A Treatise On The Principles And Practice Of Dock Engineering

124 DOCK ENGINEERING. local conditions, or whether they arise from causes of a more general and widespread nature. The writer has seen to the construction of a good deal of concrète work, executed without any spécial précautions, the whole of which during a number of ensuing years has been exposed either to constant immersion or to tidal alternations, without the slightest sign whatever of détérioration. Indeed, from specimens which have been cut out of the solid mass, he is convinced that a harder and more compact material for its purpose would be difficult to tind. At the same time, the evidence in faveur of adopting certain measures, of the nature of preventives against possible degeneration, is so weighty and backed by such influential authority that it cannot be lightly disregarded or passed over without due con- sidération. In order then to present sonie evidence bearing on the question, a typical instance will be taken in which concrete, composed of Portland cement and a mineral aggregate, has proved abortive and exhibited un- doubted signs of disintegration and decay. The case is that of the entrance walls of a graving dock, at Aberdeen, opened in 1885. They were built as a “homogeneous mass of concrete,’ deposited inside frames, composed of 1 of cement, 2 of sand, and 3 of stone’ for one-third of the depth of the frame, and of 1 of cement, 3 of sand and 4 of stone, in the upper two-thirds.” It had also been intended to provide the wall with a facing of 2 of cement, 3 of sand, and 4 of stone, but this was omitted and the surface was plastered instead. The sand used was clean, sharp, quartzose sand, screened through a sieve of 40 meshes to the inch,* and containing a small proportion of minute, water-worn pebbles The stones consisted of smooth water-worn pebbles, granite, trap or whin- stone, macadam, and granite chips. Shortly after the opening of the dock symptoms of disruption appeared and in June, 1887, Mr. Wm. Smith, the engineer at that period, reported thar “the Portland cement concrete entrance walls have expanded 24 inches on the height of the walls, their surfaces have cracked and bulged, and the joints of the caisson quoin stones have opened up, causin« considerable leakage.” Professor Brazier, of Aberdeen University, Mr. P. J. Messent, M. Inst. C.E., and Mr. Pattinson, a Public Analyst, were consulted on the subject. The first-named reported as follows :— The analyses of the series of decomposed cements show a remarkable difterence to the original cement, inasmuch as that in all these samples there is found a large quantity of magnesia, and a large proportion of the lime in the form of carbonate. I believe this alteration is brought about entirely by the action of sea-water upon the cement. There is no other source for either the magnesia or the carbonic acid.” Although not specifically stated, apparently the linear inch is intended and accordmgly there would be 1,600 meshes to the square inch, the more generally accepted