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VII-GENERAL DISCUSSION. Qualities of the Aggregates Nothing was brought out in the tests to show marked differences in strengths because of the differences in the nature of the surface of the grains and the effect of smoothness or of differences in the porosity of the grain on the adhesion of the cement to the grain. There was some indication that the smooth and non-porous grain of the Ottawa sand gave slightly lower strength results. J. Singleton-Green
With the gradual raising of concrete engineering into a definite branch of science, the need for accuracy in every detail and, therefore, for testing at each successive step, has brought about the introduction of quite a large number of appliances used in controlling operations. Such checking of operations is required not only in connection with the primary materials-cement, sand, the larger aggregates and water-but the secondary adjuncts, including reinforcements, waterproofing or other ingredients, and also the mixtures at various stages, as well as the completed work, whether this be in the form of mass construction or moulded units.
0NE of the problems of civilisation to-day is the prevention of waste, and most countries of the world, if they adopted scientific methods, could live on what they throw away. One good example of this is the fact that millions of tons of ash and clinker, from steam boiler, destructor and other furnaces, for example, together with blast furnace slag, are wasted every year, whereas all this material could be converted, at a handsome profit, into high grade bricks. A Correspondent