A Treatise On The Principles And Practice Of Dock Engineering

Il8 DOCK ENGINEERING. The above definition and classification do not include three compositions, commonly called concrète, but which differ fundamentally therefrom in tliat no chemical action is required to solidify them. Apart from this, their use in constructive work is very limited, and they are quite unimportant. The compositions are as follows :— Tar concrete, made of broken stones and tar. Iron concrete, composed of iron turnings, asphalte, bitumen, and pitch ; and head concrete, which consists of broken bricks immersed in lead. Reverting to the first and most prévalent conception of concrete, we will discuss its composition a little more in detail. The aggregate should be clean and perfectly free from impurities, such as dust, dirt, and greasy matter. Ballast, therefore, which has been carried as such by a ship should not be used. The material should also be sharp and contain as many angular fragments as possible. Rough, porous surfaces are better adapted for the adherence of the matrix than those which are smooth and vitreous. Hence brick and gravel offer certain advantages over shingle and flints, though these latter are often preferred for a reason given below. Fragments of different size should be employed, so that the smaller material may fill up the interstices in the larger, and it is to be noted in this connection that equal measures of large and small stone, when combined, make less than double the volume of either. No individual fragment should have a dimension exceeding 4 inches, and the material is often specified to pass through a ring of ij inches diameter. Weight is a desirable feature in dock walls, aud accordingly for this class of work preference should be given to aggregates °' high spécifie gravity. The amount of sand and cement will evidently be governed by the volume of the remaining cavities to be filled. These may be estimated from the following table, quoted from Mr. Sandeman’s paper on “ Portland Cement and Concrete ” :—* TABLE VI. Weight of Material. Ratio of Interstices. 1. Broken limestone, the greater part of which would be gauged by a 3-inch ring, 2. Gravel, screened free from sand, varying in size between small pebbles and pieces gauged by a 24-inch ring, 3. The above limestone and gravel, well mixed in equal proportions, 4. Sandstone varying in size between pieces gauged by a 4-inch ring and pieces gauged by an 8-inch ring, . 5. Sandstone varying in size between sand and pieces gauged by a 4-inch ring, 6. The above sandstones mixed in equal proportions, . Lbs. per cubic foot. 95 1114 1134 74 92 914 Per cent. 50-9 336 33'6 50-0 34-0 360 * Min. Proc. Inst. C.E., vol. cxxi.