Air Content and Unit Weight

Tests for air content and unit weight of freshly mixed concrete are made frequently both in the field and in the laboratory. Procedures are outlined in various ASTM standards which are straightforward and well within the capabilities for performance and understanding of the average concrete technician. Further, a minimum of training is required before proficiency in the testing technique is acquired. Nevertheless, an understanding of the limitations of the various methods, together with an appreciation for carefully following details of the procedures, is essential to obtaining and interpreting results. Tests for air content and unit weight are made of fresh concrete to provide a control on these properties in the hardened concrete and also to determine volume of concrete being produced from a given batch and its unit cement content. The significance of air content and unit weight in hardened concrete is discussed in detail in this publication by Helms. Air content most commonly is determined to insure the presence of small amounts of air prescribed in concrete in order to obtain its beneficial effects on resistance to freezing and thawing and effects of deicing agents applied to concrete for snow and ice removal. Tests for unit weight are made to control weight per se of both lightweight and high-density concretes. For all concrete, unit weight determinations are used to establish the volume of batch produced and to establish actual cement contents. The interrelation between air content and unit weight should be obvious; an increase in volume of air results in a lower unit weight, whereas a reduction in air content increases unit weight. Therefore, testing procedures for these properties are combined in ASTM methods and should be performed in sequence in the field.