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Both “lightweight concrete” and “lightweight aggregate” are general terms which include a wide variety of products and are frequently subject to varying definitions. Concrete lighter in weight than that usually obtained with “normal weight” aggregates (gravel, air-cooled blast-furnace slag, and crushed stone) may be produced by several methods: 1. Cellular or foam concrete, where the light weight is caused primarily by inclusion of a large amount of air or gas, usually 25 per cent or more. A suitable foam or foaming agent is combined with cement and water, either with or without other ingredients such as sand or pozzolanic fines. 2. No-fines or “popcorn” concrete, with the light weight produced by high air contents or specially graded aggregates having little or no fine material or both. Either lightweight or normal weight aggregates may be used. 3. Lightweight aggregate concrete, which is comparable to conventional concrete except for the use of lightweight materials as all or part of the aggregate. Lightweight concretes produced by these various methods may range in unit weight from 15 or 20 lb/ft3 to weights only slightly less than those of conventional concretes. Applications include: (a) insulation or fill uses where light weight and a high degree of insulation are required, with strength being of lesser importance, (b) masonry units requiring moderate strength with light weight and good insulation properties, and (c) structural concrete for use in buildings and bridge decks, where weight reduction produces design economies and other characteristics desired are the same as for conventional concretes made with normal weight aggregates. Lightweight aggregates of various types have been used for many years, but it is only recently that they have become an important factor in the concrete construction industry. Both mineral (inorganic) and vegetable (organic) materials of many varieties have been considered and tested for use as lightweight aggregates. The inorganic or mineral materials are the only ones that have been used to any significant extent and are the only materials covered by current ASTM specifications for lightweight aggregates. The most common method of making lightweight concrete in the United States is the use of lightweight mineral aggregates, with some use of cellular concretes for insulating and fill purposes.
Lewis, D. W.
Chief engineer, Nat. Slag Assn., Washington, D. C.