Question: We occasionally have clients who want us to do concrete placements throughout the winter months. We try to accommodate them, but sometimes it's hard to explain why we delay a pour when the weather is marginal. One of the things I always take into consideration is the wind chill, but I don't understand it well enoughto explain my thinking. When the temperature is above 32F but the wind chill is well below 32F, does that mean the concrete could freeze?
Answer: The short answer is no. When the air temperature is above freezing, no amount of wind will make water freeze, whether it's in concrete, someother material, or just a puddle. However, once the temperature goes below 32F, wind chill can become a factor in how quickly water freezes when evaporation can contribute to the chilling effect. For example, wind chill does not affect inanimate objects such as car radiators because they cannot cool below the actual air temperature. In the case of exposed skin, when wind picks up the body is cooled at a faster rate due to evaporative cooling, and the skin's temperature drops.
Today the wind chill reported by the National Weather Service is based on the effect moving air has on heat loss from human skin, but it has not always been so. Theoriginal wind chill index was based on research done in 1945 by two Antarctic explorers who measured the wind's effect on the cooling rate of a pail of water hung outside on a tall pole. But because water freezes faster than flesh, their calculations overestimated the wind chill effect with regard to humans.
In November 2001, the weather service started using a new wind chill temperature index that more accurately reflects how cold air feels on human skin. It differs slightly from the older index for several reasons. First, it calculates wind speed at an average height of 5 feet, rather than the 33-foot height of a standard anemometer. Also, it is based on a human face model and incorporates modern heat transfer theory. The new index also considers the calm wind threshold as 3 mph, rather than the 4 mph previously used. Finally, it uses a consistent standard for skin tissue resistance and assumes no warming effect from the sun.
All of this information is great for helping to keep your workers safe as temperatures drop and the wind picks up. But there is no direct correlation given for the wind's chilling effect on fresh concrete. Going back to the definition of the wind chill index, it's related to the chilling effect of evaporation. So if the concrete is protected from the drying effects of the wind, which can be severe in their own right, it also will be protected against the thermal effects of wind chill. However,as in all cases when there's the possibility of subfreezing temperatures, the use of insulating blankets or other thermal protection is recommended.