Year-round concrete construction requires that "young" concrete be protected. If the winter is long and rugged, heated enclosures are justified. Where the winter season is milder, other methods are more economical and equally satisfactory. Protection of concrete against cold has two aims- prevention of early freezing and attainment of early strength. During winter construction concrete is generally placed at or above some minimum temperature usually set forth in job specifications. Furthermore, as concrete hardens, heat is generated. This internal heat may be retained through use of insulation. Hardening may be accelerated through addition of heat during or after mixing, by chemical accelerators, by the use of high-early- strength cement, or by the use of additional amounts of normal cement. One or more of these methods may be used to shorten setting time and to accelerate gains in strength. The minimum temperature of concrete at placement is usually specified at about 50 degrees F. Often concrete is delivered during winter at 10 or 15 degrees higher. As temperatures rise above these levels more mixing may be necessary to maintain the required slump; or rapid loss of moisture from the surface may cause plastic shrinkage cracks. Since both of these problems can be overcome, it is possible to place concrete satisfactory at 85 to 90 degrees or higher. It is often more feasible, however, to place at somewhat lower temperatures and retain the internal heat or apply heat externally. The periods of protection can be shortened by acceleration of the mixtures. Acceleration speeds up hydration. This does two things: (1) it speeds up the gain in strength, and (2) it generates additional heat. The latter further accelerates gain in strength. The addition of extra amounts of normal cement will also shorten the times for reaching specified levels. Adding more cement without increasing the water decreases the water/cement ratio and thus increases concrete strength at all ages.