Tendons:
Tendons are a single or group of
prestressing
elements and their anchorage
assemblies, which impart a compressive force to a
structural member. Also
included are ducts, grouting attachments and grout. The main prestressing
element is usually a high strength steel member made up of a number of
strands,
wires or bars.
However,
sometimes engineers (mistakenly) use the word
“Tendon” for main prestressing
element (i.e strands/wires or bars) only
Wires: A
prestressing wire is a single unit made of steel. The nominal diameters
of
the wires commonly used are 2.5, 3.0, 4.0, 5.0, 7.0 and 8.0 mm. The
different
types of wires are as follows.
1)
Plain
wire: No indentations on the surface.
2)
Indented
wire: There are circular or elliptical indentations on the
surface.
Strands: Strand
is an 
assembly of several high
strength steel wires wound together (on a
stranding machine). Strands usually
have six outer wires wound in long-pitch helix around a single straight
wire of a similar diameter.
Different
types of strands are as follows:
- Two-wire
strand: Two wires are spun together to form the strand.
- Three-wire strand:
Three wires are spun together to form the strand.
- Seven-wire strand:
In this type of strand, six wires are spun around a
central wire. The central wire is larger than the other wires.
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What
does seven ply strand mean?
Dictionary
meaning of Ply is one of the strands twisted together to make
yarn or
rope or thread; often usedin combination; "three-ply cord";
"four-ply yarn". Similarly seven
ply strand means that seven wires
have been spun together to form the strand.
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| Strand
Pack |
Strand,
Wedge & Anchor
head |
Cables: With
wire tendons and strands, it may be desirable to form a cable to 
cope with
the stressing requirements of large post-tensioning applications.
Cables are
formed by arranging wires or strands in bundles with the wires
or strands
parallel to each other. In use the cable is placed in a preformed duct
in the
concrete member to be stressed and tensioned by a suitable
post-tensioning
method. Figure on right shows rolls of prestressing cable.
Bars: A
tendon can be made up of a single steel bar. Post-tensioning bars are
high
strength steel bars. The diameter of a bar is much larger than that of
a wire.
The normally available sizes are from 10mm to 44mm (5/8 to
1-3/4in)
diameter with 10, 12, 16, 20, 22, 25, 28 and 32 mm dias being more
common. The
bars are usually threaded with very coarse thread.
These
bar type tendons are used in certain types of post-tensioning systems.
The
alloy steel from which these bars are made has a yield stress in the
order of
620 MPa. This gives bar tendons a lower strength to weight ratio than
either
wires or strands, but when employed with threaded anchorages has the
advantages
of eliminating the possibility of pull-in at the anchorages and of
reducing
anchorage costs.
(Reference: California
Prestressing Manual)
