Concrete, while plastic, exerts a great pressure on the confining
walls, necessitating rigid tying and bracing of the forms to keep them
from bulging out of alignment. The effect of the bulging of a form is
corrected only at a considerable expense; hence it is advisable to pour
the concrete to a depth of not more than 2-1/2 or 3 feet, allowing it to set
or harden before pouring more.
The form most used in concrete construction is that for a straight wall.
The methods of building such a form apply in general to the forms for
most structural work, though modifications may be necessary to meet
particular conditions.
[Illustration: Fig. 4.--Form for basement or cellar wall. The earth may
be used as an outside form if it is sufficiently firm.]
The straight wall form may be built continuous (Figs. 4 and 5), or in
panels of a size convenient to handle, and from stock lengths of lumber
(Fig. 6). Generally the face boards are placed horizontally and secured
to studs or posts. The face boards may be 1 or 2 inches thick and from 6
to 10 inches wide, preference being given to the narrower widths, which
are less liable to cup or warp. The thickness depends upon the spacing
of the studs, the number of times the forms are to be used, and the
depth of pouring. Ordinary sheathing, if the joints are made tight, is
satisfactory for foundations of dwellings, etc., and the studs, if 2 by
4 inches, should be spaced 18 inches on centers. The studs for a long,
high form had best be 4 by 4 inches or 2 by 6 inches, spaced from 2 to 3
feet center to center. The studs of the inside and outside forms must be
tied together to prevent spreading; this is conveniently done with No. 10
wire, as shown in Figure 4, or with one-half or three-quarter inch bolts,
which is the more expensive method. Bolts should be greased to facilitate
removal. Temporary spacers of wood, 1 by 2 inches, of a length equal to
the thickness of the wall, should be used to prevent drawing the forms
together when the wire or bolt is tightened. They should be spaced at the
ties, but need not be at every wire, and are knocked out and removed as
the concreting progresses.
[Illustration: Fig. 5.--Straight wall form for level ground.]
The ties should be spaced on each stud about 2-1/2 feet vertically. If
more than 3 feet of concrete is poured at one time the ties should be
closer together, vertically, at the bottom of each pouring. The thickness
of the wall does not affect the number of ties. On removing the forms the
wires should be clipped close to the face of the concrete and punched
back, unless the surface is to be stuccoed. If a pit hole is caused by
punching back the wire it should be pointed up with mortar, which then
should be rubbed to make it blend with the general surface.