Prestress loss

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AHSANULLAH UNIVERSITY OF SCIENCE & TECHNOLOGY

Course No : CE-416.Course Title : Prestress Concrete Design

Sessional.

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Prestress Loss

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Welcome To Our Presentation

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Group Members

10.01.03.062 - Shahriar Mujthahid Hossain.10.01.03.063 - H.M.Suruzzaman.10.01.03.065 - Sabbir Bin Delwar.10.01.03.066 - Md. Imran Islam.10.01.03.069 - Tasfia Ahmed.10.01.03.072 - Mohammad Shakib Rahman.10.01.03.075 - Munshi Md. Rasel.10.01.03.076 - Md.Moinul Islam.

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What is Pre-stressed Concrete?

– Internal stresses are induced to counteract external stresses.

– In 1904, Freyssinet attempted to introduce permanent acting force in conc. to resist elastic forces under loads and was named

“Pre stressing”.

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Introduction

• In prestressed concrete applications, most

important variable is the prestress.

• Prestress does not remain constant with time.

• Even during prestressing of tendons, and transfer

of prestress, there is a drop of prestress from the

initially applied stress.

• Reduction of prestress is nothing but the loss in

prestress.

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Prestress Loss

• loss in prestress is the difference between initial prestress and

the effective prestress.

• Loss of prestress affects

– the strength of member and

– member’s serviceability [ Stresses in Concrete, Cracking, Camber and Deflection ]

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Types

Loss of prestress is classified into two types:

1. Immediate Losses

immediate losses occur during prestressing of tendons,

and transfer of prestress to concrete member.

2. Time Dependent Losses

Time dependent losses occur during service life of

structure.

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Prestress Losses

ImmediateTime

Dependent

Relaxation

Anchorage Slip

Elastic Shortening

Friction

ShrinkageCreep

Types According To Time

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Prestress Losses

Concrete Steel

Relaxation

ShrinkageElastic

ShorteningCreep

Anchorage Slip

Friction

Types According To Material

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Types of Prestressing Systems

I . Pre-tensioning: In Pre-tension, the tendons are tensioned before the

concrete is place. After the concrete hardened, the tension force is released.

II . Post tensioning: In Post tension, the tendons are tensioned after the

concrete has hardened.

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Example of Pre-tensioning

Fig : Pre-tensioned electric pole

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Example of Post-tensioning

Fig : Post-tensioning of a box girder

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Losses in Various Prestressing Systems

Type of Loss Pre-tensioning Post-tensioning

1. Elastic Shortening Yes

i. No, if all the cables are simultaneously tensioned.

ii. If the wires are tensioned in stages loss will exist.

2. Anchorage Slip No Yes3. Friction Loss No Yes

4. Creep and Shrinkage of Concrete Yes Yes

5. Relaxation of Steel Yes Yes

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Immediate Loss

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Elastic Shortening

• It is the shorten of concrete member, when the prestress is

transferred to concrete, the member shortens and the prestressing

steel also shortens in it. Hence there is a loss of prestress.

Original length of member at transfer of prestress

Length after elastic shortening

Pi

P0

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Elastic Shortening at Pre-tensioned Members

When the tendons are cut and the prestressing

force is transferred to the member, concrete

undergoes immediate shortening due to prestress.

Tendon also shortens by same amount, which leads

to the loss of prestress.

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Pre-tensioning of a member

Prestressing bed

Elastic Shortening at Pre-tensioned Members

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Elastic Shortening at Post-tensioned Members

If there is only one tendon, there is no loss because

the applied prestress is recorded after the elastic

shortening of the member.

For more than one tendon, if the tendons are

stretched sequentially, there is loss in a tendon

during subsequent stretching of the other tendons.

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Casting bed

Duct

jackAnchorage

Post-tensioning of a member

Elastic Shortening at Post-tensioned Members

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Anchorage Slip

• In most Post-tensioning systems when the prestress force is transferred from the jack to the anchoring ends, the wedges slip over a small distance.

• Loss of prestress is due to the consequent reduction in the length of the tendon.

• Amount of slip depends on type of anchorage system.

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Force variation diagrams for various stages

a) The initial tension at the right end is high to compensate for the anchorage slip. It corresponds to about initial prestress. The force variation diagram (FVD) is linear.

b) After the anchorage slip, the FVD drops near the right end till the length lset.

Note : Effect of anchorage slip is present up to a certain length, called the setting length lset.

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Force variation diagrams for various stages

c) The initial tension at the left end also corresponds to about initial prestress. The FVD is linear up to the centre line of the beam.

d) After the anchorage slip, the FVD drops near the left end till the length lset. It is observed that after two stages, the variation of the prestressing force over the length of the beam is less than after the first stage.

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Typical values of anchorage slip

Anchorage System Anchorage Slip (Δs)

Freyssinet system 12 - 5mm Φ strands 12 - 8mm Φ strands

4 mm 6 mm

Magnel system 8 mm

Dywidag system 1 mm

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Frictional Loss• The friction generated at the interface of

concrete and steel during the stretching of a curved tendon in a post-tensioned member.

• The friction in the jacking anchoring system is generally small.

• More serious frictional loss occurs between the tendon and its surrounding material.

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Frictional loss occurs only in Post-tensioned Members

• The loss due to friction does not occur in pre-tensioned members because there is no concrete during the stretching of the tendons.

• Friction is generated due to curvature of tendon, and

vertical component of the prestressing force.

A typical continuous post-tensioned member

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Frictional Loss

Frictional Loss is the summation of– Friction Loss Due to length Effect.– Friction Loss Due to Curvature Effect.

• Length Effect: If the profile of cable is linear, the loss will be due to straightening or stretching of the cables.

• Curvature Effect: If the profile is curved, there will be loss in stress due to friction between tendon and the duct or between the tendons themselves.

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Length & Curvature Effects

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Methods available to “Reduce” the frictional losses

1. Cables should pass through metal tubes.2. The bends should be through as small an

angle as possible.3. Radius of curvature for bends should be

large.4. Prestressing the wire from both ends.5. Over-tensioning the wires.

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Time Dependent Losses

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Creep of Concrete

• The Continuous deformation of concrete with time under sustained load.

Factors affecting creep of concrete• Age

• Applied Stress level

• Density of concrete

• Cement Content in concrete

• Water-Cement Ratio

• Relative Humidity and

• Temperature

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Condition for calculating the loss of prestress due to creep.

• Creep is due to sustained (permanent) loads only. Temporary

loads are not considered in calculation of creep.

• Since the prestress may vary along the length of the

member, an average value of the prestress is considered.

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Shrinkage of Concrete

• Shrinkage of concrete is defined as the contraction due to loss of moisture.

• Due to the shrinkage of concrete, the prestress in the tendon is reduced with time.

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Shrinkage of Concrete

• For pre-tensioned members, transfer commonly takes place after 24 hours after casting and nearly all shrinkage takes place after that.

• For post-tensioned members, stressing may takes place after one day or much later, thus a large percentage of shrinkage may already taken place by them.

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Relaxation

Relaxation is the reduction in stress with time at

constant strain.

– decrease in the stress is due to the fact that some

of the initial elastic strain is transformed in to

inelastic strain under constant strain.

– Percentage of relaxation varies from 1 to 5%.

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Factors effecting Relaxation :

• Time

• Initial stress

• Temperature and

• Type of steel.

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Method Available to Reduce The Loss due to Relaxation

• Choice of proper steel helps to reduce this loss.

• Prestressed wires have lesser creep. • Galvanised wires also have no creep.• overstressing steel about 10% above its initial

stress and then releasing it to the initial stress

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Calculation of Total Amount of Loss

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Loss of Prestress=Initial Stress-Effective Stress

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Initial Prestress

• Deducting the loss due to anchorage take-up and friction, initial prestress is obtained.

• If prestress is measured at the time of pulling the wire, the stress is termed as the jacking stress.

• if jacketing stress is treated as the initial stress, effective stress is jacketing stress minus all losses.

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Effective Prestress

• Initial Prestress in steel minus the losses is known as the effective or design prestress.

Effective prestress=Initial prestress-Losses

Note: For Pre-Tension system , Pretension Losses are used instead of losses.

For Post-Tension system , Post-Tension Losses are usedinstead of losses.

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Total Amount Of Losses According To Tensioning System

• Total pretension losses=Loss due to creep +Elastic shortening + Shrinkage +Steel Relaxation.

• Total post-Tension Loss=Loss due to creep +

Elastic shortening + Shrinkage +Steel Relaxation +Anchorage slip +

Friction.

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Total Losses

• It is difficult to generalize the amount of loss of prestress, because it is dependent on so many Factors :

The properties of concrete & steel.Curing & moisture condition.Magnitude & time of application of prestress.Process of prestress.

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Method of Loss Estimation

• There are two methods that can be used to estimate losses in prestressed concrete: (a)lump sum approximations;

(b)refined estimations.

One should keep in mind that all estimates for prestress loss are just that – ESTIMATIONS .

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Lump Sum Estimation For Prestress Loss

• First introduce by the ACI-ASCE Committee 423 in 1958.

Table : AASHTO Lump Sum Losses. Total Loss

Types of prestressing steel

f’c=28 MPa f’c=35 MPa

Pretensioning strand

310 MPa

Posttensioning wire or strand

220 MPa 230 Mpa

Bars 150 MPa 160 Mpa

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AASHTO-LRFD Specifications

For Lump-Sum estimates following conditions should met :1. Members that are post-tensioned must be non-segmental

members with spans less than 160 feet and concrete stressed an age of 10-30 days.

2. Members that are pretensioned must be stressed at an age where the concrete strength is not less than 3,500-psi.

3. Members must be made from normal weight concrete.4. Members cannot be steam-cured, nor moist-cured.5. The prestressing steel must be normal or low-relaxation.6. There must be Average exposure conditions at the site.

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Thumb rule of Losses

• For average steel and concrete properties ,the tabulated percentages may be taken as representative of the average losses.

Pretensioning ,% Posttensioning. %

Elastic shortening & bending of concrete

4 1

Creep of concrete 6 5

Shrinkage of concrete 7 6

Steel relaxation 8 8

Total Loss 25 20

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Comparison Between RCC & Prestress Concrete

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Thank You