The sizes of the inclined braces depend upon their length, their inclination, and their number. An approximate rule for the size of the braces, derived from experience, is that the number of square inches in the cross section of a brace should equal its length in feet. If thin plank of any considerable length are used for braces, they should be in pairs and be stiffened by cross pieces nailed to the two planks.
The advantage of outside braces is that the interior of the forms is left entirely unobstructed; but on high walls the amount of timber required for exterior braces is very great, and not infrequently the exterior braces seriously interfere with the handling of the materials for the concrete and also of the concrete itself. Further, exterior braces are usually less secure and more expensive, both in material and in time, to erect than interior rods or wire.
Horizontal Ties. The posts on opposite sides of the forms may be tied together by rods or wires running from side to side across the form. The wire is passed around the stud on each side of the form through the crack between two planks, and tightened by being twisted with a stick. After the concrete is in position and the form is taken down, the wire is cut off a little under the surface of the concrete, and the hole is plastered up with mortar. To keep the sides of the forms the proper distance apart, a piece of wood of the right length should be placed beside the wire, and should be left there until the concrete reaches that height, when the strut is to be removed. The trouble of keeping these pieces of wood in place and the difficulty of getting them removed at the proper time, are a serious objection to tying the forms together with wire; and, besides, the first cost of the wire ;s considerable, and a surprisingly large amount of time is required to put it into place.
When the sides of the forms are tied together with rods, it is customary to pass them through a pipe, so that the rod may be drawn out when the forms are taken down. For an example, see Fig. 120, page 530. The pipe should be cut a little shorter than the distance between the sides of the form, and should be held in position by placing at each end a 2-inch block with a hole through it. When the rod is withdrawn the block is removed and the hole is plastered up with mortar. The rod is better than the wire since the latter is nearly sure to stretch and allow the forms to get out of line, and sometimes the wire breaks and causes serious trouble. The rods and pipes cost a little more than the wire, but cost less to put into place and are much more substantial. Sometimes the pipes are dispensed with, and pieces of wood or small beams of concrete are used to keep the sides of the forms the proper distance apart, in which case the rods are either greased or wrapped in greased paper to facilitate their removal after the concrete has set. However, it is nearly impossible to secure a water-tight wall when rods without pipes are used for ties. Sometimes rods are used having sleeve nuts near each end, see Fig. 119, page 529; and after the forms are removed the end of the rod is unscrewed, Whatever the form of the tie, the metal left in the concrete should not come nearer to the surface than 2 inches, as otherwise rust stains are likely to appear.
The Plank. The plank used in the forms should be reason ably free from knots. Green lumber is preferable to that which is thoroughly seasoned, because it is less affected by the water in the concrete. If a good surface is required on the finished concrete, the plank should be surfaced on one side; and some contractors prefer to use surfaced lumber in all cases, as less concrete adheres to the lumber and hence it requires less labor to clean the plank for use again. If the concrete is wet when it is placed in the forms, the plank should be tongued and grooved to insure tight forms. Some times the edges of the plank are beveled, so the thin edge will crush as the plank absorbs water from the concrete and swells, thus preventing the plank from buckling and marring the surface of the completed structure.
The forms should be built reasonably tight; but any joints or holes may be closed with plaster of paris or putty, or may be covered with building paper or a thin sheet of steel. If the forms are to be removed before the concrete becomes hard, the plank should be coated with heavy oil, soft soap, or some greasy substance to prevent the concrete from adhering to the plank. Soap is better than oil or grease, since it is soluble in cold water and hence is comparatively easily removed from the concrete surface. Soft soap thinned with water and spread with a whitewash brush or a broom is efficient and is usually quite cheap. If the forms are not to be removed until the concrete has become hard, the concrete will not adhere seriously if the forms are simply wet thoroughly with water just before the concrete comes against them.
It is very important that the forms for copings, water tables, etc., be so designed that they can be removed without damaging the corners or edges of the concrete. Sometimes a panel of planks or a rectangle of half-round strips is nailed against the inside of the form to give a little ornamentation to the finished surface of the concrete. For an example of the former, see Fig. 118, page 528. The latter method was used in the abutment represented in Fig. 133, page 543, but is not shown in the figure. Occasionally beveled strips are nailed on the inside of the form to give the concrete the appearance of cut stone masonry having chamfered joints, but this is indefensi ble from the artistic standpoint. For an example of this construc tion, see the arch ring in Fig. 181, page 602.