economical but only last for a couple of concrete
pours. Concrete surfaces, since they can be reused
repeatedly, are more practical.
When building casting surfaces, you should keep
the following points in mind:
The subbase should be level and properly
The slab should be at least 6 inches thick and
made of 3,000 psi or higher reinforced
concrete. Large aggregate, 2 1/2 inches to 3
inches maximum, may be used in the casting
If pipes or other utilities are to be extended up
through the casting slab at a later date, they
should be stopped below the surface and the
openings temporarily closed. For wood, cork,
or plastic plugs, fill almost to the surface with
sand and top with a thin coat of mortar that is
finished flush with the casting surface.
It is important to remember that any
imperfections in the surface of the casting slab
will show up on the cast panels. When
finishing the casting slab, you must ensure
there is a flat, level, and smooth surface
without humps, dips, cracks, or gouges. If
possible, cure the casting surface keeping it
covered with water (pending). However, if a
curing compound or surface hardener is used,
make sure it will not conflict with the later use
of bond-breaking agents.
The material most commonly used for edge forms
is 2-by lumber.
The lumber must be occasionally
replaced, but the steel or aluminum angles and
charnels may be reused many times. The tops of the
forms must be in the same plane so that they maybe
used for screeds. They must also be well braced to
remain in good alignment.
Edge forms should have holes in them for rebar or
for expansion/contraction dowels to protrude. These
holes should be 1/4 inch larger in diameter than the
bars. At times, the forms are spliced at the line of
these bars to make removal easier.
The forms, or rough bucks, for doors, windows,
air-conditioning ducts, and so forth, are set before the
steel is placed and should be on the same plane as the
Bond-breaking agents are one of the most
important items of precast concrete construction. The
most important requirement is that they must break
the bond between the casting surface and the cast
Bond-breaking agents must also be
economical, fast drying, easily applied, easily
removed, or leave a paintable surface on the cast
panel, if desired. They are broken into two general
types: sheet materials and liquids.
There are many commercially available
bond-breaking agents available. You should obtain
the type best suited for the project and follow the
manufacturers application instructions. If
commercial bond-breaking agents are not available,
several alternatives can be used.
Paper and felt effectively prevent a bond with a
casting surface, but usually stick to the cast
panels and may cause asphalt stains on the
When oiled, plywood, fiberboard, and metal
effectively prevent a bond and can be used
many times. The initial cost, however, is high
and joint marks are left on the cast panels.
Canvas gives a very pleasing texture and is
used where cast panels are lifted at an early
stage. It should be either dusted with cement
or sprinkled with water just before placing the
Oil gives good results when properly used, but
is expensive. The casting slab must be dry
when the oil is applied, and the oil must be
allowed to absorb before the concrete is placed.
Oil should not be used if the surface is to be
painted, and crankcase oil should never be
Waxes, such as spirit wax (paraffin) and
ordinary floor wax, give good-to-excellent
One mixture that may be used is 5
pounds of paraffin mixed with 1 1/2 gallons of
light oil or kerosene. The oil must be heated to
dissolve the paraffin.
Liquid soap requires special care to ensure that
an excess amount is not used or the surface of
the cast panel will be sandy.
Materials should be applied after the side forms
are in place and the casting slab is clean but before