This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
It is only in quite recent years that the custom of ancient Rome and Byzantium, of casting edifices in concrete, has come into use to any extent in England.
Engineers have for a long time realised the great constructive value of this material, but architects have considered it to be a material unworthy of their notice, chiefly on account of its poor appearance; yet there can be no reason why buildings in this material should not rival in beauty edifices in the handsomest stones.
Whatever the artistic value of concrete may be, there remains no doubt that great economy may be effected by its use under some circumstances, and that its strength is considerably greater than that of masonry or brickwork, while its homogeneity makes it a far more suitable material for many purposes than any material of an aggregate nature.
The method of constructing concrete walls is shown in Fig. 199. A large mould is formed within which to cast the wall. Boards from 1 to 2 inches thick are supported with their inner faces spaced to the desired thickness of the wall, against upright timbers, which on commencing a wall are supported by means of inclined struts. The uprights are arranged in pairs along the wall at distances of from 6 to 12 feet, and are bolted together at frequent intervals by means of iron bolts passing through wooden cones shaped to a conical form to facilitate their removal when the work is completed. These bolts and distance pieces are illustrated at A, Fig. 199.
The planks which form the face of the mould are usually fixed together in groups of two or three, with their edges shot and placed close together. If the wall is to have a neat appearance their inner faces should be wrought. The boards or "shutters" are bolted together, and spaced by means of the conical distance pieces already mentioned, so that the face of the wall, when finished, may be true and even. The appearance of rusticated ashlar is sometimes given to concrete walls by nailing fillets of wood into the faces of the shutters, as shown in Fig. 200.
When the wall is carried up to such a level that the shutters and upright can no longer be supported from the ground, the latter are bolted at the lower ends through distance pieces higher up the wall, care being taken to set them perfectly vertically, and the work is continued as before.
When mouldings have to be formed in concrete they are cast in wooden moulds built up of pieces of wood cut to the reverse of the desired moulding and nailed at intervals on to stout support, as shown in Fig. 201. If the mouldings are undercut the moulds must be built up so as to take to pieces, and so enable the mould to be removed when the concrete is set. Undercut mouldings are best cast on to blocks and set in the building as ordinarybuilding stones.
The interiors of moulds, the faces of shutters, and all fastenings which come in contact with the concrete should be coated with a solution of soft soap to prevent them from adhering to the concrete.
One of the great objections to concrete is that it often expands or contracts for a considerable time after it has been set . in the work. When tested in a small quantity a volume of concrete may not appear to alter in size, but when a length of wall contracts all at once the total amount of movement may be very considerable, and cause unsightly gaps in the wall.
To guard against these disfigurements, wooden boxes are nailed on to the shutters at intervals vertically up the face of the wall, producing cavities in the latter as shown at B in Fig. 202. Above the centre of these cavities a piece of flat sheet iron of the shape shown at D (Fig. 202) is cast into the wall as at A. When the concrete has set the sheet iron is withdrawn, producing a vertical joint. If, now, the wall contracts sufficiently to cause it to crack, the opening occurs between the vertical joints formed by the sheet iron. When all expansion and contraction has ceased a block of concrete is fixed in the cavity with strong cement, and the vertical joints are flushed or grouted with cement, so that no disfigurement appears on the face of the work.
The composition of the concrete depends upon the degree of fineness desired for the face of the wall. The strongest wall is produced when the aggregate is fine and hard, and has its interstices thoroughly filled with mortar. This condition will be fulfilled when the concrete consists of one part of cement, two parts of sharp sand, and three or four parts of fine aggregate. If the face of the concrete work is to have the appearance of stonework the aggregate should be broken up small enough to pass through a 1/2-inch mesh, and should be of an even colour; but when the wall is to be rendered, an aggregate in larger pieces forms a surface when set in the wall, which offers a better key to the rendering.
The materials should be most thoroughly mixed for concrete walls, in order that they may have an even texture. The constituents of the concrete should be gauged upon a level wooden platform and turned over together twice with shovels while still in the dry state. The mixture should now be turned over twice more while water is sprinkled upon it from a rose, care being taken to water it uniformly.
Concrete is more thoroughly and more economically mixed in large quantities by means of a concrete mixing machine.
Great care should be taken when laying the concrete on the wall to make it come in contact with the mould in every place. To ensure this the concrete should be shovelled into position and rammed until it is thoroughly consolidated. The walls should be carried up evenly all over the site to prevent vertical cracks from appearing where the newer work is joined to the old, and this is done by laying the concrete in horizontal layers of from 9 to 8 inches deep. After ramming a layer the upper surface should be made rough so as to bond more thoroughly with the next layer.
Walls are frequently built with facings of brickwork and hearting of concrete. In Fig. 203 a wall of this description is shown, where the wall has been built up a certain distance in brickwork only, and then continued as a brick-faced concrete wall, with the facings composed of one row of headers and five rows of stretchers alternating, the headers on one face coming midway between the headers on the other face to ensure better bonding. The method of building up the wall above the plinth is as follows: Lay the first three courses of brickwork both at front and back, and fill up the hearting to the level of the top course. Lay three more courses and level up the hearting, and so on until the wall is completed. It will be seen that each layer of concrete is 9 inches deep, which is a convenient depth for a wall of the thickness shown in Fig. 203. The reason why a deeper layer is not laid at a time is that it cannot be satisfactorily packed under the heading courses. The face may of course show any form of bond, but all the headers which tail into the wall should be kept at the same levels, so that the concrete may be laid in even layers while the headers, if any, in the courses between the tailing bricks should be false or "snap" headers. It is better, however, to adopt stretching bond, as shown in Fig. 202, for the five courses which intervene between the heading courses on each face.
When it is desired to give concrete walls the appearance of tooled masonry the face is roughly cut whilst the concrete is still soft, and finished more carefully when it has set hard so as to give it the right texture.
Concrete work, when fresh, should be kept moist by plying a fine spray of water on to its surfaces in hot or windy weather, otherwise it will become covered with innumerable small cracks due to the imperfect setting caused by the evaporation of the moisture.
Concrete is now used largely in conjunction with iron, in which form it is known as Armoured Concrete, which will be dealt with in a later Volume of this work.