Using High Early-Strength Cement
High early-strength, Type II cement produces much higher hydration temperatures, which can offset some of the
cold weather effects. Other benefits include early reuse of forms and shore removal, cost savings in heating and
protection, earlier flatwork finishing, and earlier use of the structure.
Do not substitute accelerators for proper curing and frost protection. Also, do not try to lower the freezing point
of concrete with accelerators (antifreeze compounds or similar products) because the large quantities required
seriously affect compressive strength and other concrete properties. However, you can use small amounts of
additional cement, or such accelerators as calcium chloride, to speed up concrete hardening in cold weather as
long as you use no more than 2 percent of calcium chloride by weight of cement. But be careful in using
accelerators containing chlorides where an in-service potential of corrosion exists, such as in prestressed concrete
or where aluminum inserts are planned. When sulfate-resisting concrete is required, use an extra sack of cement
per cubic yard rather than calcium chloride.
Preparing for Placement
Never place concrete on a frozen subgrade because severe cracks due to settlement usually occur when the
subgrade thaws. If only a few inches of the subgrade is frozen, you can thaw the surface by burning straw, by
steaming, or, if the grade permits, by spreading a layer of hot sand or other granular material. Be sure to thaw the
ground enough to ensure that it will not refreeze during the curing period.
Concrete in forms or covered by insulation seldom loses enough moisture at 40 to 50...F to impair curing. Forms
distribute heat evenly and help prevent drying and overheating. Leave them in place as long as practicable.
However, when using heated enclosures during the winter, you must moisten curing concrete to offset the drying
effects. Keep the concrete at a favorable temperature until it is strong enough to withstand both low temperatures
and anticipated service loads. Concrete that freezes shortly after placement is permanently damaged. But if
concrete freezes only once at an early age, favorable curing conditions can restore it to nearly normal, although it
will neither weather as well nor be as watertight as concrete that is never frozen. Air-entrained concrete is less
susceptible to freeze damage than non-air-entrained concrete (see TM 5-349 for details of cold weather
concreting). Three methods of maintaining proper curing temperatures are described below:
Live steam. When fed into an enclosure, live steam is an excellent and practical curing aid during
extremely cold weather because its moisture offsets the rapid drying that occurs when very cold air is
heated. You can use a curing compound after removing the protection if the air temperature is above
Insulation blankets or bats. The manufacturers of these materials can usually provide information on how
much insulation is necessary to protect curing concrete at various temperatures. Because the concrete's
corners and edges are the most likely to freeze, be sure to check them frequently to determine the
effectiveness of the protective covering.
You can use wood, canvas, building board, plastic sheets, or
other materials to enclose and protect curing concrete at below-freezing temperatures. You
can also build a wood framework and cover it with tarpaulins or plastic sheets. Make sure