The mixed concrete should be deposited in the forms within from 20 to 30 minutes from the time the water is added to the cement, as it begins to set or harden after this time. To disturb the concrete after the set has begun is risky, as it will lose some of its strength, the extent of the injury depending upon the seriousness of the disturbance.

Concrete which has set before it can be placed in the forms should not be tempered or softened with water, but should be discarded.

To prevent delay in placing, all forms should be examined before the mixing is begun to see that they are properly braced, that all chips or loose particles are removed, that the surface of concrete which has set has been properly roughened and wetted to assure a bond, as described on page 20, and that all reinforcement, bolts, inserts, etc., are properly located and secured.

At the lunch' period, or at the end of a day's work, the mixing board and equipment should be thoroughly washed, for if this is not done many pounds of heavy concrete are needlessly carried around by the men and the addition of a pound in the weight of tools will lower the efficiency of the workers. Moreover, it will save time and wear and tear of equipment incident to cutting the surplus concrete away with a cold chisel.

In depositing concrete in the forms care should be taken that the materials do not separate.

If the mixing is done close to the place of depositing, the concrete may be shoveled into the forms directly or through a chute. If it is necessary to lift or transport the concrete, buckets and wheelbarrows are convenient containers. The concrete should be deposited in horizontal layers, preferably not over 6 inches thick, and a spade or paddle should be worked up and down against the forms to push the coarse material away from the surface, as illustrated in Figure 13. The object of the spading is to eliminate impounded air that may form pockets in the mass and to insure a smoother and more impervious surface. In addition to being spaded, stiff concrete should be rammed until water flushes to the surface. Tapping the forms with a hammer is a very effective way of securing a smooth surface. Figure 14 shows the result of improper spading.

Fresh concrete will riot bond readily to concrete that has hardened and a seam may be formed that will permit water to trickle through. When bonding fresh concrete to that which has been in place for a short time it is usually sufficient to roughen the hardened surface with a pick or by other means so as to expose the gravel or stone, and to clean off all loose particles. The hardened concrete should be soaked with water, the excess water removed, and the surface then given a coat of grout (a mixture of cement and water) of the consistency of cream just before the new concrete is deposited.

When pouring of a wall is to be discontinued for some time, provision for the bonding of future work should be made. This may be done by placing short steel dowels in the concrete when it is poured, or a rebated joint or groove may be made, as shown in Figure 15. In bonding a new wall to old concrete, holes should be drilled for the dowels, which should be grouted in, and the old surface should be roughened, cleaned, and wetted; or a groove may be cut in the old wall to receive the new concrete.

PLACING UNDER WATER.

Concrete can be placed under still water if proper precautions are taken. It should never be placed, while soft, in running water unless a form or cofferdam is used, as the cement will be washed out. When concrete is to be placed under water a form of tube or chute, known as a tremie (Fig. 16), may be used advantageously. The tube should be of sheet metal, about 8 inches in diameter, with a hopper on top, and means should be provided for quickly raising and lowering it without jolts, so that the concrete will feed out at the bottom without breaking the seal. The lower end of the tube should rest on the bottom or on the concrete as it is built up and a continuous flow of concrete, mixed somewhat soft so that it will flow easily, should be maintained.

Scum or laitance is likely to form on concrete when placed under water, and unless all of the concrete is! poured in one operation and brought to a little above the water surface, seams or planes of weakness will occur.