A Treatise On The Principles And Practice Of Dock Engineering

DOCK ENGINEERING. 128 are not, inert. In his opinion no conclusive evidence has been adduced to prove that the precipitates from sea-water induce disintegration, even of fissured or porous concrete, when sound cement is used. Had such evidence been forthcoming it would throw doubts on the durability of all such structures in the sea. In the Aberdeen experiments it was demonstrated that free caustic lime had been washed out of the concrete, and magnesia, as magnesium hydrate, precipitated, with the formation of calcium chloride and sulphate. The analyses prove nothing beyond the fact that the caustic lime present was the cause of such precipitation, and that the lime in this form is an unstable and soluble body. The inference, that by similar action long continued a dangerous portion of the lime may be dissolved out of the cement present in a concrete structure, is without proof. The precipitation of magnesian or other salts from sea-water is merely the deposition, without active chemical change and conséquent change of volume, of bodies which already exist there in solution. Sum- mmg up the facts, of which undoubted evidence has been produced, it may be stated that an excess of caustic lime or caustic magnesia causes (1) disintegration by the expansion due to hydration; and (2) being soluble, when conditions permit of their washing out, they leave the concrete in a honeycombed state.” It would be impossible to close so vexed a question without a quotation ot the views of that eminent specialist, Dr. Wilhelm Michaelis, of Berlin. He states his opinion that— “The magnesia,* which is deposited during the action of sea-water upon hydraulic niortar, is a preservative agent which tends to close the pores of the mass. It would be more correct to speak of the injurious action of the sulphates in sea-water, than to attribute such action to the magnesia salts, although it is true that magnesium sulphate is the special salt which acts in sea-water. The sulphates of lime or of alkalies, in fact, any soluble sulphate have the same destructive action, but do not act with the same degree of energy.” “ fhe main points t to be considered in erecting permanent structures in sea-water, with the aid of hydraulic cements—in other words, concrete_______ are— “ 1. From the physical point of view, completely impermeable mixtures should be made, composed of one part of cement with two or, at the most, two and a-half parts of sand of mixed grain, of which at least one third must be very fine sand. To this the requisite quantity of gravel and ballast should be added. This impermeable layer should surround the porous kernel on all sides in sufficient thickness, even underneath. It would, perhaps, be unnecessary waste of material in the case of thick walls to use the impermeable mixture throughout; but, so far as possible, the * Michaelis on “Sea-water and Hydraulic Cements,” Min. Proc. Inst. C.E. vol. cxxix. + Michaelis on “ Portlaud Cement in Sea-water,” Min. Proc. Inst. C.E., vol. evii.