There are a variety of techniques that can safely speed up winter concreting and play an important role in the success and profitability of the finished project. Several of these are discussed in the following paragraphs. Fifty degrees F is the critical figure in cold weather concreting. When the temperature of freshly placed concrete drops below this figure during the initial curing cycle, the strength and safety of the finished work are severely threatened. There are three common methods of shortening the curing time on this slab at 50 degrees F. Except where tensioning cables are being used, add a small amount of calcium chloride to the mix. Use a high early strength cement in the concrete and add heat to increase the slab temperature. When air temperature drops below 50 degrees F, chemicals or special cements are not enough and other measures must be taken: the water and other concrete components can be heated before mixing, the area in which concrete is placed can be heated, concrete with a higher cement content can be used, and after concrete is placed, a stable temperature of at least 50 degrees F can be maintained during the initial curing cycle. In many instances the concrete will generate enough heat of hydration to serve as the only source of heat necessary during a three day curing cycle- if the heat is trapped and held. In some areas, contractors are still using the old-fashioned fodder formula: if enough hay is piled on top of the fresh concrete, it can provide a degree of insulation and accelerate cold weather curing. Unfortunately, this method becomes a bad fire hazard on the site, can be undone by wind or unexpected precipitation, and won't work on vertical surfaces. Fiber filled curing blankets are a better answer. Usually these contain fiberglass encased in a moisture proof cover of coated plastic fabric. Such blankets virtually eliminate fire hazard and work equally well on vertical or horizontal surfaces.