PSC QUESTION BANK

BHARATH UNIVERSITY

DEPARTMENT OF CIVIL ENGINEERING

(Declared Under Section 3 of UGC Act, 1956)

Selaiyur, Chennai-73

Department: CIVIL ENGINEERING           Year: IV

Branch:CIVIL                                                            Semester: VII

Subject code: BCE070

Subject Name: PRE-STRESS CONCRETE

UNIT – 1

Marks: 2

1)    What is the basic principle of prestressed concrete?

2)    Name the various types of tensioning devices used in prestressed concrete.

3)    Distinguish between pre-tensioned and post-tensioned members.

4)    List the various types of losses in prestressed concrete.

5)    What are the advantages of pre-stressed concrete over the conventional reinforced concrete?

Marks: 16

1)    A rectangle concrete beam of cross section 30cm deep and 20cm wide, is pre-stressed by means of 10 wires diameter located 75mm from the bottom of the beam and 3 wires of 5mm diameter located 500mm from the top of the beam. Assuming the pre-stress in the steel as 840 N/mm² , calculate the stresses at the extreme fibres of the mid span section when the beam is supporting its own weight over a span of 6m. If a uniformly distributed load of 6KN/m is imposed, calculate the maximum working stress in concrete. The density of concrete is 24KN/m³.

2)    Explain in detail the different losses that occur in prestressed concrete members.

3)    A concrete beam with a cross sectional area of 32X10³ mm² and radius of gyration of 72mm is prestressed by a parabolic cable carrying an effective stress of 1000n/mm². The span of the beam is 8m. The cable, composed of 6 wires of 7mm diameter, has an eccentricity of 50mm at the centre and zero at the support. Neglecting all losses, find the central deflection of the beamas follows:

a)     Self-weight + prestress and

b)    Self-weight + prestress + live load of 2KN/m.

4)    A concrete beam, 100mm wide and 300mm deep, is pre-stressed by straight wires carrying an effective force of 150KN at an eccentricity of 50mm. The modulus of elasticity of steel and concrete are 210 and 35KN/mm² respectively. Estimate the percentage loss of prestress in steel due to elastic deformation of concrete if the area of steel wires is 188mm².

UNIT – II

Marks: 2

1)    List the different types of prestressing?

2)    What are the various methods generally used for the investigation of anchorage zone stress?

3)    Why wnd block is used in prestress concrete?

4)    What do you mean by loss due to elastic shortening?

5)    What is mean by anchorage zone?

Marks: 16

1)    A concrete beam with a cross sectional area of 32X10³ mm² and radius of gyration of 72mm is pre-stressed by a paradolic cable carrying an effective stress of 1000N/mm². The span of the beam is 8m. The cable composed of 6 wires of 7mm diameter has an eccentricity of 50mm at the centre and zero at the support. Neglecting all losses, find the central deflection for a load of 2 KN/m. Calculate also the long term deflection assuming a creep coefficient of 1.8 and a loss ratio of 0.8.

2)    What are the different types of flexural failure modes observed in prestressed concrete beams? Explain with sketches.

3)    A pretensioned prestressed concrete beam having a rectangular section, 150mm  wide and 350mm deep has an effective cover of 50mm. If f_ck = 40N/mm², f_p = 1600N/mm², and the area of prestressing steel A_p =461mm², calculate the ultimate flexural strength of the section using IS:1343 code provisions.

4)    The end block of a post-tensioned prestressed concrete beam, 300mm wide and 300mm deep, is subjected to a concentric anchorage force 832800M by freyssinet anchorage ofarea 11720mm². Design and detail the anchorage reinforcement for the end block.

UNIT – III
Marks: 2

1)    Mention any two losses that occur in prestress concrete.

2)    What is meant by partial prestressing?

3)    Give two application of circular prestressing.

4)    List the different types of joints used between walls and floor slab of PSC tanks.

5)    Sketch the different shapes of PSC water tanks.

Marks: 16

1)    A cylindrical prestressed concrete water tank of internal diameter 30m is required to store water over a depth of 7.5m. The permissible compressive stress in concrete at transfer is 13N/mm² and the minimum compressive stress under working pressure is 1N/mm². The loss ratio is 0.75 wires of 5mm diameter with an initial stress of 1000N/mm² are available for circumferential winding and freyssinet cables made up of 12 wires of 8mm diameter stressed to 1200N/mm² are to be used for vertical prestressing. Design the tank walls assuming the base as fixed. The cube strength of concrete is 40N/mm².

2)    A partially prestressed pre-tensioned mast is to be designed to suit the following data:

Spacing of the pole = 100m

Free – standing height of the pole above the ground = 10m

The pole is to carry twin-conductor high voltage lines (60cm apart) on a cross tree at 9m above ground level.

Conductor size: effective over all diameter = 10mm

Tension in each conductor = 5KN

Poles are to be located in Mangalore

Wind pressure for this zone = 10N/mm²

28-days cube strength of concrete = 50N/mm²

Modulus of elasticity of concrete = 40.5KN/m²

Modulus of rapture of concrete = 5N/mm²

High-tensile wires of 5mm diameter available.

Ultimate tensile strength =1600N/mm²

Loss ratio = 0.7

Permissible stress in concrete under service loads:

Compressive stress in concrete, f_cw = 18N/mm²

Tensile stress in concrete, f_tw = 5N/mm²

3)    Write a note on prestressed concrete poles with its advantages.

4)    Explain in details with sketches about the design consideration of sleepers.

UNIT – IV

Marks : 2

1)    Define the term End block?

2)    Give any two advantages of composite construction.

3)    What is mean by differential shrinkage?

4)    Explain the effect of creep on deflection of prestress concrete member.

5)    Name the types of composite construction.

Marks: 16

1)    A precast pre-tensioned beam of rectangular section beam of rectangular section has a breadth of 100mm and depth of 200mm. The beam, with an effective span of 5m, is prestressed by tendon with their centroids coinciding with the bottom kern. The initial force in tendons is 150KN. The loss of prestress may be assumed to be 15%. The beam is incorporated in a composite T-beam by casting a top flange of breadth 400mm and thickness 40mm. If the composite beam supports a live load of 8KN/m², calculate the resultant stresses developed in precast and in situ cast concrete assuming the pre-tensioned beams as: (a) unpropped and (b) propped during the casting of the slab. Assuming the same modulus of elasticity for concrete in precast beam and in situ slab.

2)    The cross-section of a composite beam is of T-section having a pretensioned rib, 80mm wide and 240mm deep, and an in situ cast slab, 350mm wide and 80mm thick, the pre-tensioned beam is reinforced with eight wires of 5mm diameter with an ultimate tensile strength of 1600N/mm², locate 60mm from the soffit of beam. The compressive strength of concrete in situ cast and precast elements is 20 and 40N/mm² respectively. If adequate reinforcement is provided to prevent shear failure at the interface, estimate the flexural strength of the composite section.

3)    What are the advantages of using composite construction with prestressed and in situ concrete in structural members.

4)    A composite beam of rectangular section is made up of a precast prestressed T-beam having a rib, 100mm by 780mm, and a slab, 450mm wide and 200mm thick. The in situ cast concrete has a thickness of 800mm and a wide of 400mm. The precast T-beam is reinforced with high tensile wires (f_pu = 1600N/mm²) having an area of 800mm² and locate 100mm from the soffit of the beam. If the cube strength of concrete in the in situ cast slab and prestressed beam is 20 and 40N/mm² respectively, estimate the flexural strength of the composite section. The compression zone of the composite beam comprises the precast and the in situ cast element of which the former is about 25 percent and the latter 75 percentage.

UNIT – V

Marks: 2

1)    Give any two advantages of prestressed concrete bridges.

2)    Sketch the typical post tensioned bridge deck commonly used in the construction of bridges.

3)    Name the type of bridge deck use for longer span in bridges. Give reason.

4)    Sketch typical the pre tensioned bridge deck commonly used in the construction of bridges.

5)    What is mean by transmission length?

Marks: 16

1)    What are the advantages of prestressed concrete bridges?

2)    Write notes on prestressed concrete bridge decks.

3)    Explain in detail pre tensioned prestressed concrete bridges with sketches.

4)    Explain in detail post tensioned prestressed concrete bridges with sketches.

5)    Write the various steps involved in design of post tensioned prestressed concrete slab bridge deck.