Portland cement concrete if made with a non-porous aggregate is impervious to moisture, and yet at the same time, if not hydraulically compressed, will take up a sufficient quantity of moisture from the air to prevent condensation upon the surface of the walls. It not only resists the disintegrating influences of the atmosphere, but becomes even harder with the lapse of time. It may also be made in several different colors, and can be finished off to nearly a polished surface or can be left quite rough. Walls built of this material may be made so hard that a nail cannot be driven into them, or they can be made sufficiently soft to become a fixing for joinery, and, if a non-porous aggregate be used, no damp course is required. Further than this, if land be bought upon which there is sufficient gravel, or even clay that can be burnt, the greatest portion of the building material may be obtained in excavating for the cellar; and in seaside localities, if the (salt) shingle from the beach be used, sound and dry walls will be obtained.
The use of concrete as a material for building will be found to meet all the defects set forth by practical people, as it may be made fire-proof, vermin-proof, and nail-proof, and in dwellings for the poor will therefore resist the destructive efforts of the "young barbarian." Nothing, therefore, can be better as a building material. The system ordinarily employed to erect structures in concrete consists of first forming casings of wood, between which the liquid concrete is deposited, and allowed to become hard, or "to set." The casings are then removed, the cavities and other imperfections are filled in, and the wall receives a thin facing of a finer concrete. If mouldings or other ornament be required, they are applied to this face by the ordinary plasterer's methods. This system finds favor in engineering construction, and also in very simple forms of architectural work, but with very complicated work the waste in casings is very great. Besides this, however, the face is found sometimes to burst off, especially if it has been applied some time after the concrete forming the body of the wall has set, and the method of applying ornament is not economical.
A system of building in concrete has recently been invented by Messrs. F. & J.P. West, of London, illustrations of which we now present. To this system Messrs. West have given the name of "Concrete Exstruction," from the Latin "exstructio," which they consider to be a more appropriate word than "constructio," as applied to concrete building in general. In Messrs. West's system of building in concrete, instead of employing wood casings, between which to deposit the concrete or beton, and removing them when the beton has become hard, casings of concrete itself are employed. These casings are not removed when the beton has set, but they become a part of the wall and form a face to the work. In order to form the casings, the concrete is moulded in the form of slabs. Figs. 1 to 18 of our engravings show various forms of the slab, which may be manufactured with a surface of any dimensions and of rectangular (Fig. 1), triangular, hexagonal (Figs. 2, 14, and 15), and indeed of any other form that will make a complete surface, while for thickness it may be suited to the work to which it is to be applied, that used for heavy engineering work differing from that employed in house construction.
It is found that the most convenient height for the rectangular slab (Fig. 1) is 12 inches and the breadth 18 inches, as the parts of a structure built with slabs of these dimensions more often correspond with architectural measurements. The hexagonal slab (Fig. 2) is made to measure 12 inches between its parallel sides. Where combinations of these slabs will not coincide with given dimensions, portions of slabs are moulded to supply the deficiency. The moulds in which the slabs are made are simple frames with linings having a thin face of India-rubber or other suitable material, by the use of which slabs with their edges as shown, and also of the greatest accuracy, can be manufactured. That portion of the back of the slab which is undercut is formed by means of soft India-rubber cores. The moulds for making portions of the slabs have a contrivance by which their length may be adjusted to suit given dimensions.
During the process of casting the slabs, and while they are in a plastic state, mouldings (if required) or other ornaments, having a suitable key, are inserted in the plastic surface, which is finished off to them (Figs. 7, 8, and 10). The slabs may also be cast with ornaments, etc., complete at one operation (Fig. 11), but it is more economical to have separate moulds for the mouldings and other ornaments, and separate moulds for the slabs, and to apply the mouldings, etc., during the process of casting the slab. Corbels (Fig. 9), sets off (which would be somewhat similar to the plinth course slab No. 10), and other constructive features may also be applied in a similar way, or may be provided for during the casting of the slab. A thin facing of marble or other ornamental solid or even plastic material may be applied to the face of the slabs during the process of casting, thus enabling the work to be finished as it is carried up, or a key may be formed on the face of the slab to enable the structure to be plastered afterward.
FIG. 19. FIG 20.
In Fig. 20, the structure from the bottom of the trenches is shown with the sides of the trenches removed. It will be seen that the footings are constructed in the most economical manner by not being stepped. As no damp-course is required in concrete work, when the aggregate is of a non-porous material, one is not shown. Upon the top of the footings is generally laid a horizontal slab, called the wall-base slab, the special feature of which is that it enables the thickness of the wall to be gauged accurately, and also provides a fixing for the first course of slabs. Figs. 4 and 5 show such slabs for internal and external angles, and Fig. 6 shows one for straight work. The use of a wall-base slab is not essential, although it is the more accurate method of building, for in cases where it is desirable to economize labor, or from other causes, the slabs forming the first course may be made with a thicker base, and may be fixed by a deposition of concrete, which is allowed to set behind them. The second course of slabs is laid upon the first course with breaking joints of half-slab bond, each course being keyed to the other by means of a quick-setting cementing material poured into the key-holes provided in the edges of the slab for that purpose, a bituminous cement being preferred.
The key-holes are made in several ways, those shown in the illustrations being of a dovetail shape; circular, square, or indeed holes of any other shape formed in the edges of the slab and in an oblique direction are also employed. Special slabs for cants, or squint-quoins (Figs. 17 and 18) and angles (Figs. 12, 13, 14, 15, and 16) are manufactured, the angle occurring (if we omit the hexagonals and take the 18 inch slab) at three-quarters the length of each slab. This gives a half-slab bond to each course, as on one face of the quoin in one course will appear a quarter slab and in the course above a three-quarter slab superimposed upon it, or vice versa. Thus are the walls in Figs. 19 and 20 built up. For openings, the jambs and lintels (and in window-openings the sill) are made solid with a provision for a key-hole to the mass of concrete filling behind them. That portion of the jambs against which the slabs butt has a groove coinciding with a similar one in the edge of the slab, for the purpose of forming a joggle joint by squeezing the bedding material into them or by joggling them in with a cement grout.
All the slabs are joggled together in a similar way.