120981999 Prestressed Concrete

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SpokesSpokes

Figure 1-1.2 Pre-tensioning the spokes in a bicycle wheel

For concrete, internal stresses are induced (usually, by means of tensioned steel) for

the following reasons.

The tensile strength of concrete is only about 8% to 14% of its compressive

strength.

Cracks tend to develop at early stages of loading in flexural members such as

beams and slabs.

To prevent such cracks, compressive force can be suitably applied in the

perpendicular direction.

Prestressing enhances the bending, shear and torsional capacities of the flexural

members.

In pipes and liquid storage tanks, the hoop tensile stresses can be effectively

counteracted by circular prestressing.

1.1.2 Early Attempts of Prestressing

Prestressing of structures was introduced in late nineteenth century. The following

sketch explains the application of prestress.

Place and stretch mild steel rods, prior to concreting

Release the tension and cut the rods after concreting

Place and stretch mild steel rods, prior to concreting

Release the tension and cut the rods after concreting

Figure 1-1.3 Prestressing of concrete beams by mild steel rods


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Mild steel rods are stretched and concrete is poured around them. After hardening of

concrete, the tension in the rods is released. The rods will try to regain their original

length, but this is prevented by the surrounding concrete to which the steel is bonded.

Thus, the concrete is now effectively in a state of pre-compression. It is capable of

counteracting tensile stress, such as arising from the load shown in the following sketch.

Figure 1-1.4 A prestressed beam under an external load

But, the early attempts of prestressing were not completely successful. It was observed

that the effect of prestress reduced with time. The load resisting capacities of the

members were limited. Under sustained loads, the members were found to fail. This

was due to the following reason.

Concrete shrinks with time. Moreover under sustained load, the strain in concrete

increases with increase in time. This is known as creep strain. The reduction in length

due to creep and shrinkage is also applicable to the embedded steel, resulting in

significant loss in the tensile strain.

In the early applications, the strength of the mild steel and the strain during prestressing

were less. The residual strain and hence, the residual prestress was only about 10% of

the initial value. The following sketches explain the phenomena.


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Original length of steel rod (L1)

Original length of concrete beam (L2

)

Original length of steel rod (L1)

Original length of concrete beam (L2

)

a) Beam before applying prestress

Reduced length of concrete beam (L3)Reduced length of concrete beam (L3)

b) Beam at transfer of prestress

Final length of prestressed beam (L4)Final length of prestressed beam (L4)

c) Beam after long-term losses of prestress

Figure 1-1.5 Variation of length in a prestressed beam

The residual strain in steel = original tensile strain in steel compressive strainscorresponding to short-term and long-term losses.

Original tensile strain in steel = (L2 L1)/L1

Compressive strain due to elastic shortening of beam = (L2 L3)/L1

(short-term loss in prestress)

Compressive strain due to creep and shrinkage = (L3 L4)/L1

(long-term losses in prestress)

Therefore, residual strain in steel = (L4 L1)/L1

The maximum original tensile strain in mild steel = Allowable stress / elastic

modulus

= 140 MPa / 2105MPa

= 0.0007


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The total loss in strain due to elastic shortening, creep and shrinkage was also close to

0.0007. Thus, the residual strain was negligible.

The solution to increase the residual strain and the effective prestress are as follows.

Adopt high st rength steelwith much higher original strain. This leads to the

scope of high prestressing force. Adopt high strength concreteto withstand the high prestressing force.

1.1.3 Brief History

Before the development of prestressed concrete, two significant developments of

reinforced concrete are the invention of Portland cement and introduction of steel in

concrete. These are also mentioned as the part of the history. The key developments

are mentioned next to the corresponding year.

1824 Aspdin, J., (England)

Obtained a patent for the manufacture of Portland cement.

1857 Monier, J., (France)

Introduced steel wires in concrete to make flower pots, pipes, arches and slabs.

The following events were significant in the development of prestressed concrete.

1886 Jackson, P. H., (USA)

Introduced the concept of tightening steel tie rods in artificial stone and concrete

arches.

Figure 1-1.6 Steel tie rods in arches

1888 Doehring, C. E. W., (Germany)

Manufactured concrete slabs and small beams with embedded tensioned steel.


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1908 Stainer, C. R., (USA)

Recognised losses due to shrinkage and creep, and suggested retightening the

rods to recover lost prestress.

1923 Emperger, F., (Austria)

Developed a method of winding and pre- tensioning high tensile steel wiresaround concrete pipes.

1924 Hewett, W. H., (USA)

Introduced hoop-stressed horizontal reinforcement around walls of concrete

tanks through the use of turnbuckles.

Thousands of liquid storage tanks and concrete pipes were built in the two decades to

follow.

1925 Dill, R. H., (USA)

Used high strength unbonded steel rods. The rods were tensioned and anchored

after hardening of the concrete.

Figure 1-1.7 Portrait of Eugene Freyssinet

(Reference: Collins, M. P. and Mitchell, D.,Prestressed Concrete Structures)

1926 Eugene Freyssinet (France)

Used high tensile steel wires, with ultimate strength as high as 1725 MPa and

yield stress over 1240 MPa. In 1939, he developed conical wedges for end

anchorages for post-tensioning and developed double-acting jacks. He is often

referred to as the Father of Prestressed concrete.


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1938 Hoyer, E., (Germany)

Developed long line pre-tensioning method.

1940 Magnel, G., (Belgium)

Developed an anchoring system for post-tensioning, using flat wedges.

During the Second World War, applications of prestressed and precast concrete

increased rapidly. The names of a few persons involved in developing prestressed

concrete are mentioned. Guyon, Y., (France) built numerous prestressed concrete

bridges in western and central Europe. Abeles, P. W., (England) introduced the

concept of partial prestressing. Leonhardt, F., (Germany), Mikhailor, V., (Russia) and

Lin, T. Y., (USA) are famous in the field of prestressed concrete.

The International Federation for Prestressing (FIP), a professional organisation in

Europe was established in 1952. The Precast/Prestressed Concrete Institute (PCI) was

established in USA in 1954.

Prestressed concrete was started to be used in building frames, parking structures,

stadiums, railway sleepers, transmission line poles and other types of structures and

elements.

In India, the applications of prestressed concrete diversified over the years. The first

prestressed concrete bridge was built in 1948 under the Assam Rail Link Project.

Among bridges, the Pamban Road Bridge at Rameshwaram, Tamilnadu, remains a

classic example of the use of prestressed concrete girders.


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Figure 1-1.8 Pamban Road Bridge at Rameshwaram, Tamilnadu

(Reference: http://www.ramnad.tn.nic.in)

1.1.4 Development of Building Materials

The development of prestressed concrete can be studied in the perspective of

traditional building materials. In the ancient period, stones and bricks were extensively

used. These materials are strong in compression, but weak in tension. For tension,

bamboos and coir ropes were used in bridges. Subsequently iron and steel bars were

used to resist tension. These members tend to buckle under compression. Wood and

structural steel members were effective both in tension and compression.

In reinforced concrete, concrete and steel are combined such that concrete resists

compression and steel resists tension. This is a passive combination of the two

materials. In prestressed concrete high strength concrete and high strength steel are

combined such that the full section is effective in resisting tension and compression.

This is an activecombination of the two materials. The following sketch shows the use

of the different materials with the progress of time.


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Compression (C) Tension (T) C and T

Stones, Bricks Bamboo, Ropes Timber

Structural steelSteel bars, wires

Reinforced

Concrete

Prestressed

Concrete

Passive combination

High StrengthSteel

High StrengthConcrete

Active combination

Concrete

Compression (C) Tension (T) C and T

Stones, Bricks Bamboo, Ropes Timber

Structural steelSteel bars, wires

Reinforced

Concrete

Prestressed

Concrete

Passive combination

High StrengthSteel

High StrengthConcrete

Active combination

Concrete

Figure 1-1.9 Development of building materials

(Reference: Lin, T. Y. and Burns, N. H.,

Design of Prestressed Concrete Structures)


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1.2 Advantages and Types of Prestressing

This section covers the following topics.

Definitions

Advantages of Prestressing

Limitations of Prestressing

Types of Prestressing

1.2.1 Definitions

The terms commonly used in prestressed concrete are explained. The terms are placed

in groups as per usage.

Forms of Prestressing Steel

Wires

Prestressing wire is a single unit made of steel.

Strands

Two, three or seven wires are wound to form a prestressing strand.

Tendon

A group of strands or wires are wound to form a prestressing tendon.

Cable

A 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.

The different types of prestressing steel are further explained in Section 1.7,

Prestressing Steel.

Nature of Concrete-Steel Interface

Bonded tendon

When there is adequate bond between the prestressing tendon and concrete, it is called

a bonded tendon. Pre-tensioned and grouted post-tensioned tendons are bonded

tendons.
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Unbonded tendon

When there is no bond between the prestressing tendon and concrete, it is called

unbonded tendon. When grout is not applied after post-tensioning, the tendon is an

unbonded tendon.

Stages of LoadingThe analysis of prestressed members can be different for the different stages of loading.

The stages of loading are as follows.

1) Initial : It can be subdivided into two stages.

a) During tensioning of steel

b) At transfer of prestress to concrete.

2) Intermediate : This includes the loads during transportation of the

prestressed members.

3) Final : It can be subdivided into two stages.

a) At service, during operation.

b) At ultimate, during extreme events.

1.2.2 Advantages of Prestressing

The prestressing of concrete has several advantages as compared to traditional

reinforced concrete (RC) without prestressing. A fully prestressed concrete member is

usually subjected to compression during service life. This rectifies several deficiencies

of concrete.

The following text broadly mentions the advantages of a prestressed concrete member

with an equivalent RC member. For each effect, the benefits are listed.

1) Section remains uncracked under service loads

Reduction of steel corrosion

Increase in durability.

Full section is utilised

Higher moment of inertia (higher stiffness)

Less deformations (improved serviceability).


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Increase in shear capacity.

Suitable for use in pressure vessels, liquid retaining structures.

Improved performance (resilience) under dynamic and fatigue loading.

2) High span-to-depth ratios

Larger spans possible with prestressing (bridges, buildings with large column-free

spaces)Typical values of span-to-depth ratios in slabs are given below.

Non-prestressed slab 28:1

Prestressed slab 45:1

For the same span, less depth compared to RC member.

Reduction in self weight

More aesthetic appeal due to slender sections

More economical sections.

3) Suitable for precast construction

The advantages of precast construction are as follows.

Rapid construction

Better quality control

Reduced maintenance

Suitable for repetitive construction

Multiple use of formwork

Reduction of formwork

Availability of standard shapes.

The following figure shows the common types of precast sections.


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Double T-sectionT-section

Hollow core Piles

Double T-sectionDouble T-sectionT-sectionT-section

Hollow core Piles

L-section Inverted T-section I-girdersL-section Inverted T-section I-girders

Figure 1-2.1 Typical precast members

1.2.3 Limi tations of Prestressing

Although prestressing has advantages, some aspects need to be carefully addressed.

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 equipments.

There is need for quality control and inspection.

1.2.4 Types of Prestressing

Prestressing of concrete can be classified in several ways. The following classifications

are discussed.

Source of prestressing force

This classification is based on the method by which the prestressing force is generated.

There are four sources of prestressing force: Mechanical, hydraulic, electrical and

chemical.


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External or in ternal prestressing

This classification is based on the location of the prestressing tendon with respect to the

concrete section.

Pre-tensioning or post-tensioning

This is the most important classification and is based on the sequence of casting theconcrete and applying tension to the tendons.

Linear or circular prestressing

This classification is based on the shape of the member prestressed.

Full, limited or partial prestressing

Based on the amount of prestressing force, three types of prestressing are defined.

Uniaxial, biaxial or multi-axial prestressing

As the names suggest, the classification is based on the directions of prestressing a

member.

The individual types of prestressing are explained next.

Source of Prestressing Force

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.

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 wire-winding machines. This type of prestressing is adopted for

mass scale production.


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Electrical Prestressing

In this type of prestressing, the steel wires are electrically heated and anchored before

placing concrete in the moulds. This type of prestressing is also known as thermo-

electric prestressing.

External or Internal PrestressingExternal 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.

Figure 1-2.2 External prestressing of a box girder

(Reference: VSL International Ltd.)

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.


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Figure 1-2.3 Internal prestressing of a box girder

(Courtesy: Cochin Port Trust, Kerala)

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 pre-tensioned electric

poles.

Figure 1-2.4 Pre-tensioned electric poles

(Courtesy: The Concrete Products and Construction Company, COPCO, Chennai)


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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 post-tensioned box girder of a bridge.

Figure 1-2.5 Post-tensioning of a box girder


The details of pre-tensioning and post-tensioning are covered under Section 1.3, Pre-

tensioning Systems and Devices, and Section 1.4, Post-tensioning Systems and

Devices, respectively.

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.


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Figure 1-2.6 Linearly prestressed railway sleepers

(Courtesy: The Concrete Products and Construction Company, COPCO, Chennai)

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. The following figure shows the containment

structure for a nuclear reactor which is circularly prestressed.

Figure 1-2.7 Circularly prestressed containment structure, Kaiga Atomic PowerStation, Karnataka

(Reference: Larsen & Toubro Ltd, ECC Division, 60 Landmark Years)


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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 (Type 1, as per IS:1343 - 1980).

Limited PrestressingWhen 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 (Type 2).

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 (Type 3).

Uniaxial, Biaxial or Multiaxial Prestressing

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.

Duct forprestressingtendon

Non-prestressed reinforcement

Duct forprestressingtendon

Non-prestressed reinforcement

Figure 1-2.8 Biaxial prestressing of a slab

(Courtesy: VSL India Pvt. Ltd., Chennai)


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Multiaxial Prestressing

When the prestressing tendons are parallel to more than two axes, it is called Multiaxial

Prestressing. For example, prestressing of domes.


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1.3 Pre-tensioning Systems and Devices


Introduction

Stages of Pre-tensioning

Advantages of Pre-tensioning

Disadvantages of Pre-tensioning

Devices

Manufacturing of Pre-tensioned Railway Sleepers

1.3.1 Introduction

Prestressing systems have developed over the years and various companies have

patented their products. Detailed information of the systems is given in the product

catalogues and brochures published by companies. There are general guidelines of

prestressing in Section 12of IS:1343 - 1980. The information given in this section is

introductory in nature, with emphasis on the basic concepts of the systems.

The prestressing systems and devices are described for the two types of prestressing,

pre-tensioning and post-tensioning, separately. This section covers pre-tensioning.

Section 1.4, Post-tensioning Systems and Devices, covers post-tensioning. In pre-

tensioning, the tension is applied to the tendons before casting of the concrete. The

stages of pre-tensioning are described next.

1.3.2 Stages of Pre-tensioning

In pre-tensioning system, the high-strength steel tendons are pulled between two end

abutments (also called bulkheads) prior to the casting of concrete. The abutments are

fixed at the ends of a prestressing bed.

Once the concrete attains the desired strength for prestressing, the tendons are cut

loose from the abutments.
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The prestress is transferred to the concrete from the tendons, due to the bond between

them. During the transfer of prestress, the member undergoes elastic shortening. If the

tendons are located eccentrically, the member is likely to bend and deflect (camber).

The various stages of the pre-tensioning operation are summarised as follows.

1) Anchoring of tendons against the end abutments

2) Placing of jacks3) Applying tension to the tendons

4) Casting of concrete

5) Cutting of the tendons.

During the cutting of the tendons, the prestress is transferred to the concrete with elastic

shortening and camber of the member.

The stages are shown schematically in the following figures.

Prestressing bed

Steel tendon

Endabutment

Jack

Prestressing bed

Steel tendon

Endabutment

Jack

(a) Applying tension to tendons

(b) Casting of concrete

Cutting of tendonCutting of tendon

(c) Transferring of prestress

Figure1-3.1 Stages of pre-tensioning

1.3.3 Advantages of Pre-tensioning

The relative advantages of pre-tensioning as compared to post-tensioning are as

follows.

Pre-tensioning is suitable for precast members produced in bulk.


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In pre-tensioning large anchorage device is not present.

1.3.4Disadvantages of Pre-tensioning

The relative disadvantages are as follows.

A prestressing bed is 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.

1.3.5 Devices

The essential devices for pre-tensioning are as follows.

Prestressing bed

End abutments

Shuttering / mould

Jack

Anchoring device

Harping device (optional)

Prestressing Bed, End Abutments and Mould

The following figure shows the devices.

Prestressing bed

Mould

Endabutment

Jack

Anchoringdevice

Prestressing bed

Mould

Endabutment

Jack

Anchoringdevice

Prestressing bed

Mould

Endabutment

Jack

Anchoringdevice

Figure1-3.2 Prestressing bed, end abutment and mould

An extension of the previous system is the Hoyer system. This system is generally

used for mass production. The end abutments are kept sufficient distance apart, and

several members are cast in a single line. The shuttering is provided at the sides and


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between the members. This system is also called the Long Line Method. The

following figure is a schematic representation of the Hoyer system

Prestressing bed

A series of moulds

Prestressing bed

A series of moulds

Figure 1-3.3 Schematic representation of Hoyer system

The end abutments have to be sufficiently stiff and have good foundations. This is

usually an expensive proposition, particularly when large prestressing forces are

required. The necessity of stiff and strong foundation can be bypassed by a simpler

solution which can also be a cheaper option. It is possible to avoid transmitting the

heavy loads to foundations, by adopting self-equilibrating systems. This is a common

solution in load-testing. Typically, this is done by means of a tension frame. The

following figure shows the basic components of a tension frame. The jack and the

specimen tend to push the end members. But the end members are kept in place by

members under tension such as high strength steel rods.

P

Free bodiesPlan or Elevation

TestspecimenHigh

strengthsteel rods

Loading

jack

P

Free bodies

P

Free bodiesPlan or Elevation

TestspecimenHigh

strengthsteel rods

Loading

jack

Plan or Elevation

TestspecimenHigh

strengthsteel rods

Loading

jack

Figure 1-3.4 A tension frame

The frame that is generally adopted in a pre-tensioning system is called a stress bench.

The concrete mould is placed within the frame and the tendons are stretched and

anchored on the booms of the frame. The following figures show the components of astress bench.


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Jack

Threaded rodElevation

Plan

Mould Strands

Jack


Jack


Plan

Mould Strands

Plan

Mould Strands

Figure 1-3.5 Stress bench Self straining frame

The following figure shows the free body diagram by replacing the jacks with the applied

forces.

Plan

Load by jack

Tension instrands

Plan

Load by jack

Tension instrands

Figure 1-3.6 Free body diagram of stress bench

The following figure shows the stress bench after casting of the concrete.

Elevation

Plan

Elevation

Plan

Figure 1-3.7 The stress bench after casting concrete


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Jacks

The jacks are used to apply tension to the tendons. Hydraulic jacks are commonly used.

These jacks work on oil pressure generated by a pump. The principle behind the design

of jacks is Pascals law. The load applied by a jack is measured by the pressure

reading from a gauge attached to the oil inflow or by a separate load cell. The following

figure shows a double acting hydraulic jack with a load cell.

Figure 1-3.8 A double acting hydraulic jack with a load cell

Anchor ing Devices

Anchoring devices are often made on the wedge and friction principle. In pre-tensioned

members, the tendons are to be held in tension during the casting and hardening of

concrete. Here simple and cheap quick-release grips are generally adopted. The

following figure provides some examples of anchoring devices.


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Figure 1-3.9 Chuck assembly for anchoring tendons

(Reference: Lin, T. Y. and Burns, N. H.,

Design of Prestressed Concrete Structures)

Harping Devices

The tendons are frequently bent, except in cases of slabs-on-grade, poles, piles etc.

The tendons are bent (harped) in between the supports with a shallow sag as shown

below.

Harping point Hold up device

a) Before casting of concrete

Harping point Hold up device

a) Before casting of concretea) Before casting of concrete

b) After casting of concreteb) After casting of concrete

Figure 1-3.10 Harping of tendons

The tendons are harped using special hold-down devices as shown in the following

figure.


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Figure 1-3.11 Hold-down anchor for harping of tendons

(Reference: Nawy, E. G., Prestressed Concrete: A Fundamental Approach)

1.3.6 Manufacturing of Pre-tensioned Railway Sleepers

The following photos show the sequence of manufacturing of pre-tensioned railway

sleepers (Courtesy: The Concrete Products and Construction Company, COPCO,

Chennai). The steel strands are stretched in a stress bench that can be moved on

rollers. The stress bench can hold four moulds in a line. The anchoring device holds

the strands at one end of the stress bench. In the other end, two hydraulic jacks push a

plate where the strands are anchored. The movement of the rams of the jacks and the

oil pressure are monitored by a scale and gauges, respectively. Note that after the

extension of the rams, the gap between the end plate and the adjacent mould has

increased. This shows the stretching of the strands.


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Meanwhile the coarse and fine aggregates are batched, mixed with cement, water and

additives in a concrete mixer. The stress bench is moved beneath the concrete mixer.

The concrete is poured through a hopper and the moulds are vibrated. After the

finishing of the surface, the stress bench is placed in a steam curing chamber for a few

hours till the concrete attains a minimum strength.

The stress bench is taken out from the chamber and the strands are cut. The sleepers

are removed from the moulds and stacked for curing in water. After the complete curing,

the sleepers are ready for dispatching.

(a) Travelling pre-tensioning stress bench


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Wedge andcylinderassembly atthe dead end

Wedge andcylinderassembly atthe dead end

(b) Anchoring of strands

Hydraulic jack atstretching end

Initial gap

Endplate

Hydraulic jack atstretching end

Initial gap

Endplate

(c) Stretching of strands


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Extension of ram

Final gap

Threaded

rod

Extension of ram

Final gap

Threaded

rod

(d) Stretching of strands

Coarse aggregate

Fine aggregate

Coarse aggregate

Fine aggregate

(e) Material storage


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Automatedbatchingby weight

Automatedbatchingby weight

(f) Batching of materials

Hopper belowconcrete mixerHopper belowconcrete mixer

(g) Pouring of concrete


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(h) Concrete after vibration of mould

(i) Steam curing chamber


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(j) Cutting of strands

(k) Demoulding of sleeper


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(l) Stacking of sleeper

(m) Water curing


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(n) Storage and dispatching of sleepers

Figure 1-3.12 Manufacturing of pre-tensioned railway sleepers


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1.4 Post-tensioningSystems and Devices

This section covers the following topics

Introduction

Stages of Post-tensioning

Advantages of Post-tensioning

Disadvantages of Post-tensioning

Devices

Manufacturing of a Post-tensioned Bridge Girder

1.4.1 Introduction

Prestressing systems have developed over the years and various companies have

patented their products. Detailed information of the systems is given in the product

catalogues and brochures published by companies. There are general guidelines of

prestressing in Section 12of IS 1343: 1980. The information given in this section is

introductory in nature, with emphasis on the basic concepts of the systems.

The prestressing systems and devices are described for the two types of prestressing,

pre-tensioning and post-tensioning, separately. This section covers post-tensioning.

Section 1.3, Pre-tensioning Systems and Devices, covers pre-tensioning. In post-

tensioning, the tension is applied to the tendons after hardening of the concrete. The

stages of post-tensioning are described next.

1.4.2 Stages of Post-tensioning

In post-tensioning systems, the ducts for the tendons (or strands) are placed along with

the reinforcement before the casting of concrete. The tendons are placed in the ducts

after the casting of concrete. The duct prevents contact between concrete and the

tendons during the tensioning operation.

Unlike pre-tensioning, the tendons are pulled with the reaction acting against the

hardened concrete.
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If the ducts are filled with grout, then it is known as bonded post-tensioning. The grout

is a neat cement paste or a sand-cement mortar containing suitable admixture. The

grouting operation is discussed later in the section. The properties of grout are

discussed in Section 1.6, Concrete (Part-II).

In unbonded post-tensioning, as the name suggests, the ducts are never grouted andthe tendon is held in tension solely by the end anchorages. The following sketch shows

a schematic representation of a grouted post-tensioned member. The profile of the duct

depends on the support conditions. For a simply supported member, the duct has a

sagging profile between the ends. For a continuous member, the duct sags in the span

and hogs over the support.

Figure 1-4.1 Post-tensioning (Reference: VSL International Ltd.)

Among the following figures, the first photograph shows the placement of ducts in a box

girder of a simply supported bridge. The second photograph shows the end of the box

girder after the post-tensioning of some tendons.

Figure 1-4.2 Post-tensioning ducts in a box girder



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Figure 1-4.3 Post-tensioning of a box girder


The various stages of the post-tensioning operation are summarised 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.

The stages are shown schematically in the following figures. After anchoring a tendon

at one end, the tension is applied at the other end by a jack. The tensioning of tendons

and pre-compression of concrete occur simultaneously. A system of self-equilibrating

forces develops after the stretching of the tendons.


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Casting bed

Duct

Side viewCasting bed

Duct

Side view

(a) Casting of concrete

JackJack

(b) Tensioning of tendons

AnchorAnchor

(c)Anchoring the tendon at the stretching end

Figure 1-4.4 Stages of post-tensioning (shown in elevation)

1.4.3 Advantages of Post-tensioning

The relative advantages of post-tensioning as compared to pre-tensioning are as

follows.

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.

1.4.4 Disadvantage of Post-tensioning

The relative disadvantage of post-tensioning as compared to pre-tensioning is the

requirement of anchorage device and grouting equipment.

1.4.5 Devices

The essential devices for post-tensioning are as follows.

1) Casting bed

2) Mould/Shuttering

3) Ducts


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4) Anchoring devices

5) Jacks

6) Couplers (optional)

7) Grouting equipment (optional).

Casting Bed, Mould and DuctsThe following figure shows the devices.

Casting bed

Mould

Duct

Casting bed

Mould

Duct

Figure 1-4.5 Casting bed, mould and duct

Anchor ing Devices

In post-tensioned members the anchoring devices transfer the prestress to the concrete.

The devices are based on the following principles of anchoring the tendons.

1) Wedge action

2) Direct bearing

3) Looping the wires

Wedge action

The anchoring device based on wedge action consists of an anchorage block and

wedges. The strands are held by frictional grip of the wedges in the anchorage block.

Some examples of systems based on the wedge-action are Freyssinet, Gifford-Udall,

Anderson and Magnel-Blaton anchorages. The following figures show some patented

anchoring devices.


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Figure 1-4.6 Freyssinet T system anchorage cones

(Reference: Lin, T. Y. and Burns, N. H., Design of Prestressed Concrete Structures)

Figure 1-4.7 Anchoring devices

(Reference: Collins, M. P. and Mitchell, D., Prestressed Concrete Structures)

Figure 1-4.8 Anchoring devices (Reference: VSL International Ltd)


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Direct bearing

The rivet or bolt heads or button heads formed at the end of the wires directly bear

against a block. The B.B.R.V post-tensioning system and the Prescon system are

based on this principle. The following figure shows the anchoring by direct bearing.

Figure 1-4.9 Anchoring with button heads


Looping the wires

The Baur-Leonhardt system, Leoba system and also the Dwidag single-bar anchorage

system, work on this principle where the wires are looped around the concrete. The

wires are looped to make a bulb. The following photo shows the anchorage by looping

of the wires in a post-tensioned slab.

Figure 1-4.10 Anchorage by looping the wires in a slab

(Courtesy : VSL India Pvt. Ltd.)

The anchoring devices are tested to calculate their strength. The following photo shows

the testing of an anchorage block.


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Figure 1-4.11 Testing of an anchorage device

Sequence of Anchoring

The following figures show the sequence of stressing and anchoring the strands. The

photo of an anchoring device is also provided.


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Figure 1-4.12 Sequence of anchoring

(Reference: VSL International Ltd.)

Figure 1-4.13 Final form of an anchoring device

(Reference: VSL International Ltd)

JacksThe working of a jack and measuring the load were discussed in Section 1.3, Pre-

tensioning Systems and Devices. The following figure shows an extruded sketch of the

anchoring devices.


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Figure 1-4.14 Jacking and anchoring with wedges


Couplers

The couplers are used to connect strands or bars. They are located at the junction of

the members, for example at or near columns in post-tensioned slabs, on piers in post-

tensioned bridge decks.

The couplers are tested to transmit the full capacity of the strands or bars. A few types

of couplers are shown.

Figure 1-4.15 Coupler for strands

(Reference: VSL International Ltd)


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Figure 1-4.16 Couplers for strands

(Reference: Dywidag Systems International)

Figure 1-4.17 Couplers for strands

(Reference: Dywidag Systems International)


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GroutingGrouting can be defined as the filling of duct, with a material that provides an anti-

corrosive alkaline environment to the prestressing steel and also a strong bond between

the tendon and the surrounding grout.

The major part of grout comprises of water and cement, with a water-to-cement ratio ofabout 0.5, together with some water-reducing admixtures, expansion agent and

pozzolans. The properties of grout are discussed in Section 1.6, Concrete (Part-II).

The following figure shows a grouting equipment, where the ingredients are mixed and

the grout is pumped.

Figure 1-4.18 Grouting equipment

(Reference: Williams Form Engineering Corp.)

1.4.6 Manufacturing of Post-tensioned Bridge Girders

The following photographs show some steps in the manufacturing of a post-tensioned I-

girder for a bridge (Courtesy: Larsen & Toubro). The first photo shows the fabricated

steel reinforcement with the ducts for the tendons placed inside. Note the parabolic

profiles of the duct for the simply supported girder. After the concrete is cast and cured

to gain sufficient strength, the tendons are passed through the ducts, as shown in the

second photo. The tendons are anchored at one end and stretched at the other end by

a hydraulic jack. This can be observed from the third photo.


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(a) Fabrication of reinforcement

(b) Placement of tendons


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(c) Stretching and anchoring of tendons

Figure 1-4.19 Manufacturing of a post-tensioned bridge I-girder

The following photos show the construction of post-tensioned box girders for a bridge

(Courtesy: Cochin Port Trust). The first photo shows the fabricated steel reinforcement

with the ducts for the tendons placed inside. The top flange will be constructed later.

The second photo shows the formwork in the pre-casting yard. The formwork for the

inner sides of the webs and the flanges is yet to be placed. In the third photo a girder is

being post-tensioned after adequate curing. The next photo shows a crane on a barge

that transports a girder to the bridge site. The completed bridge can be seen in the last

photo.

(a) Reinforcement cage for box girder


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(b) Formwork for box girder

(c) Post-tensioning of box girder


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(d) Transporting of box girder

(e) Completed bridge

Figure 1-4.20 Manufacturing of post-tensioned bridge box girders


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1.5 Concrete (Part I)


Constituents of Concrete

Properties ofHardened Concrete(Part I)

1.5.1 Constituents of Concrete

Introduction

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. Only the

information pertinent to prestressed concrete design is presented here.

The following figure shows a petrographic section of concrete. Note the scattered

coarse aggregates and the matrix surrounding them. The matrix consists of sand,

hydrated cement and tiny voids.

Figure 1-5.1 Petrographic section of hardened concrete

(Reference: Portland Cement Association, Design and Control of Concrete Mixtures)

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 light-weight concrete.
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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.

The important properties of aggregate are as follows.

1) Shape and texture2) Size gradation

3) Moisture content

4) Specific gravity

5) Unit weight

6) Durability and absence of deleterious materials.

The requirements of aggregate is covered in Section 4.2 of IS:1343 - 1980.

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.

The deleterious substances that should be limited in aggregate are clay lumps, wood,

coal, chert, silt, rock dust (material finer than 75 microns), organic material, unsound

and friable particles.

Cement

In present day concrete, cement is a mixture of lime stone and clay heated in a kiln to

1400 - 1600C. The types of cement permitted by IS:1343 - 1980 (Clause 4.1) for

prestressed applications are the following. The information is revised as per IS:456 -

2000, Plain and Reinforced Concrete Code of Practice.

1) Ordinary Portland cement confirming to IS:269 - 1989, Ordinary Portland Cement,

33 Grade Specification.

2) Portland slag cement confirming to IS:455 - 1989, Portland Slag Cement

Specification, but with not more than 50% slag content.

3) Rapid-hardening Portland cement confirming to IS:8041 - 1990, Rapid Hardening

Portland Cement Specification.


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WaterThe water should satisfy the requirements of Section 5.4of IS:456 - 2000.

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.

Admixtures

IS:1343 - 1980 allows to use admixtures that conform to IS:9103 - 1999, Concrete

Admixtures Specification. 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.

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

These are cementitious and pozzolanic materials.

1.5.2 Propert ies of Hardened Concrete (Part I)

The concrete in prestressed applications has to be of good quality. It requires the

following attributes.

1) High strengthwith low water-to-cement ratio

2) Durabilitywith low permeability, minimum cement content and proper mixing,

compaction and curing


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3) Minimum shrinkageand creepby limiting the cement content.

The following topics are discussed.

1) Strength of concrete

2) Stiffness of concrete

3) Durability of concrete

4) High performance concrete5) Allowable stresses in concrete.

Strength of Concrete

The following sections describe the properties with reference to IS:1343 - 1980. The

strength of concrete is required to calculate the strength of the members. For

prestressed concrete applications, high strength concrete is required for the following

reasons.

1) To sustain the high stresses at anchorage regions.

2) To have higher resistance in compression, tension, shear and bond.

3) To have higher stiffness for reduced deflection.

4) To have reduced shrinkage cracks.

Compressive Strength

The compressive strength of concrete is given in terms of the characteristic

compressive strengthof 150 mm size cubes tested at 28 days ( fck). The characteristic

strength is defined as the strength of the concrete below which not more than 5% of the

test results are expected to fall. This concept assumes a normal distribution of the

strengths of the samples of concrete.

The following sketch shows an idealised distribution of the values of compressive

strength for a sizeable number of test cubes. The horizontal axis represents the values

of compressive strength. The vertical axis represents the number of test samples for a

particular compressive strength. This is also termed as frequency. The average of the

values of compressive strength (mean strength) is represented as fcm. The characteristic

strength (fck) is the value in the x-axis below which 5% of the total area under the curve

falls. The value of fckis lower than fcmby 1.65, where is the standard deviation of the

normal distribution.


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fck fcm

1.65

Frequency

28 day cube compressive strength

5% area fck fcm

1.65

Frequency

28 day cube compressive strength

5% area

Figure 1-5.2 Idealised normal distribution of concrete strength

(Reference: Pillai, S. U., and Menon, D., Reinforced Concrete Design)

The sampling and strength test of concrete are as per Section 15of IS:1343 - 1980.The grades of concrete are explained in Table 1of the Code.

The minimum grades of concrete for prestressed applications are as follows.

30 MPa for post-tensioned members

40 MPa for pre-tensioned members.

The maximum grade of concrete is 60 MPa.

Since at the time of publication of IS:1343 in 1980, the properties of higher strength

concrete were not adequately documented, a limit was imposed on the maximum

strength. It is expected that higher strength concrete may be used after proper testing.

The increase in strength with age as given in IS:1343 - 1980, is not observed in present

day concrete that gains substantial strength in 28 days. Hence, the age factor given in

Clause 5.2.1should not be used. It has been removed from IS:456 - 2000.

Tensile Strength

The tensile strength of concrete can be expressed as follows.

1) Flexural tensile strength: It is measured by testing beams under 2 point loading

(also called 4 point loading including the reactions).

2) Splitting tensile strength: It is measured by testing cylinders under diametral

compression.


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3) Direct tensile strength: It is measured by testing rectangular specimens under

direct tension.

In absence of test results, the Code recommends to use an estimate of the flexural

tensile strength from the compressive strength by the following equation.

cr ck f = f0.7 (1-5.1)

Here,

fcr = flexural tensile strength in N/mm2

fck = characteristic compressive strength of cubes in N/mm2.

Stiffness of Concrete

The stiffness of concrete is required to estimate the deflection of members. The

stiffness is given by the modulus of elasticity. For a non-linear stress (fc) versus strain

(c) behaviour of concrete the modulus can be initial, tangential or secant modulus.

IS:1343 - 1980 recommends a secant modulusat a stress level of about 0.3fck. The

modulus is expressed in terms of the characteristic compressive strength and not the

design compressive strength. The following figure shows the secant modulus in the

compressive stress-strain curve for concrete.

c

fc

fck

Ec

fc

c

fc

fck

Ecc

fc

fck

Ec

fcfc

Figure 1-5.3 a) Concrete cube under compression, b) Compressive stress-strain

curve for concrete

The modulus of elasticity for short term loading (neglecting the effect of creep) is given

by the following equation.

c cE = f5000 k (1-5.2)

Here,

Ec = short-term static modulus of elasticity in N/mm2

fck = characteristic compressive strength of cubes in N/mm2.


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The above expression is updated as per IS:456 - 2000.

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.

In recent years emphasis has been laid on the durability issues of concrete. This is

reflected in the enhanced section on durability (Section 8) in IS:456 - 2000. It is

expected that the revised version of IS:1343 will also have similar importance on

durability.

The durability of concrete is defined as its ability to resist weathering action, chemical

attack, abrasion, or any other process of deterioration. The common durability

problems in concrete are as follows.

1) Sulphate and other chemical attacks of concrete.

2) Alkali-aggregate reaction.

3) Freezing and thawing damage in cold regions.

4) Corrosion of steel bars or tendons.

The durability of concrete is intrinsically related to its water tightness or permeability.

Hence, the concrete should have low permeability and there should be adequate cover

to reinforcing bars. The selection of proper materials and good quality control are

essential for durability of concrete.

The durability is addressed in IS:1343 - 1980 in Section 7. In Appendix A there are

guidelines on durability. Table 9 specifies the maximum water-to-cement (w-c) ratio

and the minimum cement content for different exposure conditions. The values for

moderate exposure condition are reproduced below.

Table 1-5.1 Maximum water-to-cement (w-c) ratio and the minimum cement content

for moderate exposure conditions (IS:1343 - 1980).

Min. cement content : 300 kg per m3of concrete

Max w-c ratio* : 0.50

(*The value is updated as per Table 5of IS:456 - 2000.)


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Figure 1-5.4 End-blocks in a bridge deck


Al lowable Stresses in Concrete

The allowable stresses are used to analyse and design members under service loads.

IS:1343 - 1980 specifies the maximum allowable compressive stresses for different

grades of concrete under different loading conditions in Section 22.8.

Allowable Compressive Stresses under Flexure

The following sketch shows the variation of allowable compressive stresses for different

grades of concrete at transfer. The cube strength at transfer is denoted as fci.

M30 M60 M40 M60

0.51fci0.44fci

0.54fci0.37fci

Post-tension Pre-tension

M30 M60 M40 M60

0.51fci0.44fci

0.54fci0.37fci

Post-tension Pre-tension

Figure 1-5.5 Variation of allowable compressive stresses at transfer

The following sketch shows the variation of allowable compressive stresses for different

grades of concrete at service loads.


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0.34fck

0.41fck

0.27fck

0.35fck

M 30 M 60

Zone I

Zone II

Figure 1-5.6 Variation of allowable compressive stresses at service loads

Here, Zone I represents the locations where the compressive stresses are not likely to

increase. Zone II represents the locations where the compressive stresses are likely to

increase, such as due to transient loads from vehicles in bridge decks.

Allowable Compressive Stresses under Direct Compression

For direct compression, except in the parts immediately behind the anchorage, the

maximum stress is equal to 0.8 times the maximum compressive stress under flexure.

Allowable Tensile Stresses under Flexure

The prestressed members are classified into three different types based on the

allowable tensile stresses. The amount of prestressing varies in the three types. The

allowable tensile stresses for the three types of members are specified in Section 22.7.

The values are reproduced below.

Table 1-5.2 Allowable tensile stresses (IS:1343 - 1980)

Type 1 No tensile stress

Type 23 N/mm2.

This value can be increased to 4.5 N/mm2for temporary loads.

Type 3 Table 8 provides hypothetical values of allowable tensile stresses.

The purpose of providing hypothetical values is to use the elastic analysis method for

Type 3 members even after cracking of concrete.


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1.6 Concrete (Part II)


Properties ofHardened Concrete(Part II)

Properties of Grout

Codal Provisions of Concrete

1.6.1 Propert ies of Hardened Concrete (Part II)

The properties that are discussed are as follows.

1) Stress-strain curves for concrete

2) Creep of concrete

3) Shrinkage of concrete

Stress-strain Curves for Concrete

Curve under uniaxial compression

The stress versus strain behaviour of concrete under uniaxial compression is initially

linear (stress is proportional to strain) and elastic (strain is recovered at unloading). With

the generation of micro-cracks, the behaviour becomes nonlinear and inelastic. After the

specimen reaches the peak stress, the resisting stress decreases with increase in strain.

IS:1343 - 1980recommends a parabolic characteristic stress-strain curve, proposed by

Hognestad, for concrete under uniaxial compression (Figure 3in the Code).

c0 cu

fc

fck

fc

c0 cu

fc

fck

c0 cu

fc

fck

fcfc

Figure 1-6.1 a) Concrete cube under compression, b) Design stress-strain curve for

concrete under compression due to flexure
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The equation for the design curve under compression due to flexure is as follows.

For c0

c cck ck

f = f -

2

0 0

2 (1-6.1)

For c< ccu

fc= fck (1-6.2)Here,

fc = compressive stress

fck = characteristic compressive strength of cubes

c = compressive strain

0 = strain corresponding to fck= 0.002

cu= ultimate compressive strain = 0.0035

For concrete under compression due to axial load, the ultimate strain is restricted to

0.002. From the characteristic curve, the design curve is defined by multiplying the

stress with a size factor of 0.67 and dividing the stress by a material safety factor of m=

1.5. The design curve is used in the calculation of ultimate strength. The following

sketch shows the two curves.

0 cu c

fc

fck

0.447 fck

Characteristic curve

Design curve

0 cu c

fc

fck

0.447 fck


Design curve

Figure 1-6.2 Stress-strain curves for concrete under compression due to flexure

In the calculation of deflection at service loads, a linear stress-strain curve is assumed

up to the allowable stress. This curve is given by the following equation.

fc= Ecc (1-6.3)

Note that, the size factor and the material safety factor are not used in the elastic

modulus Ec.


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For high strength concrete (say M100 grade of concrete and above) under uniaxial

compression, the ascending and descending branches are steep.

0 c

fcfck

Es

Eci

0 c

fcfck

Es

Eci

Figure 1-6.3 Stress-strain curves for high strength concrete under compression

The equation proposed by Thorenfeldt, Tomaxzewicz and Jensen is appropriate for high

strength concrete.

c

c ck nk

c

n

f = f

n - +

0

0

1

(1-6.4)

The variables in the previous equation are as follows.

fc = compressive stress

fck = characteristic compressive strength of cubes in N/mm2

c = compressive strain

0 = strain corresponding to fck

k = 1 for c0

= 0.67 + (fck/ 77.5) for c>0. The value of kshould be greater than 1.

n = Eci/ (Eci Es)

Eci = initial modulus

Es = secant modulus at fck= fck/0.

The previous equation is applicable for both the ascending and descending branches of

the curve. Also, the parameter k models the slope of the descending branch, which

increases with the characteristic strength fck. To be precise, the value of 0 can be

considered to vary with the compressive strength of concrete.


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Curve under uniaxial tension

The stress versus strain behaviour of concrete under uniaxial tension is linear elastic

initially. Close to cracking nonlinear behaviour is observed.

fc

c

fcfc

c

fc

c

fcfc

(a) (b)

Figure 1-6.4 a) Concrete panel under tension, b) Stress-strain curve for concrete

under tension

In calculation of deflections of flexural members at service loads, the nonlinearity is

neglected and a linear elastic behaviour fc= Eccis assumed. In the analysis of ultimate

strength, the tensile strength of concrete is usually neglected.

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

If a concrete specimen is subjected to slow compressive loading, the stress versus

strain curve is elongated along the strain axis as compared to the curve for fast loading.

This can be explained in terms of creep. If the load is sustained at a level, the increase

in strain due to creep will lead to a shift from the fast loading curve to the slow loading

curve (Figure 1-6.5).


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c

fcFast loading

Slow loading

Effect of creep

c

fcFast loading

Slow loading

Effect of creep

Figure 1-6.5 Stress-strain curves for concrete under compression

Creep is quantified in terms of the strain that occurs in addition to the elastic strain due

to the applied loads. If the applied loads are close to the service loads, the creep strain

increases at a decreasing rate with time. The ultimate creep strain is found to be

proportional to the elastic strain. The ratio of the ultimate creep strain to the elastic

strain is called the creep coefficient.

For stress in concrete less than about one-third of the characteristic strength, the

ultimate creep strain is given as follows.

cr,ult el = (1-6.5)

The variation of strain with time, under constant axial compressive stress, is

represented in the following figure.

strain

Time (linear scale)

cr, ult= ultimate creep strain

el= elastic strainstrain

Time (linear scale)

cr, ult= ultimate creep strain

el= elastic strain

Figure 1-6.6 Variation of strain with time for concrete under compression

If the load is removed, the elastic strain is immediately recovered. However the

recovered elastic strain is less than the initial elastic strain, as the elastic modulus

increases with age.

There is reduction of strain due to creep recovery which is less than the creep strain.

There is some residual strain which cannot be recovered (Figure 1-6.7).


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strain

Time (linear scale)

Residual strain

Creep recovery

Elastic recovery

Unloadingstrain

Time (linear scale)

Residual strain

Creep recovery

Elastic recovery

Unloading

Figure 1-6.7 Variation of strain with time showing the effect of unloading

The creep strain depends on several factors. It increases with the increase in the

following variables.

1) Cement content (cement paste to aggregate ratio)

2) Water-to-cement ratio

3) Air entrainment4) Ambient temperature.

The creep strain decreases with the increase in the following variables.

1) Age of concrete at the time of loading.

2) Relative humidity

3) Volume to surface area ratio.

The creep strain also depends on the type of aggregate.

IS:1343 - 1980gives guidelines to estimate the ultimate creep strain in Section 5.2.5. It

is a simplified estimate where only one factor has been considered. The factor is age of

loading of the prestressed concrete structure. The creep coefficient is provided for

three values of age of loading.

Table 1-6.1 Creep coefficient for three values of age of loading

Age of Loading Creep Coefficient

7 days 2.2

28 days 1.6

1 year 1.1


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It can be observed that if the structure is loaded at 7 days, the creep coefficient is 2.2.

This means that the creep strain is 2.2 times the elastic strain. Thus, the total strain is

more than thrice the elastic strain. Hence, it is necessary to study the effect of creep in

the loss of prestress and deflection of prestressed flexural members. Even if the

structure is loaded at 28 days, the creep strain is substantial. This implies higher loss of

prestress and higher deflection.

Curing the concrete adequately and delaying the application of load provide long term

benefits with regards to durability, loss of prestress and deflection.

In special situations detailed calculations may be necessary to monitor creep strain with

time. Specialised literature or international codes can provide guidelines for such

calculations.

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

The following figure shows the variation of shrinkage strain with time. Here, t0is the time

at commencement of drying. The shrinkage strain increases at a decreasing rate with

time. The ultimate shrinkage strain (sh) is estimated to calculate the loss in prestress.

Shrinkage

strain

t0 Time (linear scale)

sh

Shrinkage

strain

t0 Time (linear scale)

sh

Figure 1-6.8 Variation of shrinkage strain with time


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Like creep, shrinkage also depends on several factors. The shrinkage strain increases

with the increase in the following variables.

1) Ambient temperature

2) Temperature gradient in the members

3) Water-to-cement ratio

4) Cement content.

The shrinkage strain decreases with the increase in the following variables.

1) Age of concrete at commencement of drying

2) Relative humidity

3) Volume to surface area ratio.

The shrinkage strain also depends on the type of aggregate.

IS:1343 - 1980gives guidelines to estimate the shrinkage strain in Section 5.2.4. It is a

simplified estimate of the ultimate shrinkage strain (sh).

For pre-tension

sh= 0.0003 (1-6.6)

For post-tension

(1-6.7)

( )sh =

log t +10

0.0002

2

Here, tis the age at transfer in days. Note that for post-tension, tis the age at transfer

in days which approximates the curing time.

It can be observed that with increasing age at transfer, the shrinkage strain reduces. As

mentioned before, curing the concrete adequately and delaying the application of load

provide long term benefits with regards to durability and loss of prestress.

In special situations detailed calculations may be necessary to monitor shrinkage strain

with time. Specialised literature or international codes can provide guidelines for such

calculations.


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1.6.2 Properties of Grout

Grout is a mixture of water, cement and optional materials like sand, water-reducing

admixtures, expansion agent and pozzolans. The water-to-cement ratio is around 0.5.

Fine sand is used to avoid segregation.

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.

IS:1343 - 1980specifies the properties of grout in Sections 12.3.1and Section 12.3.2.

The following specifications are important.

1) The sand should pass 150 m Indian Standard sieve.

2) The compressive strength of 100 mm cubes of the grout shall not be less than 17

N/mm2at 7 days.


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1.6.5 Codal Provisions of Concrete

The following topics are covered in IS:1343 - 1980under the respective sections. These

provisions are not duplicated here.

Table 1-6.2 Topics and sections

Workability of concrete Section 6

Concrete mix proportioning Section 8

Production and control of concrete Section 9

Formwork Section 10

Transporting, placing, compacting Section 13

Concrete under special conditions Section 14

Sampling and strength test of concrete Section 15

Acceptance criteria Section 16

Inspection and testing of structures Section 17


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1.7 Prestressing Steel


Forms of Prestressing Steel

Types of Prestressing Steel

Properties of Prestressing Steel

Codal Provisions of Steel

1.7.1 Forms of 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. The

following material describes the types and properties of prestressing steel.

In addition to prestressing steel, conventional non-prestressed reinforcement is used for

flexural capacity (optional), shear capacity, temperature and shrinkage requirements.

The properties of steel for non-prestressed reinforcement are not covered in this section.

It is expected that the student of this course is familiar with the conventional

reinforcement.

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.

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.
http://prestressing%20steel/http://forms%20of%20prestressing%20steel/http://types%20of%20prestressing%20steel/http://properties%20of%20prestressing%20steel/http://prestressing%20steel/http://prestressing%20steel/http://properties%20of%20prestressing%20steel/http://types%20of%20prestressing%20steel/http://forms%20of%20prestressing%20steel/http://prestressing%20steel/


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Tendons

A group of strands or wires are placed together to form a prestressing tendon. The

tendons are used in post-tensioned members. The following figure shows the cross

section of a typical tendon. The strands are placed in a duct which may be filled with

grout after the post-tensioning operation is completed (Figure 1-7.1).

Duct

Grout

Duct

Grout

Figure 1-7.1 Cross-section of a typical tendon

Cables

A group of tendons form a prestressing cable. The cables are used in bridges.

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.

The following figure shows the different forms of prestressing steel.

Reinforcing barsPrestressing wires,strands and barsReinforcing bars

Prestressing wires,strands and bars

Figure 1-7.2 Forms of reinforcing and prestressing steel


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1.7.2 Types of Prestressing Steel

The steel is treated to achieve the desired properties. The following are the treatment

processes.

Cold working (cold drawing)The cold working is done by rolling the bars through a series of dyes. It re-aligns the

crystals and increases the strength.

Stress relieving

The stress relieving is done by heating the strand to about 350 C and cooling slowly.

This reduces the plastic deformation of the steel after the onset of yielding.

Strain tempering for low relaxation

This process is done by heating the strand to about 350 C while it is under tension.

This also improves the stress-strain behaviour of the steel by reducing the plastic

deformation after the onset of yielding. In addition, the relaxation is reduced. The

relaxation is described later.

IS:1343 - 1980specifies the material properties of steel in Section 4.5. The following

types of steel are allowed.

1) Plain cold drawn stress relieved wire conforming to IS:1785, Part 1, Specification

for Plain Hard Drawn Steel Wire for Prestressed Concrete, Part I Cold Drawn

Stress Relieved Wire.

2) Plain as-drawn wire conforming to IS:1785, Part 2,Specification for Plain Hard

Drawn Steel Wire for Prestressed Concrete, Part II As Drawn Wire.

3) Indented cold drawn wire conforming to IS:6003, Specification for Indented Wire

for Prestressed Concrete.4) High tensile steel bar conforming to IS:2090, Specification for High Tensile Steel

Bars used in Prestressed Concrete.

5) Uncoated stress relieved strand conforming to IS:6006. Specification for

Uncoated Stress Relieved Strand for Prestressed Concrete.


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1.7.3 Properties of Prestressing Steel

The steel in prestressed applications has to be of good quality. It requires the following

attributes.

1) High strength

2) Adequate ductility3) Bendability, which is required at the harping points and near the anchorage

4) High bond, required for pre-tensioned members

5) Low relaxation to reduce losses

6) Minimum corrosion.

Strength of Prestressing Steel

The tensile strength of prestressing steel is given in terms of the characteristic tensile

strength (fpk).

The characteristic strength is defined as the ultimate tensile strength of the coupon

specimens below which not more than 5% of the test results are expected to fall.

The ultimate tensile strength of a coupon specimen is determined by a testing machine

according to IS:1521 - 1972, Method for Tensile Testing of Steel Wire. The following

figure shows a test setup.

Extensometer

Wedge grips

Coupon specimen

Extensometer

Wedge grips

Coupon specimen

(a) Test set-up


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(b) Failure of a strand

Figure 1-7.3 Testing of tensile strength of prestressing strand

The minimum tensile strengths for different types of wires as specified by the codes are

reproduced.

Table 1-7.1 Cold Drawn Stress-Relieved Wires (IS: 1785 Part 1)

Nominal Diameter (mm) 2.50 3.00 4.00 5.00 7.00 8.00

Minimum Tensile Strength fpk

(N/mm2)

2010 1865 1715 1570 1470 1375

The proof stress (defined later) should not be less than 85% of the specified tensile

strength.

Table 1-7.2 As-Drawn wire (IS: 1785 Part 2)

Nominal Diameter (mm) 3.00 4.00 5.00

Minimum Tensile Strength fpk(N/mm2) 1765 1715 1570

The proof stress should not be less than 75% of the specified tensile strength.

Table 1-7.3 Indented wire (IS: 6003)

Nominal Diameter (mm) 3.00 4.00 5.00

Minimum Tensile Strength fpk(N/mm2

) 1865 1715 1570

The proof stress should not be less than 85% of the specified tensile strength.

For high tensile steel bars (IS: 2090), the minimum tensile strength is 980 N/mm2. The

proof stress should not be less than 80% of the specified tensile strength.


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Stiffness of Prestressing Steel

The stiffness of prestressing steel is given by the initial modulus of elasticity. The

modulus of elasticity depends on the form of prestressing steel (wires or strands or

bars).

IS:1343 - 1980provides the following guidelines which can be used in absence of testdata.

Table 1-7.4 Modulus of elasticity (IS: 1343 - 1980)

Type of steel Modulus of elasticity

Cold-drawn wires 210 kN/mm2

High tensile steel bars 200 kN/mm2

Strands 195 kN/mm2

Al lowable Stress in Prestress ing Steel

As per Clause 18.5.1, the maximum tensile stress during prestressing (fpi) shall not

exceed 80% of the characteristic strength.

pi pf 0.8 kf (1-7.1)

There is no upper limit for the stress at transfer (after short term losses) or for the

effective prestress (after long term losses).

Stress-Strain Curves for Prestressing Steel

The stress versus strain behaviour of prestressing steel under uniaxial tension is initially

linear (stress is proportional to strain) and elastic (strain is recovered at unloading).

Beyond about 70% of the ultimate strength the behaviour becomes nonlinear and

inelastic. There is no defined yield point.

The yield point is defined in terms of the proof stress or a specified yield strain. IS:1343

- 1980recommends the yield point at 0.2% proof stress. This stress corresponds to an

inelastic strain of 0.002. This is shown in the following figure.


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0.002

Proofstress

p

fp

0.002

Proofstress

p

fp

Figure 1-7.4 Proof stress corresponds to inelastic strain of 0.002

The characteristic stress-strain curves are given in Figure 5 of IS:1343 - 1980. The

stress corresponding to a strain can be found out by using these curves as shown next.

0.002 0.005

0.95fpk

0.9fpk

p

fp

0.002 0.005

0.95fpk

0.85fpk

p

fp

Stress relieved wires,strands and bars

As-drawn wires

0.002 0.005

0.95fpk

0.9fpk

p

fp

0.002 0.005

0.95fpk

0.9fpk

p

fp

0.002 0.005

0.95fpk

0.85fpk

p

fp

0.002 0.005

0.95fpk

0.85fpk

p

fp

Stress relieved wires,strands and bars

As-drawn wires

Figure 1-7.5 Characteristic stress-strain curves for prestressing steel

(Figure 5, IS:1343 - 1980)

The stress-strain curves are influenced by the treatment processes. The following figure

shows the variation in the 0.2% proof stress for wires under different treatment

processes.

low relaxation

stress relieved

as-drawn

p

fplow relaxation

stress relieved

as-drawn

p

fp

Figure 1-7.6 Variation in the 0.2% proof stress for wires under different treatment

processes


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The design stress-strain curves are calculated by dividing the stress beyond 0.8fpkby a

material safety factor m=1.15. The following figure shows the characteristic and design

stress-strain curves.

0.8fpk

p

fp


Design curve0.8f

pk

p

fp


Design curve

Figure 1-7.7 Characteristic and design stress-strain curves for

prestressing steel

Relaxation of SteelRelaxation of steel is defined as the decrease in stress with time under constant strain.

Due to the relaxation of steel, the prestress in the tendon is reduced with time. Hence,

the study of relaxation is important in prestressed concrete to calculate the loss in

prestress.

The relaxation depends on the type of steel, initial prestress and the temperature. The

following figure shows the effect of relaxation due to different types of loading conditions.

p

fp

Fast loading

With sustained loadingEffect of relaxation

p

fp

Fast loading

With sustained loadingEffect of relaxation

Figure 1-7.8 Effect of relaxation due to different types of loading conditions

The following figure shows the variation of stress with time for different levels of

prestressing. Here, the instantaneous stress (fp) is normalised with respect to the initial

prestressing (fpi) in the ordinate. The curves are for different values of fpi/fpy, where fpyis

the yield stress.


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100

90

80

70

60

5010 100 1000 10,000 100,000

Time (hours)

fp

fpi p i

p y

f=

f0.6

0.7

0.8

0.9

100

90

80

70

60

5010 100 1000 10,000 100,000

Time (hours)

fp

fpi p i

p y

f=

f0.6

0.7

0.8

0.9

Figure 1-7.9 Variation of stress with time for different levels of prestressing

It can be observed that there is significant relaxation loss when the applied stress is

more than 70% of the yield stress.

The following photos show the test set-up for relaxation test.

Load cell

Specimen

Load cell

Specimen

(a) Test of a single wire strand


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SpecimenSpecimen

(b) Test of a seven-wire strand

Figure 1-7.10 Set-up for relaxation test

The upper limits of relaxation loss are specified as follows.

Table 1-7.5 Relaxation losses at 1000 hours (IS:1785, IS:6003, IS:6006, IS:2090)

Cold drawn stress-relieved wires 5% of initial prestress

Indented wires 5% of initial prestress

Stress-relieved strand 5% of initial prestress

Bars 49 N/mm2

In absence of test data, IS:1343 - 1980 recommends the following estimates of

relaxation losses.

Table 1-7.6 Relaxation losses at 1000 hours at 27C

Initial Stress Relaxation Loss (N/mm2)

0.5fpk 0

0.6fpk 350.7fpk 70

0.8fpk 90

Fatigue

Under repeated dynamic loads the strength of a member may reduce with the number

of cycles of applied load. The reduction in strength is referred to as fatigue.


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In prestressed applications, the fatigue is negligible in members that do not crack under

service loads. If a member cracks, fatigue may be a concern due to high stress in the

steel at the location of cracks.

Specimens are tested under 2 x 106 cycles of load to observe the fatigue. For steel,

fatigue tests are conducted to develop the stress versus number of cycles for failure (S-N) diagram. Under a limiting value of stress, the specimen can withstand infinite number

of cycles. This limit is known as the endurance limit.

The prestressed member is designed such that the stress in the steel due to service

loads remains under the endurance limit. The following photo shows a set-up for

fatigue testing of strands.

Figure 1-7.11 S

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