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Samuel Helms discussed the unit weight and air content of hardened concrete in each of the three preceding editions of this ASTM Special Technical Publication. This present edition makes much use of Helms' clear and concise work, particularly in the area of unit weight. The discussion of air content is more extensive here, however, largely in response to the great deal of new research since the last major update. It has also become evident that the further in time that the industry has come from the landmark contributions of the early workers in air-void systems and frost resistance in concrete, the less understood are the fundamental principles laid down in the late 1940s and early 1950s. Following the pattern set by Helms, the present edition therefore reviews much of this key literature to explain the origins of today's state of the art. Finally, this edition appears when there is a proliferation of the use of the ASTM Practice for Microscopical Determination of Air-Void Content and Parameters of the Air-Void System in Hardened Concrete (C 457) microscopical analysis procedure, which was once exclusively performed by a small cadre of experts. While expansion of the use of the test has produced more data, it has in some cases produced more confusion where less experienced operators have used nonstandardized equipment and procedures. The chapter has therefore been written in part to point out pitfalls and sources of error in performing and interpreting the test.
Professor of Structural Engineering, Cornell University, Ithaca, NY