MATERIALS AND MANUFACTURE OF CONCRETE What Concrete Is. Concrete is an artificial or manufactured stone consisting of aggregates (sand, gravel, etc.) and cementing materials, mixed together with water. The ingredients to be used, as well as their proportions, vary with the kind of work for which the concrete is in tended.
If a mixture could be made containing abso lutely no voids or air-spaces—a mixture in which every part of the surface of every particle was covered by cement paste in intimate contact with it, and every intervening space between the particles completely filled with the cement paste —that mixture would be an ideal mixture. And provided the ingredients in this ideal mixture were of the kind and size and proportions best adapted to the work in view, and were of uni form good quality throughout, the resulting mass would be an ideal concrete.
A concrete which would be ideal for one pur pose or location might not be at all suitable for another. The particular use that is to be made of the concrete is a factor which must always be considered in the selection and proportioning of its ingredients.
In practice it is impossible to attain the abso lute perfection of these theoretical ideals. The air-spaces in concrete can be reduced in quan 35 tity, but cannot be entirely eliminated. Even with the utmost practicable care and thorough ness in proportioning and mixing, and notwith standing the use of enormous power in com pressing the mass, some small, infinitesimal voids will still remain. The fallibility, also, of human perception and judgment in the choosing and testing of materials, and the greater or less imperfection that characterizes all devices and processes of man's making, will introduce at least some slight variation from the theoretical standards of absolute perfection. An ideal con crete is no more possible in actual practice than is an ideally perfect wood or stone or brick or steel or other structural material. We may know perfectly well what the ideal requirements are, but their complete realization will always be beyond us. The utmost that we can do is to approximate them more or less closely. They are the goal to be aimed at in all endeavor, and our work will be perfect only in proportion as we approach them. Nature herself the only examples that are to be found of absolute perfection—in the handicraft of the Master Workman.
In spite, however, of theoretical limitations, and the fact that not all the problems of concrete have yet been finally worked out, nor all its de tails standardized, it can be said that the modern art of working in concrete has now reached such a stage of development that a degree of perfec tion sufficient for all practical purposes is quite within the reach of everyone who will give the matter a little careful study.
For many years, concrete has been regarded as an ideal building material for the heavier and more massive kinds of work, such as aqueducts and viaducts, dams and piers, sewers, retaining walls, etc.; but it is only very recently that any great degree of attention has been given to its use in the smaller construction about the home and on the farm. Even here, however—among suburbanites and home-owners, villagers and farmers—it is now a subject of steadily growing interest and increased study, and is demonstrat ing its possibilities and advantages as a material of construction in many different ways.
The wonderful adaptability of concrete to various forms and combinations of construction is based on its property of changing from a plas-` tic condition—in which it can be moulded to any shape or design—into a firm, rigid, rock-like mass, through the setting and hardening of the cement paste. No other building material that the world has ever known can compare with con crete in this respect. In addition, however, to this peculiar property, concrete possesses cer tain qualities which have irresistibly compelled the structural world to study carefully its al leged advantages as a substitute for wood, cut stone, and other old-time building materials, and which have naturally caused its claims to be looked upon with more or less favor. These qualities are its durability and accumulative strength, its non-corrosion in moist places and freedom from rotting under any conditions, its cleanliness and sanitary value, its resistance to fire and the action of the elements, its protection of steel from corrosion, its deadening to sound, its ease of manipulation, and its moderate and steadily lowered cost as compared with the in creasingly higher cost of lumber, due to the gradual exhaustion of our natural resources of forest wealth.