In depositing concrete under water, some means must be taken to prevent the separation of the materials while passing through the water. The three principal methods are as follows:

(1) By means of closed buckets;

(2) By means of cloth or paper bags;

(3) By means of tubes.

164. Buckets

For depositing concrete by the first method, special buckets are made with a closed top and hinged bottom.

Concrete deposited under water must be disturbed as little as possible, and in tipping a bucket the material is apt to be disturbed. Several different types of buckets with hinged bottoms have been devised to open automatically when the place for depositing the concrete has been reached. In one type, the latches which fasten the trap doors are released by the slackening of the rope when the bucket reaches the bottom, and the doors are open as soon as the bucket begins to ascend. In another type, in which the handle extends down the sides of the bucket to the bottom, the doors are opened by the handles sliding down when the bucket reaches the bottom. The doors are hinged to the sides of the bucket, and when opened permit the concrete to be deposited in one mass. In depositing concrete by this means, it is found rather difficult to place the layers uniformly and to prevent the formation of mounds.

165. Bags

This method of depositing concrete under water is by means of open, woven bags or paper bags, two-thirds to three-quarters filled. The bags are sunk in the water and placed in courses, if possible, header and stretcher system, arranging each course as laid. The bagging is close enough to keep the cement from washing out, and at the same time, open enough to allow the whole to unite into a compact mass. The fact that the bags are crushed into irregular shapes which fit into each other, tends to lock them together in a way which makes even an imperfect joint very effective. When the concrete is deposited in paper bags, the water quickly soaks the paper; but the paper retains its strength long enough so that the concrete can be deposited properly.

166. Tubes

The third method of depositing concrete under water is by means of long tubes, 4 to 14 inches in diameter. The tubes extend from the surface of the water to the place where the concrete is to be deposited. If the tube is small, 4 to 6 inches in diameter, a cap is placed over the bottom, the tube filled with concrete, and lowered to the bottom. The cap is then withdrawn; and as fast as the concrete drops out of the bottom, more concrete is put in at the top of the tube, and there is thus a continuous stream of concrete deposited.

When a large tube is used to deposit concrete in this manner, it will be too heavy to handle conveniently if filled before being lowered. The foot of the tube is lowered to the bottom, and the water rises into the chute to the same level as that outside; and into this water the concrete must be dumped until the water is wholly replaced or absorbed by the concrete. This has a tendency to separate the cement from the sand and gravel, and will take a yard or more concrete to displace the water in the chute. There is a danger that this amount of badly washed concrete will be deposited whenever it is necessary to charge the chute. This danger occurs not only when the charge is accidentally lost, but whenever the work is begun in the morning or at any other time. Whenever the work is stopped, the charge must be allowed to run out, or it would set in the tube. The tubes are usually charged by means of wheelbarrows, and a continuous flow of concrete must be maintained. When the chute has been filled, it is raised slowly from the bottom, allowing a part of the concrete to run out in a conical heap at the foot.

This method has also been used for grouting stone. In this case, a 2-inch pipe, perforated at the bottom, was used. The grout, on account of its great specific gravity, is sufficient to replace the water in the interstices between the stones, and firmly cement them into a mass of concrete. A mixture of one part cement and one part sand is the leanest mixture than can be used for this purpose, as there is a great tendency for the cement and sand to separate.