In any line of work fallacies develop which are bound to receive acceptance. The fallacy often seems easier to grasp, accept and remember than the truth. The field of concrete construction is no exception. Fallacy one: concrete hardens as a result of drying out. The chemical process which we call hydration, and which is wholly responsible for the hardening of concrete can only take place while moisture is available. Hydration not drying out hardens it. This is the entire reason why curing is so important in determining the ultimate strength and quality of concrete. Fallacy two: aggregates are inert. Recent research has shown that aggregates can be the source of a number of difficulties experienced in concrete construction. The most widely discussed phenomenon associated with chemically reactive aggregates is alkali-aggregate reactivity. This highly unpredictable phenomenon hinges upon the chemical makeup of the aggregate and the cement. It results in great enlargement of the aggregate particle which in turn results in severe cracking and deterioration of the concrete. Fallacy three: the strength of concrete determines its quality. Strength specifications ignore many highly important factors. For example, in floor construction they fail to take into consideration the abrasion resistance of aggregates, the amount of bleeding that is to be tolerated, the rate of set, and the temperature of the concrete when delivered. All of these factors have a profound influence on the quality of the floor in place. Fallacy four: type III portland cement concrete eventually loses strength. Actually, concrete make with any portland cement does not lose strength, even at advanced ages unless outside forces attack the concrete. Type III cement, however, develops the lowest ultimate strengths of the five major types of portland cement, but its strength does not retrogress at any stage. This is only a partial list of concrete fallacies. Other fallacies are: the slump test measures workability; calcium chloride is a concrete anti-freeze; the use of retarders results in lower early strengths; concrete mixes should be designed to eliminate segregation during handling and placing; reinforcement reduces cracking of concrete; raveling of joints being sawed indicates improper timing; prestressing cable is reinforcement; test cylinders indicate strength of concrete-in-place; air content is determined by amount of air-entraining agent used; vibrate reinforcement to consolidate concrete and slab-on-grade floor reinforcement fabric should be pried up to assure correct positioning. The explanations are given in the article.