Civil Engineering Reference
In-Depth Information
salvage other than perhaps a board or timber here or there. Such a practice still exists in
some parts of the world where labor costs are very low. In a high-labor-cost area, such as
the United States, the trend for several decades has been toward reusable forms made
from many different materials. Wood is still the most widely used material, however,
and no matter what types of materials are used, some wood will still probably be re-
quired, whether lumber or plywood.
The lumber used for formwork usually comes from the softwood species such as
southern pine, ponderosa pine, Douglas fir, and spruce. These woods are relatively light
and are commonly available. Southern pine and Douglas fir are two of the strongest of
these woods, are usually available, and thus are most often used for formwork. Form lum-
ber should be only partially seasoned because if it is too green, it shrinks and warps in hot
dry weather. If it is too dry, it swells a great deal when it becomes wet. The lumber should
be planed if it is going to be in contact with the concrete, but it is possible to use rough
lumber for braces and shoring.
Lumber is graded according to the rules established by various agencies and pub-
lished in the American Softwood Standard. It can be graded visually in accordance with
ASTM-D245, “Methods for Establishing Standard Grades and Related Allowable Proper-
ties for Visually Graded Lumber.” It may also be graded by a machine with nondestruc-
tive testing.
The introduction of plywood made great advances in formwork possible. Large
sheets of plywood save a great deal of labor in the construction of the forms and at the
same time result in large areas of joint-free concrete surfaces with corresponding reduc-
tions in costs of finishing and rubbing of the exposed concrete. Plywood also has con-
siderable resistance to changes in shape when it becomes wet, can withstand rather
rough usage, and can be bent to a certain degree, making it possible to construct curved
surfaces.
Steel forms are extremely important for today's concrete construction. Not only are
steel panel systems important for buildings, but steel bracing and framing are important
where wood or plywood forms may be used. If steel forms are properly maintained,
they may be reused many times. In addition, steel forms with their great strength may
be used in places where other materials are not feasible, as in forming long spans.
Forms for some other types of structures are frequently made with steel simply as a
matter of expediency. Falling into this class are the forms for round columns, tunnels,
and so on.
The Sonotube paper-fiber forms are another type of form often used for round
columns. These patented forms are lightweight one-piece units that can be sawed to fit
beams, utility outlets, and other items. They have built-in moisture barriers that aid in cur-
ing. Because they are disposable, there are no cleaning, reshipping, or inventory costs.
They can be quickly stripped with electric saws or hand tools.
Two other materials often used for concrete forms are glass-fiber-reinforced plastic
and insulation boards. The glass-fiber-reinforced plastic can be sprayed over wooden base
forms or can be used to fabricate special forms such as the pans for waffle-type floors.
Quite a few types of insulation board used as form liners are on the market. They are fas-
tened to the forms, and when the forms are removed, the boards are left in place either
bonded to the concrete or held in place by some type of clips.
