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Here’s a surprise: concrete placed in cold weather doesn’t really care about the weather forecast. ACI 306, Guide to Cold Weather Concreting, defines cold weather as “when air temperature has fallen to, or is expected to fall below, 40°F during the protection period.” At the recent meeting of ACI Committee 306, during a discussion on a revised version of the ACI 306 specification for concrete work in cold weather, Ken Hover, professor at Cornell University and popular World of Concrete speaker, stated bluntly that “I would very much like to get ourselves out of the weather forecasting business.” He pointed out that “Who cares what label we put on the weather? When the concrete is too cold, the concrete is too cold!” He recommends instead that we “state minimum concrete temperture and maximum temperature and decide how to measure air temperature on the job site and leave it at that.”

So there are some decisions to be made in order to accomplish that, like how and where to measure air temperature. The temperature that really matters is the air temperature to which the concrete is exposed. Is the concrete tented? Covered with insulating blankets? And how do you measure the air temperature under insulating blankets or right next to the concrete?

Dennis Purinton showed off an invention he made from two pieces of Styrofoam with a little cavity on the side placed against the concrete and a hole into it from above to insert a thermometer to measure the temperature of the concrete surface. The committee was interested in this device, but of course it needs to be tested and verified before it shows up in an ACI document. Hopefully that can be accomplished in less time than writing a new specification, which hasn’t been done for cold weather since 1990.