Seth Jai Parkash Polytechnic Damla, Yamunanagar Name of the faculty: - Amit Kumar (Lecturer in Civil Engg. Deptt.)

Subject Notes: - PRESTRESSED CONCRETE (120766-C)

Semester: - 6th Semester

PRESTRESSED CONCRETE STRUCTURES History… Internal stresses are induced to counteract external stresses. In 1904, Freyssinet attempted to

introduce permanent acting forces in conc. to resist elastic forces under loads and was named

“Pre stressing”.

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.

1938 Hoyer, E., (Germany)

Developed ‘long line’ pre-tensioning method.

1940 Magnel, G., (Belgium)

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

Eugene Freyssinet (France)

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.

Pamban Road Bridge at Rameshwaram, Tamilnadu

Reinforced concrete:-

1. Concrete is strong in compression weak in tension.

2. Steel in strong in tension

3. Reinforced concrete uses concrete to resist compression and to hold bars in position and uses

steel to resist tension.

4. Tensile strength of concrete is neglected (i.e. zero )

5. R.C beams allows crack under service load.

UNIT-1

INTRODUCTION

What is Prestressed Concrete?

It is a method of applying pre-compression to control the stresses resulting due to external loads below the neutral axis of the beam.

Pre-compression resulting either no tension or compression.

Pre stressed concrete: - is the one in which internal stresses are induced in a planned manner, so

that the stresses resulting from external loads are counteracted to a desired level.

OR

Pre stress:- means to induce compressive stress in the zones where external loads would normally

cause tensile stress.

Pre stressed concrete is better as compared to RCC because of two major causes:

i) Minute cracks observed in RCC can be eliminated.

ii) High tensile steel can be used economically.

Basic Concept of pre-stressing:-

Prestressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from the external loads are counteracted to a desired degree.

Pre-stress is introduced by stretching steel wire and anchoring them against concrete.

The concept of pre stressing was invented years ago when metal brands were wound around wooden pieces to form barrels.

The metal brands were tightening under tensile stress which creates compression between the staves allowing them to resist internal liquid pressure.

Principle of pre-stressing:- 1. Pre-stressing is a method in which compression force is applied to the reinforced concrete section. 2. The effect of pre stressing is to reduce the tensile stress in the section to the point till the tensile

stress is below the cracking stress. Thus the concrete does not crack. 3. It is then possible to treat concrete as a elastic material. 4. The concrete can be visualized to have two compressive force:- a. Internal pre-stressing force. b. External forces (Dead Load, Live Lode etc ) 5. These two forces must counteract each other.

Advantages of prestressed concrete in comparison with RCC:-

1. It needs about 1/3rd the quantity of steel and 1/4th the quantity of concrete as compared to RCC.

2. Lighter and slender members can be used.

3. Factory made members are possible in pre tensioning.

4. Members like railway sleepers, electric poles, boundary pillars, gantary girders can be made.

5. Long span structures are possible so that saving of wt is significant & thus it become economical.

6. Pre-stressed member are tested before use.

7. Dead loads are get counter balanced by eccentric pre-stressing

8. Cracks can be eliminated in tension zone.

9. It has high fatigue resistance.

10. It has high ability to resist the impact.

11. It has high live load carrying capacity.

12. Use the entire section to resist the load

13. It free from cracks from service loads and enables entire section to take part in resisting moments.

14. Take full advantages of high strength concrete and high strength steel

15. Need less materials

16. Smaller and lighter structure

17. Better corrosion resistance

18. Good for water tanks and nuclear plant

19. Very effective for deflection control

20. Better shear resistance

Disadvantages of pre stressed concrete are:-

1. Initial cost of equipment is very high.

2. Requires skilled supervision.

3. Very long slender members are difficult to transport.

4. Requires high tensile steel which is 2.5 to 3.5 times costlier than mild steel.

5. Pre-stressed concrete is less fiber resistant

6. Required skilled builders & experienced engineers.

7. It requires high strength concrete & steel.

8. Need higher quality materials

9. More complex technically

10. More expensive

11. Harder to re-cycle

12. Availability of experienced engineers is less.

13. Required complicated formwork.

Application

1. Bridges

2. Slabs in buildings

3. Water Tank

4. Concrete Pile

5. Thin Shell Structures

6. Offshore Platform

7. Nuclear Power Plant

8. Repair and Rehabilitations

Unit-2 Prestressing Materials

IS Specifications for materials used in pre-stressed concrete:

As per Indian standard code of practice for pre-stressed concrete (IS: 1343) following specifications

for materials must be kept in mind:

The two main materials in pre-stressed concrete are: A) Concrete B) Steel

Concrete: -The concrete to be used in pre-stressed concrete member should be strong enough so

that full strength can be utilized. 1. As per IS: 456-2000, a minimum grade of M40 for pre-tensioned system and M30 for post-

tensioned system should be used. High strength concretes are preferred for pre-stressing works because:

2. Water cement ratio should be about 0.45 i.e. 10 bags of cement per m3 of concrete is required.

3. Small cross-sections are possible by using high strength concrete.

4. Rich mix concrete has high value of modulus of elasticity which helps in reducing deflection and

early release of pre-stressing equipment.

5. Creep and shrinkage is less in high strength concrete and causes less loss of pre-stressing force.

Pre-stressing steel: - Steel to be used for pre stressing must have high tensile strength, good

surface condition and good bonding with concrete.

The steel used for pre-stressing available in three forms:-

i) Single wires (also called as tendons) ii) Group of wires (also termed as strands or cables) iii) Alloy steel round bars

1. Single wires (tendons):- Hard drawn high tensile steel wire of diameter ranging from 1.5mm

to 8mm and having tensile steel and other properties as specified in following clauses may be

used.

2. Wire strands (cables):- Hard drawn steel wires may be used in the form of cables known as

wire strands. The diameter of strand cable varies from 7mm to 17mm.

3. Round bars: - High tensile alloy steel bars are used in pre-stressing systems. It is available in

10mm to 32mm diameter.

Some of the important properties of high tensile steel are mentioned below:

Ultimate tensile strength:- Cold drawn high tensile strength steel wires used for pre-stressed

concrete shall conform to the specifications shown in table below: Minimum ultimate tensile strength of high strength steel wires:- Diameter (in mm) 1.5 2.0 2.5 3.0 4.0 5.0 7.0 8.0

Minimum ultimate strength (N/mm2)

2350 2200 2050 1900 1750 1600 1500 1400

Pre stressing equipments:

Equipments required for pre – stressing are:

i) Tensioning equipment – hydraulic jacks.

ii) Temporary gripping device – Double cone, wedge

iii) Releasing device – for gradual and uniform release.

iv) End anchorage – strong enough to hold stress.

Methods of pre stressing:- briefly discuss in unit no-3

i) Pre tensioning – steel is tensioned before the casting of concrete.

ii) Post tensioning – steel is tensioned after the casting of concrete.

Pre tensioning method is best suitable for factory production

whereas post tensioning method is suitable for both cast in situ and pre cast members.

Systems of pre stress:- briefly discuss in unit no-3

Systems depending upon their patents and end anchorage systems are:

A. Freyssinet system

B. Magnel blaton system

C. Le Mc Call system

Principle of pre-stressing:- 1. Pre-stressing is a method in which compression force is applied to the reinforced concrete section.

2. The effect of pre stressing is to reduce the tensile stress in the section to the point till the tensile

stress is below the cracking stress. Thus the concrete does not crack.

3. It is then possible to treat concrete as a elastic material.

4. The concrete can be visualized to have two compressive force:-

a. Internal pre-stressing force.

b. External forces (Dead Load, Live Lode etc )

5. These two forces must counteract each other.

Principle of Pre-stressing:- Analysis by Stress Concept

Stress in concrete when pre stressing is applied at the C.G of the section

Stress in concrete when pre stressing is applied eccentrically with respect to the C.G

of the section.

Tendon Profile:- The deflection due to prestress depends on the profile of the c.g.s. line

Unit-3 Prestressing Methods

Types of Pre-stressing

1. External or internal pre-stressing:- It is based on the location of the pre-stressing tendons with respect to concrete section.

2. Pre-tensioning or post-tensioning:- It based on the sequence of casting the concrete and applying tension to the tendons.

3. Linear or circular pre-stressing:-It based on the shape of the member pre-stressed.

4. Full, limited or partial pre-stressing:- It based on the pre-stressing force.

5. Uniaxial, biaxial or multi-axial pre-stressing:- It based on the direction of the pre-stressing member.

Pre-stressed Concrete Methods: - There are two basic methods of applying pre-stress to a

concrete member.

1. Pre tensioning – steel is tensioned before the casting of concrete.

2. Post tensioning – steel is tensioned after the casting of concrete.

1. Pre-tensioning:- In Pre-tension, the tendons are tensioned against some abutments before the

concrete is place. After the concrete hardened, the tension force is released. The tendon tries to

shrink back to the initial length but the concrete resists it through the bond between them, thus,

compression force is induced in concrete. Pretension is usually done with precast members.

Suitability/ Advantages of Pre tensioning method-:

1. This Method is best suitable for factory production.

2. This method is very simple.

3. Many members can be casted at one time on same casting bed.

4. This method is economical.

Disadvantages of pre-tensioning method:-

1. Size of member is restricted because long slender members are more difficult to transport.

2. Loss of pre-stress is more. (18-20%)

Procedure followed for the process of pre-tensioning :

1) Anchoring the tendons against the end abutments.

2) Placing of jacks.

3) Applying tension to the tendons.

4) Casting of concrete.

5) Cutting of the tendons.

2. Post tensioning:-

In Post tension, the tendons are tensioned after the concrete has hardened. Commonly, metal or

plastic ducts are placed inside the concrete before casting. After the concrete hardened and had

enough strength, the tendon was placed inside the duct, stressed, and anchored against concrete.

Grout may be injected into the duct later. This can be done either as precast or cast-in-place.

Suitability/Advantages of Post tensioning method:-

1. This is suitable for both cast in situ and pre cast members.

2. Loss of pre-stress is less.(15-18%)

3. There is no limit of casting as the method can be appllied at site also.

Disadvantages of Post tensioning method:-

1. This method is costly because of sheathing and grouting.

Methods of post-tensioning:

1) Casting of concrete.

2) Placement of tendons.

3) Placement of the anchorage block and jack.

4) Applying tension to the tendons.

5) Seating of the wedges.

6) Cutting the tendons.

Differentiate between pre-tensionng and post-tensioning methods:

S.NO: Pre-tensioning method Post-tensioning method

1. Method is best suitable for factory production under controlled conditions.

This method is suitable for both cast-in-situ and pre-cast members.

2. Loss of pre-stress is more. (18-20%) Loss of pre-stress is less.(15-18%)

3. Size of member is restricted because

large members are more difficult to

transfer.

Size of member is unrestricted therefore any

size of member can be casted.

4. This method is economical. This method is costly because of use of

sheathing and grouting.

5. Minimum grade of concrete to be used

is M40.

Minimum grade of concrete to be used is

M30.

Systems of pre stressing :-

For pre-tensioning following system are adopted:

A. Hoyer’s system

For post-tensioning following system are adopted:

A. Freyssinet system

B. Magnel blaton system

C. Le Mc Call system

1. Pre-tensioning system-

HOYER SYSTEM: - Hoyer system or long line method is often adopted in pre-tensioning.

Large scale production

Two bulk heads or abutments independently anchored to the ground are provided several meters

apart, say, and 100m. Wires are stretched between the bulkheads. Moulds are placed enclosing the

wires.

The concrete is now poured so that a number of beams can be produced in one line.

After the concrete has hardened, the wires are released from bulkheads and are cut off.

The prestress is transferred through the bond between tendons and concrete.

Uneconomical for larger spans:-

2. Post tensioning system:

A. Freyssinet System: - Introduced by the French Engineer Freyssinet and it was the first method

to be introduced. High strength steel wires of 5mm or 7mm diameter, about 12 in number are grouped into a cable with a helical spring inside. Spring keeps proper spacing for the wire, and thus provides a channel which can be cement grouted. It further assists to transfer the reaction to the concrete Cable is inserted in the duct. Anchorage device consists it concrete cylinder with a concentric conical hole and

Corrugations on its surface and a conical plug carrying grooves on its surface.

These cylinders are kept in proper position and the conical plugs are pushed into holes after cables

are tightened.

The central whole passing axially permits cement grout to be injected through.

Freyssinet system

Advantages:

1. Securing the wires is not expensive

2. Desired stretching force is obtained quickly

3. The plugs may be left in concrete and they do not project beyond the ends of the member

Disadvantages:

1. Stresses in wires may not be exactly same ( all the wires are stretched together)

2. Jacks used are heavy and expensive

3. The greatest stretching force available is 250kN to 500kN, which is not sufficient.

B. MAGNEL BLATON SYSTEM:-

Anchorage of Magnel Blaton System:

1. This method was introduced by a famous engineer, Prof. Magnel of Belgium.

2. In Freyssinet system, several wires are stretched at a time. In Magnel Blaton system, two wires are

stretched at a time.

3. Cable of rectangular section is provided, which contains layers of wires 5 to 8mm diameter.

4. A cable consists of wires in multiples of 8 wires. Cables with as much as 64 wires are also used

under special conditions.

5. Wires in two adjacent layers are separated with a clearance of 4mm.

6. Wires are maintained in form by providing spacers at regular intervals throughout the length of

cable.

7. Wires are anchored by wedging, 2 at a time into sandwich plates. These plates are 25mm thick

and are provided with two wedge shaped grooves on its two faces.

8. The wires are taken two in each groove and tightened. A jack is used to tighten the wires.

9. A steel wedge is driven between the tightened wires to anchor them against the plate.

Advantages:-

1. This method saves the cost of sheathing as ducts ate formed by rubber cores.

2. Wires are placed in layers with proper horizontal and vertical spacing by providing spacers.

3. Only two wires are stretched at a time thus uniform stress is induced in every wire.

C. Lee-Mc-call system: In this method high tensile alloy steel bars are (silico magnesia steel) are

instead of wire with tensile strength varying from minimum 950N/mm2 to maximum of

2100N/mm2. These steel rods are provided in 22mm, 25mm, 28mm and 30mm diameter and length

upto 20m. The anchoring of the bar is done by screwing special threaded units. This system is best

suitable for span 12-15m.

Advantages:-

1. This system is very simple.

2. The member can be stressed and distressed as desired.

3. Loss of pre-stress can be overcome by re-stressing with steel rods.

4. Equal stressing in bars is possible than using members of wires.

5. Stressing can be done in steps in this system by tightening the nut at any stage.

Disadvantages:-

1. Large size members cannot be stressed suitable only 12-15m spans.

2. High pre-stressing intensities cannot be employed.

3. Large size bars cannot be used in all members.

4. To stress a bar of greater diameter, heavy jacks are required.

Circular Prestressing and its Application:-Circular Prestressing” is employed to denote the

prestressing of circular structures such as pipes and tanks where the prestressing wires are wound in circles. In contrast to this term, “linear prestressing” is used to include all other types of prestressing, where the cables may be either straight or curved, but not wound in circles around a circular structure. In most prestressed circular structures, prestress is applied both circumferentially and longitudinally, the circumferential prestress being circular and the longitudinal prestress actually linear.

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.

Circularly prestressed containment structure, Kaiga Atomic Power Station, Karnataka

Application of Circular Prestressing:-

1. Circular cylindrical tank 2. Conical tank 3. Water tower with conical tank 4. Water tower of doubly curved shell

Unit-4 Bending and Shear capacity

What Is Shear Force: - Shear forces are unaligned forces pushing one part of a body in one direction,

and another part the body in the opposite direction. Shear force acting on a substance in a direction

perpendicular to the extension of the substance.

Shear Mechanism: - In a simply supported rectangular beam, self weight & super imposed loads

act downward, reaction acts upward. Resultants of all these vertical forces generates vertical shear in

a member.

Normal Concrete Vs Pre-stressed Concrete: - Comparatively smaller sectioned member

needed for load carrying, so less self weight i.e. less shear.

D1

RCC Beam

D2

Prestress Concrete Member

D1>D2 i.e. for same load carrying

Sagged tendon in most case provide additional shear but opposite direction.

Prestressing prevents the occurrence of shrinkage cracks which could conceivably

destroy the shear resistance.

Modes of Failure in Prestressed Beam

Flexure-Compression (FC): Flexure compression failures are the result of having a beam with

higher shear strength than flexural strength. Failure occurs at the point of maximum flexural stress

where the compressive strain exceeds its capacity.

Flexure-Shear Failure: - A flexure-shear failure is the result of a crack which begins as a flexural

crack, but as shear increases, the crack begins to “turn over” and incline towards the loading point.

Failure finally occurs when the concrete separates and the two planes of concrete slide past one

another. This mode of failure is common in beams which do not contain web reinforcement.

Shear-Compression Failure: - Shear compression failures, shown in Figure, typically occur in

beams which contain adequate web reinforcement. In this mode, the crack propagates through the

section until it begins to penetrate the compression zone. This crack causes a redistribution of

compressive forces in the compression zone onto a smaller area. When the compressive strength is

exceeded, a shear compression failure occurs. This type of failure is common in deep beams, where

arch action is prevalent. The compressive strut caused by arch action prevents a diagonal tension

crack from propagating into the compression zone.

Web-shear Failure:- Before a section cracks from flexure, it is possible to exceed the tensile

strength of the concrete at the point of maximum shear stress. This mode is primarily observed in

sections with thin webs. Failure occurs at the location of peak shear stress, as shown in Figure. While,

the mechanics of this failure are identical to flexure-shear, failure is brittle and occurs with little or no

warning.

Factors Influencing Shear Strength:- Axial Force: Shear failures are commonly due to tensile failure of the concrete. Axial compression can

delay the onset of critical tension in the section, axial tension can hasten the failure. Compression

such as provided by an axial force or prestressing tendons, provides an increase in shear strength.

Tensile Strength of Concrete: As the tensile strength of the concrete is increased, there is a

corresponding increase in the shear strength of the section.

Longitudinal Reinforcement Ratio: Low amount of steel may result in wider flexural cracks, resulting

in reduced dowel action and aggregate interlock.

Shear Span-to-Depth Ratio: High values of require a larger compression zone, raising the amount of

shear which can be transferred by the uncracked concrete shear transfer mechanism, thus increasing

shear strength

Shear Carrying of Concrete & Tendon on Different Tendon Profile:-

Some Important Notes about Shear in Prestressed Concrete:- 1. Prestressed beam never fail under direct shear or punching shear. They fail as a result of tensile stress

produced by shear. 2. In some rare instance the transverse component of prestress increases the shear in concrete. 3. By following load balancing approach, it is theoretically possible to design a beam with no shear in concrete

under a given condition of loading.

Development of Shear Cracking:-

Steps of Shear Design: - For a Simply Supported Beam Section with UDL loading,

Step -1: Calculate the moment of inertia of the section.

Step -2: Calculate Support reaction.

Step -3: Calculate Moment at desire beam section from x distance from support.

Step -4: Calculate ‘a’ and then the eccentricity of tendon at desire (x) distance from

support i.e. ex

For Flexural Shear Crack:-

1. Calculate

Calculate

2. Calculate Flexural Cracking Moment

3. Calculation of cracking flexural shear

4. Calculation of Nominal flexural shear

For Web Shear Crack:-

1. Calculate

2. Calculation of Nominal web shear

3. Calculate ultimate load

4. Calculate factored shear at a section x distance from support

Shear Reinforcement Spacing:-

Smallest spacing among S1, S2, S3 should be chosen as stirrup spacing.

BOND” in Prestressed Concrete

Interlocking between two properties e.g. pre-stressed tendon and concrete.

Main Types of Internal Prestressed Concrete

1. Pre-Tension Concrete: Pre-stressing steel is tension stressed prior to the placement of the concrete and unloaded after concrete has harden to required strength.

2. Bonded post-tensioned concrete: Unstressed pre-stressing steel is placed within the concrete and then tension stressed after concrete has harden to required strength

3. Un-bonded post-tensioned concrete: Differs from bonded post-tensioning by providing the pre-stressing steel permanent freedom of movement relative to the concrete.

Bending/Deflection

In general, Deflection is the degree to which a structural element is displaced under a load.

Types of Deflection Short-term deflection occurs immediately upon the application of a load.

Long-term deflection takes into account the long-term shrinkage and creep movements

Causes of Deflection in PSC Beams Due to external loads

Due to prestress force

Tendon Profile:- The deflection due to prestress depends on the profile of the c.g.s. line

Methods of Calculation:- 1. Double Integration Method 2. Moment Area Method 3. Conjugate Beam Method 4. Principle of Virtual Load

Calculations of the Short-term Deflection The usual loading which should be investigated in calculating deflections are:

1. Prestress plus dead load

2. Prestress plus maximum service load

3. Prestress plus minimum service load

UNIT-5 Losses in Prestressing

Syllabus Topic -:

(A) Types of losses in prestress – 1. Friction loss 2. Creep loss 3. shrinkage loss of concerete, 4. Elastic shortening loss 5. Stress relaxation in prestress steel.

(B) Computation of losses for simple beam problems

Loss in Pre-stress:- The pre-stressed force applied to the member does not remain constant but

decrease with passage of time due to various losses.

Causes for loss of pre-stress:- 1. Loss of pre stress due to friction:-This type of loss occurs (during the tensioning process) only in the

post tensioned members. The major losses due to friction occur between the tendons and its surrounding material (i.e. duct or spacer)These losses are due to length and curvature effect. To reduce the loss due to friction cables can be lubricated, metal tubes may be provided at ends and stress may be applied from both ends.

2. Loss due to creep of concrete:-Creep is a time dependent deformation which takes place due to continued compression of concrete. Pre tensioned member experiences more loss of pre stress due to creep of concrete then post tensioned members and amounts to 5%-10%.

3. Loss due to shrinkage of concrete:-Shrinkage in concrete is its contraction due to drying and chemical changes. Loss of pre stress due to shrinkage of concrete may range from 4% to 6% for post tensioned members and 3% to 4% for pre tensioned members.

4. Loss due to elastic shortening (deformation) of concrete: When pre stress is transferred to concrete, elastic stress and strains are induced in it. Due to this concrete members get shortened along with shortening of steel. The loss due to elastic shortening of concrete may range from 3% to 6% in pre tensioned members and 4% in post tensioned members. .

5. Loss due to Stress relaxation in prestress steel: Under a constant strain, there is a loss of stress in steel which is called relaxation. Loss of pre stress due to relaxation of steel amounts to 2% to 8% of the initial stress.

6. Loss due to slippage of tendons and anchorage system:-When the jacks are released a slight loss of pre stress occurs due to slippage of tendon and end anchorage system. Longer the length of tendon lesser will be the percentage loss. But in shorter tendons length this value may be greater importance. This slippage generally varies from 2-5mm.

NOTE: - Loss of pre stress is 18 – 20% in post tensioning and 15 – 18% in pre tensioning system.