happens within the first 24 hours. In addition, whenever air
temperature at the time of concrete placement is below 5°C (40°F) and
freezing temperatures within the first 24 hours after placement are
expected, the following general issues should be considered:
(1) Adjustment of construction schedule regarding loads imposed on the
new concrete structure
(2) Placing and curing temperatures to produce
quality concrete
The exposure of concrete to cold weather will extend the time required
for it to gain strength. In structures that will carry large loads at
an early age, concrete must be maintained at a minimum of 10°C (50°F)
to accommodate stripping of forms and shoring and to permit loading of
the structure. In many cases, achieving the required durability will
require a protection period of more than 24 hours. This may not be an
issue with residential applications where applied loads are typically
small and may be applied in small increments over several days or
weeks.
In no case should concrete be allowed to freeze during the first 24
hours after it has been placed. Since cement hydration is an
exothermic reaction, the concrete mixture produces some heat on its
own. Protecting that heat from escaping the system may be all that is
required for good concrete quality, while more severe temperatures may
require supplemental heat.
