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384 Barry’s Advanced Construction of Buildings
Shallow oval indents on each side
Figure 6.31 Prestressing wires.
Crimped wire
End of wire wedges
into concrete
Figure 6.32 Anchorage to stressing wire.
to take up the prestress caused by releasing the wires from the anchorages. The bond
between the stretched wires and the concrete is maintained by the adhesion of the cement
to the wires, by frictional resistance and the tendency of the wires to shorten on release
and wedge into the concrete. To improve frictional resistance, the wires may be crimped
or indented, as illustrated in Figure 6.31. When stressing wires are cut and released from
the anchorages in the stressing frame, the wires tend to shorten, and this shortening is
accompanied by an increase in diameter of the wires which wedge into the ends of the
member, as illustrated in Figure 6.32. Pre-tensioning of concrete is mainly confined to the
manufacture of precast large-span members such as floor beams, slabs and piles. The stressing
beds required for this work are too bulky for use on site.
Post-tensioning
After the concrete has been cast inside moulds or formwork and has developed sufficient
strength to resist the stress, stressing wires are threaded through ducts or sheaths cast in
along the length of the member. These prestressing wires are anchored at one end of the
member and are then stretched and anchored at the opposite end to induce the compressive
stress. The advantage of post-tensioning is that the stressing wires or rods are stressed
against the concrete and there is no loss of stress as there is in pre-tensioning due to the
shortening of the wires when they are cut from the stressing bed. The major part of
the drying shrinkage of concrete will have taken place before it is post-tensioned and
this minimises loss of stress due to shrinkage of concrete. The systems of post-tensioning
used are Freyssinet, Gifford–Udall–CCL, Lee–McCall, Magnel–Blaton and the PSC one
wire system.