Chapter-2 Materials For Prestressed Concrete CE7005 Prestressed Concrete
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Chapter-2
Materials For Prestressed Concrete
CE7005 Prestressed Concrete
High strength concrete
High tensile steel
Outline
Higher strength is necessary for prestressed
concrete for following reasons:
Anchorages for prestressing steel always
designed on the basis of high strength
concrete.
High strength concrete offers high resistance
in tension and shear ,as well as bond and
bearing.
High strength concrete
High strength concrete is lees liable to the
shrinkage cracks.
It also has a higher modulus of elasticity and
smaller creep strain ,resulting in smaller loss of
prestress.
By using high strength concrete the c/s area
required for member will be reduced resulting
less dead weight moment.
High strength concrete
Cold working: rolling the bars through a series of
dyes.
Stress relieving: heating the strand to about
350°C and cooling slowly. Improves plastic
deformation of steel
Strain tempering for low relaxation: heating the
strand to about 350°C while it is under
tension .Improves the stress-strain behaviour of
steel . Also the relaxation is reduced.
High strength steel
High Strength
Adequate Ductility
Bendability
High Bond
Low Relaxation
Minimum Corrosion
Properties of prestressing steel
Chapter-3
Systems of prestressing
CE7005 Prestressed Concrete
Out line
Systems in pre-tensioning
Systems in post-tensioning
Systems of prestressing
As system of prestressing involves the
process of tensioning the tendons and
securing them firmly to the concrete.
Pre- tensioning
Hoyer long line system
Freyssinet system
Magnel balton system
Gifford -Udall system
P.S.C mono wire system
Lee-mc-call system
Electrical prestressing
Chemical prestressing
Post-tensioning
Hoyer long line system
The Hoyer system is usually adopted for the
production of pre –tensioned members on a large
scale.
It consists of stretching wires between two
abutments at a large distance apart.
With this Hoyers process, several members can be
produced along one line, by providing suitable
shuttering.
After concrete has hardened ,the wires are released from abutment
The prestress is transferred to the concrete through bond between the tendons and the concrete
Hoyer long line system
• Freyssinet system was first to be introduced among
the post –tensioning systems.
• Anchor element consisting in two parts, cylindrical
piece with tapered hole (sleeve)and a cone(wedge).
• The cylindrical piece is a helically reinforced
concrete unit cast with M60 concrete.
Freyssinet system
• The tapered hole in the cylindrical piece is lined
with closely wound helix of 2.5mmǿ steel wires
of strength of 200N/mm2 is that it should be
stronger than the wires pulled through the cone.
• The cone is made with M100 concrete
• A small tube is inserted inside the cone to
facilitate grouting.
Freyssinet system
• The surface of the cone is grooved with grooves
depending on the number of wires to be stretched.
• As the wires are stretched and stressed ,released, cone
slips in to the sleeves.
• The space between the wires will be filled with the grout.
• This provides additional restraint against the slipping of
the tendons.
• Normally Freyssinet system is used for pulling 5mm wires
12 to18Nos.
Freyssinet system
The wires are arranged with four wires per layer.
The wires in the same layer and the wires in
adjacent layers are separated with a clearance of
4mm spacers.
The wires are anchored by wedging two at a
time in to sandwich plates.
The sandwich plates are about 25mm thick and
are provided with two wedge-shaped grooves on
its two faces.
Magnel Balton System
The wires are taken two in each groove and
tightened.
Then a steel wedge is driven between the
tightened wires to anchor them against the
plate.
A complete anchorage unit may consists of one to
eight sandwich plates.
Each plate can anchor eight wires.
Two wires are tensioned at a time
Magnel Balton System
Magnel Balton System
• Gifford Udall system is another wedge system.
• Small wedge is slit in to 2 units and the cut
pieces have teeth on the inner face.
• The wires are stressed and anchored one by
one in a separate cylinder using small
wedging grips.
• Theses wedge grips are called Udall grips.
Gifford Udall System (method-1)
Tube anchorage consists of a bearing plate,
anchor wedges and anchor grips.
Anchor plate may be square or circular and have
8 or 12 tapered holes to accommodate the
individual prestressing wires.
These wires are locked into the tapered holes by
means of anchor wedges.
Gifford Udall System(method-2)
In addition, grout entry hole is also provided in
the bearing plate for grouting.
• Anchor wedges are split cone wedges carrying
wires on its flat surface.
There is a tube unit which is a fabricated steel
component incorporating a thrust plate, a steel
tube with a surrounding helix.
Gifford Udall System
This method is used to prestress steel bars.
The diameter of the bar is between 12 and
28mm.
bars provided with threads at the ends are
inserted in the performed ducts.
After stretching the bars to the required
length, they are tightened using nuts against
bearing plates provided at the end sections of
the member
Lee mc call system
Wires are tensioned individually
The anchorage consists of a single piece
collect sleeve wedging in a conical hole
A steel truncated guide leads each wire from
the cable to the anchorage point along a
gentle curvature
In addition to the guide a central block is also
provided to anchor the central wires.
P.S.C Monowire system
in this method, reinforcing bars is coated with thermoplastic material
such as sulphur or low melting alloy and buried in the concrete.
After the concrete is set, electric current of low voltage but high
amperage is passed through the bar.
Electric current heats the bar and the bar elongates. Bars provided
with threads at the other end are tightened against heavy washers,
after required elongation is obtained.
When the bar cools, prestress develops and the bond is restored by
resolidification of the coating.
Electrical Prestressing
Chemical prestressing is done using expanding cement. Prestressing can be applied embedding steel in concrete made of expanding cement.
Steel is elongated by the expansion of the concrete and thus gets prestressed.
Steel in turn produces compressive stress in concrete.
Chemical Prestressing
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