A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
reinforcement. Bursting reinforcement is used where tensile stresses are
induced during prestressing operation and the maximum bursting stress
occurs where the stress trajectories are concave towards the line of action
of the load. Reinforcement is needed to resist these lateral tensile forces.
For equilibrium reinforcement, it is required where there are several
anchorages in which prestressing loads are applied sequentially.
During prestressing, spalling stresses are generated in the region behind
the loaded faces of anchor blocks [14]. At the zone between two
anchorages, there is a volume of concrete surrounded by compressive
stress trajectories. Forces are induced in the opposite direction to the
applied forces and it forces the concrete out of the anchor block. On the
other hand, the spalling stresses are set up owing to the strain compatibility
relating to the effect of Poisson’s ratio.
7. What are the functions of grout inside tendon ducts? (PC2)
Grout in prestressing works serves the following purposes:
(i) Protect the tendon against corrosion.
(ii) Improve the ultimate capacity of tendon.
(iii) Provide a bond between the structural member and the tendon.
(iv) In case of failure, the anchorage is not subject to all strain energy.
8. In prestressing work, if more than one wire or strand is included in
the same duct, why should all wires/strands be stressed at the same
time? (PC1)
If wires/strands are stressed individually inside the same duct, then those
stressed strand/wires will bear against those unstressed ones and trap
them. Therefore, the friction of the trapped wires is high and is undesirable.
9. What is the optimum size of cable duct for prestressing? (PC1)
The cross sectional area of duct is normally 2.5 times that of the area of
prestressing steel. The size of ducts should be not designed to be too small
because of the followings:
(i) Potential blockage by grout
(ii) Excessive development of friction
(iii) Difficulty in threading prestressing tendon
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