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.

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 SLABS
Two 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

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.

MATRIALS &
EQUIPMENTMATERILS
• 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

TENDONS
Tendons 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
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.

EQUIPMENTS
1.Stressing jack
2.Swaging jack
3.Strand threading
machine
4.Strand cutters or
shears
5.Grout mixer&pump
Monojack
Prestressing jack.
STRESSING JACKS

Tensioning of strand cables using a monostrand jack

ADVANTAGES OF PT
1.Longer span
2.Overall structural cost
3.Reduced floor to floor
height.
4.Deflection &cracks
control
5.Waterproof slabs
6.Early formwork
stripping
7.Materials handling
8.Fast construction

SAVINGS
Exponential saving for spans > 7.3 m
ECONOMICS

REDUCED CONSTRUCTION TIME: 1000m2 every 5 days

ESSENTIAL ADVANTAGES OF PRESTRESSING:
ƒ Reduce the Quantity of Concrete;
ƒ Reduce Quantity of Reinforcement;
ƒ Simplify Reinforcement Works;
This will lead to:
ƒ 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

post tension slabs in residential building

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