A Self-Learning Manual - Institution of Engineers Mauritius

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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 60
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 10. What is stress corrosion of prestressing steel? (PC1) Stress corrosion is the crystalline cracking of metals under tensile stresses in the presence of corrosive agents [44]. The conditions for stress corrosion to occur are that the steel is subjected to tensile stresses arising from external loading or internally induced stress (e.g. prestressing). Moreover, the presence of corrosive agents is essential to trigger stress corrosion. One of the main features of stress corrosion is that the material fractures without any damage observed from the outside. Hence, stress corrosion occurs without any obvious warning signs. 61
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A Self-Learning Manual - Institution of Engineers Mauritius

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