A Treatise On The Principles And Practice Of Harbour Engineering

102 HARBOUR ENGINEERING. bardens. Of course, the more the time is prolonged the greater difficulty will be experienced in preserving the soft concrete from the chafing action of waves ; but, on the other band, it seems to be pretty clearly established that the slower the setting action the greater the ultimate strength attained. By way of completing this brief review of the subject, a model spécification is appended, drawn up from a harbour engineer’s point of view, and there- fore containing several stipulations of a special character inapplicable to con- ditions elsewhere. The quality of the Portland cement, however, is strictly in accordance with the terms of the Engineering Standards Committee’s spéci- fication, the clauses extracted from which are indicated by quotation marks. Spécification. The aggregats shall consist of gravel and broken stone of varying size mixed with sand, the quantity of sand being sufficient to fill completely the interstices in the larger material. The precise proportion of sand is to be ascertained by gauging the volume of water contained in a vessel which has been packed with the maximum amount of gravel and stone it can contain up to, and flush with, the level of the brim. No fragment shall measure more than 4 linear inches in any direction, and every piece must be capable of passing through a ring 2| inches in internal diameter. The length, breadth, and depth of the larger pieces must not be greatly unequal, i.e. there must be an absence of long, flat, slaty slips, as also of smooth, water-worn pebbles. The stone must be heavy, weighing in the solid mass not less than 150 Ibs. per cubic foot. Slag from ironworks may be used in place of, or in conjunction with, stone, provided it conform to the same conditions of weight and size and is not brittle or friable in any part. Both gravel and stone or slag must be perfectly clean and free from admixture with any foreign substance, whether mineral or vegetable, and no gravel which has corne as ships’ ballast will be accepted.1 The sand must also be clean; sharp, and not too fine, i.e. it should all be retainable on a sieve of 32 S.W.G., having 900 meshes to the square inch. Dust and powder, as well as earthy and greasy matter generally, must be rigidly exeluded. Concrete described as æ to 1 shall be understood to mean x parts by measure of gross aggregate as detailed above, combined with 1 part of Portland cement. [Assuming that there is on an average some 35 per cent, of interstitial space2 in the mixed stone and gravel, and allowiug 5 per cent, margin to cover extreme cases, the quantity of sand required will be 40 per cent. The proportion of sand to cement should not exceed 3 to 1. Therefore the mini- mum amount of cement will be 13 parts in 40. 1 On account of the liability ofa ship’s hold to greasiness when used for mixed cargoes, especially in the case of oil in barreis, etc. 2 Substantiated by experiment.