1. 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.
2. 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.
3. 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.
4. 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%.
5. The post tensioned members in dry atmospheric conditions, the shrinkage may be increased by ___________
a) 50%
b) 70%
c) 30%
d) 100%
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.
6. 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.
7. 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?
a) 6.3%
b) 5.3%
c) 4%
d) 2.3%
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%.
8. Which of the following results in the reduction of stress in steel used for prestress?
a) Shrinkage
b) Friction
c) Creep
d) Anchorage slip
Explanation: The sustained prestress in the concrete of a prestressed members results in creep of concrete which effectively reduces the stress in high tensile steel and the progressive inelastic strains due to creep in a concrete are likely to occur under the smallest sustained stresses at ambient temperatures, shrinkage and creep of concrete are basically similar in origin, being largely the result of migration of water in the capillaries of cement paste.
9. The loss of stress due to creep of concrete can be estimated by ___________
a) Ultimate creep strain
b) Ultimate load
c) Ultimate creep stress
d) Ultimate creep tension
Explanation: The loss of stress in steel due to creep of concrete can be estimated if the magnitude of ultimate creep strain or creep coefficient is known and the values of creep coefficient which is the ratio of ultimate creep strain to the elastic strain is 2.2 at 7 days of loading, 1.6 at 28 days and 1.1 when the age at loading is 1 year.
10. The value of creep coefficient ‘f’ depends upon ___________
a) Humidity
b) Frost
c) Uv rays
d) Ice
Explanation: The value of creep coefficient ‘f’ depends upon various factors such as humidity duration of load applied, age of loading and effective section thickness and the effective section thickness is defined for uniform sections as twice the cross sectional area divided by the exposed perimeter, it can be assumed about 4060 and 80 percent, respectively of the final creep develops during the first, six and 30 months under load when concrete is exposed to conditions of constant relative humidity