1 Third Annual Report, Boston Transit Commission.
541. This principle was employed by Mr. Daniel W. Mead in placing concrete in a small shaft in ninety feet of water.1 An eight inch, wrought iron pipe was screwed together in sections, and provided with a hopper at the upper end and a wooden plug at the lower end. After lowering the pipe into the shaft, the pipe was filled with concrete and it was expected that its weight would force out the plug at the bottom when the pipe was raised. On the first attempt, however, the plug failed to drop out, and on raising the pipe the cause was apparent. The plug had evidently leaked, and as the first concrete was dropped into the pipe it had separated, the broken stone being at the bottom, the sand next, and the cement above had so plugged the pipe as to support the weight of the concrete. The second attempt, when a tighter wooden plug was used and a small pipe placed inside the larger one to assist in loosening the plug if necessary, was successful.
Since in submerged work the concrete should be deposited in as large masses as possible, the use of a large skip will probably give better results than the tremie. A box form may be used with hinged lids at the top to permit filling, and two hinged doors at the bottom which may be opened from the surface by a tripping rope when the box has reached the place for depositing the concrete.
A convenient form of skip is made in two halves, each half having a cross-section either of "a right angled triangle or a quadrant of a circle. The two boxes are hinged at their upper inside corners and the pieces through which the hinge rod passes are prolonged upward, the lowering cables being attached to their ends. Two opening cables are fastened to the outer corners of the boxes. Two sheets of iron may be used as covers to the boxes, being attached to the hinge rod that serves for the two halves of the skip.
It is seen that the skip will work on the principle of a pair of ice tongs. While being filled with concrete the box is supported by the lowering cables, and the hinged lids are kept up by some simple contrivance. When full, the lids are closed and the skip lowered till it rests on the bottom; the skip being, then hoisted slowly by means of the opening cables, the concrete is gently deposited in place. Such skips are supplied by the makers of concrete machinery.
1 Trans. Assn. of Civil Engineers of Cornell University, 1898.
543. In depositing concrete by means of skips it is well to have the latter of large size, holding not less than a cubic yard, and preferably two cubic yards or more. The larger quantity will compact itself better on account of the greater weight, and the surface which is subjected to wash will have a lesser area in proportion to the volume of the mass. The skip should be completely filled with concrete and tightly closed while it is being lowered. It. is important also that the skips be lowered slowly, in order that the inclosed air may be replaced by water without commotion.
Mr. Wm. Shield 1 devised a bag for depositing concrete under water which is said to work very satisfactorily. The top of the bag is closed, and has a three-quarter inch wrought iron bar fastened across the end with a loop to receive the hook of the lowering line. The mouth of the bag is slightly larger than the upper end, to facilitate the discharge of the concrete. The bag is inverted to be filled, and the mouth is then secured by a turn of a line provided with loops through which a small tapering pin is passed. This pin is attached to a tripping line, and when the bag has reached the place of deposition, a pull of the tripping line releases the pin; when the bag is gently lifted, the concrete is deposited in place with such slight commotion that but little cement is said to be lost.
For deposition under water the materials for concrete are sometimes mixed dry, but this is not good practice. The mere soaking of water into cement does not form a compact mortar; the moistened materials need to be thoroughly mixed and, if possible, rubbed together in order to obtain perfect adhesion. Then, too, if the dry materials are lowered to place and water is suddenly allowed access to the mass, much of the cement will be washed away in the disturbance caused by the sudden inrush of water.
M. Paul Alexandre 2 found by experimenting on mortars of "dry" (stiff), "wet" and "ordinary consistency," that mortars mixed "dry" suffered the greatest decrease in strength by immersion in running water. Mortars mixed "wet" suffered the least loss, though their resistance was less than those mixed to the ordinary consistency, since when not subject to the current of water, the wet mortars gave much lower results than those of ordinary consistency.
1 "Subaqueous Foundations," London Engineering, 1892. Abstract in Engineering News, Vol. xxviii, p. 379.
2 " Recherches Experimentales sur les Mortiers Hydrauliques" par M. Paul Alexandre, pp. 93-96.
546. In order to avoid the washing out of the cement, the concrete is sometimes allowed to partially set before deposition. Mr. Robert W. Kinipple has used this method and advocates its adoption.1 In employing this method, the concrete, which should be deposited when of the consistency of stiff clay, requires careful watching that it does not set so hard as not to reunite after being broken up. Under ordinary supervision, this will probably not prove as successful as some of the other devices, but it may be found valuable under certain circumstances. The writer made a few experiments with this method on a small scale in swiftly running shallow water. Much of the cement appeared to be washed out by the current, but the results were somewhat better than were obtained when the concrete was deposited fresh. (See § 456.) M. Paul Alexandre made some short time experiments on this point, which indicated that but little advantage was gained in allowing the cement to partially set before deposition.