COMPARISON OF THE AIR CONTENTS OF FRESHLY MIXED AND HARDENED CONCRETES

This study compares the air contents of freshly mixed and hardened concretes. At the fresh stage, pressure meters (Types A and B) and a volumetric meter were used to determine the air content. At the hardened stage, the air content was calculated using the linear traverse method described in ASTM C 457, which is a microscopical procedure. The unit weight and compressive strength of the concretes were also determined. The results show that, at the ranges commonly used in the construction of pavements and bridges, the air content of fresh concrete measured by pressure meters and that determined by the microscopical method for essentially the same concrete after hardening are, for practical purposes, the same. The air content obtained by a volumetric meter as normally run in the field is generally lower than that obtained for the same hardened concretes by the microscopical method. The unit weight and compressive strength correlate well with the air content. It was also shown that adding water to concrete can significantly increase the air content as well as the slump. Thus, a higher air content in hardened concretes than that indicated by initial measurements with a pressure meter is likely to be present if water is added during placement.