Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

408 Chapter Fourteen
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chisel. It may be desirable to use a thin layer of colored plaster next to
the concrete as a warning, to prevent cutting into the concrete during
the chipping operation.
Because concrete will adhere to the untreated surface of plaster, it
is necessary to paint the contact surface of a waste mold with shellac
prior to placing the concrete. Any pointing of joints or patchwork
should be sanded smooth and painted with shellac. Also, all contact
surfaces of a mold should be greased with a light coat of grease, which
may be thinned with kerosene if it is too thick. Any surplus must be
wiped off.
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Plastic Molds
Manufacturers of plastic products have perfected methods of making
plastic molds for use with architectural concrete. The designs,
which are attached to back-up sheathing as form liners, may vary in
individual sizes from less than 1 ft square to approximately 4 by 7 ft.
Several liner sheets may be installed adjacent to each other to produce
larger designs.
These liners are light in weight, easy to handle, rustproof, dent
proof, and sufficiently strong to withstand the weight of, and the
pressure from, the poured concrete without distortion. The smooth
surface of the plastic liners, combined with the flexibility of the material,
will permit the liners to release readily from the hardened concrete.
Because, with reasonable care, each liner may be reused 10 or
more times, the cost per use can be quite reasonable.
The concrete placed against these molds should contain an airentraining
agent to improve its workability, and it should be internally
vibrated to release all air bubbles and to assure full contact
between the concrete and the surfaces of the molds. The liners should
be left in place for 48 to 72 hours.
Metal Molds
Metal forms and molds are sometimes used for architectural concrete.
Because concrete tends to adhere to galvanized steel, black iron
should be used for formwork. The surface in contact with the concrete
should be oiled with a bond breaker.
Corrugated metal sheets can be used to form fluting on pilasters,
piers, and spandrel beams, as illustrated in Figure 14-8. It should be
noted that butt joints, rather than laps of the metal, are used between
adjacent sheets to eliminate or reduce the effects of joints.
Figure 14-9 illustrates methods of using metal forms for architectural
concrete. The metal is stiffened by wood blocks or collars sawed
to the desired curvature and spaced not more than 12 in. apart, depending
on the gauge of metal used.
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