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.

J.D. COLLEGE OF ENGINEERING, NAGPUR

Basic Concept

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.

J.D. COLLEGE OF ENGINEERING, NAGPUR

History…

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. 1938 Hoyer, E., (Germany)

Developed ‘long line’ pre-tensioning method.

1940 Magnel, G., (Belgium) Developed an anchoring system for post-tensioning, using flat wedges.

J.D. COLLEGE OF ENGINEERING, NAGPUR

History… 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

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.

Materials for pre-stress concrete member

1. Cement,

2. Concrete,

3. Steel.

# Cement:

# Ordinary portland cement,

# Portland slag cement,

# Rapid hardening portland cement,

# High strength ordinary portland cement.

# Concrete:

Pre-stress concrete requires high strength concrete, which has high compressive strength comparatively higher tensile strength than ordinary concrete.

The concrete is a material should be compose of gravels or crushed stones, sand, cement.

In pre-stress concrete minimum grade of concrete M20.

#Steel:

High tensile steel, tendons, strands.

In pre-stress concrete high tensile steel with tensile strength around 2000MPa.

According to IS: 1343-1980 prestress concrete is design.

Types of Pre-stressing1. 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-tensioning: A method of prestessing concrete in which tendons are tensioned before the concrete is placed.

The operation commonly a 5-step process.

Post-tensioning: A method of prestessing concrete in which tendons are tensioned after the concrete has place.

The operation commonly a 6-step process.

Methods 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.

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.

Popular post-tensioning system: 1) Freyssinet system

2) Magnel blaton system

3) Gifford-udall system

4) Lee-mccall system

Section for Pre-tensioning

Section for Post-tensioning

Advantages of pre-stressed concrete. Factory products are possible. Long span structure are possible so that saving of wt

is significant & thus it become economical. Pre-stressed member are tested before use. Dead load are get counter balanced by eccentric pre-

stressing It has high ability to resist the impact. It has high fatigue resistance. It has high live load carrying capacity. It free from cracks from service loads and enable

entire section to take part in resisting moments. Member are free from the tensile stresses.

Disadvantages of pre-stressed concrete. Required skilled builders & experienced engineers. Initial equipment cost is very high. Availability of experienced engineers is less. Required complicated formwork. It requires high strength concrete & steel. Pre-stressed concrete is less fiber resistant.

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BY: PARAG KAMLAKAR PAL.