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4000.043
Editors of Concrete Products
Concrete Products
Article
Maclean Hunter Publishing
1962
February
39-42, 45
N
The versatility of concrete is changing the skyline of America, and today almost any architectural form can be translated into concrete.SHELL CONCRETE is one development that has most recently stimulated greater use of concrete. This type of construction has appeared in the shape of domes, barrel shells, undulating curves and hyperbolic paraboloids.
One can visualize the strength of shell concrete by comparing it, oddly enough, to a tiny chicken egg shell. Such an egg shell can withstand an end pressure of 75 lb.; the secret of its strength lies in a curved, three-dimensional shape that distributes stresses over the entire area. When this same principle is applied to concrete, roofs that are 3 in. thick or less can be used to cover spans of several hundred feet.
PRECAST CONCRETE STRUCTURAL MEMBERS, too, have stimulated the potential market for concrete because of a considerable rise in popularity.
LIGHTWEIGHT AGGREGATES — although not yet entirely common in some of the structural members that we have mentioned — have been a boon for concrete technology that are just now beginning to make an impressive impact on building design. These aggregates are used in more than half of all the block produced in the United States today.
Several different types of lightweight aggregates are being produced. Pumice, perlite and vermiculite are ultra-light and suitable for insulation concrete, masonry block and roof fill, rather than for structural concrete. Expanded shales, clays and slates, formed by crushing and heating raw materials to around 2,000 deg. F. until the pellets bloat, are proving to be excellent for structural use. The resulting concrete is strong but light, 85 to 110 lb. per cu. ft. as against 146 lb per cu. ft. for ordinary concrete.
Expanded slags, another type of lightweight aggregate, are produced by shooting small amounts of water into molten slag as it comes from the blast furnace. These aggregates are apt to be harsh, and require more air in the concrete mix for workability and more cement for strength.
THE CONCRETE BLOCK FIELD — partly through the development of lightweight aggregates — has had some striking changes lately. This building material has evolved into a wide assortment of units that vary in size, shape, color and texture.