1 From London Engineering, quoted in Engineering News, Vol. xxvii, p. 551.
It is evident that this method of depositing concrete in large sacks is peculiarly suited to forming a foundation on a soft bottom, since, if the bags are made to project well beyond the sides of the molded concrete to be deposited above, they act in the double capacity of a mattress to prevent scour, and a foundation for the upper part of the structure.
In the construction of the Merchants' Bridge at St. Louis, bags of concrete were used to check the scour which occurred beneath the upstream cutting edge of one of the caissons while it was being grounded. The bags were thrown into the river at such a distance above the pier that they settled to the bottom at the point where the scour was taking place.
Burlap bags were used at St. Marys Falls Canal for laying concrete under water next the face of the form to prevent washing of the cement in building concrete superstructure for canal walls. As the bags were placed by hand they were made to hold only about two cubic feet of concrete.
Paper sacks are sometimes employed instead of jute bags. Dr. Martin Murphy 1 describes the methods employed in filling steel cylinders for the substructure of the Avon Bridge as follows: "Bags made of rough brown paper well stiffened with glucose, were employed and slipped into the water over the required place of deposition. Each bag held about one cubic foot of concrete; smaller ones were used between dowels. The bags were quickly made up and dropped one after another, so that the one following was deposited before the cement escaped from the former one. The paper was immediately destroyed by submersion, and the cement remained; it could not escape. The bags Cost one dollar thirty-five cents per hundred, or thirty-five cents per cubic yard." The success of this method will depend upon the character of the sacks, for in some experiments on a small scale with sacks of stiff manila paper the author found that the bags were not destroyed, and that no adhesion took place between the separate sacks.
" Bridge Substructure and Foundations in Nova Scotia," by Martin Murphy. Trans. A. S. C. E., Vol. xxix, p. 629.
The advantage of the block system of construction lies in the fact that the individual blocks may be made with the greatest care, and as they are allowed to harden thoroughly before being put in place, the loss of cement incident to the other systems is avoided. There is, however, the difficulty of forming a joint between adjacent blocks. The joints are of great importance when small blocks are employed, since the latter may not have sufficient weight to escape being washed out of the work. Large blocks may make a very solid structure by being simply superimposed, but special hoisting machinery will be required to place such blocks.
Sometimes a large bed of mortar is laid in coarse sacking and carefully lowered and spread on the block last laid, the next block being placed upon it immediately. A very rich mortar should be used for this purpose. Usually, however, it is not attempted to place mortar in the horizontal joints in concrete block work laid under water, but it is considered that all vertical joints should be filled with rich Portland cement mortar when the work is to be exposed to wave action. If settlement is anticipated, and large blocks are used, no attempt should be made to break joints in the direction of the longer dimension of the work, but the blocks should bond in a direction transverse to the wall. Concrete blocks may be advantageously employed to form the faces of a structure built under water or exposed to wave action, the concrete hearting or backing being built in situ.
556. For convenience in handling, a groove to receive a chain or cable should be left down two sides and across the bottom of the blocks to enable them to be placed close together and to facilitate the withdrawal of the hoisting chain. These grooves may afterward be filled with concrete; such recesses are sometimes molded for the sole purpose of filling them with fresh concrete when in place, and thus binding the work together. The molds for forming the blocks should be carefully made in order that the finished blocks may have good bearings one upon another. If the corners are rounded, they are less likely to be chipped off in handling or by having an undue strain come upon the corner when in place.
1 Report Chief of Engineers, 1897, p. 2624.
If any recesses are desired in the blocks, the pieces placed in the mold to form them should be trapezoidal in cross-section with the longer parallel face against the side of the mold. If such filling pieces are made rectangular, difficulty will be experienced in removing them when the concrete has set. The molds should, of course, be so constructed as to be readily taken apart to be used again. The opposite sides may be kept from spreading by rods which pass through the mold, but such rods are an inconvenience in packing the concrete into the mold, and it is therefore better to truss the mold outside. If such tie rods are used, they may be left imbedded in the concrete, or removed with the mold, as desired.
An illustration of the use of the. block method is furnished in the United States breakwater at Marquette.1 The general plan of this breakwater has already been briefly noted and two methods of laying a two foot layer of subaqueous concrete, as a foundation for monolithic blocks forming the superstructure proper, have been described. A third method was to mold footing blocks, seven feet by five feet in section and two feet high, which were afterward laid flush with the lake side of the substructure cribs and filled in behind with concrete laid in place. The footing blocks thus assured a good quality of concrete beneath the toe of the monolithic block on the lake side where it was most necessary to provide a good foundation, and also served as a protection behind which the remainder of the two foot layer could be placed with greater facility.
Many of these blocks were built during the winter in a shed artificially heated, the materials being thawed out as required. The molds were of six by six inch and four by four inch pine, lined with two by eight inch plank dressed on one side. Strips of trapezoidal cross-section, nailed inside the mold, provided for two parallel grooves on the bottom and two sides of the block to receive hoisting chains. A dovetail at the back of the block was also formed by three wedge-shaped pieces placed against the back face of the mold. The Cost per cubic yard of making forty blocks is as follows: —
1.42 bbls. Portland cement, at $2.75........$3.90
.45 cu. yd. sand, at $0.45.............20
Cost materials in concrete per cu. yd..... $5.20
Superintendence, labor and watchman.......$2.21
10 per cent, of Cost of warehouse and molds.....52
Total Cost of making per cu. yd........ 3.04
Total Cost per cu. yd. of blocks ready to place in work............... $8.24