A Treatise On The Principles And Practice Of Harbour Engineering

98 HARBOÜR ENGINEERING. in themselves, are a source of weakness, but when their use is inseparably combined with an intricate system of keyage and bonding, they prove doubly unsatisfactory and afford but an indifferent sense of security. What masonry, with its vertical and horizontal breaks and intersections, and its costly chisel- work, fails to ensure, is readily achieved by concrete, easily moulded while in a plastic condition to any required shape or outline, and deposited in position by the simplest means, with a minimum expenditure of time, money, and labour. Concrete, as the term is generally understood amongst engineers, is an admixture of various mineral substances which, under chemical action, become incorporated into a solid body. Of its ingredients one group is inert, the other is active. The inert group is called the aggregate, and it comprises any number of the following substances: slag, shingle, burnt clay or earthenware, broken stone, broken brick, gravel, and sand. The active elements are hydraulic lime or cement and water. These constitute what is called the matrix. The uses and applications of concrete are manifold. We have, however, to confine our attention to those points alone which are of pre-eminent im- portance and value in regard to maritime work, leaving other features and adaptations for treatment elsewhere. The first thing to be noticed—which has already been animadverted upon in the earlier part of this chapter—is the necessity for an aggregate of high spécifie gravity. The reasons need not be repeated. From this point of view, heavy materials, such as slag and broken granite, are preferable to broken brick and sandstone. Secondly, in order to ensure close adhesion, the aggregate should be rough and angular. Porous surfaces are admirable in this respect, but they do not generally appertain to heavy substances. However, the coarse crystalline texture of granite offers a sufliciently marked advantage over the smooth polished surfaces of flints and pebbles to constitute an excellent instance of what is meant by compliance with this requirement. Thirdly, in order to reduce the number and volume of interstices, fragments of different sizes should be employed so that the smaller material may fill up the voids in the larger. At the same time, it is not desirable to use fragments of greater linear dimension than 4 inches, nor sand so fine as to be dust-like. It is usual to specify that the stone shall pass through a fj- or 2-inch ring, and that the sand shall be coarse and sharp. If fine sand be used, the grains cohere when watered and impede the introduction of the cement, besides requiring a greater quantity to effect the same complète envelopment. Lastly, the aggregate should be perfectly clean and free from grease, clay, mud, and any other impurity whatever. Such substances have no adhesive value; they intervene between the parts which should corne in contact, and are themselves readily soluble and removable by water, leaving the