Prestressing Concept, Materials
and Prestressing System
Presented by:
Faria Haider(10.01.03.057)
Swat Hossain(10.01.03.058)
Mana Bala(10.01.03.059)
Shafkat Islam(10.01.03.061)
Md...
A prestressed concrete structure is different from a
conventional reinforced concrete structure due to the
application of ...
Force-fitting of metal bands on wooden barrels
The metal bands induce a state of initial hoop compression, to
counteract t...
Pre-tensioning the spokes in a bicycle wheel
The pre-tension of a spoke in a bicycle wheel is applied to
such an extent th...
Basic Concepts of Prestressing
There are three basic concepts of prestressing:
First Concept: Prestressing to transform
co...
First Concept
Prestressing to transform
concrete into an Elastic
Material.
Second Concept:
Prestressing for Combination of
High Strength Steel with
Concrete.
Third Concept:
Prestressing to Achieve
load Balancing.
Advantages of
Prestressing
.
1) Section remains uncracked under service
loads
 Reduction of steel corrosion
 Full section is utilised
Higher moment o...
2) High span-to-depth ratios
Larger spans possible with prestressing
buildings with large column-free spaces)

(bridges,

...
Limitations of Prestressing
 Prestressing needs skilled technology. Hence,
it is not as common as reinforced concrete.
T...
Types of Prestressing
Source of Prestressing Force
1)Hydraulic Prestressing
This is the simplest type of prestressing,
pr...
2)Mechanical Prestressing
In this type of prestressing, the devices includes
weights with or without lever transmission, g...
External or Internal Prestressing
External Prestressing
When the prestressing is achieved by elements
located outside the...
External prestressing of a box girder
Internal Prestressing
When the prestressing is achieved by elements
located inside the concrete member (commonly,
by embed...
Internal prestressing of a box girder
Pre-tensioning or Post-tensioning
Pre-tensioning
The tension is applied to the tendons before
casting of the concrete. Th...
Pre-tensioned electric poles
Post-tensioning

The tension is applied to the tendons (located
in a duct) after hardening of the concrete. The
pre-compre...
Post-tensioning of a box girder
Linear or Circular Prestressing
Linear Prestressing
When the prestressed members are straight or
flat, in the direction o...
Linearly prestressed railway sleepers
Circular Prestressing

When the prestressed members are curved, in the
direction of prestressing, the prestressing is
call...
Circularly prestressed containment structure
Full, Limited or Partial Prestressing
Full Prestressing
When the level of prestressing is such that no tensile
stress is ...
Uniaxial,
Biaxial
Prestressing

or

Multiaxial

Uniaxial Prestressing
When the prestressing tendons are parallel to one a...
Pre-tensioning Systems and Devices
In pretensioning, the tension is applied to
the tendons before casting of the concrete.
Stages of Pre-tensioning
The various stages of the pre-tensioning operation
are summarized as follows.
1) Anchoring of ten...
Stages of pre-tensioning
Advantages
The relative advantages of pre-tensioning as
compared to post-tensioning are as follows.
 Pre-tensioning is su...
Disadvantages of Pre-tensioning
The relative disadvantages are as follows.
 A prestressing bed is required for the preten...
Travelling pre-tensioning stress bench

Anchoring of strands
Stretching of strands
Pouring of concrete

Steam curing chamber
Demoulding of sleeper

Storage
Post-tensioning Systems and Devices
In posttensioning, the tension is applied to the
tendons after hardening of the concre...
Stages of Post-tensioning
The various stages of the post-tensioning operation
are summarized as follows.
1) Casting of con...
Advantages of Post-tensioning
The relative advantages of post-tensioning as
compared to pre-tensioning are as follows:
1)P...
Devices
The essential devices for post-tensioning are as
follows.
1) Casting bed
2) Mould/Shuttering
3) Ducts
4) Anchoring...
Materials
Concrete
Concrete is a composite material composed of
gravels or crushed stones (coarse aggregate), sand
(fine aggregate) ...
Concrete
Aggregate
The coarse aggregate are granular materials
obtained from rocks and crushed stones. They
may be also obtained fr...
The nominal maximum coarse aggregate size is
limited by the lowest of the following
quantities.
1) 1/4 times the minimum t...
Cement
In present day concrete, cement is a mixture of
lime stone and clay heated in a kiln to 1400 –
1600 ºC.
Water
Water used for mixing and curing shall be clean
and free from injurious amounts of oils, acids,
alkalis, salts, suga...
Admixtures
The admixtures can be broadly divided into two
types: chemical admixtures and mineral admixtures.
The common ch...
The common mineral admixtures are as
follows.
1) Fly ash
2) Ground granulated blast-furnace slag
3) Silica fumes
4) Rice h...
Properties of Hardened Concrete
1) High strength
2) Durability
3) Stiffness
4) Minimum shrinkage and creep
High strength
The maximum grade of concrete is 60 MPa.
The minimum grades of concrete for
prestressed applications are as ...
Stiffness of Concrete
The stiffness of concrete is required to estimate
the deflection of members. The stiffness is
given ...
Durability of Concrete
The durability of concrete is of vital importance
regarding the life cycle cost of a structure. The...
Creep of Concrete
Creep of concrete is defined as the increase in
deformation with time under constant load. Due to
the cr...
Shrinkage of Concrete
Shrinkage of concrete is defined as the contraction
due to loss of moisture. The study of shrinkage ...
Grout
Grout is a mixture of water, cement and optional
materials like sand, water-reducing admixtures,
expansion agent and...
The desirable properties of grout are as
follows.
1) Fluidity
2) Minimum bleeding and segregation
3) Low shrinkage
4) Adeq...
Prestressing Steel
The development of prestressed concrete was
influenced by the invention of high strength
steel. It is a...
Wires
A prestressing wire is a single unit made of
steel. The nominal diameters of the wires are
2.5, 3.0, 4.0, 5.0, 7.0 a...
Strands
A few wires are spun together in a helical form to
form a prestressing strand. The different types of
strands are ...
Tendons
A group of strands or wires are placed together
to form a prestressing tendon. The tendons are
used in post-tensio...
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. Bars are ava...
Thank
You
for
Yours
Patience
Prestressing Concept, Materials and Prestressing System - Section B, Group 1
Prestressing Concept, Materials and Prestressing System - Section B, Group 1
Prestressing Concept, Materials and Prestressing System - Section B, Group 1
Prestressing Concept, Materials and Prestressing System - Section B, Group 1
Prestressing Concept, Materials and Prestressing System - Section B, Group 1
Prestressing Concept, Materials and Prestressing System - Section B, Group 1
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Prestressing Concept, Materials and Prestressing System - Section B, Group 1

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Prestressing Concept, Materials and Prestressing System - Section B, Group 1

  1. 1. Prestressing Concept, Materials and Prestressing System
  2. 2. Presented by: Faria Haider(10.01.03.057) Swat Hossain(10.01.03.058) Mana Bala(10.01.03.059) Shafkat Islam(10.01.03.061) Md. Rifat Hassan(09.01.03.008) Shariful Islam(09.01.03.086) Jawad Khalil(09.02.03.096) Istiaq Arefin Tanim(09.02.03.098)
  3. 3. A prestressed concrete structure is different from a conventional reinforced concrete structure due to the application of an initial load on the structure prior to its use. The initial load or ‘prestress’ is applied to enable the structure to counteract the stresses arising during its service period. The prestressing of a structure is not the only instance of prestressing. The concept of prestressing existed before the applications in concrete.
  4. 4. Force-fitting of metal bands on wooden barrels The metal bands induce a state of initial hoop compression, to counteract the hoop tension caused by filling of liquid in the barrels. Force-fitting of metal bands on wooden barrels
  5. 5. Pre-tensioning the spokes in a bicycle wheel The pre-tension of a spoke in a bicycle wheel is applied to such an extent that there will always be a residual tension in the spoke. Spokes in a bicycle wheel
  6. 6. Basic Concepts of Prestressing There are three basic concepts of prestressing: First Concept: Prestressing to transform concrete into an Elastic Material. Second Concept: Prestressing for Combination of High Strength Steel with Concrete. Third Concept: Prestressing to Achieve load Balancing.
  7. 7. First Concept Prestressing to transform concrete into an Elastic Material.
  8. 8. Second Concept: Prestressing for Combination of High Strength Steel with Concrete.
  9. 9. Third Concept: Prestressing to Achieve load Balancing.
  10. 10. Advantages of Prestressing .
  11. 11. 1) Section remains uncracked under service loads  Reduction of steel corrosion  Full section is utilised Higher moment of inertia (higher stiffness) 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.
  12. 12. 2) High span-to-depth ratios Larger spans possible with prestressing buildings with large column-free spaces) (bridges, 3) Suitable for precast construction  Rapid construction  Better quality control  Reduced maintenance  Suitable for repetitive construction  Availability of standard shapes.
  13. 13. Limitations of Prestressing  Prestressing needs skilled technology. Hence, it is not as common as reinforced concrete. The use of high strength materials is costly. There is additional cost in auxiliary equipment. There is need for quality control and inspection.
  14. 14. Types of Prestressing Source of Prestressing Force 1)Hydraulic Prestressing This is the simplest type of prestressing, producing large prestressing forces. The hydraulic jack used for the tensioning of tendons, comprises of calibrated pressure gauges which directly indicate the magnitude of force developed during the tensioning.
  15. 15. 2)Mechanical Prestressing In this type of prestressing, the devices includes weights with or without lever transmission, geared transmission in conjunction with pulley blocks, screw jacks with or without gear drives and wirewinding machines. This type of prestressing is adopted for mass scale production. 3)Electrical Prestressing In this type of prestressing, the steel wires are electrically heated and anchored before placing concrete in the molds. This type of prestressing is also known as thermoelectric prestressing.
  16. 16. External or Internal Prestressing External Prestressing When the prestressing is achieved by elements located outside the concrete, it is called external prestressing. The tendons can lie outside the member (for example in I-girders or walls) or inside the hollow space of a box girder. This technique is adopted in bridges and strengthening of buildings. In the following figure, the box girder of a bridge is prestressed with tendons that lie outside the concrete.
  17. 17. External prestressing of a box girder
  18. 18. Internal Prestressing When the prestressing is achieved by elements located inside the concrete member (commonly, by embedded tendons), it is called internal prestressing. Most of the applications of prestressing are internal prestressing. In the following figure, concrete will be cast around the ducts for placing the tendons.
  19. 19. Internal prestressing of a box girder
  20. 20. 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. The following figure shows manufactured pretensioned electric poles.
  21. 21. Pre-tensioned electric poles
  22. 22. 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). The following figure shows a posttensioned box girder of a bridge.
  23. 23. Post-tensioning of a box girder
  24. 24. Linear or Circular Prestressing Linear Prestressing When the prestressed members are straight or flat, in the direction of prestressing, the prestressing is called linear prestressing. For example, prestressing of beams, piles, poles and slabs. The profile of the prestressing tendon may be curved. The following figure shows linearly prestressed railway sleepers.
  25. 25. Linearly prestressed railway sleepers
  26. 26. Circular Prestressing When the prestressed members are curved, in the direction of prestressing, the prestressing is called circular prestressing. For example, circumferential prestressing of tanks, silos, pipes and similar structures.
  27. 27. Circularly prestressed containment structure
  28. 28. Full, Limited or Partial Prestressing Full Prestressing When the level of prestressing is such that no tensile stress is allowed in concrete under service loads, it is called Full Prestressing. Limited Prestressing When the level of prestressing is such that the tensile stress under service loads is within the cracking stress of concrete, it is called Limited Prestressing. Partial Prestressing When the level of prestressing is such that under tensile stresses due to service loads, the crack width is within the allowable limit, it is called Partial Prestressing.
  29. 29. Uniaxial, Biaxial Prestressing or Multiaxial Uniaxial Prestressing When the prestressing tendons are parallel to one axis, it is called Uniaxial Prestressing. For example, longitudinal prestressing of beams. Biaxial Prestressing When there are prestressing tendons parallel to two axes, it is called Biaxial Prestressing. The following figure shows the biaxial prestressing of slabs. Multiaxial Prestressing When the prestressing tendons are parallel to more than two axes, it is called Multiaxial Prestressing. For example, prestressing of domes.
  30. 30. Pre-tensioning Systems and Devices In pretensioning, the tension is applied to the tendons before casting of the concrete.
  31. 31. Stages of Pre-tensioning The various stages of the pre-tensioning operation are summarized as follows. 1) Anchoring of tendons against the end abutments 2) Placing of jacks 3) Applying tension to the tendons 4) Casting of concrete 5) Cutting of the tendons.
  32. 32. Stages of pre-tensioning
  33. 33. Advantages The relative advantages of pre-tensioning as compared to post-tensioning are as follows.  Pre-tensioning is suitable for precast members produced in bulk.  In pre-tensioning large anchorage device is not present.
  34. 34. Disadvantages of Pre-tensioning The relative disadvantages are as follows.  A prestressing bed is required for the pretensioning 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.
  35. 35. Travelling pre-tensioning stress bench Anchoring of strands
  36. 36. Stretching of strands
  37. 37. Pouring of concrete Steam curing chamber
  38. 38. Demoulding of sleeper Storage
  39. 39. Post-tensioning Systems and Devices In posttensioning, the tension is applied to the tendons after hardening of the concrete.
  40. 40. Stages of Post-tensioning The various stages of the post-tensioning operation are summarized as follows. 1) Casting of concrete. 2) Placement of the tendons. 3) Placement of the anchorage block and jack. 4) Applying tension to the tendons. 5) Seating of the wedges. 6) Cutting of the tendons.
  41. 41. Advantages of Post-tensioning The relative advantages of post-tensioning as compared to pre-tensioning are as follows: 1)Post-tensioning is suitable for heavy cast-inplace members. 2)The waiting period in the casting bed is less. 3)The transfer of prestress is independent of transmission length.
  42. 42. Devices The essential devices for post-tensioning are as follows. 1) Casting bed 2) Mould/Shuttering 3) Ducts 4) Anchoring devices 5) Jacks 6) Couplers (optional) 7) Grouting equipment (optional).
  43. 43. Materials
  44. 44. Concrete Concrete is a composite material composed of gravels or crushed stones (coarse aggregate), sand (fine aggregate) and hydrated cement (binder). It is expected that the student of this course is familiar with the basics of concrete technology.
  45. 45. Concrete
  46. 46. Aggregate The coarse aggregate are granular materials obtained from rocks and crushed stones. They may be also obtained from synthetic material like slag, shale, fly ash and clay for use in lightweight concrete. The sand obtained from river beds or quarries is used as fine aggregate. The fine aggregate along with the hydrated cement paste fill the space between the coarse aggregate.
  47. 47. The nominal maximum coarse aggregate size is limited by the lowest of the following quantities. 1) 1/4 times the minimum thickness of the member 2) Spacing between the tendons/strands minus 5 mm 3) 40 mm.
  48. 48. Cement In present day concrete, cement is a mixture of lime stone and clay heated in a kiln to 1400 – 1600 ºC.
  49. 49. Water Water used for mixing and curing shall be clean and free from injurious amounts of oils, acids, alkalis, salts, sugar, organic materials or other substances that may be deleterious to concrete and steel.
  50. 50. Admixtures The admixtures can be broadly divided into two types: chemical admixtures and mineral admixtures. The common chemical admixtures are as follows. 1) Air-entraining admixtures 2) Water reducing admixtures 3) Set retarding admixtures 4) Set accelerating admixtures 5) Water reducing and set retarding admixtures 6) Water reducing and set accelerating admixtures.
  51. 51. The common mineral admixtures are as follows. 1) Fly ash 2) Ground granulated blast-furnace slag 3) Silica fumes 4) Rice husk ash 5) Metakoline
  52. 52. Properties of Hardened Concrete 1) High strength 2) Durability 3) Stiffness 4) Minimum shrinkage and creep
  53. 53. High strength The maximum grade of concrete is 60 MPa. The minimum grades of concrete for prestressed applications are as follows. 1)30 MPa for post-tensioned members 2)40 MPa for pre-tensioned members.
  54. 54. Stiffness of Concrete The stiffness of concrete is required to estimate the deflection of members. The stiffness is given by the modulus of elasticity.
  55. 55. Durability of Concrete The durability of concrete is of vital importance regarding the life cycle cost of a structure. The life cycle cost includes not only the initial cost of the materials and labour, but also the cost of maintenance and repair.
  56. 56. Creep of Concrete Creep of concrete is defined as the increase in deformation with time under constant load. Due to the creep of concrete, the prestress in the tendon is reduced with time. Hence, the study of creep is important in prestressed concrete to calculate the loss in prestress. The creep occurs due to two causes. 1. Rearrangement of hydrated cement paste (especially the layered products) 2. Expulsion of water from voids under load
  57. 57. Shrinkage of Concrete Shrinkage of concrete is defined as the contraction due to loss of moisture. The study of shrinkage is also important in prestressed concrete to calculate the loss in prestress. The shrinkage occurs due to two causes. 1. Loss of water from voids 2. Reduction of volume during carbonation
  58. 58. Grout Grout is a mixture of water, cement and optional materials like sand, water-reducing admixtures, expansion agent and pozzolans. The water-tocement ratio is around 0.5. Fine sand is used to avoid segregation.
  59. 59. The desirable properties of grout are as follows. 1) Fluidity 2) Minimum bleeding and segregation 3) Low shrinkage 4) Adequate strength after hardening 5) No detrimental compounds 6) Durable.
  60. 60. Prestressing Steel The development of prestressed concrete was influenced by the invention of high strength steel. It is an alloy of iron, carbon, manganese and optional materials. In addition to prestressing steel, conventional non-prestressed reinforcement is used for flexural capacity (optional), shear capacity, temperature and shrinkage requirements.
  61. 61. Wires A prestressing wire is a single unit made of steel. The nominal diameters of the wires are 2.5, 3.0, 4.0, 5.0, 7.0 and 8.0 mm. The different types of wires are as follows. 1) Plain wire: No indentations on the surface. 2) Indented wire: There are circular or elliptical indentations on the surface.
  62. 62. Strands A few wires are spun together in a helical form to form a prestressing strand. The different types of strands are as follows. 1) Two-wire strand: Two wires are spun together to form the strand. 2) Three-wire strand: Three wires are spun together to form the strand. 3) Seven-wire strand: In this type of strand, six wires are spun around a central wire. The central wire is larger than the other wires.
  63. 63. Tendons A group of strands or wires are placed together to form a prestressing tendon. The tendons are used in post-tensioned members. Cables A group of tendons form a prestressing cable. The cables are used in bridges
  64. 64. 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. Bars are available in the following sizes: 10, 12, 16, 20, 22, 25, 28 and 32 mm.
  65. 65. Thank You for Yours Patience

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