This set of Design of RC Structures Multiple Choice Questions & Answers (MCQs) focuses on “Prestressed Concretes Basics”.
1. Prestressing is the application of predetermined force or moment to a structural member.
a) True
b) False
View Answer
Explanation: Prestressing can be defined as the application of a predetermined force or moment to a structural member in a way that the combined internal stresses in the member resulting from this force or moment and from any anticipated condition of external loading will be confined within specific limits. It involves the imposition of stresses opposite in sign to those which are caused by the application of loads.
2. Which of the following is not a limit state of collapse under which the assumed concrete section is checked for design of prestressed concrete?
a) Fatigue
b) Vibration
c) Torsion
d) Flexure
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Explanation: The limit state of design of a prestressed concrete beam consists of selecting and proportioning a concrete section, determination of amount of prestressing force and eccentricity required for a given section. The designer assumes a concrete section which should be checked under the limit state of collapse and serviceability. Limit state of collapse are limit state of flexure, shear, torsion, fire resistance and fatigue.
3. The assumed concrete section for prestressed concrete is not checked under limit state of maximum compression.
a) True
b) False
View Answer
Explanation: The limit state of serviceability includes limit states for cracking, deflection, maximum compression, durability and vibration. Limit state of serviceability in maximum compression is an additional limit state in which a prestressed concrete beam must be checked. It is not applicable to reinforced concrete beams. This limit state ensures that maximum compressive stresses remain within permissible limits under service loads.
4. Which of the following is not an advantage of prestressed concrete?
a) Free of cracks under loads
b) No corrosion of steel
c) Its behavior can be explained on rational basis
d) Uses high strength concrete and steel
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Explanation: The most significant feature of prestressed concrete is that it is free of cracks under service loads and enables the entire section to take part in resisting the moments. It also eliminates corrosion of steel when the structure is exposed to weather. It is more predictable than ordinary reinforced concrete in several aspects. Since there is no cracking in prestressed concrete its behavior can be explained on a more rational basis.
5. In prestressed concrete structure sections are much smaller than those in reinforced concrete structures.
a) True
b) False
View Answer
Explanation: In prestressed concrete structure sections are much smaller than those in reinforced concrete structures. This is because the dead load moments are neutralized by the prestressing moments and shear forces are reduced. The reduced self weight of the structure further saves the cost of foundation.
6. Prestressed concrete do not have ability to resist fatigue and impact.
a) True
b) False
View Answer
Explanation: Some features of prestressed concrete are its high ability to resist impact, high fatigue resistance and high live load carrying capacity. It is possible to assemble precast prestressed elements thus saving cost of shuttering and centering, and time besides maintaining high quality control.
7. Which of the following is not a disadvantage of prestressed concrete construction?
a) Uses high strength concrete and steel
b) Requires end anchors
c) Requires bearing plates
d) Requires less complicated form work
View Answer
Explanation: The disadvantages of prestressed concrete construction are:
i. It uses high strength concrete and steel.
ii. It requires end anchors and bearing plates.
iii. It may require complicated form work.
iv. Labor cost may be greater.
v. It requires better quality control.
8. The limit state of collapse for fire resistance provides time to occupants to escape.
a) True
b) False
View Answer
Explanation: The limit state of collapse for fire resistance provides for sufficient time to allow any occupants to escape. The fire resistance of a concrete structure is mainly determined by the concrete-cover to the steel since it is the strength of steel which deteriorates with increasing temperature.
9. The limit state of collapse for fatigue is not important for prestressed concrete structures subjected to cyclic loading.
a) True
b) False
View Answer
Explanation: The limit state of collapse in fatigue is important for structures subject to cyclic loading, especially in the case of prestressed concrete beams used in bridges and prestressed concrete sleepers used in railway lines. The major areas where fatigue failure can occur are in concrete in compression, the bond between the steel and concrete, and the prestressing steel.
10. Which of the following is not an example of pre-tensioning process of prestressing?
a) Railway sleepers
b) Electrical poles
c) Floor slab
d) Strengthening of existing structures
View Answer
Explanation: In pre-tensioning, lengths of steel wire, cables, or ropes are laid in the empty mold and then these are stretched and anchored. After the concrete has been poured and allowed to set, the anchors are released. This technique is very economical for mass production of smaller elements for example railway sleepers, electrical poles/lighting pole, floor slab, etc.
11. Which of the following is not an advantage of post-tensioning?
a) Cracks are held tightly together
b) Eliminates shrinkage cracking
c) Less complexity of work
d) Allows building of slabs on expansive soils
View Answer
Explanation: The advantages of post-tensioning process are:
i. It eliminates shrinkage cracking.
ii. Cracks formed are held tightly.
iii. It allows slabs to be thinner.
iv. It allows slabs to be built on expansive or soft soils.
v. Lesser usage of materials.
vi. Flexibility in design.
vii. Durability.
12. Which of the following is not a method of post-tensioning process of prestressing?
a) Gifford-Udall system
b) Lee-McCall system
c) Partial prestressing
d) Magnel system
View Answer
Explanation: Some popular methods of post-tensioning are as follows:
i. Gifford-Udall system
ii. Lee-McCall system
iii. Magnel system
iv. Leonhardt system
v. Freyssinet system
13. The cost of pre-tensioning system is smaller as compared to cost of post-tensioning system.
a) True
b) False
View Answer
Explanation: The cost of pre-tensioning system is generally small as compared to the cost of post-tensioning system. This is due to larger amount of labor required in placing, stressing and grouting post-tensioned tendons and to the cost of metal sheathing, spacers, special anchorage devices and jacks which are required in a post-tensioning system.
14. Which of the following is the minimum grade of concrete mix suggested by IS code for pre-tensioned systems?
a) M30
b) M40
c) M45
d) M60
View Answer
Explanation: The concrete to be used for prestressed construction should be of very good quality and of very high strength. The IS code suggests a minimum mix of M40 for pre-tensioned systems and M30 for post-tensioned systems. The advantage of using rich concrete mix is that its elastic shrinkage and creep strains are smaller.
15. The yield strength of mild steel bars and deformed bars used in prestressed concrete construction are 250 MPa and 415 MPa.
a) True
b) False
View Answer
Explanation: If the steels of 250 MPa and 415 MPa are used in prestressed concrete very little prestress will be left. So it becomes essential to use high ensile steel so that there is sufficient prestress is available to maintain a practical design. High tensile steel is generally available in the form of wires which are either stress relieved, as-drawn or cold-drawn from high tensile steel bars.
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