Problems that originate with the mix can be the toughest to diagnose. One question answered by the article is the effect of calcium chloride in concrete floors. Possible disadvantages are that it may increase shrinkage and may cause discoloration. It has also been reported that not all types of aggregate are compatible with calcium chloride. Its use sometimes carries a long-term risk of corrosion of reinforcement if the steel has less than three-fourths of an inch cover and the concrete is subject to repeated wetting and drying or exposed to the weather. It should be noted that calcium chloride is not an antifreeze. Another question answered by the article is how to handle false-setting concrete. Premature stiffening or false set is a result of a reduction in the effective content of gypsum available in the cement to act as a retarder. Although this reduction may be caused by a fault in manufacture it can also be caused by conditions to which the cement has been subjected prior to or during delivery. For this reason the occurrence of false set, which is comparatively rare with modern cements, should be reported to the cement supplier so that the cause can be traced back to its source. Another topic dealt with is the age when freezing is safe. Most authorities agree that the first 24 hours is the most critical stage. Adequate curing during this period results in sufficient hardening so that subsequent freezing will not result in permanent damage. One or two cycles of freezing and thawing are not likely to damage concrete if they occur after a strength of 500 psi has been attained but since actual strength level of any particular concrete at very early ages is usually it is wise to provide protection whenever there is any possibility of freezing.