Prestressed Concrete Structures Multiple Choice Questions on “Pretensioning Systems”.
1. The tendons in the pretensioning system are tensioned between ___________
a) Rigid anchorages
b) Hydraulic jacks
c) Concrete beds
d) Variable beams
Answer: a
Clarification: In the prestressing system, the tendons are first tensioned between rigid anchorage blocks cast on the ground or a column or a unit mould type pretensioning bed, prior to the casting of concrete in the moulds.
2. When the concrete attains sufficient strength, which elements are released?
a) Jacks
b) Casting bed
c) Tendons
d) Beams
Answer: a
Clarification: High early strength concrete is often used in factory to facilitate early stripping the reuse of moulds and when the concrete attains sufficient strength, the jacking pressure is released and the edge of tendon at its either side is formed to an abutment and its other edge is to be pulled with the application of jack.
3. Which is one of the systems used for pretensioning?
a) Magnel-Balton system
b) Freyssinet system
c) Gifford-Udall system
d) Hoyer’s long line method
Answer: d
Clarification: Hoyer’s long line method is the system used in pretensioning and the other systems like Freyssinet, Gifford-Udall, and Magnel-Balton are post tensioning systems and large numbers of beams are produced in an individual alignment.
4. Hoyer’s system of pre tensioning is generally adopted for ___________
a) Small scale members
b) Large scale members
c) Middle span members
d) End members
Answer: b
Clarification: Hoyer’s system is generally recommended when the production of pretensioned members is required on a large scale is the principle which are used to precast the beams the post tensioning system was considered to pretensioning system when used for large spans, due to this reason the pre tensioning system was replaced by post tensioning system.
5. The transfer of prestress of concrete is achieved by ___________
a) Plates
b) Rings
c) Steel bars
d) Jacks
Answer: d
Clarification: The transfer of prestress to concrete is usually achieved by large hydraulic or screw jacks by which all the wires are simultaneously released after the concrete attains the required compressive strength generally strands of up to 18mm diameter and high tensile wires of up to 7mm diameter anchor themselves satisfactorily with the help of surface bond and the interlocking of the surrounding matrix in the micro indentations on the wires.
6. The bond of prestressing wires in Hoyer’s system can be formed by ___________
a) Helical crimping
b) Tangential crimping
c) Circular crimping
d) Diode crimping
Answer: a
Clarification: Bond of prestressing wires may be considerably improved by forming surface indentations and by helical crimping of wires in Hoyer’s system, strands have considerable better bond characteristics than plain wires of equal cross sectional area supplementary anchoring devices are required when single wires of larger diameter are used in the pretensioned units.
7. The Hoyer’s method of prestressing is done by ___________
a) Pulling out of wires
b) Pushing wires
c) Heating of wires
d) Stressing of wires
Answer: a
Clarification: Hoyer’s system of prestressing involves pulling out of wires between two bulkheads which are separated at large distances in order to produce a larger number of beams in an individual alignment, the concrete is to be poured by providing appropriate shuttering between the beams and then the wires are united after hardening of concrete and cutoff.
8. Hoyer’s system of pretensioning can be done for beams.
a) 1
b) More than 2
c) Less than 2
d) 2
Answer: b
Clarification: United beams more than 2 are hardened in Hoyer’s method of pretensiong in case of large distances between each beam, the most commonly used devices are the Weinberg clip developed in France and the Dorland clip developed in the united states these clips are clamped on to the tensioned wires close on the end diaphragms of the units before concreting operations.
9. The Hoyer’s system of prestressing proves to be economical for ___________
a) Pre tensioning system
b) Post tensioning system
c) Beam casting
d) Bed casting
Answer: b
Clarification: The post tensioning system was considered as economical when compared to pre tensioning system, when used for larger spans due to this reason pre tensioning system was replaced by post tensioning system, for mass production of pretensioned elements, the long line process developed by Hoyer is generally used in the factory and in this method the tendons are stretched several hundred meters apart so that a number of similar units may be cast along the same group of tensioned wires.
10. In Hoyer’s system the projection of plugs left in concrete exceeds beyond ___________
a) Middle of member
b) End of member
c) First of member
d) Transfer part of member
Answer: b
Clarification: One of the disadvantages of Hoyer’s system is the projection of plugs which are left in concrete exceeds beyond the end of the member, application of heavy jacks which are uneconomical, projection of plugs which are left in concrete exceeds beyond the ends of the member, additional reinforcement is required to prevent failure of shear.