INDUSTRIAL AND ENGINEERING CHEMISTRY
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where z
time, days unit stress in concrete at loading unit stress in concrete at x days after loading p = proportion of steel cross-sectioned area in the gross column cross section E, = modulus of elasticity of steel = 30,000,000 pounds/ square inch n = modular ratio E,/E, e = Kaperian base =
= fCz = .fc
If we consider a volume of cross-sectional area of 100 square inches having 2 per cent ( p = 0.02) of steel in the cross section and the modular ratio n of 12, under a concentric load of 61,000 pounds, and take the values 0.130 and 2.9 for C and a, respectively, and 365 days for 2, we have by the above formulaf, = 500 pounds per square inch, fc. = 306, f. = 6000, and fsz = 10,160 pounds per square inch. This means that the unit stress in the steel has advanced by 69 per cent. If the steel proportion is changed to 0.5 per cent, the values will change to f c = 578 pounds per square inch, f C z = 510, f v = 6940, and f.. = 20,500 pounds per square inch. Here the increase in the unit stress in the steel is 190 per cent in one year. Another theoretical aspect is that of the release of the stresses and load due to a sustained strain rather than a sustained stress or loading. Whitney (88) gives this expression: jcz
L eEcY
If the value of y from the power expression is introduced, the expression becomes : fcz =
.fc
eEcCI;/;
If the conditions of the two examples shown for sustained load are used, fez at one year after loading will be 415 pounds per square inch for the 2 per cent steel, and the total load will reduce to 16,070 pounds. For the 0.5 per cent column f C z will again be 415 pounds per square inch, and the load will have reduced from 61,000 to 7070 pounds. It will be noticed that f s will remain constant for a sustained strain and will be 6000 pounds per square inch for both cases. From these equations and examples it will be seen that other estimates and calculations can be made for the easement of secondary stresses in frames and arches, and that deflections of beams and other distortions may be more closely estimated than heretofore; these estimations mill aid the engineer in his judgment and may thus save in materials and avoid damage to the working of machinery which cannot allow undue distortion of supports. Bibliography Bingham, E. C., and Reiner, M., Physics, 4,88-96 (1933). Clemmer, H. F., Proc. Am. SOC.Testma Materials, 23,339 (1923). Daris, R. E., Proc. Am. Concrete Inst.. 24,303 (1928). Davis, R. E., and Davis, H. E., Ibid., 27, 837 (1931). Davis, R. E., and Davis, H. E., Proc. Am. SOC.Testing M u terials, 30, 707 (1930). (6) Davis, R. E., Davis, H. E., and Hamilton, J. S., Ibid., 34, 11, 354 (1934). (7) Faber, Oscar, Proc. Inst. Civil Engrs. (London), 225, Part I (1927-28). (8) Fuller, 8.H., and More, C . C., Proc. Am. Concrete Inst., 12, 302 (1916). (9) Glanville, W. H., Dept. Sei. Ind. Research (Brit.), Building Research, Tech. Paper 12 (1930). (10) Goldbeck, A. T., and Smith, E. B., Proc. Am. Concrete Inst.. 12, 324 (1916). (11) H a t t , W. K., PTOC. Am. Soc. Testin0 Materials, 7, 421 (1907). (1) (2) (3) (4) (5)
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(12) Kluge, R. W., and Wilson, W. M., Univ. Ill., Bull. 269 (1935). (13) Kreidler, C. L., and Lyse, Inge, Proc. Am. Concrete Inst., 28, 317 (1932). 114) Lord. A. R.. Ibid.. 13. 45 11917). i15j Lyna‘m, C . ‘G., “Growth and Movement in Portland Cement Concrete,” Oxford University Press, 1934. (16) McMillan, F. R., Trans. Am. Soc. Civil Engrs., 80,1743 (1916). (17) McMillan, F. R., Univ. Minn., Studies in Engineering, Bull. 3 (1915). (18) Richart, F. E., and Stachle, C. C., Proc. Am. Concrete Inst., 28, 279 (1932). (19) Smith, E. B., Ibid., 12 (1916). (20) Smith, E. B.. Ibid.. 13,99 (1917). (21) Straub, Lorens, Trans. Am. SOC.Czvd Engrs., 95,613 (1931) (22) Whitney, C. S., Proc. Am. Concrete Inst., 28,479 (1932). RECEIVED-4pril 27, 1935.
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Portland Cement and Its Possibilities Simple descriptions are given of the changes which take place in limestone and clay as they pass through the kiln and grinding mills to produce Portland cement, and in the cement as it hydrates to produce hardened concrete. The limited demands made upon concrete by the civil engineer indicate that the strength-giving quality of cement can be used only in small measure because of the properties of volume change and brittleness in the concrete. The major factors which influence volume change of concrete are given, and suggestions are made as to possible methods of controlling volume changes. One of these suggestions is to increase the efficiency of the cement so that less can be used per cubic yard of concrete.
R . W. CARLSON Massachusetts Institute of Technology, Cambridge, Mass.
SSEKTIALLY the making of cement consists in combining the oxides of calcium, silicon, iron, and aluminum into crystalline compounds by the application of heat. Limestone and clay are ground together in predetermined proportions until about 90 per cent of the mixture passes the 200-mesh sieve, making the average particle size about 0.001 inch. This ground “raw mix” is fed into the upper end of a rotating kiln to work its way gradually toward the flame a t the lower end. First the moisture is evaporated. As the temperature rises and reaches about 1800” F., the limestone decomposes into calcium oxide and carbon dioxide, the latter escaping as a gas through the flue. -4s soon as the limestone decomposes, the
SEPTEMBER. 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
calcium oxide and clay begin to combine and form compounds, tending to fuse into semi-molten lumps. The final compounds of most importance are tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite, but two of these are not formed in a single step. Certain aluminates of lime are first formed, and. as the temperature is raised, more lime combines with these aluminates to produce only the tricalcium aluminate. Similarly, the first combination of lime and silica results in dicalcium silicate which takes up more lime to form tricalcium silicate only as higher temperatures are reached and as lime is available for the reaction. The tetracalcium aluminoferrite probably forms in a single step. As the semi-molten clinker passes the point of maximum temperature in the kiln (having reached a temperature above 2500" F.),it begins to cool and the compounds begin to crystallize out, starting with the tricalcium silicate and ending with the iron compound. The conditions under which cooling takes place help to determine the quality of the clinker. Under some conditions the compounds break down partially, and in any case more or less mechanical strain is introduced which affects grindability. The clinker leaves the kiln in the form of lumps varying in size up to several inches in diameter. When the clinker is cooled, and usually after aging for a t least a few days, it is ground into cement with about 3 per cent gypsum to retard the set. As in the case of the raw mix, the average particle size is again about 0.001 inch, with a fair percentage of particles only 0.0001 inch in diameter. The cement is now ready for use. The cement is mixed with perhaps five times its weight of rock and sand and about two-thirds its weight of water to make concrete. For the first few hours after the water is added, the cement behaves (as far as the observer can see) as though it were made up of inert particles of rock dust, but actually the water is attacking the surface of the grains. It is believed that tricalcium aluminate goes into solution and combines with the gypsum to form a sulfoaluminate. The tricalcium silicate hydrolyzes to form calcium hydroxide and calcium silicate gel; the gel remains attached to the particle, but the calcium hydroxide diaperses through t,he liquid to crystallize in the pore spaces. The other two major compounds apparently do not react sufficiently in the first few hours to require consideration. When the supply of gypsum is exhausted, tricalcium aluminate begins to crystal!ize in hydrated form. Up to this time only a small amount of heat has been liberated, but as the tricalcium aluminate crystallizes there is a rapid evolution of heat, often sufficient to raise the temperature of the concrete 10" F. in a half-hour. Rapid hardening occurs a t this time also, but it is attributed mainly to the dehydrating of the tricalcium silicate gel because of the removal of free water by the crystallization of the tricalcium aluminate in hydrated form. Unless the concrete dries out or the temperature rises, the volume changes very little as hardening progresses even though the cement and water occupy as much as one per cent less absolute volume than before the rapid reaction occurred. That is, the overall dimensions of the concrete remain practically the same when correction is made for expansion due to temperature rise, but minute voids are formed within the gel of the cement as it hydrates and expands into the small spaces which previously contained mater. Owing to the fact that the pore spaces in the gel are almost impermeable, the hydration and gelation of cement are a most effective means of sealing the somewhat larger pores between particles which are highly permeable.
Volume Changes It has been said that unless concrete dries out or the temperature rises, it5 volume changes very little. However, hy-
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drated cement is largely gelatinous and, if the average concrete does dry out in air of average temperature and humidity until it loses no more weight, it will contract about 0.05 per cent in length, or about 0.5 inch per 100 feet of length. Although this does not seem to be a large amount, if the concrete were prevented from contracting it would require a tensile strength of 1500 pounds per square inch to keep it from cracking. Since the tensile strength of concrete is usually not more than one-fifth this amount, it cracks. Temperature changes may cause concrete to crack also. illthough the gain in strength is gradual, it may be assumed that concrete solidifies a t a time when the temperature is well above normal because of the heat of hydration. Since it will return sooner or later to the normal temperature, it tends to contract, and if restrained from contracting it may crack. A temperature drop of about 15" to 20" F. is required to crack concrete in such a case. Since the dissipation of heat from thin sections is rapid, temperature rise mill be small in thin sections; heat generation is therefore not important for members having thicknesses of less than 1 or 2 feet but has considerable effect on members 10 feet or more in thickness.
Cost and Efficiency of Cement Although raw materials are quarried, ground to powder, fired in a kiln to more than 2500" F., and ground again to powder, all under controlled conditions, finished cement is delivered to the consumer a t a cost of about a half-cent a pound. Furthermore, it is such an efficient material that it can be diluted with five to ten parts of rock and sand and still produce compressive strengths of several thousand pounds per square inch. Perhaps it is fortunate that the cement can be diluted because otherwise the cost of structures would be greatly increased. Furthermore, the so-called neat cement (without aggregate) would be an unsatisfactory material for moat purposes. For example, if neat cement paste mere cast in a structural member of any size, the temperature of the interior mould approach the boiling point of water, and large cracks would develop upon cooling which would make the member of doubtful value for supporting load even if smdl samples of the material might show strengths of 6000 pounds per square inch. Besides giving trouble due to temperature changes, the neat cement member would contract upon drying t o the extent of about 3 inches per 100 feet of length.
Latent Possibilities of Cement The civil engineer has found that, in general, concrete develops cracks, however small, without the application of load, so that he does not employ concrete for tension regardless of what the tensile strength of small specimens may be. When a member must support tension, he casts enough steel into the concrete for that purpose. He has also found that volume changes upset his precise predictions of compressive stress. rllthough concrete can readily be produced which has an ultimate strength of 4000 pounds per square inch, he dares ask only that it carry perhaps 800 pounds per square inch useful load. In massive structures where maximum computed stresses are often only 100 or 200 pounds per square inch, he demands a quality of concrete developing a strength of perhaps 2000 pounds, because he feels that leaner concrete may not be sufficiently durable or impermeable. When a thin member 2 or 3 inches thick would easily support the expected load, he specifies a minimum thickness of 4 to G inches, mtinly because the brittleness of the concrete makes a more substantial thickness advisable. In general, he resorts to inefficient designs of structures because he can make use of only a fraction of the potential strength.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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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