PRESTRESSED CONCRETETYPES OF PRESTRESSINGBy LAKSHMINARAYANAN M S7C1, Roll No. 25

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OVERVIEW PRETENSIONING SYSTEMS POST- TENSIONING SYSTEMS

THERMO-ELECTRIC

PRESTRESSING CHEMICAL PRESTRESSING2

PRETENSIONING SYSTEMSTensioning of Tendons is Done Before Casting

INTRODUCTION

STAGES OF PRETENSIONING ADVANTAGES OF PRETENSIONING DISADVANTAGES DEVICES Hoyers System3

INTRODUCTION Tensioning of Tendons is Done Before Casting

Prestress transferred to concrete due to the bond Occurs only over a definite length from ends called

Transmission Length The member undergoes elastic shortening. If the tendon is located at an eccentricity-

The member bends up (camber).4

STAGES OF PRETENSIONING1) Anchoring of tendons against end abutments

(bulkheads)2) Placing the jacks

3) Applying tension4) Casting the concrete member

5) Cured until desired strength is gained6) Cutting of tendons5

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ADVANTAGES Suitable for precast members produced in bulk

Eg: Railway sleepers, electric poles etc Large anchorage devices not required

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DISADVANTAGES Suitable prestressing bed is required; Long waiting period; Strength depends on transmission length.

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DEVICES1. Prestressing bed 2. End abutments 3. Mould 4. Jack 5. Anchoring device 6. Harping device10

Prestressing Bed

Abutments should be stiff and should have goodfoundation.

Which is usually expensive.So a better solution is an self-equilibrating system.11

Self- equilibrating Frame

P/2

P

P/2

Tension Frame12

FBD of Tension Frame

Hoyers System

Also called Long Line Method End abutments are kept sufficient distance apart Several members are cast in a single line. Shuttering provided at sides and as separators. Hydraulic jacks, travelling stress bench, steel wedge anchorages are generally used.13

Anchoring Devices Usually wedge and friction principle. In pretensioning, the tendons are held in tension for a

longer period Usually, cheap quick-release grips are used.

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Chuck assembly for anchoring tendons15

Harping Devices Hold down anchors Jack action

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POST-TENSIONING SYSTEMS INTRODUCTION

STAGES ADVANTAGES DEVICES SYSTEMS BASED ON ANCHORING DEVICES17

INTRODUCTION The ducts are placed along with the reinforcement The tendons are placed in the duct after casting Thus, there is no contact between tendons and

concrete The prestress is transferred by end reactions at the 2

ends18

STAGES OF POSTTENSIONING1) Casting of concrete 2) Placement of ducts/ tendons 3) Placement of anchorage blocks and jack 4) Applying tension to the tendons 5) Seating of the wedges 6) Cutting of the tendons19

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ADVANTAGES Suitable for heavy cast-in-situ members.

Waiting period is less Transfer of prestress is independent of transmission

length.The only DISADVANTAGES are the

Requirement of anchorage and grouting devices21

DEVICES1. Casting bed

2. Mould3. Ducts

4. Anchoring devices5. Jacks

6. Couplers7. Grouting equipments22

Anchoring Devices Based on 3 principles: o Wedge Action o Direct Bearing o Looping the Wires

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Wedge Action Consists of Anchorage block and Wedges. Strands are held by friction grip of the wedges

The popular patented systems based on wedge action are: Freyssinet anchorages Magnel-Blaton anchorages Gifford-Udall anchorages Lee McCall anchorages24

Freyssinet Consists of two pieces (wedges). Piece 1:- Hollow Cylinder with Conical Interior. Piece 2:- Conical Plug lined longitudinally

- has groves to house the wires. The tendons pass through the hollow cylinder. The

plug wedges the tendons against its walls by friction. Advantage: Many wires can be tensioned

simultaneously by a double acting jack.

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Freyssinet T System Anchorage Cones

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Conical Plug/Wedge

Hollow Cylinder

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Magnel-Blaton System Modification of Freyssinet.

Consists of sandwich plates

and flat wedges; both

grooved.Sandwich Plate Each plate can hold 8 wires. The space between the wires

Wedge28

is maintained by the spacers.

Gifford-Udall Consists of:- Metallic ducts,

- Steel Split Cone, and - Cylindrical Female-Cone Anchorages. These house the steel wires bearing on a steel plate

(at the end). Each wire is tensioned separately and anchored by

forcing a sleeve wedge into a cylindrical grip29

(resting against the steel grip).

Gifford-UdallTube Anchorages Plate Anchorages

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Lee McCall For wires of large diameters (bars);

diameter varies from 12 t 40 mm The bars are threaded at ends

After tensioning they anchored by

tightening a nut and washer against

the end plate (at the threaded end).Disadvantage: Cannot be adopted for curved tendons32

Direct Bearing The tendons are anchored by bearing action

The popular systems include:

B.B.R.V Prescon

system system

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B.B.R.V Suitable for transmitting very large forces. A B.B.R.V. tendon consists of several parallel wires The ends of each of this terminate with a

cold-formed button-head with a machined anchorage fixture. In case of strands (not wires), split sleeve cones are

used in addition, to anchor the button heads to the34

machined fixture.

forces upto12000 kN 90-170 wires

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Prescon Modification of the BBRV system In addition,o Steel wires are threaded with washers tightened before

the button headso Steel shims are inserted after stressing for better

bearing.

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Looping of Wires Double tendons are wrapped around the end block,

which is D-shaped Theses end blocks (outside the structural member) are then forced to move by jack action. The final gap is concreted.o Baur-Leonhardt system. o Leoba system

o Dwidag single-bar anchorage system

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THERMO-ELECTRIC & CHEMICAL PRETENSIONING

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THERMO ELECTRIC Wires are coated with thermoplastic material. Then they are embedded in concrete as usual After hardening, the wires are heated by low voltage

high current. Temperature rises to 300-400C in 3-5 min. Causes elongation of bars by 0.4-0.5 % On cooling, the bars shorten; but are checked by end

anchors. The prestress is transferred by reaction at end anchors.39

CHEMICAL Makes use of expanding cement. Linear expansion of the cement is 3-4 % Prestress is developed when the steel wires check the

expansion of concrete. Initial prestress upto 4-6.5 N/mm2 (Compressive)

Suitable for pipes, thin walls & slabs, shells etc.

Disadvantage: Low value of prestress40