Question: In the February Problem Clinic a question was submitted regarding cold weather concreting. Within the response, the writer asks “What is the coldest temperature at which to place concrete?” The writer then answers the question by referring to ACI 306 Table 3.1 and states “...it is unwise to place thin sections or slabs less than 12 inches thick during weather colder than 55° F.” This is a misinterpretation of the ACI document and simply not true. Line 1 of Table 3.1 provides recommended minimum temperatures for the concrete (not ambient air) as placed and maintained during a recommended protection period no matter what the ambient air temperature is.
Answer: The objectives for cold weather concreting are to 1) prevent damage due to freezing at early ages, 2) assure the concrete develops the required strength, 3) maintain curing conditions for normal strength development, and 4) limit rapid temperature changes. In order to meet these objectives, the concrete must be protected to maintain the recommended temperatures. The ambient air temperature along with the service category and type of cementitious material used in the concrete will dictate how long the concrete requires protection from the cold.
At the end of the protection period, the concrete should be cooled gradually to reduce the possibility of thermal cracking. Table 5.5 of the document provides recommendations for the maximum allowable drop during the first 24 hours after the end of protection. This should not be misinterpreted as a temperature difference between the concrete and ambient air temperature as presented in the response.
This still leaves us with the question, what is the coldest temperature at which to place concrete? Nowhere in the ACI 306 document does the committee provide a minimum ambient air temperature because there is not one. The weather conditions (snow, ice, high winds, etc.) will govern the possibility of concrete placement on any cold day. Of course the workers may have a different answer for that, but as long as the concrete is protected so that it does not freeze, and there is someone willing to pay the extra cost associated with cold weather concreting, then let's place concrete and keep the project on schedule.
In closing, everyone involved with cold weather concreting should refer to ACI 306 “Cold Weather Concreting” to develop an understanding for what is needed to provide quality concrete during the colder months of the year.
Eric D. King, Summit Testing & Inspection Co., Cuyahoga Falls, Ohio