January, 1926
INDUSTRIAL A,YD ElVGIiVEERISG CHEJIISTRI-
of the Pyrotechnic Division, I embodied the specifications for these signals in a patent application which was assigned to the Government. It was felt necessary at the time that at least temporary patent protection should b e provided, and there was no intention on my part to claim any of the
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credit for the invention of these devices. Except for very minor changes in the mechanical construction made necessary in adapting these smoke signals to certain uses, there were no improvements in the developments made by Captain Ray and his assistants.
Relation between Water Permeability and Water Absorption of Concrete' By Evald Anderson WESTERSPRECIPITATION Co., Los ANOELES, CALIF.
HE tests used ordinaExperimental Water absorption and water permeability tests for rily for the determinaconcrete are described and experiments are reported, Either of these two methtion of the comparathe results of which indicate that there is, in general, ods should give a direct meastive waterproof qualities of no direct relation between tbe per cent water absorpure of the p e r m e a b i l i t y . concrete-or better, perhaps, tion of a cement mortar specimen and its water perOne modification of the secits comparative water-tightmeability or water-tightness. ond method was used in these ness-are of two main types. The fact that the cement in concrete must be comtests. I n this modification, I n one case a direct determiposed partly of hydrate-like compounds is pointed out which has been described in nation of the water permeand the consequences of this view discussed, and it is detail in a recent paper,'the ability is made; in the other concluded that if this view is correct neither the abcement mortar specimens are the water absorption of a sorption tests nor other tests depending upon the diffuin the form of disks, 8 cm. dried concrete specimen is sion of water vapor from the cement mortar specimen (3.25 inches) in diameter and determined and this is then could serve as a measure of the water permeability or 13 mm. (0.5 inch) thick. .assumed to be proportional to water-tightness of concrete. These are placed in holders the water permeability. CerIt is recommended that for the determination of the so arranged that one side of .tain considerations led to the water permeability of concrete some method be emthe disks can be subjected to 'belief that this latter asployed which directly measures this property. a constant water pressure, sumption might not be cora n d t h e a m o u n t of water rect, and some experiments were therefore undertaken, using both types of tests on which is forced through can be determined. The accompanying sketch shows the principle of this metht h e same specimen, for the purpose of studying the relaod. I n practice, however, there is ordinarily not sufficient tions between the absorption and the permeability. I n determining this water absorption by the standard water forced through the specimens to measure conveniently method,2 the concrete specimens are first dried at not less by this method, but for these manipulative details the than 110' C. to constant weight. They are then immersed in original article may be consulted. In these tests the water pressure was 2 atmospheres (30 boiling water for 5 hours to drive out the air in the pores, a n d then cooled in water to a temperature of 10' to 15" C. pounds per square inch). The amount of water which passed They are then removed from the water, superficially dried through the disks under these conditions was measured in with towel or blotting paper, and a t once re-weighed. The grams per square centimeter per day. When the permeability of the specimens had been so depercentage of increase in weight when so saturated over termined, the disks were dried and the percentage absorption that when thoroughly dried is termed the "absorption" of the same specimens was then determined by the standard of the specimen. Here the underlying theory is evidently that a piece of method just described. concrete is made up of water-impervious masses, interspersed In making these experiments it was decided to compare with pores or capillaries, and that the more pores or capillaries three different cements, designated A , B, and C. Cement hhe concrete has the greater will be its water absorption and mortar disks or specimens were made with three different the less its water-tightness; or, in other words, that the chief proportions of cement and sand, 1: 2 , 1 : 4 ,and 1:5, from each reason for the permeability of concrete is its porosity and that one of the cements, except that no 1:2 disks were made from the water absorption of the dried specimen is a correct measure cement C. Standard Ottawa sand was used in the 1:2 of this porosity. disks and ungraded plaster sand in the others. Six disks I n determining the water permeability directly, two general were made in each case so that the figures given are the avermethods are available. I n one, exemplified by the standard age of six individual determinations. hydrostatic test for cement pipe,a the specimens are suba n d Water Absorption of C e m e n t jected to an increasing water pressure until the water is Table I-Water Permeability Mortars forced through, and the pressure a t which this occurs is Cement-sand Permeability Absorption Samule ratio Grams Per cent taken as a measure of the permeability. I n the other method, A' 1:2 41 5.6 the cement specimens are subjected to a constant water B 1:2 380 5.2 pressure, and the weight of water forced through a unit area A 1:4 81 8.4 B 1:4 1655 7.0 in unit time is determined, and becomes the measure of the C 216 1:4 10.1 permeability of the specimens. A 1:5 69 10.0 B
July 23, 1925. Materials, Standard, 1924, p. 705. Ibid., 1920, p 695.
1 Received
C
1:s 1:s
* Am. SOC.Testing J
4
Anderson, Concrcfr, 26, 195 (1925).
$080 I 980
8.3 9.7
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Table I shows the comparative results. Here the permeability is expressed in grams per square centimeter per year for 13-mm. (0.5-inch) disks a t 2 atmospheres (30 pounds) pressure and the absorption as the per cent increase in weight of the water-saturated over the dried specimen. Discussion of Result8
It seems evident from these data that there is no direct relation between the water absorption and the water permeability. Indeed, with cements A and B taken together in each group, just the opposite is the case to that ordinarily supposed, in that here the greater the absorption the less the permeability. The data for cement C , however, show that this is not always so. As a matter of fact the data for cements A and B can be used to support the contention that t h e permeability varies either directly or inversely as the absorption. For, by grouping together the data for each of the cements given, it is seen that both the permeability and absorption increase with the amount of sand in the specimen, while with the groupings given in the table i t is evident that in each case cement A has a smaller permeability and greater absorption than has cement B. One would therefore conclude from the results of these Diagrammatic S k e t c h Showing Method e x p e r i m e n t s t h a t for Determining- the Water Permeability of there is, in general, Cement no direct relation between the water permeability and the water absorption, and that the water permeability of concrete cannot be correctly determined by measuring its water absorption. Consideration of the fundamental nature of the cement in concrete and of the conditions specified in the standard absorption test would seem to indicate that the foregoing results should be expected. As has already been mentioned, the theory underlying the absorption test for concrete and its use as ~tmeasure of the water permeability seems to be that these absorption tests correctly measure the porosity of the concrete and that the greater the porosity the greater the permeability. This Nolt-In discussing this question i t would seem desirable to distinguish between water permeating the concrete and its water permeability, a s this term is ordinarily used. In order t o avoid misunderstanding i t would be better t o use the word “water-tight” rather than “impervious to water.” For example, a dry homogeneous wooden vessel, free from cracks, is ordinarily water-tight, but the dry wood itself is far from being impervious to water.
theory appears faulty in many respects. The fact that the cement in concrete is partly a mixture of hydrates or hydrated colloids, each of which must have a more or less definite vapor pressure, seems often to be overlooked. Now, it is a fundamental property of hydrates that they lose water when the pressure of the water vapor in contact with them falls below the equilibrium pressure, and regain this water up t o their saturation point when the pressure of the surrounding water vapor reaches or exceeds this equilibrium
Vol. 18, No. 1
pressure. Therefore, when the cement in a piece of concrete absorbs water from the surrounding atmosphere, the compounds having the lowest vapor pressure must become saturated first, so that the vapor pressure of such a piece of concrete must depend on its degree of saturation and must be, in fact, the vapor pressure of the compound last saturated. If all the cement in concrete were always thoroughly hydrated, and if the thoroughly hydrated cement were perfectly stable under the strenuous drying conditions of this test, and if, further, the concrete capillaries were always equally interconnected so as to form clear, though constricted, passages through the concrete, then the per cent water absorption of a concrete specimen might serve as a measure of its water-tightness. But these conditions are not ordinarily fulfilled. When a concrete specimen is thoroughly dried at the high temperatures specified in the absorption test, a t least a part of the water of hydration must be driven off. When the specimen is again placed in water this water of hydration is re-absorbed. The total increase in weight of the specimen is thus made up not only of the water which fills the pores, but also of the water which has been chemically absorbed by the cement. A measure of this total absorption is therefore obviously not a measure of the porosity of the concrete. Nor does it necessarily follow that a very porous specimen of concrete will be very permeable to water in the sense that it will permit the water to pass through it. If the pores are not large and freely interconnected, there may be a tendency, due to the capillary force, to absorb water from the outside, but the general direction of this force will always be towards the center of the specimen and will itself counteract any force which would tend to cause water to pass through. The real water permeability of concrete or the inverse function, the water-tightness, must necessarily depend on the resistance to the flow of water through the specimens, and this resistance to flow would seem to be directly measured by means of either of the water permeability tests described. One wouId therefore not expect to find any direct relation between the water absorption and permeability, and the results of the experiments herein reported which showed that, in these cases at any rate, there was no such direct relation bear out that point of view. Other methods have been proposed for the determination of the water permeability of cement, which depend on measuring the rate of evaporation from one surface of a specimen while the other side is kept in contact with water. There would seem to be little more justification for these methods than for the standard absorption method, for the watertightness-i. e., the resistance to flow of water through the specimen-is not determined. As long as direct methods are available for measuring the permeability, or real watertightness, of concrete, the absorption method and other indirect methods are probably not to be recommended.
New York Museum Gets First Sample of Synthetic Gold George F. Kunz, famous gem expert, has obtained the first sample of synthetic gold which has reached this country. It will form a part of the collection of elements a t the American Museum of Natural History in New York. To Dr. Kunz has fallen the difficult task of assembling as many of the ninety-two elements as are procurable. The sample of synthetic gold comes from the laboratory of Prof. Hantaro Nagaoka of the Tokyo Imperial University, and is but a tiny speck. Professor Nagaoka obtained microscopic quantities of what he reports to be artiiicial gold from mercury by running a mercury lamp for a long period of time and employing very high voltage. Dr. Kunz also has in his collection the first crystals of pure fluoride of hafnium and metallic hafnium, but does not have samples of the two new elements rhenium and masurium.
