Correpondence on Capillary canals in concrete by E. B. Moulin

DEAR SIR, In the March, 1922, Journal of the Institute, wherein we have the very excellent paper by E. B. Moulin, M.A., on Capillary Canals in Concrete, there is, I believe, one point which has been overlooked when dealing with the large number of spherical cavities found when a cement block is cut through and its surface ground to a dead flat. These cavities are stated to be probably due to air bubbles carried into the mass while mixing, or, according to the Author, by excess water in the form of drops. I think it highly probable that both theories are, to a certain extent, correct, but the main cause of their formation is the air absorbed by water at atmospheric pressure. When the mass sets this air is left behind in innumerable small air bubbles in exactly similar fashion to the formation found in a block of ice when this is made of undistilled water, the ice being quite white in appearance from the small air cavities formed throughout its mass. To form crystal or clear ice only distilled water is used, or in special cases the freezing takes place from the bottom of the block and the water is gently agitated to free it of air as the mass solidifies. Neither of these methods are applicable to concrete work; at the same time it is obvious that the smaller the quantity of water used in mixing the fewer spherical cavities will be formed and the more dense will be the mixture. Yours faithfully, T. D. KEY.