1. The creep coefficient varies from a minimum value of ___________
a) 2.0
b) 1.5
c) 2.5
d) 1.0
Explanation: 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.
2. The creep coefficient is high for ___________
a) Pretensioned
b) Post tensioned
c) Chemical tensioned
d) Electrical tensioned
Explanation: 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
3. 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
Explanation: 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.
4. 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
Explanation: 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
5. 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
Explanation: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.
6. 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
Explanation: A = 3×104mm2, p = (200×1200) = 240000n, e = 50mm,
Stress in concrete at the level of steel: (240000/30000) = 8n/mm2.
7. 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
Explanation: 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
8. The phenomena of reduction of stress in steel at a constant strain are known as ____________
a) Relaxation of stress
b) Shrinkage of concrete
c) Creep of concrete
d) Anchorage slip
Explanation: In high tension strength steel, it is noticed that at a particular time the stress in steel reduced at a constant value of strain and this phenomena of reduction of stress in steel at a constant strain is known as relaxation of stress.
9. The relaxation of stress in steel changes according to the variation in ____________
a) Stress
b) Strain
c) Creep
d) Shrinkage
Explanation: The phenomena of relaxation of stress occur as a result of creep in steel and the relaxation of stress in steel changes according to the variation of percentage of creep, the phenomenon of creep is influenced by the chemical composition, micro structures, grain size and variables in the manufacturing process, which results in changes in the internal crystal structure
10. The codes provided for the loss of stress due to relaxation of steel are expressed as a percentage of ____________
a) Initial stress
b) Final stress
c) Major stress
d) Minor stress
Explanation: Most of the codes provided for the loss of stress due to relaxation of steel are expressed as a percentage of initial stresses in steel and the high tensile steel tendon in a prestressed concrete member does not remain strictly under a condition of either stress or strain, the most severe condition occurs generally at the stage of initial stressing subsequently the strain in the steel reduces as the concrete under the prestressing force