Q.: In addition to the regular air temperatures reported in winter weather forecasts we keep hearing about the wind chill as well. How does that apply to concrete? When the air temperature is above 32 degrees but the wind chill is well below 32 degrees, does this mean that the concrete could freeze and must be protected accordingly?
A.: The quick answer is no. The concrete will not freeze when the air temperature (ambient temperature) is above 32 degrees no matter how frightening the wind chill is reported to be and no matter how hard the wind blows. At any ambient temperature below 32 degrees, however, the chance of concrete freezing increases with an increase in wind velocity.
The wind chill only relates to the loss of heat caused by cold air moving over a warm surface. It compares the rate at which heat is lost from a warm object, when the colder air surrounding it is still, to the rate at which heat is lost when there is a wind, with the ambient temperature the same in both cases.
The wind chill, as reported by the weather service, is based on the effect of cold temperatures and wind velocity on the human skin from research done by the U.S. Army. If there were a wind chill for concrete, however, the principle would be similar. For example, if concrete at a temperature of 60 degrees were exposed to an ambient temperature of 35 degrees, and if there were no wind, the concrete would begin to cool slowly. If a wind began to blow, a wind chill factor would be created and the concrete would cool more rapidly. If the wind chill for concrete were reported as 25 degrees, it would merely mean that the rate at which the concrete would cool at a 35-degree ambient temperature with the wind blowing at a specific velocity, is the same as it would be if the ambient temperature were 25 degrees and there were no wind. In neither case can the concrete be cooled below the ambient temperature of 35 degrees. With the wind blowing it only cools faster.
When the ambient temperature is below 32 degrees, however, wind velocity does affect the likelihood of concrete freezing. With exposure to low temperatures, hydration is slow, and little heat is generated from within the concrete. With a below freezing ambient temperature, rapid cooling could very well bring the concrete down to the freezing temperature, with the surface being especially vulnerable. These facts were known for many years before wind chill was popularized to provide an ominous note of interest in television's winter weather news and to add to the national confusion index (NCI).