Prestressed Concrete Structures Multiple Choice Questions on “Loss Due to Creep of Concrete”.
1. Which of the following results in the reduction of stress in steel used for prestress?
a) Shrinkage
b) Friction
c) Creep
d) Anchorage slip
Answer: c
Clarification: 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.
2. 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
Answer: a
Clarification: 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.
3. The value of creep coefficient ‘f’ depends upon ___________
a) Humidity
b) Frost
c) Uv rays
d) Ice
Answer: a
Clarification: 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.
4. The creep coefficient varies from a minimum value of ___________
a) 2.0
b) 1.5
c) 2.5
d) 1.0
Answer: b
Clarification: The creep coefficient varies from a minimum value of 1.5(for wet conditions) to a maximum value of 4.0(for dry condition), for design purposes it is convention to differentiate between deformation due to externally applied stress generally referred as creep and the deformation which occurs without externally applied stresses referred as shrinkage as the increase in strain under a sustained stress is several times larger than the strain on loading, it is of considerable importance in prestressed structural members.
5. The creep coefficient is high for ___________
a) Pretensioned
b) Post tensioned
c) Chemical tensioned
d) Electrical tensioned
Answer: a
Clarification: Creep coefficient is generally low for post tensioned members and high for pre tensioned members, the various factors influencing creep of concrete are relative humidity, stress level, strength of concrete, age of concrete at loading, duration of stress, water cement ratio, and type of cement and aggregate in the concrete, for stress up to half of the crushing strength of concrete.
6. The creep coefficient equation is given as ___________
a) Creep strain/elastic strain
b) Creep stress/elastic stress
c) Elastic strain/creep strain
d) Elastic stress/creep stress
Answer: a
Clarification: The magnitude of the creep coefficient ϕ varies depending upon the humidity, concrete quality, duration of applied loading and the age of concrete when loaded,
Creep coefficient = (creep strain / elastic strain), ϕ = ℇc/ℇe, ℇc = ϕ ℇe = ϕ(fs/Ec),
Loss of stress in steel = ℇcEcϕEs = ϕ(fs/Ec)Es = ϕfcαe.
7. The loss of stress in steel due to creep of concrete is ___________
a) ℇcc fc Es
b) ℇc fc Ec
c) ℇc fe αe
d) ℇcc fc ϕ
Answer: a
Clarification: The loss of stress in steel due to creep of concrete is ℇcc fc Es,
ℇcc = ultimate creep strain for a sustained unit stress, fc = compressive stress in concrete at the level of steel, Es = modulus of elasticity of steel.
8. A concrete beam of rectangular section 100mm wide, area is 3×104, initial stress is 1200n/mm2 of 7mm diameter located at an eccentricity 50mm. Estimate the loss of stress due to creep of concrete using the ultimate creep strain method?
a) 88
b) 40
c) 50
d) 120
Answer: a
Clarification: Es = 210kn/mm2, I = 225×106mm4, Ec = 35n/mm2, a = 3×104mm2, p = (1200×38.5×5) = 23×104n, ℇcc = 41×10-6mm/mm per n/mm2, αe = (Es/Ec) = 6, ϕ = 1.6, fc = (23×104/3×104+23×104×50×50/225×106) = 10.2n/mm2, Ultimate creep strain method loss = ℇcc fc Es = (41×10-6)(10.2)(210×103) = 88n/mm2.
9. A post tensioned concrete beam of rectangular section 200mm wide and 300mm deep is stressed by a parabolic cable with eccentricity 50mm at centre of span, area is 3×104mm2, initial stress is 1200n/mm2 Find the stress in concrete at the level of steel?
a) 10n/mm2
b) 15n/mm2
c) 6n/mm2
d) 8n/mm2
Answer: d
Clarification: A = 3×104mm2, p = (200×1200) = 240000n, e = 50mm,
Stress in concrete at the level of steel: (240000/30000) = 8n/mm2.
10. The British code for structural concrete recommends design values of ultimate creep strain of pretensioning is ___________
a) 48×10-6
b) 53×10-6
c) 40×10-6
d) 38×10-6
Answer: a
Clarification: The British code for structural concrete recommends design values of ultimate creep strain of 48×10-6 for pretensioning and 36×10-6 for post tensioning and these values have to be increased in inverse proportion if the compressive strength of concrete at transfer is less than 40n/mm2.