462. Change In Volume During Setting

Cement mixtures shrink somewhat when hardened in air, while specimens stored in water expand a trifle during hardening. Although several experiments have been made on this subject the specimens used have been so small that the results obtained by various authorities do not agree, and the effect of variations in the character of the mixtures has not been thoroughly investigated. The importance of the question is found in the necessity of providing expansion joints in long walls or sheets, and in the effect of such changes in volume in producing initial stresses in concrete or steel where these materials are used in combination.

Certain general conclusions are well established and may be stated as follows: 1st. The shrinkage of mortar and concrete hardening in air is considerably greater than the expansion of similar specimens hardening in water; 2d. The amount of change in volume increases with the proportion of cement used in the mixture; 3d. The change in volume is continuous up to one year, but about one half of the change occurs in the first week, and it is very slow after 3 to 6 months.

The following values of the change in linear dimensions are derived from the results of several experimenters, and show in a general way what changes are to be expected at the end of three months.1 Variations in the character of the cement and the consistency of the mortar will affect the result.

1 For more detailed results the reader is referred to the following authorities:—Dr. Tomei, Trans. A. S. C. E., Vol. xxx, p. 16. Mr. John Grant, Proc. Inst. C. E., Vol. lxii, p. 108. Prof. Bauschinger, Trans. A. S. C. E. Vol. xv, p. 722.

463. The Coefficient Of Expansion Of Cement And Concrete

Concerning the coefficient of expansion of cement mortars of various compositions, we know but little. The result obtained by M. Bonniceau, giving the coefficient of neat Portland cement as about .000006 per degree Fahr., is frequently quoted. This is very nearly the value for iron and steel, and has formed a theoretical basis for combining these materials. In the case of cement mortars and concretes, however, it is highly probable that the coefficient follows quite closely the behavior of the sand and stone used in the mixture, and is much less dependent upon the coefficient of the cement. This was indicated by the results of M. Bonniceau who obtained a value of about .000008 for concrete.

464. A number of experiments to determine the coefficient of expansion of cement concretes were carried out under the direction of Prof. Wm. D. Pence by students of Purdue University.1 As a mean of seven tests with one-two-four concrete of Bedford oolitic and Kankakee limestones combined with Portland cements of two well-known brands, the mean result for the coefficient was .0000055, the lowest result being .0000052, and the highest result .0000057. The coefficient of a bar cut from the Kankakee limestone was .0000056, the same result as obtained from the mean of three tests of concrete containing broken stone of this variety.

The average result of four tests of gravel concrete composed of one part Portland cement, two parts sand and four parts screened gravel, or one part Portland cement to five parts unscreened gravel, gave .0000054 as the coefficient of expansion.

These values differ from the coefficient of steel enough to indicate that in positions where the range in temperature is great, the resulting stresses in the concrete and steel may be considerable, and worthy of attention.

465. The Fire-Resisting Qualities Of Concrete

The value of concrete as a material to be used in the construction of the walls and floors of buildings, is largely dependent on its fire-resisting qualities. That its use for such purposes is rapidly extending, is some evidence that these qualities are as satisfactory as in other classes of materials devoted to the same use.

1 Paper read before the Western Society of Engineers, Engineering News, Nov. 21, 1901.

Under favorable circumstances, a fire in a building filled with combustible materials may reach a temperature of 2,000° to 2,300° Fahr. If a small specimen of cement mortar or concrete is subjected to a temperature approaching this intensity, the cement loses its water of crystallization and becomes friable. If cooled suddenly in water, the specimen cracks and disintegrates. If cooled gradually, the outer edge of the specimen crumbles away. From such tests on small specimens some very erroneous conclusions have been drawn as to the value of concrete as a fire-resisting material. Such conclusions have done much to prejudice the public mind against concrete, and to retard its introduction in buildings designed to be fireproof.

466. Conductivity

The great value of concrete as a fire resistant is due to its low conductivity of heat, and while the surface of a mass of concrete exposed to an intense flame for some time is ruined, and may be flaked off by the application of a strong stream of water from a fire hose, the depth to which the heat penetrates is very limited. Steel is said to lose ten per cent, of its strength at about 600° Fahr. and fifty per cent, at about 750° Fahr. The importance of protecting the steel framework of a building, not only from warping and complete destruction due to flames, but from loss of strength from overheating, is therefore evident.

Among engineers and architects it is recognized that the term "fireproof construction" is only relative, although the lay mind is apt to give a definite and literal meaning to the term. It is well known that fireproofing tile, whether hard or porous, will fall to pieces if subjected to a temperature above that employed in its manufacture. The practical question then is, what type of construction will withstand long continued intense flame, and subsequent quenching with water, with the least injury to the strength of the structure. The results of fire tests that have been conducted in several places, and notably those made by the Department of Buildings of New York City, have shown that floor arches properly constructed of concrete-steel are equal to any style of floor with which they come in competition.

The low conductivity of concrete is shown by the fact, stated by Mr. Howard Constable in connection with the discussion of fire tests of concrete floor arches,1 "that in some thirty-five cases where the temperature ranged from 1,500 to 2,400 degrees, the time of exposure being from one to six hours, the temperature of the upper flanges of six-inch to ten-inch beams might be approximately placed at not much above 200 degrees." He also says "in one case, where the beam was protected by three inches of concrete, the fire was maintained for five hours, and the temperature went as high as 2,300 degrees, and there was no practical or permanent set produced in the beams".