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What do these facts mean to the chemical engineer? His reaction may be to apply his science to reducing the shrinkage of concrete. He will study the researches which have been made and find that, within the ordinary range of Portland cement compositions, all cements give roughly the same shrinkage. He will find that keeping the concrete moist for longer periods of time reduces shrinkage somewhat but that concrete dried after being immersed in water for a year shrinks almost as much as concrete which is kept moist for only a week. He will find that concrete in thick sections dries out and tends to shrink heavily at the surface but not in the interior; there he sees some hope. He will iind that concrete cured moist a t low temperatures before being allowed to dry will shrink excessively when dried, while concrete cured moist a t higher temperature before being allowed to dry will shrink very little. He will iind that aggregates play a most important part and that, although with some aggregates concrete will shrink half as much as neat cement, with other aggregates it will shrink only one-Bth as much. These are a few high lights that suggest lines of attack in the attempt to control shrinkage of concrete. Perhaps most promising is the possibility of preventing concrete from drying out and thus preventing shrinkage. This suggests protective coatings such as paints and varnishes. Thus far, most coatings have been ineffective, but some coatings such as coal tar have prevented concrete from shrinking. A coating material which will be decorative and durable as well as effective remains t o be found.
Control of Shrinkage Another possibility for controlling shrinkage lies in making cement still more efficient than a t present. Just as concrete shrinks less than neat cement, so does a leaner concrete generally shrink less than a richer concrete. Here the question of water requirement plays an important part, and an efficient cement must be one which requires a minimum of mixing water in the concrete. Mechanical vibrators for placing concrete have contributed greatly toward reducing the water requirement, but there is still room for continuing to improve the efficiency of cement as it has been improved during the last twenty years. The aim should be to obtain not necessarily higher strength but ample strength with emphasis on low water requirement and other factors which lead to lower shrinkage without sacrificing durability. The improvement in the efficiency of cement may be partly through control of particle size and partly through chemical means. The indications are at present that neither the very coarse particles in cement nor the extremely fine particles are as efficient as those a little less than 0.001 inch in diameter, although no one would recommend a cement of uniform particle size. The chemical means of improving cement efficiency may be through securing and maintaining in the kiln a higher percentage of the desired compounds, or perhaps through adding chemical reagents to the grinding mill or to the cement after grinding. Cements have already been made in some plants with as much as 85 per cent of combined tricalcium and dicalcium silicate, compared with the usual 75 per cent or less. Even in such a case, however, it is doubtful if the full percentage of silicates as computed was maintained in the cooling cycle of the kiln. Space does not permit a full discussion of all the possibilities, but great strides may be made through better knowledge of how to insure more perfect formation and retention of compounds in the kiln, of how to control alkalies and adulterants in the clinker, and of how to employ heat treatment of the clinker to modify crystal size and grindability. Studies on concrete will undoubtedly aid in perfecting the cement. For example, little is known of the part the minute pores in the gel and also the larger pores between the smaller
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grains of cement and aggregate in a concrete must play in the phenomenon of shrinkage. The void space in a hardened concrete is not nearly as great as in a fresh concrete because the water and cement on combining occupy more volume than the cement alone, although less than the combined cement and water. These pores first make shrinkage possible by providing paths of escape for the moisture; in addition they provide the yielding space necessary for the contraction to take place. Any studies, to be of value, must distinguish between the submicroscopic pores in the gel and the larger pores between particles. A study of these pores and their relation to shrinkage has hardly begun. Perhaps the chemical engineer may turn his efforts to controlling the temperature changes in concrete. Temperature problems become more important than drying shrinkage in massive concrete structures, not only because drying shrinkage is less in thick sections, but because temperature rise due to heat of hydration is greater. The known facts indicate that some hopes for the control of temperature rise are identical with those for the control of drying shrinkage. Just as leaner mixes shrink less because of drying, leaner mixes also rise in temperature less because of heat of hydration, since there is less cement to generate heat per unit of concrete. The same problem suggests itself-that is, to increase the efficiency of cement so that less can be used without sacrificing durability. Since strength is usually greatly in excess of that required, it is not so important. Again, chemical composition and fineness are among the variable factors. Although fineness of cement has little or no effect on the ultimate heat of hydration, a smaller amount of finer cement has to be used per cubic yard of concrete to provide durability and other required properties. The chemical composition desired should be that which results in durable concrete with the minimum heat of hydration per cubic yard of satisfactory concrete. The cement which gives the least heat of hydration per gram of cement is not necessarily the best. While the two silicate compounds in cement contribute about equally to long time strength, the tricalcium silicate generates almost twice as much heat as does the dicalcium silicate. Thus for mass concrete, cements of high dicalcium silicate content are now being specified. There have been thorough investigations of the effect of chemical composition on heat of hydration and these have been very useful; but there is still opportunity for going further, in attempting to find not only the cement which gives the least heat of hydration but also the cement which makes durable concrete of satisfactory strength with the minimum temperature rise. Advantage should be taken of the fact that less of some cements has to be used than of others. Thus, the richness of the concrete mix which is required for a given cement enters into the consideration and, for the sake of completeness, the thermal properties of the aggregates as well. It is not intended to advocate the general use of leaner mixes but rather the development of a superior cement at a superior price if necessary, so that less can be used per unit of concrete to make the concrete more suitable and extend its use. Although a material reduction in volume changes will result in a tremendous increase in the utility of concrete, there are also great possibilities in developing concrete for special uses where light weight, sound absorption, heat insulation, or other properties are the controlling factors. The present discussion is limited to the ordinary uses of concrete. The purpose has been to call to the attention of the chemical engineer the manner in which volume change of concrete is preventing its better and more universal application to ordinary types of construction. RECEIVEDApril 27, 1935.
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ard units, sold by the usual merchandising methods, and on research investigation in the field of structural gypsum products suitable for house construction. This work has erected by the usual run of labor found on any job. The year 1930 saw the successful development of a product been guided by a belief that, regardless of the progress made which has taken the form of a piece of tongued and grooved in the mass production of the small, standard, modernistic type house, a large demand will still remain for the home wood lumber. One of the several standard units manufacwhich expresses the individuality of the owner. This means tured, characteristic of this new type of gypsum product, that any construction devised must necessarily have unlimited is the so-called GyDsteel Plank. It consists of a factoryflexibility. AS a part, molded unit, 2 in&& of this program certhick, 15 inches wide, t a i n ideas were and e i t h e r 6 or 10 recently p u t to a feet long, bound and severe practical test r e i n f o r c e d on the in the construction of s i d e s a n d e n d s by an extremely c o m tongued and grooved pli c a t ed eight-room galvanized, copperhouse, at Montclair bearing-steel c h a n Heights, N. J. The nels 2 i n c h e s d e e p . essence of this probThe gypsum core is lem can best be conreinforced with steel templated by visualmesh. This gypsum izing the c o m p l e t e plank unit possesses structural core of the all of the great flexib u i l d i n g ( t h a t is, bility inherent in walls, partitions, wood lumber a n d f l o o r s , r o o f s , and which has made the stairs) built of a firep l a n k f o r m a basic resistant, permanent, element of construcvermin-proof material tion from time imwith a h i g h d e g r e e memorial. It can be EXPERIMENTAL G Y P S U M HOUSEPARTIALLY COhlPLETED of insulation, such as s a w e d , nailed, and bored with equal gypsum. T h e house was facility t o wood. started in late October in order to bring the erection into I n erecting, the gypsum plank is laid on top of supports the severe fall and winter weather. The framework consists spaced up to approximately 7 feet for roof decks, and 4 feet of 4 X 4 inch reinforced gypsum timbers or studs, spaced for floors. It is laid a t random-that is, without regard to up to 4.5 feet on centers in the standard arrangement termed the position of the plank ends in relation to the supports. “balloon frame” when constructed in wood. The exterior The gypsum plank is attached securely to the top of the supsurface consists of 2-inch gypsum plank, and the interior ports by means of a simple clipping device nailed to the plank. of 1.25-inch gypsum plank, both laid horizontal and nailed This type of construction gives a high-class, incombustible direct to the vertical supports. Floors and roofs are roof deck, with a pleasing appearance and a considerable of 2-inch gypsum plank supported on metal joists. Partidegree of insulation a t a reasonable cost. It can be erected tions consist of 2-inch gypsum plank placed vertical. The with great speed and is independent of weather conditions. stairs are gypsum block laid on an angle iron frame. All The Gypsteel Plank has been used in the construction of finish materials, such as the heavy slate roofing, gutters, expractically every type of building and is constantly being terior and interior trim, wood flooring, etc., are applied by put to new uses never imagined in its original development, nailing direct to the gypsum core in the usual manner. All such as the construction of fire doors, stairs, air ducts, partiinterior wall and partition surfaces were plastered direct. tions, wall sheathing, furring, and even transformer fencing. The exterior wall finish will be a synthetic stucco which is in It has a wide use for industrial repairs; many plants stock the the process of development. Its application is being dematerial and erect it with their own men as occasion demands. layed pending observation of the weather-resisting qualities The success of these structural gypsum products is due not of the unprotected gypsum exterior wall surfaces. only to the form in which they are available to the industry This house was constructed by a typical, small-house but also bo the noteworthy properties of gypsum itself: builder; all of the gypsum units were furnished to the job in 1. Gypsum is highly fire-resistant and is generally regarded standard sizes. They were cut and erected by ordinary caras the most dependable protection for structural steel. penters who had never seen the material before. Every 2. I t is the best known insulator among important fire-resistant structural building materials; its thermal conductivity effort was made to duplicate the usual small-house operation. is 1.66 B. t. u. per hour per square foot per F. per inch of thickThis scheme of construction appears to be practical from ness. other angles than construction alone. It makes it possible for 3. It is one of the lightest of structural materials; its weight the manufacturer to turn out a few standard units from which per cubic foot is in the range of 48 to 65 pounds. 4. Since it is itself a sulfate, gypsum is quite resistant t o a house can be built and which can be distributed through the many acid fumes commonly encountered in industry, particularly great distribution system of the building supply dealers in to sulfuric and sulfurous fumes. exactly the same manner as lumber. This places the material 5. Exhaustive tests by the Termite Investigation Committee in reach of the small-house constructor who usually requires have determined that termites cannot destroy or multiply in processed gypsum. short-term financing by his dealer and, as the material is readily erected by a standard trade (carpenter), it fits into his picture Gypsum in Modern, Better House Construction with the least possible upset to the present conditions with which he is familiar. The net result is that by these methods The American Cyanamid 8: Chemical Corporation has a superior house can be built which is economical to heat, recognized the growing need for better houses a t lower costs, fire-safe, and permanent, with a low maintenance cost for a and during the last tn-o years has spent a considerable sum
