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The term concrete can be construed to include a considerable variety of products made from portland cement or other cementing media, but in this publication the term concrete usually refers to a material which was at first a plastic mixture (or mixture that became plastic as a result of manipulation, especially vibration) of portland cement, water, air, and mineral aggregate. Therefore, this discussion of the nature of concrete will have the scope indicated by that description. A writer's concept of the nature of concrete can hardly be revealed in a few words, but his treatment of certain topics and his definitions are indicative. Here are some examples: in 1878 Trautwine, in the 11th edition of his Pocket Book for Civil Engineers, said “Cement concrete, or beton, is … cement mortar mixed with gravel or broken stone, brick, oyster shells, etc., or with all together.” He described mortar as sand containing a volume of cement equal to the volume of voids in the sand. In 1907, L. C. Sabin in a book on concrete said, “Concrete is simply a class of masonry in which the stones are small and of irregular shape. The strength of concrete depends largely on the strength of the mortar; in fact, this dependence will be much closer than in the case of other classes of masonry, since it may be stated as a general rule, the larger and more perfectly cut are the stone, the less will the strength of the masonry depend on the strength of the mortar.” Feret in 1896 considered water and air to be definite components of mortar (and presumably also of concrete), but it is not clear that he thought of cement paste as an entity. Zielenski, once head of the Hungarian Association for Testing Materials, in 1910 called concrete a conglomerate body; he considered the conglomerate to be composed of mortar and coarse aggregate, and the mortar to be composed of paste and sand, with or without air voids. Taylor and Thompson, authors of perhaps the best of early books on concrete, in the 1912 edition said, “Concrete is an artificial stone made by mixing cement, or some similar material which after mixing with water will set or harden so as to adhere to inert material, and an aggregate composed of hard, inert, materials of varying size, such as a combination of sand or broken stone screenings, with gravel, broken stone, cinders, broken brick, or other coarse material.” D. A. Abrams, in the first bulletin from the Structural Materials Research Laboratory, Lewis Inst., Chicago, Ill. (1918) emphasized the significance of the ratio of water to cement in concrete, and he abandoned the notion that concrete is a mixture of mortar and coarse aggregate, pointing out that the whole aggregate, fine and coarse combined, should be considered as one, even though fine and coarse aggre-gates are proportioned separately. F. R. McMillan in his book, Basic Principles of Concrete Making (1929) said, “Expressed in the simplest terms, concrete is a mass of aggregates held together by a hardened paste of portland cement and water … the paste is the active element.” In their textbook on concrete, Troxell and Davis (1956) wrote, “Concrete is a composite material which consists essentially of a binding medium within which are embedded particles or fragments of a relatively inert mineral filler. In portland cement concrete the binder or matrix, either in the plastic or in the hardened state, is a combination of portland cement and water.” This definition was adopted by the American Concrete Inst. (ACI) Committee 116 on Nomenclature in 1964. The Encyclopedia Britannica, 1963 edition, says, “Concrete is a building material consisting of a mixture in which a paste of portland cement and water binds inert aggregates into a rock-like mass as the paste hardens through chemical reaction of cement with water.” Although it is possible to discern an evolution of concepts in the above definition and descriptions, the early concept of concrete as a mixture of mortar and coarse aggregate tends to persist, despite Abrams' contention that the total aggregate functions as a unit. The idea lingers with us, perhaps, because it has simplicity and plausibility and partly because it is not altogether unrealistic, especially when there is a gap between the largest size in the sand and the smallest size in the coarse aggregate. However, gap gradings are not common, and in any case the correctness of Abrams' conclusion can hardly be questioned.
Powers, T. C.
Research counselor, Portland Cement Assn., Skokie, Ill.