Plywood, fiberboard, or metal joints, when oiled,
can effectively break a bond and can be used
many times. However, the initial cost 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 concrete.
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 used.
Waxes, such as spirit wax (paraffin) and ordinary
floor wax, give good-to-excellent results. 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 any
reinforcing steel is placed. To ensure proper adhesion
of the concrete, keep all bond-breaking materials off
the reinforcing steel.
REINFORCEMENTS AND INSERTS.
Reinforcing bars (rebar) should be assembled into
mats and placed into the forms as a unit. This allows
for rapid assembly on a jig and reduces walking on the
casting surface, which has been treated with the
Extra rebars must be used at openings. They
should be placed parallel to and about 2 inches from
the sides of openings or placed diagonally across the
corners of openings.
The bars may be suspended by conventional
methods, such as with high chairs or from members
laid across the edge forms. However, high chairs
should not be used if the bottom of the cast panel is to
be a finished surface. Another method is to first place
half the thickness of concrete, place the rebar mat, and
then complete the pour. However, this method must be
done quickly to avoid a cold joint between the top and
When welded wire fabric (WWF) is used, dowels
or bars must still be used between the panels and
columns. WWF is usually placed in sheets covering
the entire area and then clipped along the edges of the
openings after erection.
If utilities are going to be flush-mounted or
hidden, pipe, conduit, boxes, sleeves, and so forth,
should be put into the forms at the same time as the
reinforcing steel. If the utilities pass from one cast
panel to another, the connections must be made after
the panels are erected but before the columns are
poured. If small openings are to go through the panel,
a greased pipe sleeve is the easiest method of placing
an opening in the form. For larger openings, such as
air-conditioning ducts, forms should be made in the
same manner as doors or windows.
After rebar and utilities have been placed, all other
inserts should be placed. These will include lifting and
bracing inserts, anchor bolts, welding plates, and so
forth. You need to make sure these items are firmly
secured so they will not move during concrete
placement or finishing.
POURING, FINISHING, AND CURING. With
few exceptions, pouring cast panels can be done in the
same manner as other pours. Since the panels are
poured in a horizontal position, a stiffer mix can be
used. A minimum of six sacks of cement per cubic yard
with a maximum of 6 gallons of water per sack of
cement should be used along with well-graded
aggregate. As pointed out earlier though, you will have
to reduce the amount of water used per sack of cement
to allow for the free water in the sand. Large aggregate,
up to 1 1/2 inches in diameter, maybe used effectively.
The concrete should be worked into place by spading
or vibration, and extra care must be taken to prevent
honeycomb around outer edges of the panel.
Normal finishing methods should be used, but
many finishing styles are available for horizontally
Scast panels. Some finishing methods include
patterned, colored, exposed aggregate, broomed,
floated, or steel-troweled. Regardless of the finish
used, finishers must be cautioned to do the finishing
of all panels in a uniform manner. Spots, defects,
uneven brooming, or troweling, and so forth, will be
highly visible when the panels are erected.
Without marring the surface, curing should be
started as soon as possible after finishing. Proper
curing is important, so cast panels should be cured just
like any other concrete to achieve proper strength.
Curing compound, if used, prevents bonding with
other concrete or paint.