503. The construction, placing and removal of forms frequently represent a considerable percentage, from five to thirty per cent., of the total Cost of the concrete, and it is therefore evident that an improper design may result in a considerable waste of money, as well as in marring the appearance of the work. The character of the form will of course depend on the character of the work; in the construction of a large number of small blocks of the same shape, where one mold may be used over and over, the thickness of the pieces should not be stinted, and the ease of knocking down the mold should be carefully considered. When a form can be used but once, the size of pieces should be no larger than necessary to give the requisite stiffness, and the ease of first construction is a main consideration. Forms should be left in place forty-eight hours to allow the concrete to set, and in the case of arches and beams a much longer time is necessary, so that the concrete may assume considerable strength before it is called upon to support its weight.
forms for massive walls of monolithic construction usually have vertical posts, with iron ties across, or braced by battered posts outside. The sheathing planks are then placed horizontal. In a few cases horizontal wales have been placed within the posts and vertical sheathing laid against the wales.
The strength of the sheathing must be sufficient to stand the pressure transmitted to it through the concrete when the rammer is used close to the face of the mold. The concrete is seldom built up fast enough to bring upon the sheathing a great head of fluid pressure, but the ramming brings a heavy local pressure upon it. If supported at intervals of four feet, two-inch lumber dressed to one and three-quarters inches thick is usually sufficient; for spans of more than 5 feet, 2 3/4 inch lumber is required to make a perfect face. Boards seven-eighths inch thick are suitable only when supports are not more than about 2 feet apart. In placing concrete in molds under water there is more danger of bursting the mold by the weight of semi-fluid concrete, and if the work is to be built up rapidly, this must be guarded against by sufficient bracing.
505. For exposed faces, the duty to be performed by the lagging includes leaving as smooth a finish as possible on the concrete after the removal of the forms. If green lumber is employed, the boards may shrink before use, leaving openings between the sheathing that will show plainly on the face of the work. A slight tendency of this kind may be checked by keeping the boards well wet with a hose until the concrete is placed. On the other hand, thoroughly seasoned lumber will swell when the concrete is placed; to obviate this difficulty the lower edge of each sheathing plank may be beveled on the outer edge; the thin edge on the inside will then crush when the planks swell.
The use of tongue and grooved lagging has been tried, but is not usually satisfactory, as there is no opportunity to expand, and the planks are particularly hard to place a second time. To give a good face in work under water, however, tongue and groove sheathing will assist in preventing washing of the cement. Yellow pine lumber is found to be excellent for sheathing; on account of the large amount of pitch contained, it absorbs water slowly and holds its shape. For a similar reason, fir timber would be suitable.
In order that the face of the mold shall be perfectly smooth, it is necessary to size and dress the plank on at least one side and two edges.
As it is almost impossible to avoid having some line of demarcation shown in the concrete at the joints of the sheathing planks, care should be taken that the lagging is of uniform width throughout, and laid horizontal so that consecutive sections show the joint continuous. The sheathing may be placed for the entire form before concreting is commenced, or the plank may be raised on the posts as the work advances. The former method will usually give the neater appearance, but is too expensive for high walls.
The appearance of the finished concrete is much improved, and the labor of preparing the forms probably not increased, since less care may be taken in surfacing, by lining the mold with thin sheet iron. Iron of number twenty gauge (.035 inch thick, 1.42 pounds per square foot) has been used for this purpose, but where the same lining is used several times, a heavier iron is preferable. The joining of one sheet of lining to another may present greater difficulties than the joining of planks, but joints will occur less frequently.
In the construction of the Marquette Breakwater, Mr. Clarence Coleman, Asst. Engineer, used sheet steel one eighth of an inch thick for lining molds for building monolithic blocks. Concerning the use of the steel, Mr. Coleman says:1 "Very smooth surfaces were produced on the slopes of the concrete and the work of the molders was greatly facilitated on account of the comparative ease with which the concrete was compacted under the slope pieces of the molds. The steel effectually prevented the aggressive friction of the sharp particles of broken stone on the wooden surfaces of the molds, thus increasing the life of the molds and decreasing the Cost of molding the concrete".
1 Report Chief of Engineers, U. S. A., 1898, p. 2254.
Oiling or greasing the face of the mold, in order that the latter may be removed without detaching particles from the concrete face, is usually advisable. Soap, crude oil, linseed oil, bacon fat, are some of the materials that have been used for this purpose; the first mentioned probably gives the best results, and if not applied too freely will have no injurious effect upon either the finish or strength of the work. Applying shellac to the molds improves the appearance of the concrete surface. When the forms are lined with steel, the adhesion of the concrete to the lining is more difficult to overcome. In this case the ordinary oils are not entirely successful, but fat salt pork has been found to give satisfactory results.