This set of Prestressed Concrete Structures Multiple Choice Questions & Answers (MCQs) focuses on “Loss Due to Shrinkage”.
1. The shrinkage of concrete in prestressed members results in:
Explanation: The shrinkage of concrete in prestressed members is due to the gradual loss of moisture which results in changes in volume, rich mixes exhibit relatively greater shrinkage than lean mixes since the contraction of the cement gel increases with the cement content, the shrinkage also depends upon the degree of hardening of the concrete at the commencement of drying and the shrinkage of concrete in prestressed members results in a shortening of tensioned wires and hence contributes to the loss of stress.
2. The shrinkage of concrete is influenced by:
a) Type of cement
b) Type of chemical
c) Type of admixtures
d) Type of retarders
Explanation: The shrinkage of concrete is influenced by the type of cement and aggregates and the method of curing used, the rate and amount of shrinkage of the structural member under ambient conditions will depend very much upon the ratio of surface area to volume of the member, as the exchange of moisture between the concrete and the atmosphere must take place through the surface.
3. Which of the following is used to reduce shrinkage of concrete?
a) High strength concrete
d) Low water cement ratio
Explanation: The use of high strength concrete with low water cement ratios results in a reduction in shrinkage and consequent loss of prestress, aggregates of rock type having high modular of elasticity and low values of deferred strain are more effective in restraining the contraction of the cement paste and their use reduces the shrinkage of concrete.
4. The rate of shrinkage is higher at:
Explanation: The rate of shrinkage is higher at the surface of the member and the primary cause of drying shrinkage is the progressive loss of water from concrete, the phenomena of shrinkage being time dependant only the total anticipated or residual shrinkage strain is considered in the computation of loss of prestress to be used in design.
5. The differential shrinkage between the interior and surface of large members result in:
a) Surface gradient
b) Strain gradient
c) Compatibility gradient
d) Stress gradient
Explanation: The differential shrinkage between the interior and surface of large members may result in strain gradients leading to surface grading; it is the effect due to relative deformation caused by the shrinkage of cast in situ concrete and combined action of shrinkage plus creep in the prestressed concrete.
6. Which curing is adopted in case of pretensioned members to prevent shrinkage?
a) Surface curing
b) Edge curing
c) Moist curing
d) Total curing
Explanation: In the case of pretensioned members, generally moist curing is resolved to in order to prevent shrinkage until the time of transfer, the magnitude of relative strain and the stresses induced depend on the concrete composition and surrounding environment to which the composite member is exposed.
7. After the transfer of prestress, the total residual shrinkage will be larger in case of:
a) Pretensioned members
b) Post tensioned members
c) Chemical tensioned members
d) Biological tensioned members
Explanation: The total residual shrinkage strain will be larger in pretensioned members after transfer of prestress in comparison with post tensioned members, where a position of shrinkage will have already taken place by the time of transfer of stress.
8. Which of the following Indian standard code is recommended for the loss of prestress due to shrinkage?
a) IS: 1445
b) IS: 1343
c) IS: 1210
d) IS: 1550
Explanation: The Indian standard code IS:1343 is recommended for the loss of prestress due to the shrinkage of concrete, individual shrinkage strains are evaluated from British code BS 8110 the shrinkage stresses are calculated.
9. The loss of prestress due to shrinkage is obtained by multiplying:
a) Shrinkage strain and modulus of elasticity
b) Principal stress and durability of concrete
c) Curing and grade of concrete
d) Compression and tension
Explanation: The loss of prestress due to shrinkage is obtained by multiplying the shrinkage strain with modulus of elasticity of steel, Δfs = Esh×Es , Es = Residual shrinkage, Es = modulus of elasticity of steel and light weight aggregates with low modulus of elasticity may lead to higher than normal concrete shrinkage where light weight aggregates are used, the value of shrinkage should be increased by 50%.
10. The post tensioned members in dry atmospheric conditions, the shrinkage may be increased by:
Explanation: For post tensioned members in dry atmospheric conditions, the shrinkage strain may be increased by 50% however, it should not exceed the value of 300×10-4units for pretensioned members and 2.0 x 10-4/ log(t+2) for post tensioned members, where t is the age in days of the concrete at transfer.
11. A concrete beam is prestressed by a cable carrying an initial prestressing force of 300kn, area is 300mm2. Calculate the percentage of loss of stress due to shrinkage in pretensioned members?
Explanation: Initial stress in wires = (300×103/300) = 1000n/mm2,
If the beam is pretensioned the total residual shrinkage strain = 300×10-6,
Loss of stress = (300×10-6)(210×103) = 63n/mm2, % loss 0f stress = (63×1000/100) = 6.3%.
Sanfoundry Global Education & Learning Series – Prestressed Concrete Structures.
To practice all areas of Prestressed Concrete Structures, here is complete set of 1000+ Multiple Choice Questions and Answers.