This set of Prestressed Concrete Structures Multiple Choice Questions & Answers (MCQs) focuses on “Thermo Electric Prestressing”.
1. The method of pre stressing by heated tendons, achieved by passing an electric current is known as ___________
a) Chemical pre stressing
b) Thermo electric pre stressing
c) Dynamic pre stressing
d) Thermo blasting pre stressing
Explanation: In this method the tendons are heated through the high tensile wires and by allowing electric current to flow through it, this process is referred as thermo electric pre stressing, in the erstwhile U.S.S.R, the electro thermal method has been widely used since 1958 for pretensioning bar reinforcements of precast units.
2. In thermo electric pre stressing, heating the bars is done to a temperature of ___________
Explanation: The temperature in the bars is raised from 300 to 400degrees for a time duration of 3to 5 minutes, with this the bar elongates up to 0.4 to 0.5%, empirical relations for the estimation of the current, voltage and power requirements of the transformer are reported by Graduck.
3. The period of cooling in thermo electric pre stressing is observed as ___________
a) 20-30 minutes
b) 12-15 minutes
c) 5-10 minutes
d) 15-20 minutes
Explanation: After the process of cooling, the bars get shortened but which can be known with the fixed arrangement at both the ends and the period of cooling is observed as 12 to 15minutes, a steel metal sheath of enlarged diameter and sufficient length is generally used to cover the splice, screw threaded connectors are not recommended for splicing heat treated prestressing steels, which are highly suspectable for stress corrosion.
4. The thermo electric pre stressing, initial stress in tendons ranges between ___________
Explanation: Thermo electric pre stressing helps to cause on initial stress in tendons of magnitude from 500 to 600n/mm2 and concrete is placed in moulds only after the temperature of the wires falls below 90degrees.
5. In which place thermo electric pre stressing found to be more economical?
Explanation: In the erstwhile USSR, the Thermo electric pre stressing method has been widely used since 1958 for pre tensioning bar reinforcements of pre cast units and this method has been found to be more economical than conventional mechanical devices.
6. The thermo electric pre stressing is also used in ___________
a) Pre-block units
b) Pre-cast units
c) Pre-beam units
d) Pre-anchorage units
Explanation: Thermo electric pre stressing is adopted in many foreign countries as it is most economical when compared to the other conventional mechanical devices; it is also used in precast units, precast are only used within ranges of exterior and interior walls compressed in concrete and stone, creating a solid but maneuverable wall or face, precast concrete production performed on ground level, which helps with safety throughout a project.
7. The tensioning of oval section ribbed wires with an ultimate tensile strength was adopted in ___________
Explanation: Thermo electric pre stressing has also been adopted in Germany for the tensioning of oval section ribbed wires with an ultimate tensile strength of 1600n/mm2, the wrapping wire of 1mm diameter is generally used for splicing wires up to 6mm diameter and the length of splice may vary from 20-30cm and the splice formed in this manner has a strength almost equal to that of the normal wire and this type of splice is generally used for the wires of circular concrete tanks and anchorage loops.
8. The estimation of the current, voltage and power requirements of the transformer are for ___________
a) Theoretical equations
b) Empirical relations
c) Global relations
d) Quadratic equations
Explanation: A temperature of about 4600c was necessary to induce an initial stress of 55 percent of the ultimate tensile strength the heating time being 40-90s at 30v and 300-1100 A and Empirical relations for the estimation of the current, voltage and power requirements of the transformer are reported by Graduck.
9. In thermo electric pre stressing, the elongation is observed in case of ___________
a) Bar after cooling
b) Bar after heating
c) Bar after anchoring
d) Simple bar
Explanation: In case of heating the bar gets elongated and shortens after heating and after anchorages are applied on both sides which stretch the bar, clamp splices are equipped with bolts and nuts since there will be a considerable reduction in the tensile strength of up to 50 percent, these type of splice can be used only in locations where the prestressing force can be sufficiently reduced by the curvature of tendon due to friction.
10. How many wedges are considered in the case of torpedo splices?
Explanation: Tendons are spliced in case of continuous pre stressed concrete members to gain continuity and the various types of splices used as tendons are screw connector, torpedo splices, clamp splice and wrapped splice and torpedo splices consists of triple wedges for securing the wires and the entire unit is covered and protected by a sleeve and this type of splice is largely used for splicing cold drawn wires.
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