A Treatise On The Principles And Practice Of Dock Engineering

I 20 DOCK ENGINEERING. cernent portion of the concrete, which sea-water, whilst precipitating the magnesia that it contains, takes away, in an altered form, a portion of the lime from the cement.” Mr. Messent made experiments as to the quantity of water absorbed by briquettes of neat cernent, and of cernent and sand, and found that bv repeated absorption and drying, the solids contained in the sea-water were left in the briquettes, the strength of which decreased by from 37 to 70 per cent. He went on, in his report, to add :—“ I am of opinion that the cause of the damage referred to is the injurious effect of sea-water, which entered through holes in the plaster, . . . percolated the concrete of the inter- mediate portion of the wing walls, and of the mass behind the altars of the dock walls, and, in so percolating, extracted lime from, and deposited magnesia in, the cement portion of the concrete, causing it to deteriorate and expand; and that the injurious percolation was facilitated by the inappropriate relative proportions of the cement, sand, and stone, or the insufficient quantity of cement in the original composition of the deterior- ated concrete.” So much for one side of the question. The unanimity of conclusion is apparently convincing, but, at the same time, it must be admitted that other solutions of the problem are equally admissible. In the first place, there are one or two inconsistencies in the individual reports which call for notice. Mr. Pattinson asserts that much of the lime has disappeared from his samples—as much as two-thirds in one case—while an examination of the analytical tables of Professor Brazier demonstrates as remarkable an increase in that constituent. These State- ments are, of course, not necessarily conflicting. The lime may have been washed away by tidal action from Mr. Pattinson’s specimens, but the uniformity of its absence is curious and striking. Then, no explanation is offered to account for the very singular fluctuations, both above and below the normal quantity, of the amount of alumina and oxide of iron. A decrease is intelligible, but there is no manifest source of supply for an increase.* Aluminium salts are not present in ordinary sea-water, nor is oxide of iron a common constituent. Without personal knowledge of the facts and cireumstances, it is difficult to express a definite opinion, but it occurs to the author to suggest— 1. That the cement actually used in the construction of those portions of the wall in which decay occurred might have been of different com- position, and of inferior quality, to that of the original test briquette. 2. That the aggregate was impregnated with impurities of an argil- laceous nature. (One or other of these hypotheses would appear necessary to account for the large increase of alumina in some of the specimens of decomposed * Increase by difference in ratio is not supported by an examination of the tables.