Only the foolish rush blindly into a winter concreting job. Few projects can be looked upon as routine, particularly if the temperature drops much below freezing. If the job is to be successful, the contractor must heed a few simple rules and must adhere to some well established practices. Frozen ground presents a vexing situation at the very beginning. Under no circumstance should concrete be placed on a frozen subgrade; the outcome is almost certain to be poor. Artificial thawing by burning straw or using surface heaters is prudent even if the subgrade is coated only by a thin layer of frost. The supplier should provide the contractor with a mix that is suitable for placing in cold weather. This would include heating the mixing water and/or aggregates, using calcium chloride as an accelerator, and using an air-entraining agent. At air temperatures below 70 degrees F., the setting and hardening of concrete starts to slow down. The slow-down is not particularly troublesome even at temperatures as low as 50 degrees F. But when the mercury sinks into the 40's, some cold-weather precautions should be taken. Full protection becomes necessary when the mercury approaches 32 degrees F., the freezing point of water. Curing stops altogether in freezing temperatures. The time for all-out protection comes when temperatures readings are in the 20's. Heaters, enclosures, and protective coverings then become necessary to keep the concrete warm enough for proper curing and setting. Salamanders are one of the best sources of heat because they are easy to install and economical. Often a tarpaulin thrown over a framework serves adequately as an enclosure. A protective covering often serves adequately for a job that does not lend itself well to an enclosure. One of the newer protective coverings consists of blanket insulation enclosed in asphalt-coated liners. Such insulation has proven to be highly satisfactory.