Modern Gasworks Practice

46 MODERN GAS WORKS PRACTICE mixmg and good contact between the different ingredients will not be as perfect as is aesirable. 2. It 18 essential to press or pun the mixture well into position in order to elimmate voids, which tend to increase permeability to water and air and are in consequence highly dangerous. 3. A sufficient thickness of concrete should be applied to the metal. If too thia the concrete may not be sufficiently impervipus or may crack mechanically and thus admit water and air to the metal. 4. Stray electric currents must be avoided. Should the metal become anodic, rusfang would take place in consequence of the liberation of oxygen, and this, leadin^ to Cracking, would rapidly destroy the cement. 5 Substanceslikelytocontainacidsoracid-producing bodies should be avoided. Coke breeze and slags are cases in point, as they frequently contain sulphur com-pounds. 6. ^ The concrete may advantageously be coated with some waterproofing material to render it still more impervious, provided such proofmg is entirely free from acids or acid-producing substances. The author suggests that in cases where rolled steel joists are buried in concrete, such as in the construction of retort house floors, etc., the steelwork should firstly be painted with. a cement wash. made into a thin paste from ordinary Portland cement and water, and that the paste should be made thoroughly alkaline by the addition of a small quantity of lime or soda. The questioa of the life of concrete is an important one, the main destructive agents being heat, frost, carbon dioxide, and salt water. Cement or concrete beging to disintegrate under heat when a temperature of about 300° C. is reached, because the combined water is expelled. In reinforced concrete the coefficient of expansion of the concrete is practically the same as that oi the steel, and value of reinforced concrete in fire resistance is due largely to this faet. But the heat conductivity of the steel is much greater than that of the concrete. Consequently, if the steel is covered with only a thin layer of concrete at any point, in case of fire, it will become heated more rapidly, expand at a greater rate and so set up internal stresses that may be disastrous. If freezing occurs during the setting of the cement, the colloidal mass is to some degree prevented from consolidating. During the time prior to the completion . the set, a certam amount of free water exists in the interstices between the hydrat-mg grams of cement. Under normal conditions this water diffuses through the colloidal coating of the grams and is used up by continued hydration. Then as the grams contmue swelling they stick together and the mass consolidates. As reezmg retards diffusion and hydration, these processes may be delayed to such, an extent that the water will evaporate from the mortar and not leave enough for sufficient hydration when the temperature rises. This evaporation of water may occur even though the temperature remains below the freezing point. The effeet of tlus drymg is most noticeable on exposed surfaces and is very similar to that pro-duced by using a cement mortar with a dry porous brick. '