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

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Forms for Architectural Concrete 399
A releasing agent should be used to minimize stripping forces
and shorten form cleanup between pours. The release agent should
be applied before each pour.
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Sealing Form Liner Joints
All form liner joints and tie holes should be sealed to prevent localized
water loss and subsequent discoloration of the concrete. Silicon sealant
is recommended for cast-in-place jobs. Once cured, it is flexible, has
good adhesion, and will not discolor or stick to the concrete. Sealant or
putty on fastener heads will hide them in the finished concrete.
Heavy duct tape or double-sided foam tape stabilizes and seals
the joint when applied to the formwork side of the form liner. Auto
body putty is good for precast or other applications where the formwork
is not flexed or moved.
Smooth-Surfaced Concrete
Exceptionally smooth concrete surfaces are generally more difficult
to achieve than a textured surface. If a smooth surface is desired, patented
metal forms, hardboards, or high-density overlay (HDO) Plyform
sheathing are usually used. Even though the individual panels
may provide smooth flat surfaces, the effect of joints appearing
between panels may mar the surface of the concrete. Thus, special
care must be exercised during erection of the panel forms to ensure
smooth joints between the form panels.
Patented metal form panels are frequently used for continuous
straight or curved concrete surfaces. They are manufactured with
heavy-gauge metal to prevent waves in the surface. Because they are
jig built in a factory with precision dimensions, the panel joints facilitate
alignment and fit on the job. The panels are also generally large
so that there will be a minimum of linear feet of joints.
The properties of plywood are discussed in Chapter 4. Plyform is
the special product designed for use in forming concrete. It is exteriortype
plywood limited to certain species of wood and grades of veneer
to ensure high performance as a form material. It is especially useful
for curved smooth surfaces. Table 4-19 provides the minimum bending
radii for dry Plyform.
Hardboard
Hardboard, whose properties are discussed in Chapter 4, is frequently
used to line forms for architectural concrete where smooth surfaces,
entirely free from grain markings, are desired. The physical properties
are shown in Table 4-20 and the minimum bending radii are listed
in Table 4-21. Only tempered boards that have been treated to minimize
absorption should be used with formwork.
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