In actual construction work, the cost of forms is a large item of expense and offers the best field for the exercise of ingenuity. For economical work, the design should consist of a repetition of identical units; and the forms should be so devised that it will require a minimum of nailing to hold them, and of labor to make and handle them. Forms are constructed of the cheaper grades of lumber. To secure a smooth surface, the planks are planed on the side on which the concrete will be placed. Green lumber is preferable to dry, as it is less affected by wet concrete. If the surface of the planks that is placed next to the concrete is well oiled, the planks can be taken down much easier, and, if they are kept from the sun, can be used several times.

Crude oil is an excellent and cheap material for greasing forms, and can be applied with a whitewash brush. The oil should be applied every time the forms are used. The object is to fill the pores of the wood, rather than to cover it with a film of grease. Thin soft-soap, or a paste made from soap and water, is also sometimes used.

In constructing a factory building of two or three stories, usually the same set of forms are used for the different floors; but when the building is more than four stories high, two or more sets of forms are specified, so as always to have one set of forms ready to move.

The forms should be so tight as to prevent the water and thin mortar from running through, and thus carrying off the cement. This is accomplished by means of tongued-and-grooved or beveled-edge boards (Fig. 153); but it is often possible to use square lumber if it is thoroughly wet so as to swell it before the concrete is placed. The beveled-edge boards are often preferred to tongued-and-grooved boards, as the edges tend to crush as the boards swell, and beveling prevents buckling.

Lumber for forms may be made of 1-inch, 1 1/2-inch, or 2-inch plank. The spacing of studs depends in part upon the thickness of concrete to be supported, and upon the thickness of the boards on which the concrete is placed. The size of the studding depends upon the height of the wall and the amount of bracing used. Except in very heavy or high walls, 2 by 4-inch or 2 by 6-inch studs are used. For ordinary floors with 1-inch plank, the supports should be placed about 2 feet apart; with 1 1/2-inch plank, about 3 feet apart; and with 2-inch plank, 4 feet apart.

Fig. 152. Century Cement Brick Machine.

Fig. 152. "Century" Cement Brick Machine.

The length of time required for concrete to set depends upon the weather, the consistency of the concrete, and the strain which is to come on it. In good drying weather, and for very light work, it is often possible to remove the forms in 12 to 24 hours after placing the concrete, if there is no load placed on it. The setting of concrete is greatly retarded by cold or wet weather. Forms for concrete arches and beams must be left in place longer than in wall work, because of the tendency to fail by rupture across the arch or beam. In small, circular arches, like sewers, the forms may be removed in 18 to 24 hours if the concrete is mixed dry; but if wet concrete is used, in 24 to 48 hours. Forms for large arch culverts and arch bridges are seldom taken down in less than 14 days; and it is often specified that they must not be struck for 28 days after placing the last concrete. In ordinary floor construction, consisting of slabs, girders, and beams, the forms are usually left in place at least a week.