post tension slabs in residential building

POST TENSION SLAB IN

RESIDENTIAL BUILDING

WHAT IS POST TENSIONING SYSTEM?

In post-tensioning, the concrete units are first cast by incorporating ducts or grooves to house the tendons .when the concrete attains sufficient strength, the high-tensile wires are tensioned by means of jack bearing on the end face of the member and anchored by wedges or nuts.

GENERAL STRUCTURE OF PT

PRINCIPLES OF POST TENSIONING

Wedge action producing a frictional grip on the wires. Direct bearing from rivet or bolt heads formed at the

end of the wires. Looping the wires around the concrete.

IN SLABSTwo categories of PT system are

used:

UN-BONDED SYSTEM

&BONDED SYSTEM

BONDED SYSTEM

• For bonded system the post-tensioned strands are installed in galvanized steel or plastic ducts.

• The duct can be either circular-or oval-shaped.• The anchorage transfers the force from the stressing

jack into the concrete.• The duct and the strands contained within are

collectively called a tendon.• Due to the concentrated arrangement of the strands

within the ducts a high force can be applied to a small concrete section.

UN –BONDED SYSTEM

• In an unbonded system the individual steel strands are encapsulated in a polyurethane sheath & the voids between the sheath & the strand are filled with rust-inhibiting grease

• The tendon can be prefabricated off site.• The force loss due to friction is lower than for bonded tendons due to the

action of the grease. • A broken tendon cause’s prestress to be lost for the full length of that

tendon.• Careful attention is required in design to ensure against progressive

collapse.

ARRANGEMENT OF UNBONDED POST-TENSIONING SYSTEM

UNBONDED POST-TENSIONING CONSTRUCTION

DIFFERENT POST-TENSIONING SYSTEMS

FREYSSINET SYSTEM

GIFFORD-UDALL

LEE-MC CALL SYSTEM

MAGNEL- BLATON SYSTEM

BBRV PT SYSTEM

PRESCON SYSTEM

BAUR-LEONHARDT SYSTEM

DYWIDAG SINGLE-BAR SYSTEM

FREYSSINET SYSTEM

Widely used in Europe and India It consists of cylinder with a conical interior Large no of wires or strands can be

simultaneously tensioned using the double acting hydraulic jack.

GIFFORD-UDALL SYSTEM

The system developed in UK It consists of steel spilt-cone and

cylindrical female cone anchorages. In this system each wire is tensioned

separately

LEE-MC CALL SYSTEM

The tendons comprise high –tensile bars of diameter varying from 12-40 mm which are thread at ends.

After tensioning each bar is anchored by screwing a nut & washer tightly against the end plates

It has a disadvantage in that curved tendons cannot be used

MAGNEL-BLATON PT SYSTEM

Adopt metallic sandwich plates, flat wedges and a distribution plate for anchoring the wires

Sandwich plate can house up to four pairs of wires

The no. of wires in the magnel cable varies from 2-64

BBRV SYSTEM

Developed in1949 by four Swiss engineers-Birkenmeier,Brandestini,Ros&Vogt.

This system well suited for transmitting large forces

At present tendons capable of developing forces up to 12000KN are available under this system

ONE WAY & TWO WAY SLAB

• One way floors carry the applied loading primarily in one direction and are treated as beams or plane frames.

• Two-way spanning floors have the ability to sustain the applied loading in two directions.

STRUCTURAL FORMS

STRUCTURAL FORMS

MATRIALS & EQUIPMENT

MATERILS

• FORM WORK

• CONCRETE

• STRANDS

• TENDONS

• DUCTS

• ANCHORAGE

• WEDGES

FORM WORK

• The formwork for a PT floor is similar to that for a reinforced concrete floor.

Tendons arrangement inside formwork prior of casting

STRAND

• Strand is made from seven cold drawn high carbon steel wires.

• Seven individuals wires, with six wires helically wound to a long pitch around a centre wire.

• All strands should be grade 1860 Mpa• Strand is mostly available in two nominal size

12.7mm&15.7mm diameter

TENDONSTendons protection

Unbonded tendons

• unbonded tendons are protected by a layer of grease inside a plastic sheath.

UNBONDED TENDONS

• Single strand coated with corrosion inhibiting grease and encased in polyethylene sheathing• PT force is transferred to the concrete by

the anchors provided at the ends

BONDED TENDONS

BONDED TENDONS

ADVANTAGES

• Higher flexural capacity.

• Good flexural crack distribution.

• Good corrosion protection.

• Flexibility for later cutting of penetrations.

• Easier demolition.

DUCTS

• The duct is the sheath for the PT strand or bar.• The duct must be accurately fabricated, placed

and secured.• Duct materials are required to be high density

polyethylene(HDPE),polypropylene or schedule 40galvanized steel pipes.

ANCHORAGES

• The anchorage points apply additional horizontal force and vertical forces to the existing structure.

The factors that need to be considered in the design of anchorages are:• Attached to soffit.• Through a beam or column.

WEDGES

EQUIPMENTS1.Stressing jack2.Swaging jack3.Strand threading

machine4.Strand cutters or

shears5.Grout

mixer&pump

Monojack

Prestressing jack. STRESSING JACKS

Tensioning of strand cables using a monostrand jack

CURVED NOSE JACKS

ADVANTAGES OF PT

1.Longer span2.Overall structural

cost3.Reduced floor to

floor height.4.Deflection &cracks

control5.Waterproof slabs6.Early formwork

stripping7.Materials handling8.Fast construction

SAVINGS

Exponential saving for spans > 7.3 m

ECONOMICS

REDUCED CONSTRUCTION TIME: 1000m2 every 5 days

ESSENTIAL ADVANTAGES OF PRESTRESSING:

� Reduce the Quantity of Concrete;� Reduce Quantity of Reinforcement;� Simplify Reinforcement Works;

This will lead to:� Decrease cost of material for slab;

DISADVANTAGES

Since there are a number of tendons and wires spread inside the post tension slab, it can result in corrosion.

Complexity of work.

Poor workmanship can lead to accidents.

APPLICATIONS OF PT It used to fabricate large members, such as long

–span bridge decks of the box – girder type by prestressing together a number of smaller precast units

The chief merit of post –tensioning is that it allows the use of curved and stopped-off cables

Post-tensioning is variably used for strengthening concrete dams, circular prestressing of large concrete tanks & biological shields of nuclear reactors

EXAMPLES OF PT

THANK YOU FOR YOUR ATTENTION