Metal-wood forms are just like metal forms
except for the face. It is made with a sheet of B-grade
exterior plywood with waterproof glue.
Wooden forms are by far the most common type
used in building construction.
They have the
advantage of economy, ease in handling, ease of
production, and adaptability to many desired shapes.
Added economy may result from reusing form lumber
later for roofing, bracing, or similar purposes.
Lumber should be straight, structurally sound, strong,
and only partially seasoned. Kiln-dried timber has a
tendency to swell when soaked with water from the
concrete. If the boards are tight-jointed, the swelling
will cause bulging and distortion.
lumber is used, an allowance should be made for
shrinkage, or the forms should be kept wet until the
concrete is in place. Soft woods, such as pine, fir, and
spruce, make the best and most economical form
lumber since they are light, easy to work with, and
available in almost every region.
Lumber that comes in contact with concrete
should be surfaced at least on one side and both edges.
The surfaced side is turned toward the concrete. The
edges of the lumber may be square, shiplap, or tongue
and groove. The latter makes a more watertight joint
and tends to prevent warping.
Plywood can be used economically for wall and
floor forms if it is made with waterproof glue and is
identified for use in concrete forms. Plywood is more
warp resistant and can be reused more often than
lumber. Plywood is made in 1/4-, 3/8-, 1/2-, 9/16-,
5/8- and 3/4-inch thicknesses and in widths up to 48
inches. Although longer lengths are manufactured,
8-foot lengths are the most common. The 5/8- and
3/4-inch thicknesses are most economical; thinner
sections require additional solid backing to prevent
bulging. However, the 1/4-inch thickness is useful for
forming curved surfaces.
Fiber forms are prefabricated from impregnated
waterproofed cardboard and other fiber materials.
Successive layers of fiber are first glued together and
then molded in the desired shape. Fiber forms are
ideal for round concrete columns and other
applications where preformed shapes are feasible
since they require no form fabrication at the job site.
This saves considerable time and money.
Fabric forming is made of two layers of nylon
fabric. These layers are woven together, forming an
envelope. Structural mortar is injected into these
envelopes, forming nylon-encased concrete
pillows. These are used to protect the shorelines of
waterways, lakes and reservoirs, and as drainage
Fabric forming offers exceptional advantages in
the structural restoration of bearing piles under
waterfront structures. A fabric sleeve with a zipper
closure is suspended around the pile to be repaired,
and mortar is pumped into the sleeve. This forms a
strong concrete jacket.
Forms for concrete construction must support the
plastic concrete until it has hardened. Stiffness is an
important feature in forms. Failure to provide form
stiffness may cause unfortunate results. Forms must
be designed for all the weight to which they are likely
to be subjected. This includes the dead load of the
forms, the plastic concrete in the forms, the weight of
the workmen, the weight of equipment and materials,
and the impact due to vibration. These factors vary
with each project, but none should be ignored. The
ease of erection and removal is also an important
factor in the economical design of forms. Platform
and ramp structures independent of formwork are
sometimes preferred to avoid displacement of forms
due to loading and impact shock from workmen and
When concrete is placed in forms, it is in a plastic
state and exerts hydrostatic pressure on the forms.
The basis of form design, therefore, is the maximum
pressure developed by concrete during placing. The
maximum pressure developed depends on the placing
rate and the temperature. The rate at which concrete
is placed affects the pressure because it determines
how much hydrostatic head builds up in the form.
The hydrostatic head continues to increase until the
concrete takes its initial set, usually in about 90
minutes. At low temperatures, however, the initial set
takes place much more slowly. This makes it
necessary to consider the temperature at the time of