Regarding basics of prestressed such as inventor, types of prestressing systems, methods of prestressing, types of grouting, types of cables used for prestressed structure and method of construction etc..

1. PRESTRESSED CONCRETE

2. Eugene Freyssinet - Father of
Prestressed concrete.

3. • First prestressed concrete bridge - 1948 -
under the Assam Rail Link Project.

4. Pamban Road Bridge at
Rameshwaram, Tamilnadu,

5. Pre-stressing the metal bands on
wooden barrels

6. Pre-tensioning the spokes in a
bicycle wheel

7. Prestressing of concrete beams by
mild steel rods

8. A prestressed beam under an
external load

9. Variation of length in a prestressed beam

11. Linearly prestressed railway
sleepers

12. Circularly prestressed containment
structure, Kaiga Atomic Power
Station, Karnataka

13. Concrete
• Minimum grades of concrete for
prestressed applications are as follows.
– 30 MPa for post-tensioned members
– 40 MPa for pre-tensioned members.
• Maximum grade of concrete is 60 MPa.

14. Forms of Prestressing Steel
Wires - single unit made of steel.
Strands – 2,3 or 7 wires wound to form a
prestressing strand.
Tendon - group of strands or wires wound to
form a prestressing tendon.

15. Cross-section of a typical tendon

16. Cable - group of tendons form a
prestressing cable.
Bars - A tendon can be made up of a single
steel bar. The diameter of a bar is much
larger than that of a wire.

17. Forms of reinforcing and
prestressing steel

18. Properties of Prestressing Steel
• High strength
• Adequate ductility
• Bendability,
• High bond,
• Low relaxation to reduce losses
• Minimum corrosion.

19. • High tensile steel bars (IS: 2090) -
minimum tensile strength is 980 N/mm2.

20. Nature of Concrete-Steel
Interface
• Bonded tendon
• Unbonded tendon

21. Advantages of Prestressing
Section remains uncracked under
service loads
• Reduction of steel corrosion
• Increase in durability.
• Full section is utilised
• Higher moment of inertia (higher stiffness)

22. • Less deformations (improved
serviceability).
• Increase in shear capacity.
• Suitable for use in pressure vessels, liquid
retaining structures.
• Improved performance (resilience) under
dynamic and fatigue loading.

23. High span-to-depth ratios
(bridges, buildings with large column-free
spaces)

24. Span-to-depth ratios in slabs

25. • Reduction in self weight
• More aesthetic appeal due to slender
sections
• More economical sections.

26. Suitable for precast construction
• Rapid construction
• Better quality control
• Reduced maintenance

27. • Suitable for repetitive construction
• Multiple use of formwork
• Reduction of formwork
• Availability of standard shapes.

28. Typical precast members

29. Limitations of Prestressing
• Prestressing needs skilled technology. •
• use of high strength materials is costly.
• additional cost in auxiliary equipments.
• need for quality control and inspection.

30. Pre-tensioning or Post-
tensioning
Pre-tensioning
• The tension is applied to the tendons
before casting of the concrete.
• The pre-compression is transmitted from
steel to concrete through bond over the
transmission length near the ends.

31. Pre-tensioned electric poles

32. Post-tensioning
• The tension is applied to the tendons
(located in a duct) after hardening of the
concrete.
• The pre-compression is transmitted from
steel to concrete by the anchorage device
(at the end blocks).

33. Post-tensioning of a box girder

34. Stages of the pre-tensioning
operation
• Anchoring of tendons against the end
abutments
• Placing of jacks
• Applying tension to the tendons
• Casting of concrete
• Cutting of the tendons.

35. Applying tension to the tendons

36. Casting of concrete

37. Cutting of the tendons.

38. Advantages of Pre-tensioning
• Pre-tensioning is suitable for precast
members produced in bulk.
• In pre-tensioning large anchorage device
is not present.

39. Disadvantages of Pre-tensioning
• prestressing bed required for the pre-
tensioning operation.
• There is a waiting period in the
prestressing bed, before the concrete
attains sufficient strength.
• There should be good bond between
concrete and steel over the transmission
length.

40. Devices
• Prestressing bed
• End abutments
• Shuttering / mould
• Jack
• Anchoring device
• Harping device (optional)

41. Prestressing bed, end abutment
and mould

43. Chuck assembly for anchoring
tendons

44. Harping of tendons

45. Hold-down anchor for harping of
tendons

46. various stages of the post-
tensioning operation
• Casting of concrete.
• Placement of the tendons.
• Placement of the anchorage block and
jack.
• Applying tension to the tendons.
• Seating of the wedges.
• Cutting of the tendons.

47. Casting of concrete

48. Tensioning of tendons

49. Anchoring the tendon at the
stretching end

50. Advantages of Post-tensioning
• Post-tensioning is suitable for heavy cast-
in-place members.
• The waiting period in the casting bed is
less.
• The transfer of prestress is independent of
transmission length.

51. Disadvantage of Post-tensioning
• requirement of anchorage device and
grouting equipment.

52. Devices
• Casting bed
• Mould/Shuttering
• Ducts

53. • Anchoring devices
• Jacks
• Couplers (optional)
• Grouting equipment (optional).

54. Casting bed, mould and duct

55. Properties of Grout
Grout = water + cement + sand + (water-
reducing admixtures, expansion agent and
pozzolanas.
w/c = 0.5.
Fine sand - used to avoid segregation.

56. Desirable properties of grout
1) Fluidity
2) Minimum bleeding and segregation
3) Low shrinkage
4) Adequate strength after hardening
5) No detrimental compounds
6) Durable.

57. Durability
• Prestressing steel is susceptible to stress
corrosion and hydrogen embrittlement in
aggressive environments.
• Hence, prestressing steel needs to be
adequately protected.

58. • Bonded tendons - alkaline environment of
the grout provides adequate protection.
• Unbonded tendons - corrosion protection
is provided by the following methods:

59. • Epoxy coating
• Mastic wrap (grease impregnated tape)
• Galvanized bars
• Encasing in tubes.

60. Various losses in prestress