The usual method of building concrete walls, piers, arches, etc., is by setting up uprights of 4x4 or 4x6 scantlings at each side of the proposed wall or pier and securing to them boards or moulds, between which the concrete is deposited and rammed. To prevent springing the standards should be bolted together through the wall. For the moulding boards, dressed pine boards \\ inches thick are recommended; these should be brushed with a hot solution of soap each time before using. After the lower portion of the concrete has set the moulding boards may be removed and used above.
Mr. Ernest Ransome, who has had much experience in the erection of concrete buildings, has patented a movable cribbing, which consists of slotted standards, which, being placed in pairs, one on each side of the wall, and bolted together, hold in position the mould boards. These standards may be raised from time to time as the work progresses without interrupting the filling in of the concrete. In connection with the movable cribbing a series of hoisting buckets, with a traveling crane, is provided for hoisting the concrete. One man stationed upon the wall receives and empties the buckets of concrete as they are hoisted and rams the concrete into place. The crane may be moved around the wall upon the upright slotted standards, so that no scaffolding whatever is required about the wall. It is claimed that the expense of working this apparatus need not exceed a cent per cubic foot of concrete. The first cost is also small.
When mouldings are to be formed on the wall the reverse profile of the mould is stuck in wood and set in its proper place on the mould boards.
Buildings of concrete may be erected very rapidly, as the process of depositing the concrete goes on continuously all around the building, and there is no stone to cut or set, and with proper foresight there need be no waiting for materials.
Concrete Beams or Lintels. - Wherever lintels or beams occur in concrete buildings they should be formed of concrete and twisted rods or cables in the manner shown in Fig. 243. A beam like that shown, 22 inches wide and 2 feet 10 inches high, was used in a building in San Francisco, where it carries three stories of brick walls and wood floors, with a clear span of 15 feet. The twisted bars were 1 inch square. The three bars near the top were placed only over the supporting posts to give the effect of a continuous girder.
Most of the concrete buildings constructed previous to 1885 were finished on the outside with plaster or stucco in the manner described for plastering brick walls, Section 354. This finish has not proved very satisfactory, and, moreover, added considerably to the cost of the wall. This unsatisfactory surface finish undoubtedly has had much to do with the limited use of concrete for wall construction.
It has been demonstrated, however, that the natural face of the concrete can, at slight expense, be finished to closely imitate roughly dressed stonework. Such imitation, moreover, is not, as in most cases, a false pretense or sham, as such surface finish is as natural to concrete as to stone - concrete being in fact an artificial stone. The usual method of finishing the surface of concrete walls, when it is desired to imitate stonework, is by forming imitation joints in the face of the wall, and either picking or tooling the surface of the blocks thus formed, the former giving the appearance shown on the face of the wall, Fig. 242.
The joints are formed by lightly nailing to the inside face of the moulding boards cleats or strips, moulded or beveled to give the desired form to the recessed joint. After nailing on the strips the inner face of the mould or cribbing will appear something like Fig. 244, the shape and size of the blocks varying to suit the character of the work and the divisions of the wall.
In imitating rough-dressed work the mould is taken from the concrete while it is yet tender, and with small light picks the face of the stone is picked over with great rapidity, an ordinary workman finishing about 1,000 superficial feet per day. (The first and second stories of the Alabama House are finished in this way.)
For imitations of finer-tooled work the concrete should be left to harden longer before being spalled or cut, and the work should be done with a chisel.
A very neat effect may be obtained by chiseling a margin around the blocks to imitate tooled work, and then picking the centre, as shown in Fig. 245.
If the strips are properly planed and beveled the recessed joints will need no "touching up." Most natural stones (especially granite), bricks and clinkers, if crushed sufficiently fine, make excellent material for this face, but ordinary gravel will do.
Whatever is used, let it be uniform in color and of an even grade. When a very fine and close imitation of a natural stone is required, take the same stone, crush it, and mix it with cement, colored to correspond.
The finer the stone is crushed the nearer the resemblance will be upon close inspection; but for fine work it is generally sufficient to reduce the stone to the size of buckshot or fine gravel.
Rough effective work, excellent in appearance, can be obtained by using the ordinary concrete made with coarse materials. For a finer grade a better material should be used, with aggregates of coarse sand, very small gravel or finely-crushed stone. This fine grade need not extend through the mass of the concrete, but can be applied at the surface only, and by coloring in imitation of various natural stones, the most effective and pleasing results are obtained.
The large bridge in Golden Gate Park, San Francisco, was made with coarse concrete, mixed I of cement, 2 of sand and 6 of quartzite rock taken out of adjacent hills and simply broken by hammers without screening, and notwithstanding its coarseness the structure has frequently been mistaken for natural stone by the public*