Pre-tensioning ~ this method is used mainly in the factory
production of precast concrete components such as lintels, floor
units and small beams. Many of these units are formed by the long
line method where precision steel moulds up to 120„000 long are
used with spacer or dividing plates to form the various lengths
required. In pre-tensioning the wires are stressed within the mould
before the concrete is placed around them. Steam curing is often
used to accelerate this process to achieve a 24 hour characteristic
strength of 28 N/mm^2 with a typical 28 day cube strength of
40 N/mm^2. Stressing of the wires is carried out by using hydraulic
jacks operating from one or both ends of the mould to achieve an
initial 10% overstress to counteract expected looses. After curing
the wires are released or cut and the bond between the stressed
wires and the concrete prevents the tendons from regaining their
original length thus maintaining the precompression or prestress.
At the extreme ends of the members the bond between the
stressed wires and concrete is not fully developed due to low
frictional resistance. This results in a small contraction and
swelling at the ends of the wire forming in effect a cone shape
anchorage. The distance over which this contraction occurs is
called the transfer length and is equal to 80 to 120 times the wire
diameter. To achieve a greater total surface contact area it is
common practice to use a larger number of small diameter wires
rather than a smaller number of large diameter wires giving the
same total cross sectional area.
Prestressed Concrete