PRE-TENSIONING

SYSTEMS

Principle of Prestressing

Prestressing is a method by which predetermined

compression force is applied to the concrete section

before loading.

The effect of prestressing is to reduce the tensile stress

in the section to the point that the tensile stress is below

the cracking stress. Thus, the concrete does not crack!

RC vs. PSC

Reinforcement stops cracking in tension zone from opening

Prestressing stops the concrete going into tension

Advantages of PC over RC

Take full advantages of high strength concrete and high strength steel

Need less materials

Smaller and lighter structure

No cracks

Use the entire section to resist the load

Better corrosion resistance

Good for water tanks and nuclear plant

Very effective for deflection control

Better shear resistance

Applications of Prestressed Concrete

Bridges

Slabs in buildings

Water Tank

Concrete Pile

Thin Shell Structures

Offshore Platform

Nuclear Power Plant

Repair and Rehabilitations

Classification and Types

Pretensioning vs. Post tensioning

External vs. Internal

Linear vs. Circular

End-Anchored vs. Non End-Anchored

Bonded vs. Unbonded Tendon

Precast vs. Cast-In-Place vs. Composite

Partial vs. Full Prestressing

Prestressing Types

Pre-tensioning

• Tendons are stressed prior to concrete being placed

• Force transferred by action of bond

Post-tensioning

• Tendons are stressed after concrete has been placed

• Can be bonded or unbonded

Prestressing Applications

Pretensioning

Pre-tensioning

Tensioning of the tendons to a predetermined level,

after which the concrete is placed

After the concrete is 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

Pre-tensioning

Mechanism of transfer

Tension in the steel is transferred to the concrete

entirely by bond-“prestress transfer bond. ”

Three factors which contribute pretension bond

Adhesion between concrete and steel

Hoyer’s Effect

Mechanical interlocking

Adhesion

Adhesive bond is a chemical mechanism, between the

prestressing strand and concrete.

Easily destroyed in the presence of any strand slip or

cracking

Contribution of adhesion to the bond is effective only

till occurrence of end slip or cracking

Mechanical Interlock

When concrete is cast around the 7 wire strand, it

forms the helical shape.

This helical shape prevents the prestressing strand from

twisting- mechanical interlock.

At the event of a crack, slip occurs and mechanical

interlock acts to prevent further slippage of the

prestressing strand.

Effective only if there is no twisting of strand

Hoyer's Effect or wedge action

Pretension causes reduction in diameter of strand,

proportional to Poisson’s ratio

Upon release, steel restrained from regaining its

originial dimension

Restraint causes the high radial pressure on the concrete

that in turn causes high frictional resistance along the

longitudinal axis of the prestressing strand.

This frictional resistance opposes any relative movement

between the prestressing strand and the concrete

Methods of

Pretensioning

Pretensioning with Individual mould

Tendons are anchored directly

to the individual steel moulds

in which the concrete is cast.

Moulds must be designed &

constructed to withstand the

additional forces induced by

the tendons

Mould Method

Pretensioning on stressing beds

The tendons are tensioned between and

subsequently anchored to the rigid vertical steel

anchor columns

Pretensioning on stressing beds

Long Line Pretensioning

Fixed Bed Rolling Bed

Flow line pretensioning

Sectional flow line

Simultaneous flow line

Long line or Hoyer Method

When several pre-tensioned concrete members are to

be manufactured.

Tendons are stretched between 2 bulk heads, a great

distance apart, so that a number of similar units may

be cast

Hoyer Method-Contd..

When concrete attains the required strength, jacks are

released and tendon wires in-between various units are

cut

Advantage of this method is that only two stretching

mechanisms (two jacks - one at each end or one jack at

one end and one anchoring device at the other end)

Fixed bed technologies

Elements remain in the same place during the entire

production and curing cycle

Different equipment (with or without operators) moves

along the line to perform the appropriate processes on

consecutive stations.

Bridge girders and double and single tee units are

manufactured most frequently in fixed beds with full

length fixed molds

Horizontal Shear

Horizontal shear imposed - large enough to cause

cracking

pull- off mold - molds are raised or lowered from

hardened concrete but before release of prestress

Slipping or rolling molds-before release of prestress,

the finger plates are removed, thus yielding a gap which

allows for mold to move longitudinally by means of low

friction rollers

Rolling bed technologies

Prestressed concrete members are manufactured in long

steel rolling beds which are capable of temporarily

holding the whole prestressing force

Hollow core floor slabs and sandwich wall panels are

manufactured by rolling bed method

Flow line Pretensioning method

Individual self stressing molds or pallets move through

the plant from one station to another

Moving of molds is performed by roller, chain or wheel

conveyors, sliding platforms or by crane

classified according to the movement of the molds

simultaneous (synchronous) movement

sectional (asynchronous) movement.

Flow line Methods

Simultaneous

Movement of the molds is

performed along the whole

conveyor simultaneously

Highly automated technology

can be used only for mass

production of standard

products

Sectional

Each mold or group of molds is

moved independently among

the processing areas of the plant

Allows simultaneous

production of different types of

elements on the same line

Stages and Devices

Stages

Anchoring of tendons against

the end abutments

Placing of jacks

Applying tension to the tendons

Casting of concrete

Cutting of the tendons.

Devices

Prestressing bed

End abutments

Shuttering / mould

Jack

Anchoring device

Harping device (optional)

Jacks

To apply tension to the

tendons

Individually stressing or

simultaneous stressing

(by special jacks) from

one end of the stressing

bed.

Anchoring device

Used to maintain tendons in tension.

Once tensioned to required level, jack is released

and the wedges lock against the sides of tendon and

barrel as tendon contracts

The barrels bear against an anchor block which

transmits the tensioning force via PS bed to the other

end

Anchoring Devices

After hardening,

prefabricated stool is

inserted between

anchor block and jack

Anchorage relieved,

jack pressure released

and PS force transferred

Release of Tendons

Tendons are released individually either by flame

cutting, sawing or by hydraulic cutters

Tendons are released simultaneously by making use of

hydraulic rams.

Upon release of the prestressing force, cracking at the

upper face near the ends can happen. To avoid

Draped or Harped tendons

Blanketed Tendons

Harped Tendons

Deflect some of the tendons to obtain the desired cable

profile

Held in their deflected position by special hold-down

devices at the lower deflection points (also called hold

down or draping points) and by hold-up devices at the

high positions

Blanketed Tendons

Concrete is prevented from bonding for some of the

pretensioned tendons

Plastic tubing which surrounds the ”blanketed” tendons

for certain lengths measured from the ends of the

elements.

Applications

Railway sleepers

Floor joists

Beams

Floor units,

Poles

Piles etc

Prestressed Piles

The prestressing strands are the only longitudinal

reinforcement in the piles.

The prestressing process introduces compression into

the pile, which counteracts the tensile stress resulting

from handling, driving eccentricity, and stress waves

generated during driving

Prestressed Piles

Square piles-fewer strands to obtain the same effective

prestress

Octagonal piles, Strands typically are positioned in a

circular pattern confined within a circular spiral

Railway Sleepers

(a)Travelling pre-tensioning stress bench (b)Anchoring of strands

(c)Stretching of strands

Railway Sleepers

(a)Stretching of strands (b)Pouring (c)Steam curing

(a) Cutting of strands (b)Demoulding (c)Stacking

Railway Sleepers

(a)Water curing (b)Storage and dispatching of sleepers