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Accuracy in the determination of unit weight of hardened concrete can be attained using relatively simple procedures based (a) on displacement methods, such as suspended-immersed weighings of samples on sensitive scales, or (b) on weight-in-air of the sample divided by its volume as calculated from dimension measurements. The latter procedure is especially adaptable for determinations on dry specimens using precise measurements of linear dimensions within an accuracy of one per cent. Apparatus and instruments for these purposes are familiar to all, and, in regard to matters of significance, elucidation of procedure details seems unnecessary. On the other hand, any technique for determination of air content, after the concrete has set, must be more complex and would require specialized equipment if exact information is desired. Where an estimated air content will suffice to differentiate or to be indicative of the general level of air content, less discriminating techniques, predicated on precise density measurement, specific gravities, and known or determinable proportions of the ingredients, are available. While methods for unit weight are traditional, those for air content have developed since the time the advantages of entrained air became apparent, as the means for studying the effects of air content on other properties of the concrete.
Helms, S. B.
Research engineer, Lehigh Portland Cement Co., Allentown, Pa.