A Treatise On The Principles And Practice Of Dock Engineering

ACTION OF SEA-WATER UPON CONCRETE. I27 cement, and the second would also admit of an explanation for a decrease by reason of fluxion.) 3 . That the sand was much too fine for the purpose of making concrète, and was used in excessive quantities. A 1,600-mesh sieve for sifting sand is absurdly fine. In confirmation of this view, Mr. Messent’s report may be quoted, in which it is said that “ the deterioration was chiefly confined (so far as could be ascertained by examination) to the concrete which contained the largest proportion of sand—viz., 3 to 1 and upwards.” Supplementary evidence is afforded by Mr. Leedham White,* who stated that— “Twenty years ago he was in Aberdeen, and examined one of the concrete blocks made at the beginning of that particular work. Ihe block was pointed out to him as not giving satisfaction to the engineers, and, although it had been made several weeks, he had no difficulty in crumbling a piece off in his bands, part of which he took home and washed, which disclosed that the sand, which had been used very liberally, was so minute in the grain that, though sharp and clean, it was little better than dust. He was so impressed with the faulty character of the sand that lie took a sample of the cement to the manufacturer, and told him that he would certainly hear complaints of the cement, and ought to know how it had been treated. He did not know whether sand of that quality was subse- quently used in the work, but, as a manufacturer, he affirmed that if such sand was used at the Aberdeen works during successive years, it was a miracle that the concrete had ever stood at all.” Mr. Faija,f one of the greatest authorities on the subject of Portland cement, expressed himself as follows : — “ Magnesia, as precipitated from sea-water, was simply in the form of a hydrate or carbonate, and was a perfectly inert material. The lime was dissolved from the cement, and the magnesia precipitated from tlie sea- water; but the lime was not dissolved to the destruction of the cement if it was Sound, and, as the lime from the outside surface was dissolved, a crust of lime and magnesia was formed which rendered the mass impervious to further destructive action. He had passed sea-water through blocks under a head of 21 feet and found that, after a time, percolation ceased, because the pores of the concrete became filled with the deposit of carbonate of lime and magnesia, so that the briquettes through which the sea-water had percolated were stronger than those left in the sea-water without percolation. The analyses given by Mr. Smith showed that the failure at Aberdeen was due to bad cement or bad manipulation.” Mr. Carey,J who has also largely contributed to the scientific data of Portland cement, summed up the matter as follows : — “ The real point at issue is whether the salts of magnesia, which are admittedly deposited from the sea in porous concrete structures, are, or * Min. Proc. Inst. C.E., vol. cvii., p. 109. \lbid., vol. evii., p. 118. $ Carey on “ Portland Cement,” Min. Proc. Inst. G.E., vol. evii.