Prestressed Concrete Structures Multiple Choice Questions on “Tensioning Devices”.
1. In pre tensioning system, type of prestressing tendon used is ___________
a) Bars
b) Wires
c) Strands
d) Cables
Answer: c
Clarification: The strand tendons are used in both pre tensioning and post tensioning, they are made by winding seven cold drawn wires together on a stranding machine, the addition of strands in subsequent layers of wire forms strands of 19 or 37 wires, large post tensioning applications can cope with stressing requirements by use of these tendons.
2. The yield stress relieved in wires is about ___________
a) 620mpa
b) 1300mpa
c) 1250mpa
d) 600mpa
Answer: b
Clarification: The wire tendons are mainly used in post tensioning system for prestressed concrete, they are cold drawn and stress is relieved with a yield stress of about 1300mpa and the diameter generally ranges from between 5mm-8mm.
3. The use of bar tendons with threaded anchorages reduces the possibility of ___________
a) Push
b) Pull
c) Break
d) Bend
Answer: b
Clarification: The bar type tendons are used in certain post tensioning systems, the bars are of high strength alloy steel of yield strength about 620mpa and diameter up to 40mm and the use of bar tendons with threaded anchorages reduces the possibility of pull in at the anchorages and also anchorage cost.
4. The cables are formed by arrangement of wires or strands in ___________
a) Bundles
b) Rings
c) Ducts
d) Steel
Answer: a
Clarification: The cables are formed by arrangement of wires or strands in bundles and the performed duct in concrete member can be stressed and tensioned by appropriate post tensioning method by using cables, post tensioning is invariably used for strengthening concrete dams, circular prestressing of large concrete tanks and biological shields of nuclear reactors.
5. How many types of splicing arrangement are widely used in post tensioning systems?
a) 4
b) 5
c) 3
d) 2
Answer: a
Clarification: Screw connectors are normally employed to splice large diameter high tensile bars which can be threaded at ends, the torpedo splice consists of triple wedges for securing the wires and the entire unit is covered and protected by a sleeve, clamp splices are equipped with bolts and nuts with a series of clamp plates to house the tendons between them for splicing of small diameter wires of 3-6mm.
6. The commonly used mechanical devices used in the following statement.
a) Weight with lever transmission
b) Geared transmission without pulley blocks
c) Hydraulic jacks
d) Tendons
Answer: a
Clarification: The commonly used mechanical devices are weights with the level transmission, weight without lever transmission, geared transmission with pulley blocks; screw jacks without gear drives, wire winding machinery and the mechanical devices are generally used for imparting prestress in the concrete members which are prepared in large amounts in big factories.
7. The hydraulic jacks are the simplest and most widely used devices for providing prestress of ___________
a) High magnitudes
b) High discharges
c) High Bending moment
d) High tension
Answer: a
Clarification: Hydraulic jacks are the simplest and most widely used devices for providing prestress of high magnitudes and various hydraulic devices are prepared by different scientists from which the commonly used devices Freyssinet, Magnel Balton, Gifford Udall system, Baur Leonhardt, lee mc call system, Prescon system etc.
8. The applied force should be measured accurately in hydraulic devices while?
a) Tensioning the tendons
b) Tensioning the wires
c) Tensioning the cables
d) Tensioning the anchorages
Answer: a
Clarification: The prestressing members with hydraulic devices, the applied force should be measured accurately while tensioning the tendons and this can be achieved by providing pressure gauges with the jacks.
9. The electrical devices are commonly used for tensioning of ___________
a) Bend wires and steel wires
b) Deformed bars and steel wires
c) Torsion and steel wires
d) Compression wires and steel wires
Answer: a
Clarification: Electrical devices are used commonly for the tensioning of deformed bars and steel wires and the method of pre stressing involves the heating up of steel wires and anchoring them before filling the moulds with concrete.
10. The basic chemical substance used for pre stressing forces in concrete member is ___________
a) Aluminium
b) Sulphur
c) Shrinkage cement
d) Expanding cement
Answer: d
Clarification: Chemical substances are also used for developing pre stressing force in concrete members and the basic chemical substance used is expansive cement, when expansive cement is used in construction, the tendons are subjected to tension (while setting).