05 b prestressed concrete

8/10/2019 05 B Prestressed Concrete

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CONSTRUCTION TECHNOLOGY- 2

CHAPTER 2. PRESTRESSED CONCRETE


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Structural

Concrete:

Comparisonof methods


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Principles of Prestressing

The concrete has high compressive strength

and low tensile strength.

The basic concept of RCC is to include a

designed amount of steel bars to give

concrete a reasonable amount of tensile

strength.


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Principles of Prestressing


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In prestressed concrete a pre compression isinduced into the member to make full use of

its own compressive strength when loaded.


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Comparison of Reinforced and Prestressed Concrete

Reinforced concrete beam

Compression in upper fiber

Close spaced stirrups are provided to give resistance to diagonal

tension or shear near the supports.

Main reinforcing bars to provide tensile strength

Prestressed beam:

Slight initial tension in upper fibers

Hydraulic jack is used to stress tendons

Stretched and anchored tendons induces compression in trying toregain original length

Tension cracks will not occur.


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Materials for prestressing

Tendons: These are small diameter wires (2 to 7mm) round, crimped or indented format.

The wires may be single or grouped in to form acable. It may be in the form such as 7 wires (6over1) or 19 wires (9 over 9 over 1). These arecalled as strands.

The main advantages of strands are:

A large prestressing can be provided over arestricted area.

Strand can be supplied in long flexible length andcan be stored on drums.


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Prestressing Methods

There are two methods of producing pre-stressed concrete members:

Pre-tensioning 2.Post-tensioning

Pre-Tensioning

In this method the wires or cables are stressed before concrete is castaround them. The stressing wires are anchored at one end of the mould

and stressed by hydraulic jack from the other end until the required stressis obtained.

It is common practice to overstress the wire by some 10% to counteractthe anticipated losses, which will occur due to creep, shrinkage andrelaxation.

After stressing the wires the side forms of the mould are positioned and

the concrete is placed around the tensioned wires. The casting is then usually steam cured for 24 hours to obtain the desired

characteristic strengths of 28 N / mm2in 24 hours.

The bond between the stressed wires and concrete will prevent thetendon from regaining their original length thus inducing the pre-stress.


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Usually small diameter wires 2 to 5 mm are

used so that for any given total area of

stressing wire a greater surface contact area is

obtained. Mainly the manufacturer of precast

components such as floor units and slabs uses

this method


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Post-Tensioning

In this method the concrete is cast around ducts in which thestressing tendons can be housed and stressing is carried out afterthe concrete has hardened.

The tendons are stressed from one end or at both ends and whenstress required has been reached the tendons are anchored at their

ends to prevent them from returning to their original length thusinducing a compressive force.

The ducts for housing the stressing tendons can be formed by usingflexible steel tubing or inflatable rubber tubes.

The placing of the tendons can be done after the casting and curingof the concrete has been completed.

The remaining portion of the ducts is filled with grout to stop anymoisture present setting up a corrosive action and to assist thestress distribution.


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Post tensioning method is usually used where

stressing is to be carried out on site.

Curved tendons can the complete member is

to be formed by joining together a series of

precast units. This type of is used where

prestressing is done for beams of bridges or a

flyover bridge.


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The advantages and disadvantages of

prestressed concrete when compared

with conventional reinforced concrete

are:Advantages: Make full use of inherent compressive strength of concrete.

Make full use of special alloy steel s used to form prestressingtendons.

Eliminates tension crakes thus reducing the risk of corrosion of steelcomponents.

Reductions in shear stresses.

For any given span and loading condition, a smaller cross section is

sufficient thus reducing the weight of the member. Individual member can be joined together to act as a single

member.


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Disadvantages

High degree of control of materials, design andworkmanship is required.

Special alloy steel is costlier than the mild steel.

Extra cost on special equipment is required tocarry out the stressing activities.

Up to 6.000 m span traditional RCC is the mosteconomic method. Span between 6 to 9 m thetwo mediums are compatible. Over 9 m spanprestressed concrete is more economical thanRCC.


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Thank You

05 b prestressed concrete

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