Important Questions and Answers - Civil - Prestressed Concrete Structures - Theory and Behaviour

INTRODUCTION-THEORY AND BEHAVIOUR

1.     What are the advantages of PSC construction

·        In case of fully prestressed member, which are free from tensile stresses under working loads, the cross section is more efficiently utilized when compared with a reinforced concrete section which is cracked under working loads.

·        The flexural member is stiffer under working loads than a reinforced concrete member of the same length.

2.     Define Pre tensioning and Post tensioning

·        Pre tensioning: A method of Pre stressing concrete in which the tendons are tensioned before the concrete is placed. In this method, the prestress is imparted to concrete by bond between steel and concrete.

·        Post tensioning: A method of pre stressing concrete by tensioning the

tendons against hardened concrete. In this method, the prestress is imparted to concrete by bearing.

3.  What is the need for the use of high strength concrete and tensile steel in Pre stressed concrete?

·        High strength concrete is necessary for prestress concrete as the material offers highly resistance in tension, shear bond and bearing. In the zone of anchorage the bearing stresses being hired, high strength concrete is invariably preferred to minimizing the cost. High strength concrete is less liable to shrinkage cracks and has lighter modulus of elasticity and smaller ultimate creep strain resulting in a smaller loss of prestress in steel. The use of high strength concrete results in a reduction in a cross sectional dimensions of prestress concrete structural element with a reduced dead weight of the material longer span become technically and economically practicable.

·        Tensile strength of high tensile steel is in the range of 1400 to 2000 N/mm² and if initially stress upto 1400 N/mm² their will be still large stress in the high

tensile reinforcement after making deduction for loss of prestress. Therefore high tensile steel is made for prestress concrete.

4.  Define Kern Distance.

Kern is the core area of the section in which if the load applied tension will not be induced in the section

K_t = Z_b/A,              K_b = Zt/A,

If the load applied K Compresser will be the maximum at the top most fiber

and zero stress will be at the bottom most fiber. If the load applied at K_b

compressive stress will be the maximum at the bottom most fiber and zero

stress will be at the top most fiber.

5.  What is Relaxation of steel?

When a high tensile steel wire is stretch and maintained at a constant strain the initially force in the wire does not remain constant but decrease with time. The decrease of stress in steel at constant strain is termed relaxation of steel.

6.  What is concordant prestressing?

Pre stressing of members in which the cable follow a concordant profile. In case of statically indeterminate structures. It does not cause any changes in support reaction.

7.     Define bonded and non bonded prestressing concrete.

·        Bonded prestressing: Concrete in which prestress is imparted to concrete through bond between the tendons and surrounding concrete. Pre tensioned members belong to this group.

·        Non-bonded prestressing: A method of construction in which the tendons are not bonded to the surrounding concrete. The tendons may be placed in ducts formed in the concrete members or they may be placed outside the concrete section.

8.     Define Axial prestressing

Members in which the entire cross-section of concrete has a uniform compressive prestress. In this type of prestressing, the centroid, of the tendons coincides with that of the concrete section.

9.  Define Prestressed concrete.

It is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from external loads (or) counteracted to a desire degree in reinforced concrete member the prestress is commonly introduced by tensioning the steel reinforcement

10. Define anchorage.

A device generally used to enable the tendon to impart and maintain prestress to the concrete is called anchorage. e.g. Fressinet, BBRV systems,etc.,