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

406 Chapter Fourteen
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Much time can be saved in the erection and stripping of the
forms for a detail involving many pieces of run moldings if brackets
are made in the mill to a template to fit the general profile of the
detail. The section in Figure 14-6 illustrates this point. The studs for
the wall forms are cut off at the line X-X. The brackets, consisting of
pieces A, B, and C, which have been assembled in the mill, are
scabbed to the studs. The wales bearing on piece A and the lower
half of the wale bearing on piece C are put in place to hold the brackets,
which are spaced about 16 in. apart, in alignment. The cornice
members are then applied. Pieces 1, 2, 3, and 4 are moldings, and all
other pieces are ripped to size from stock lumber.
Even when wood is carefully oiled prior to its use with formwork,
there is a tendency for it to swell when it is wet. For this reason, consideration
should be given to the selection of the types, sizes, and arrangements
of the component parts when detailing wood molds. Because wide and
thick molds will swell more than narrow and thin molds, the former are
more likely to bind and damage the ornaments in the concrete. Thus, it is
desirable to use the narrowest and thinnest pieces from which the mold
can be made with adequate strength. The danger of molding swelling and
damaging the concrete details may be reduced by using longitudinal saw
cuts along the back sides of the pieces, as specified in Figure 14-6.
Recesses in concrete, made by using narrow strips of wood molding,
should always be beveled to give a width at the surface greater
than that at the back. If this is not done, it will be necessary to split the
strip with a wood chisel prior to removing it, which may damage the
concrete. Even though the strip of molding is beveled, it is good practice
to run a longitudinal saw cut along the full length of the back side
of it to facilitate stripping.
Frequently it is necessary to leave individual pieces of molding in the
concrete for several days after the molds are removed, to permit them to
dry out and shrink so that they can be removed without danger of damaging
the concrete. If this is to be done, the moldings or the parts that will
be left in place should be nailed to the backing lumber, using relatively
few small nails driven through the backing into the molding pieces.
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Plaster Waste Molds
Where the ornamental details for architectural concrete are so intricate
and complicated that it is impossible to use wood molds, plaster
molds are used. Because these molds are destroyed in stripping, they
are called waste molds. Plaster waste molds are not as common today
as they have been in the past. Casting resins, plastics, and synthetic
rubber have replaced much of the architectural concrete work that
has been done in the past by plaster waste molds.
Waste molds are made of casting plaster containing jute fiber, and
they are reinforced to prevent breakage during handling and erection.
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