DEPOSITING CONCRETE UNDER WATER. In laying concrete under water, an essential requisite is that it shall not fall from any height, but be deposited in the allotted place in a compact mass, as otherwise the cement will be separated from the other ingredients and the strength of the work will be seriously impaired. If the concrete is allowed to fall through the water, the finer or active portions of the cement are likely to be washed out and thus weaken the concrete; and, besides, the ingredients will be deposited in a series, the heaviest—the stone—at the bottom and the lightest—the cement —at the top, a fall of even a few feet causing an appreciable separa tion. Of course, concrete should not be used in running water, as the cement would be washed away.
There are three methods in common use in depositing concrete under water: (1) placing it in bags and forming a pile of bags under the water; (2) passing it through a tube in a continuous flow; and (3) lowering it in a self-dumping bucket with a crane.
1. Concrete is sometimes deposited under water by enclosing it in open-cloth bags, the cement oozing through the meshes sufficiently to unite the whole into a single mass. Concrete has also been success fully deposited under water by enclosing it in paper bags, and lowering or sliding them down a chute into place. The bags get wet and the pressure of the concrete soon bursts them, thus allowing the concrete to unite into a single mass.
2. The tube used for depositing concrete under water is called a tremie. It consists of a wooden or steel tube 12 or 14 inches in diam
eter, open at the top and the bottom, and is suspended from a crane or movable frame running on a track, by which it is moved about as the work progresses. The upper end is hopper-shaped, and is kept above the water; the lower end rests against the bottom. The concrete should he mixed plastic, but neither dry nor wet—the former is liable to clog in the chute, and the latter flows out at the lower end too easily. When the concrete is to be deposited in large masses, the tr6mie is filled by placing the lower end on the bottom and filling the tube by dropping concrete through the water; or the trrmie may be filled by placing its lower end in a box having a movable bottom, filling the tube, lowering all to the bottom, and then detaching the bottom of the box. After the tube is full it is raised a little from the bottom, and as the concrete flows out below, more is thrown in at the top.
3. There are a number of bottom-dumping buckets upon the market, designed for depositing concrete under water; but it is pos sible to make a home-made bucket that will serve the purpose. A wooden box having a V-shaped bottom is so constructed that on reaching the bottom a pin may be drawn out by a string reaching to the surface, thus permitting one or both of the sloping sides to swing open and allowing the concrete to fall out. The box is then raised to he refilled. The box should preferably have a lid.