If desired, triangular strips may be nailed to the inside of the forms in such a way as to block off the face to represent stone masonry, and in this way the marks of joints between planks or between strips of lining may be avoided. Square corners should not be allowed on exterior angles, as it is difficult to so tamp the concrete as to make the corner perfect, and they are so likely to be chipped off. Triangular strips or moldings should be tacked along the corners of the mold as a fillet to cut off the corner by a plane making equal angles with the adjacent faces. This plane may be from one inch to two inches wide.
To form water drips on projecting ledges, such as door caps and sills, abutment copings, etc., a small half-round should be nailed to the upper surface of the mold a short distance back from the projecting face. This leaves a ridge at the edge of the under side of projection so that the water must drip from the edge, and not follow back to the main wall face.
The sizes of posts and braces must be such as to make a practically unyielding support to the sheathing. With one and three-quarters inch lagging, posts may be four feet apart; if five feet four inches apart (three to each sixteen foot length), some yielding of the sheathing may be expected if it is less than two and three quarter inches. If sheathing is four inches thick, the distance between posts may be six or seven feet.
Fir, yellow pine, and Norway pine are suitable for posts. Three-inch by eight-inch is an ordinary size, and a post of these dimensions should be supported, either by ties or braces, at intervals of four to six feet. Where the posts are four inches by ten inches, supports may be six to eight feet centers, while with six-inch by twelve-inch posts, the distance between centers of supports may be eight to ten feet. Posts should be sized and dressed on the side which is to receive the sheathing, in order that the alignment may be perfect.
The general plan of the mold may vary according to conditions, the following methods having been employed on heavy work to support the vertical posts: 1st, With outside inclined braces, leaving the interior of the mold unobstructed. 2d, Tie rods across the interior of the mold connecting opposite posts at frequent intervals. 3d, Each post trussed vertically and tied across at top and bottom only. 4th, Horizontal trussed wales outside of posts, spaced four to five feet apart in the vertical and tied across at the ends.
The sizes of inclined braces depend on their lengths, the inclination to the vertical, and the amount of shoring used. An approximate rule for the size of braces under usual conditions and using ordinary dimension stuff, not boards, is that the number of square inches area of cross-section of brace should equal length of span in feet. If thin planks are employed, they should be in pairs, one on either side of the vertical post, and made to act together by cross-pieces nailed to the two planks.
The aim should be to make the whole form practically unyielding under the action of the tampers, as it has been found that this action is usually more severe than the mere pressure of the concrete in a semi-liquid condition. The sizes of pieces cannot, therefore, be accurately computed, but the above sizes are derived from the general result of experience as to what has proved satisfactory.
The advantage of the form of construction just described is. that the interior of the mold is left entirely unobstructed. On high walls, however, the amount of timber required for braces is excessive, and the braces may be almost as objectionable as tie rods, since the former prevent the laying of tracks along the side of the form.
When the vertical posts are supported by tie rods across the mold and the wall is thin, it may be possible in removing the mold to withdraw the bolts or rods if they have been thoroughly greased or wrapped with stiff paper before the concrete is placed. If it is designed to leave the rods in the concrete, they should be provided with sleave nuts near the end, which, when unscrewed, will leave the end of the rod within the concrete mass not less than two inches from the face. The hole left by the nut should be carefully filled with mortar after the mold is removed.
With vertical posts four feet apart, this method of support is objectionable, as it leaves a network of ties within the forms interfering seriously with the operation of a skip and with the ramming. It is not necessary, however, to place all of the tie rods to the top of the mold before beginning the concreting, as it is sufficient to keep one or two rods in place above the plane where tamping is being done.
A modification of this method is to use wires of large diameter with an eye at the end just inside the finished face of the concrete. A short bolt, with hook at one end and threaded at the other, passes through the post, hooks into the eye of the wire, and is tightened by a nut on the threaded end outside the post. After removing the nut, the rod is unhooked and the hole in the face filled with mortar, the wire remaining in the concrete.
The third method of support, where the posts are trussed and provided with heavy tie rods at the top, and held at the bottom either by tie rods or some other means, seems to have fewer objections than the methods just described. Less timber will usually be required to build this form than for that where inclined braces are used, and the obstruction to operations will usually be less than with either of the other styles. This mold is also very readily taken down,, though the posts are heavier and more difficult to handle.
To secure the bottoms of the posts, they may be set in the ground, or rest against sills braced to some other portion of the structure, or to piles. A suitable support may also be obtained by dumping a mass of concrete around the bottom of each post and allowing it to set. forms erected on rock may have the posts rest against blocks bolted to the rock.
The fourth method of supporting the posts is particularly applicable where the work is divided into blocks of moderate size in horizontal cross-section, say twenty feet square. In longer lengths the horizontal trussed wales become rather heavy for convenient handling. Within these limits, however, this is an excellent form. In the construction of Lock No. 2 between Minneapolis and St. Paul,1 a form of this kind was used for blocks about twelve by fifteen feet at the bottom. The sheathing was one and three-quarters inches, lined with No. 20 galvanized iron. Verticals were four by twelve, spaced about two feet centers. The trussed wales were twelve by twelve inch, trussed with one and one-quarter inch rod, the king-post being of twelve by twelve inch about two and one-half feet long, making depth of truss three and one-half feet. The ends of opposite wales were connected by one and one-quarter inch rod passing outside of the sheathing. Each pair of the longitudinal wales was just above the corresponding pair of transverse wales, so that they did not interfere at the corners. The mold was twenty-nine feet in height.
In describing this mold, Mr. Powell says: "One complete form weighs twenty-eight tons; each piece about seven tons. Each piece is moved separately by the cable-way in forty-five to sixty minutes. The operation of removing one complete form requires from three to four hours time. After being moved, a small crew of men occupy nearly a day in plumbing and bolting together the form." "The boxes containing 1.7 cubic yards of concrete are landed on top of the form by cable-way and tipped from that position. Although the jar and strain is severe, the forms have shown no ill effects therefrom, remaining tight and secure".
1 Major Frederic V. Abbot in charge. Mr. A. O. Powell, Asst Engr., Report Chief of Engineers, 1900, p. 2778.