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In the United States the “standard” specimens for strength are the 6 by 12-in. cylinder and the 6 by 6-in. cross-section beams. In many countries cubes are used to measure strength. Both cylinders and cubes have advantages and disadvantages. The chief purpose of making specimens is to determine the strength of concrete for check of mixture proportions, quality control, indication of time when loads can be applied, and acceptance for payment. Appropriate ASTM methods cover procedures for making and curing specimens, and these must be followed conscientiously. A major factor among many affecting the proper preparation of specimens is the use of competent personnel. Factors that can have an adverse effect on specimens are the use of nonspecification single-use molds, leaky reusable molds, improper sampling, improper consolidation, inadequate protection of young specimens, improper storage conditions, improper capping, and inadequate curing. Adequate curing becomes more important as the water-cement ratio decreases.
Kennedy, T. B.
Chief, U.S. Army Engineer Waterways Experiment Station, Jackson, Miss.