THE GENERAL PRESENTATION OF THIS EDITION IS similar to previous editions, with additional information on structural properties of lightweight aggregates, as well as strength making, durability, and placement characteristics of lightweight concrete to reflect the current state of the art. This edition also includes new discussions relative to the moisture dynamics and the enhanced hydration of cementitious materials brought about by internal curing when using water-entraining lightweight aggregates. Specified density concrete, concrete with densities between traditional lightweight and normal-weight concretes, is also discussed. In ASTM STP 169 this chapter was authored by R. E. Davis and J. W. Kelly. ASTM 169A and ASTM 169B were authored by D. W. Lewis. ASTM 169C was authored by T. A. Holm.