Placing concrete in cold weather often involves heating some of the concrete ingredients, protecting the freshly-placed concrete from freezing, and closely monitoring the strength gain of the concrete.
Residential Concreting: "The COLD Facts"
With the arrival of fall and cooler temperatures, the placement of residential flatwork can continue, provided that the principles of "Cold Weather Concreting" are followed. By definition (ACI 306), cold weather conditions exist when "…the air temperature has fallen to, or is expected to fall below, 40°F during the protection period."
ACCELERATED EARLY STRENGTH GAIN
To overcome delayed strength and initial set development associated with cold weather conditions, one or a combination of the following mix adjustments are recommended:
- Substituting Type I with Type III cement
- Addition of calcium chloride admixtures
- Addition of non-chloride accelerators
- Increasing the Type I cement content by 100-200 lbs.
- Do not place concrete on a frozen subgrade; upon thawing, uneven settlement and cracking are likely to occur.
- The minimum concrete temperature, as placed and maintained, must exceed 55°F. . . however, caution should be exercised with concrete temperatures above 75°F.
- Appropriate curing and cold weather protection must be incorporated to prevent the concrete from freezing.
CURING AND COLD WEATHER PROTECTION
To develop the strength, durability and permeability potential of the concrete, curing and protection during cold weather conditions are essential. The following guidelines are recommended upon placement:
- The curing period must extend a minimum of 7 days (maintaining the 55°F temperature).
- Do not seal freshly placed concrete. Sealing retains water in the concrete thereby keeping it saturated during freeze/thaw conditions.
- Cold weather protection is best provided through insulating blankets or loose straw (minimum 12 inches deep) sandwiched between a waterproof cover e.g. polyethylene.
MIX DESIGN RECOMMENDATIONS
A minimum specified compressive strength of 4000 psi at 28 days is required for exterior flatwork exposed to cyclic freezing and thawing. The slump, as placed, should not exceed 5 inches with the exception of those mixes incorporating mid or high range plasticizers. The recommended air content is 6.5 ±1.5%.
OWNER CARE - FIRST WINTER
- Deicing salts must not be applied…for traction, sand is recommended.
- Do not allow snow and ice to accumulate…this maintains the concrete in a saturated condition during freeze-thaw conditions.