1. The rate of increase in stress in the tendons of a prestress concrete member depends upon ____________
a) Bond and breakage
b) Bond and surrounding concrete
c) Bond and elasticity
d) Bond and anchorage
Explanation: The rate of increase of stress in the tendons of a prestressed concrete member under loads depends upon the degree of bond between the high tensile steel wires and the surrounding concrete, the increase of stress in steel depends on the average strain in concrete at the level of steel.
2. A prestressed concrete beam used over a span of 6m is 100mm wide and 300mm deep, live load of 4kn/m, density of concrete is 24kn/m3, modulus of elasticity of concrete is 36 and steel is 210kn/mm2. Find rotation due to loads(sagging moment)?
a) 0.005
b) 0.00525
c) 0.0024
d) 0.0045
Explanation: E = 210kn/mm2, Self weight of the beam, g = (0.1×0.3×24) = 0.72kn/m
Live load on the beam (q) = 4kn/m, Total load on the beam, Wd = (q+g) = (4+0.72) = 0.00472kn/mm
Rotation due to prestressing force θp = PeL/24EI = (100×50×6×103/24×36×210×106)
Sagging moment = 0.00525radians.
3. The stress in tendons of bonded beams is ____________
a) αe (My/I)
b) αe (My/R)
c) αe (My/L)
d) αe (My/20)
Explanation: The stresses in tendons of bonded beams is αe (My/I), αe = Modular ratio of steel to concrete, y = vertical distance of a point from centroid of concrete section, M = bending moment, I = moment of area of the section, in cases of bonded members such as pretensioned elements or post tensioned grouted members, the composite action between steel and concrete prevails and the stresses in steel are computed using the theory of composite sections up to stage of cracking.
4. The rate of increase of stress is larger in case of ____________
a) Bonded beams
b) Un bonded beams
c) Tensioned beams
d) Anchorage beams
Explanation: The rate of increase of stress is larger in case of bonded beams than in un bonded beams both in the pre cracking and post cracking stages, in case of unbounded beams, the tendons are free to elongate independently throughout their length under the action of transverse loads on the beam.
5. The development of crack widths is comparatively larger in ____________
a) Bonded beams
b) Un bonded beams
c) Localized beams
d) Strengthened beams
Explanation: In post cracking stage, while the bonded beams are characterized by small cracks, which are well distributed in the zone of the larger moments, unbounded beams develop only a few cracks, which are localized at weaker sections and the crack widths are correspondingly larger in comparison with the bonded beams.
6. Which beams are preferred mostly due to their higher flexural strength?
a) Bonded beams
b) Un bonded beams
c) Exhaustive beams
d) Extended beams
Explanation: In general bonded beams are preferable due to their higher flexural strength and predictable deformation characteristics, after the onset of cracking the stress in steel increases at a faster rate in both types of beams since steel does not reach its ultimate strength in case of unbounded beams the ultimate load supported by the beam is smaller than that of bonded beam in which the steel attains its ultimate strength at the failure stage of member
7. The soffit of the beam after the transfer of prestress to concrete will be under ____________
a) Tension
b) Compression
c) Breakage
d) Bondage
Explanation: The bending moment at which visible cracks developed in prestressed concrete members is generally referred to as the cracking moment after the transfer of prestress to concrete, the soffit of the beam will be under compression.
8. The compressive and tensile stresses developed in cracking moments are due to ____________
a) Bending loads
b) Transverse loads
c) Tensile loads
d) Compressive loads
Explanation: These compressive stresses are balanced by the tensile stresses developed due to the transverse loads on the beam, so that the resultant stress at the bottom fiber is zero, a further increase in loading results in the development of tensile stresses at the soffit of beam.
9. The micro cracks develop as soon as the tensile strain in concrete exceeds about ____________
a) 80-100×10-6 units
b) 100-150×10-6 units
c) 150-300×10-6 units
d) 300-500×10-6 units
Explanation: As concrete is weak in tension, micro cracks develop as soon as the tensile strain in concrete exceeds about 80-100×10-6 units and the parameters in concrete are influenced by various materials and their permissible values according to Indian standard codes.
10. When the loads in a concrete member are further increased than permitted, the crack widths are of an order of limit?
a) 0.01-0.02mm5
b) 0.05-0.10mm5
c) 0.03-0.05mm5
d) 0.07-0.08mm5
Explanation: If the loads are further increased, than 80-100×10-6 units visible cracks appear in the tension zone and at this stage, it is estimated that the crack widths are of an order of 0.01-0.02mm5 and these width of cracks are considered from a value of minimum to maximum not excess till permitted