537. In building a concrete structure under water where the site cannot be coffered, it must be expected that the expense of the work will be increased, and the quality of concrete poorer. The methods employed for subaqueous construction are: 1st, the laying of freshly mixed concrete in roughly prepared forms; 2d, placing the fresh concrete in bags of burlap or canvas which are deposited while the concrete is still soft; and 3d, molding in air concrete blocks which are placed in the work when well set.
In the first method some cement will certainly be washed out of the concrete, the extent of this loss depending upon the condition of the water in which the work is done (i.e., its depth and the amount of current and wave action) and the care with which the concrete is lowered to place. Tamping cannot be done with this method, and any movement of the concrete to level it will cause further loss of cement.
In the second method the loss of cement will be much less, but the adhesion between the different masses will be slight. In the third method there is no loss of cement and the concrete can be well rammed; but if small blocks are used, there may be difficulty in so placing them under water as to make a solid structure, while if large blocks are used, special hoisting machinery is required to handle them.
The first method mentioned above, depositing fresh concrete in place, is usually the cheapest and most expeditious method, though it is not likely to give the best results. When concrete is lowered through water, there is a tendency for the cement to separate from the sand and stone. This tendency seems to be exhibited in a more marked degree with some cements than with others. In connection with the construction of the concrete foundations of the Charlestown bridge, a test was devised for determining the relative values of the different lots of cement for depositing in water.1 Concrete was laid, through a small chute, in a cement barrel placed in a hogshead filled with salt water. It was found that while some specimens would retain their form after twenty-four hours when the barrel was removed, others showed but little cohesion after twenty-four to forty-eight hours. In the former, the cement and gravel remained well distributed throughout the mass, but in the latter much of the cement had separated from the gravel, and settled in the bottom of the barrel, where it remained in an inert state, while the central portion of the concrete, robbed of its cement, had many voids. As a result of this test, some lots of cement were not accepted for use.
The finest portion of the cement is very liable to separate from the remainder as the concrete passes through the water, and if subjected to the action of waves or a current, much of the cement will be washed away. In exposed situations it is especially necessary to inclose the site of the work with sheet piling or cribs, or a wall constructed by the bag or block method. When the water level outside the form is constantly changing, the flow of water through' the joints in the sheathing is especially effective in washing out the cement, and in such conditions the sheathing should be made as nearly water-tight as possible. To this end tongue and groove lagging may be used, or the face of the mold may be covered with tarred felt, or canvas, tacked in place.
539. Laitance is the term applied to the whitish spongy material that is washed out of concrete when it is deposited in water. Before settling on the surface of the concrete, which it does slowly, it gives to the water a milky appearance, hence the name. In fresh water this semi-fluid mass is composed of the finest flocculent matter in the cement, containing generally hydrate of lime. It remains in a semi-fluid condition for a long time and acquires very little hardness at the best. In sea water the laitance is more abundant and is made up of silica, lime and magnesia, with carbonic acid and alumina, its exact composition depending upon the character of the cement. This interferes seriously with the bonding of the layers of concrete, and when it has settled it should be cleaned from the surface before another layer is placed.
1 Report of Mr. William Jackson, Chief Engineer. Third Annual Report Boston Transit Commission.
A method frequently employed to prevent, as much as possible, the loss of cement, is to make use of a large tube of wood or sheet iron, made in sections so that its length is adjustable, and provided with a hopper at the upper end. Such a tube is called a tremie. The hopper is always above water, and the lower end of the tube, which may also terminate in a hopper, rests upon the bottom of the foundation.
The tremie is first filled with concrete, a box placed over the lower end serving to prevent the escape of the concrete while the tube is being lowered until the end rests upon the bottom. The tube is then lifted from the bottom sufficiently to allow the concrete to escape as fast as fresh concrete is added at the top. The surface of the concrete in the tube should be kept continuously above the water surface. The tremie may be held in position by a crane, or it may be so supported as to allow of two motions at right angles to each other. Such an arrangement was used in building the piers for the Boucicault Bridge, the tube traveling along a platform, which in turn could move on a track at right angles to the first motion. In using a tremie the thickness of a layer may be regulated at will.
In the construction of the Charlestown Bridge 1 a tube was used fourteen inches in diameter at the bottom, and about eleven inches in diameter at the neck, above which was a hopper to receive the concrete. When the attempt was made to place too thick a layer at one operation, it was found that the charge was likely to be lost, and the best results were believed to be obtained with layers two feet to two and one-half feet thick. Some experiments were made with a plug designed to keep the water from flowing up through the concrete when the tube was being refilled after a loss of the charge. This plug was made with a central core of wood and sides of canvas expanded by steel ribs. It worked fairly well, but its use was not continued.