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The Structural Engineer

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.

Publish Date - 1st May 1922

Author – N/A

Price – £9

The Structural Engineer

Coming now to the consideration of reinforced concrete beams, if we turn back to the early pioneers we find first of all that their work was done with next to no calculation; slabs and beams were tested to destruction, and in the light of such experiments buildings and parts of buildifigs were constructed, the sizes being roughly proportioned from the data thus obtained. The first attempt to formulate some method of calculations that is known to the author was that given in a book printed at Chiswick in 1877 for private circulation by Thaddeus Hyatt, the inventor of pavement lights, who came over from New York to establish a business therein in England, and had a number of experiments on reinforced concrete bearns conducted by David Hirkaldy. The method of calculations put forward in the book was a simple comparison with iron joists embedded in concrete. It was pointed out that the upper flange of a steel joist was in compression, and the lower in tension, and that the web and compression flange might be omitted, the purpose served thereby being fulfilled by the concrete. All the concrete above the middle of the steel joists was taken as being in compression. The ultimate compressive resistance of the concrete was taken as 2,000 lbs. per square inch, but this was reduced to a mean stress of 1,000 lbs. per square inch, which looks like a triangular stress distribution, but the centre of gravity is taken halfway, thus suggesting rectangular stress distribution. Mr. Hyatt acknowledged his indebtedness to Mr. Thos. Rickett, of Birmingham, formerly, as he says, in the employ of the London and North-Western Railway. Probably Mr. Rickett was responsible for the method of calculations, which are merely sketched in the book. It is interesting to note that Mr. Rickett and Mr. Hyatt did not lose sight of the possibility that the neutral axis might become displaced by reason of the different deformations of concrete and steel under a similar stress, because they record an experiment that they made to ascertain the amount of deformation, but apparently through the roughness of the apparatus they were not able to get any difference between wrought iron and cement, so assumed the same for concrete (the book states) "the effect of which in the compound beam is to keep the neutral axis at the central line of the beam, the entire tensional strain being thus thrown on the metal tie." H. Kempton Dyson

Publish Date - 1st May 1922

Author – N/A

Price – £9