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It has been
found that the last two
requirements can be met
only if the concrete is
prestressed, i.e. the
concrete is ‘‘precompressed’’
so that it will not crack
easily. Ordinary reinforced
concrete does not work
for such slender shapes.
Lintels must comply with
SABS 1504:1990; Standard
specification for prestressed
concrete lintels. Prestressing
is done, basically, as
follows:
Steel moulds, long enough
to make several lintels
end to end, are placed
side by side on a concrete
bed. The ends of the moulds
have holes in them. The
whole diameter is slightly
bigger than that of the
prestressing wire. Movable
stop-end plates, with
holes in them, are placed
in the moulds to divide
the length into individual
lintels. After the mould
has been treated with
a release agent to prevent
concrete from sticking
to it, the wires are threaded
through these holes so
that they protrude from
each end of the mould.
The wires are anchored
at one end and connected
to a powerful, usually
hydraulic, jack at the
other end. By activating
the jack the wires are
stretched to the required
extent and held in this
state (see Figure 1 below). |
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Prestressing
Equipment |
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The
following equipments are
needed: |
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Suitable
moulds (which may have
to be purpose-made). |
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A jacking
system incorporating a
means of anchoring the
wires, jacks that can
be closely controlled
(for accurately stretching
the wires) and a means
of gripping the wires.
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A design
for prestressing must
include information on
concrete strength, grade
and diameter of steel
wire, number of wires
per lintel, prestressing
force or extension, and
age of concrete when prestressed.
Suppliers of prestressing
equipment can assist with
designs. Alternatively,
a consulting structural
engineer can be appointed
to carry out a design.
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Copyright
Information | All Rights Reserved |
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