A Treatise On The Principles And Practice Of Dock Engineering

122 DOCK ENGINEERING. cement.* Bicarbonate of soda, on the other hand, retards it considerably, as also do sugar, glycerine, and sait, slightly. Integrity or Soundness.—This may be tested by Faija’s steaming apparatus or by simple immersion in water. The former is the more rapid method, occupying about as many hours as the other occupies days. In both cases thin pats are made, | inch thick at the centre and as thin as possible at the edges. Signs of cracking, blowing, or expansion indicate a cement either unsound or too hot for use. Adultérants of Cement.—Two common adultérants of Portland cement are turnace slag and Kentish ragstone, the introduction of which, though defended by some manufacturers, must be held a reprehensible practice. The first, besides being injuriously impregnated with sulphur, possesses scarcely any hydraulic properties whatever, and the second is an inferior variety of carbonate of lime. Effervescence under the action of hydrochloric acid will betray the ragstone. The slag, which is a crude mixture of silicates of lime, iron, &c., has a high specific gravity, and confers a mauve tint upon the powdered cement. The water may be either salt or fresh, unless for important surface work above ground, in which case salinity is objectionable, on account of the resulting efflorescence. The amount of water required cannot be stated with exactitude. It will depend upon the proportion of the aggregate and its porosity. It is best determined by experience in each particular case. Without being profuse enough to drown the concrete or wash away the cement, it should be used in sufficient quantity to act as an efficient inter- mediary between the matrix and the aggregate. Some autliorities advocate a very sparing use, but the author’s experience is to the effect that a plentiful supply is advantageous, for several reasons : it serves to intimately incorporate the materials; if the aggregate be very porous it prevents undue absorption of moisture from the matrix, and it allows a scum of inert or limey cement to rise to the surface and pass away with the drainage. In certain parts, such as the floors and walls of graving docks, founded on water-bearing strata, and sea piers, impermeability of the work is essential to its stability, and it has been claimed by somei that a minimum of water in mixing produces a maximum of watertightness in the mixture, but this is far from being the case, and the labour involved in manipulating the concrete under such conditions is greatly increased, since, in order to secure the complète penetration of the scanty allowance of water, the mixture has to be beaten in a manner such as would be adopted to cause moisture to appear on the surface of damp sand. For the majority of dock walls, in * Mr. E. E. Priest, of Liverpool, has been kind enough to communicate to the author the results of some experiments which he undertook in reference to this point, from which it appears that the weakening is only a transitory feature, and that at the end of four years the testing of the briquettes indicated perfeetly normal results. + Vide Deacon on “Liverpool Waterworks,” Afin. Proc. Inst. C.E., vol. oxxvi pp. 42, 43.