Q: The current project that I am bidding has a cold-weather clause in the specifications, which requires heating materials and protecting concrete from freezing during freezing or near-freezing weather. This specification is vague to me. What is near-freezing?

A.: Cold-weather clauses can be found in many specifications. For example ACI 318, "Building Code Requirements for Reinforced Concrete," says: Adequate equipment shall be provided for heating concrete materials and protecting concrete during freezing or near-freezing weather.

And ACI 301, "Specifications for Structural Concrete" says: When the mean daily outdoor temperature is less than 40º F, the temperature of the concrete shall be maintained between 50º F and 70º F for the curing period.

ACI 306, "Cold Weather Concreting," defines cold weather as a period of more than three consecutive days in which the following conditions exist:

  • The average daily air temperature is less than 40º F and
  • The air temperature is not greater than 50º F for more than one-half of any 24-hour period.
  • The average daily air temperature is the average of the highest and the lowest temperatures occurring during the period from midnight to midnight.

This specification may not always protect fresh concrete from freezing. For example, temperatures on a job located in a part of the country that has dramatic weather changes, like the Rocky Mountain states, may be 30º F one day and above 50º F the next. These weather conditions are not classified as "cold weather" because the temperature did not remain under 40º F for three consecutive days. Use good judgment and protect concrete whenever there is a possibility of freezing. During periods not defined as cold weather but when freezing temperatures can occur, protect concrete surfaces from freezing for the first 24 hours after placement. ACI 306 recommendations to protect fresh concrete from freezing can be broken down into two categories:

  • Modify the mix design so the concrete will gain the necessary strength before freezing, or
  • protect the concrete from freezing by external means (i.e., blankets, enclosures, or heaters).

For more information, see "Concrete Basics" on page 945 of this issue or the article "Cold-weather Finishing" in the November 1993 issue of Concrete Construction.