Materials ~ concrete will shrink whilst curing and it can also suffer
sectional losses due to creep when subjected to pressure. The
amount of shrinkage and creep likely to occur can be controlled by
designing the strength and workability of the concrete, high
strength and low workability giving the greatest reduction in both
shrinkage and creep. Mild steel will suffer from relaxation losses
which is where the stresses in steel under load decrease to a
minimum value after a period of time and this can be overcome by
increasing the initial stress in the steel. If mild steel is used for
prestressing the summation of shrinkage, creep and relaxation
losses will cancel out any induced compression, therefore special
alloy steels must be used to form tendons for prestressed work.Tendons † these can be of small diameter wires (2 to 7 mm) in a
plain round, crimped or indented format, these wires may be
individual or grouped to form cables. Another form of tendon is
strand which consists of a straight core wire around which
is helically wound further wires to give formats such as 7 wire
(6 over 1) and 19 wire (9 over 9 over 1) and like wire tendons strand
can be used individually or in groups to form cables. The two main
advantages of strand are:-- A large prestressing force can be provided over a restricted
area. - Strand can be supplied in long flexible lengths capable of
being stored on drums thus saving site storage and site
fabrication space.
Prestressed Concrete