The document provides information on methods of prestressing in concrete, including pretensioning and post-tensioning. It discusses: - Pretensioning involves stressing steel tendons before the concrete is cast around them. - Post-tensioning involves stressing steel tendons after the concrete has cured using jacks, then grouting the voids. - Both methods put the concrete in compression and increase its strength and durability compared to conventional reinforced concrete.

1. Seminar Presentation on
METHODS OF PRESTRESSING
IN CONCRETE
Submitted To-
Mr. S.C.Verma
Department of civil engg.
COER
Submitted By-:
Abhishek Jain
Civil Engg.
Section – E (T1)
Roll No.-03

2. METHODS OF PRESTRESSING
IN CONCRETE
PRESTENSIONING & POST- TENSIONING

3. PRESTRESSED CONCRETE
 PRINCIPLE – Using high tensile strength
steel alloys producing permanent pre-
compression in areas subjected to Tension.
 A portion of tensile stress is counteracted
thereby reducing the cross-sectional area of
the steel reinforcement .
 METHODS :- a) Pretensioning
b)Post-tensioning
 PRETENSIONING :- Placing of concrete
around reinforcing tendons that have been
stressed to the desired degree.
 POST-TENSIONING :- Reinforcing tendons
are stretched by jacks whilst keeping them in
serted in voids left pre-hand during curing of
concrete.
 These spaces are then pumped full of grout to
bond steel tightly to the concrete.
STEEL BARS BEING
STRETCHED BY JACKS

4. POST - TENSIONING
 WHAT IS POST-TENSIONING?
 Post-tensioning- is a method of reinforcing
(strengthening) concrete or other materials with high-
strength steel strands called tendons.
 Post-tensioning allows construction that would
otherwise be impossible due to either site constraints
or architectural requirements.
 Requires specialized knowledge and expertise to
fabricate, assemble and install.
 After adequate curing of concrete, reinforcing
tendons (placed in side the voids of the structure) are
tensioned/stretched by jacks on the sides & grouts
filled with appropriate mix.
 Applications – a) Structural members beams,
bridge-deck panels, Roof –Slabs, Concrete Silos Etc.

5. POST –TENSIONING METHOD

6. BENEFITS
 Concrete is very strong in compression but weak
in tension,
 This deflection will cause the bottom of the beam
to elongate slightly & cause cracking.
 Steel reinforcing bars (“rebar”) are typically
embedded in the concrete as tensile
reinforcement to limit the crack widths.
 Rebar is what is called “passive” reinforcement
however; it does not carry any force until the
concrete has already deflected enough to crack.
 Post-tensioning tendons, on the other hand, are
considered “active” reinforcing.
 Because it is prestressed, the steel is effective as
reinforcement even though the concrete may not
be cracked .
 Post-tensioned structures can be designed to
have minimal deflection and cracking, even
under full load.
Post –Tensioned Structure

7. ADVANTAGES/APPLICATIONS
 Post-tensioning allows longer clear spans, thinner
slabs, fewer beams and more slender, dramatic
elements.
 Thinner slabs mean less concrete is required. It
means a lower overall building height for the same
floor-to-floor height.
 Post-tensioning can thus allow a significant
reduction in building weight versus a conventional
concrete building with the same number of floors
reducing the foundation load and can be a major
advantage in seismic areas.
 A lower building height can also translate to
considerable savings in mechanical systems and
façade costs.
 Another advantage of post-tensioning is that beams
and slabs can be continuous, i.e. a single beam can
run continuously from one end of the building to
the other.
 Reduces occurrence of cracks .
 Freezing & thawing durability is higher than non
prestressed concrete.
This innovative form is result of
post tensioning.
Bridge decks

8.  Post-tensioning is the system of choice for parking structures
since it allows a high degree of flexibility in the column layout,
span lengths and ramp configurations.
 In areas where there are expansive clays or soils with low
bearing capacity, post-tensioned slabs-on-ground and mat
foundations reduce problems with cracking and differential
settlement.
 Post-tensioning allows bridges to be built to very
demanding geometry requirements, including complex
curves, and significant grade changes.
 Post-tensioning also allows extremely long span bridges to be
constructed without the use of temporary intermediate
supports. This minimizes the impact on the environment
and avoids disruption to water or road traffic below.
 In stadiums, post-tensioning allows long clear spans and very
creative architecture.
 Post-tensioning can also be used to produce virtually crack-free
concrete for water-tanks.
 The high tensile strength & precision of placement gives
maximum efficiency in size & weight of structural members.
 Applications of various prestressed techniques enable quick
assembly of standard units such as bridge members,building
frames, bridge decks providing cost-time savings.

9. CONSTRUCTION
 In slab-on-ground construction, unbonded tendons
are typically prefabricated at a plant and delivered to
the construction site, ready to install.
 The tendons are laid out in the forms in accordance
with installation drawings that .
 After the concrete is placed and has reached its
required
strength, usually between 3000 and 3500 psi
(“pounds per
square inch”), the tendons are stressed and
anchored.
 The tendons, like rubber bands, want to return to
their original length but are prevented from doing so
by the anchorages.
 The fact the tendons are kept in a permanently
stressed
(elongated) state causes a compressive force to act
on the
concrete.
 The compression that results from the post-tensioning
counteracts the tensile forces created by subsequent
applied loading (cars, people, the weight of the
beam itself when the shoring is removed).
 This significantly increases the load-carrying capacity
of the concrete.
 Since post-tensioned concrete is cast in place at the
job site, there is almost no limit to the shapes that can
be formed.
Limitations of Prestressing
The limitations of prestressed concrete are few and really
depend only upon the imagination of the designer and the
terms of his brief. The only real limitation where
prestressing is a possible solution may be the cost of
providing moulds for runs of limited quantity of small
numbers of non-standard units.

10. Method of post-tensioning
TENDONS
Wedges tensioned by
jacks
Tendons

11. PRESTRESSED CONCRETE
 Prestressed concrete, invented by Eugene
Frevssinet in 1928 is a method for overcoming
concrete’’s natural weakness in tension . It can
be used to produce beams , floors or bridges with
a longer span than is practical with ordinary
reinforced concrete.
 It can be accomplished in three ways: pre-
tensioned concrete, and bonded or unbonded.
Pre-tensioned concrete
 Pre-tensioned concrete is cast around already tensioned
tendons.
 This method produces a good bond between the tendon
and concrete, which both protects the tendon from
corrosion and allows for direct transfer of tension.
 The cured concrete adheres and bonds to the bars and
when the tension is released it is transferred to the
concrete as compression by static friction.
 However, it requires stout anchoring points between
which the tendon is to be stretched and the tendons are
usually in a straight line.
 Thus, most pretensioned concrete elements are
prefabricated in a factory and must be transported to the
construction site, which limits their size.
 Pre-tensioned elements may be balcony elements, lintels
, floor slabs, beams or foundation piles.

12. Bonded post-tensioned concrete
 Bonded post-tensioned concrete is the descriptive term for a
method of applying compression after pouring concrete and the
curing process (in situ).
 The concrete is cast around a plastic, steel or aluminium curved
duct, to follow the area where otherwise tension would occur in
the concrete element.
 A set of tendons are fished through the duct and the concrete is
poured. Once the concrete has hardened, the tendons are
tensioned by hydraulic jacks.
 When the tendons have stretched sufficiently, according to the
design specifications they are wedged in position and maintain
tension after the jacks are removed, transferring pressure to the
concrete.
 The duct is then grouted to protect the tendons from corrosion.
This method is commonly used to create monolithic slabs for
house construction in locations where expansive soils create
problems for the typical perimeter foundation.
 All stresses from seasonal expansion and contraction of the
underlying soil are taken into the entire tensioned slab, which
supports the building without significant flexure. Post-stressing
is also used in the construction of various bridges.
 The advantages of this system over unbonded post-tensioning
are:
DECK STEEL LAYING

13.  Large reduction in traditional reinforcement
requirements as tendons cannot destress in
accidents.
 Tendons can be easily 'weaved' allowing a
more efficient design approach.
 Higher ultimate strength due to bond
generated between the strand and concrete.
 No long term issues with maintaining the
integrity of the anchor/dead end.
Unbonded post-tensioned
concrete
 Unbonded post-tensioned concrete differs
from bonded post-tensioning by providing
each individual cable permanent freedom of
movement relative to the concrete.
 To achieve this, each individual tendon is
coated with a grease (generally lithium
based) and covered by a plastic sheathing
formed in an extrusion process.
 The transfer of tension to the concrete is
achieved by the steel cable acting against
steel anchors in the perimeter of the slab.
 The main disadvantage over bonded post-
tensioning is the fact that a cable can
destress itself and burst out of the slab if
damaged (such as during repair on the slab).
The advantages of this system over bonded
post-tensioning are:

14. External Prestressing
 This refers to the case where prestressing tendons are
placed outside the concrete section and the
prestressing force is transferred to a structural member
through end anchorages or deviators. Advantages of
external prestressing include the possibility of
monitoring and replacing tendons, ease in concreting
and hence better concrete quality and the use of
narrower webs. External prestressing is being
increasingly used in the construction of new bridges
and is a primary method for the strengthening and
rehabilitation of existing structures.
 At NUS, a three-year project on the application of
external prestressing in structural strengthening has
been completed, and this has resulted in design charts
being developed for such applications. Works were
also carried out on the use of fibre-reinforced polymer
(FRP) reinforcement as external tendons in both simply
supported and continuous beams.

15. • Fallingwater is comprised of a series of concrete cantilever
“trays” 30-ft. above a waterfall. Previous efforts failed to
permanently address excessive deflections of the cantilever
and repair the cracks. After a thorough design review, the
owner and engineer selected an external post-tensioning
solution for its durability, aesthetics and structural
unobtrusiveness.
• Construction plans called for strengthening of three support
girders spanning in the north-south direction with multistrand
post-tensioning tendons consisting of multiple 0.5” diameter
strands.
• Thirteen strand tendons were placed on each side of two
girders. One 10-strand tendon was placed on the western side
of the third girder (access on the eastern side of this girder
was not available). Eight monostrand tendons, 0.6” diameter,
were slated for the east-west direction.
•The monostrand tendons were stressed in the east-west
direction and then the multistrand tendons were stressed in the
north-south direction and grouted with a high quality, low-bleed
cementitious grout mixture.
•VSL’s scope of work also included welding steel cover plates,
attaching structural steel channels, injecting epoxy grout,
doweling reinforced cast in place concrete blocks and the
installation of near surface mounted carbon fiber rods.
Challenged with maintaining Fallingwater’s original setting,
furnishings and artwork, the project was successfully
completed in six months.
The lower and upper terraces cantilever
over the stream below. The temporary
structural steel shoring was placed beneath
the main level terrace.
Frank Lloyd Wright's Fallingwater
Mill Run, Pennsylvania
APPLICATIONS

16.  Cline Avenue Bridge
Gary, Indiana
 The Cline Avenue Bridge (SR 912) is a predominately cast-in-place post-tensioned
structure located in Gary, Indiana. The bridge mainline is over 6,000 LF, has two
adjacent segments nearly 35 feet wide each, and contains four connecting ramps. An
inspection and analysis team was assembled to perform a thorough investigation of
the bridge. The team concentrated on the existing post-tensioning system and
interior and exterior concrete cracks. The engineer retained VSL to assist with the
inspection of the tendons.
 VSL approached the Cline Avenue project with a guideline that outlines a statistically
sound method of sampling the tendons. A statistical sample pool (which consisted of
the mainline structure and the ramps) was defined by referencing the American
National Standard Institute’s (ANSI) guideline “Sampling Procedures and Tables for
Inspection by Attributes as published by the American Society for Quality Control
(1993).”
 The probable void locations throughout the structure’s mainline segments and ramps
were initially identified by VSL to appropriately distribute the sampling population.
Such areas consisted of high points, areas approaching and leaving the high points,
and couplers.
 Using non-destructive Ground Penetrating Radar (GPR) and field layout drawings,
VSL located existing post-tensioning tendons. Once the layout was performed,
specific tendons throughout the bridge and ramp structures were sampled by drilling
into the duct and exposing the tendon for visual inspection. The use of a borescope
allowed for detailed visual inspection of the tendon and also captured video footage
to share with the owner and the engineer. After review of each inspection, VSL
placed epoxy in the borescope hole to protect the tendons from air and moisture
intrusion. When voids were encountered, the project team observed and documented
the condition of the strand based on the PCI Journal guideline, “Evaluation of Degree
of Rusting on Prestressed Concrete Strand.” VSL used vacuum grouting technology
to fill the void, thereby protecting the previously exposed strand.
 The tendon inspection data was analyzed with other findings (such as crack survey
findings) to determine what type of rehabilitation was required. VSL’s goal to
establish a statistically sound sample of physically inspected tendons that provided
valid data as to the current state of the existing PT system was accomplishedGrouting of void using VSL’s specialized vacuum grouting equipment

17.  85th Street Bridge
Valley Center, Kansas
 The 85th Street North Bridge is a seven span post-tensioned
haunched slab bridge with a typical span of 26 meters for the
middle five spans, and 20 meters at the ends. This 170 meter
long bridge accommodates two lanes of traffic reaching over
the Wichita Valley Center Floodway. VSL post-tensioning
systems utilized for this project include 5-19 longitudinal
tendons as well as 6-4 transverse tendons.
 Post-tensioned haunched slab bridges are noted for ease of
construction. Once the geometry of the bridge falsework has
been obtained, prefabricated spacer frames are set into
place. The spacer frames serve as templates for profiling the
longitudinal post-tensioning tendons and aid in the placement
of the remaining conventional reinforcement. Transverse
tendons maintain mid-depth placement along the geometry of
the haunched slab and provide the minimum pre-
compression over the length of the structure.
 The fi nished product has several advantages over
conventionally reinforced concrete. Dead loads are balanced
by the use of longitudinal post-tensioning reducing the
sustained loading and associated creep. Corrosion
resistance is increased due to the encapsulation of the post-
tensioning reinforcement. Through the use of transverse
post-tensioning, added compression improves the longevity
of the structure by adding resistance to de-icing methods
such as salt and magnesium chloride. Post-tensioned
haunched slab bridges allow for a larger span to depth ratio
than that of conventionally reinforced haunched slab bridges.
The labor and material savings on mild reinforcement is
another clear advantage to using post-tensioning for this
application.
Overlooking the 85th Street Bridge prior to concrete placement

18. Colorado Convention Center
Expansion
Denver, Colorado
 The Colorado Convention Center Expansion
project is a 1.4 million square foot expansion of
the existing facility. This was a multi-level
project, which included a 1,000-car attached
parking garage.
 The garage above the street was constructed
using precast tees and columns with a cast-in-
place topping slab. In order to maintain regular
spacing for the columns in the precast section
of the garage and still maintain an
unobstructed path for the road and light rail,
large post-tensioned transfer girders were
required to support several of the columns
above. The transfer girders allowed for the
placement of columns required for the precast
design despite the restricted column locations
at the street level.
 Post-tensioning the transfer girders resulted in
smaller dimensions than a conventional
reinforced concrete design, an important factor
given the girders are over 7 feet high and up to
7 feet wide and a larger section would not fit
within the space constraints of the building.
The girders could not be stressed until after the
precast garage was fully erected and the
topping slab poured on the truck dock.
Temporary columns were placed under the
girders to support the load until stressing.
 The effective post-tensioning force required for
the beams ranged from 2176 to 5457 kips. A
multistrand bonded system was installed

19.  The Seward Silo project involved the post-tensioning of three
interconnected ash silos that are part of the Seward Re-Powering
Project in Seward, Pennsylvania. The overall project involved the
construction of a new, state-of-the-art 208 MW power plant designed to
burn low-grade coal that can not be burned in ordinary coal plants. This
is a design-build project with Drake-Fluor Daniel as the
owner/construction manager until the completed plant is turned over to
Reliant Energy, the ultimate owner.
 T.E. Ibberson Company was contracted to build three 187’-6” tall,
interconnected, in-line silos; two 82’-4” diameter fly ash silos and one
64’-8” diameter bed ash silo. The silos were built using the slip-form
method of construction and are believed to be the first interconnected
silos in the world built using post-tensioning as the primary
circumferential reinforcement.
 VSL’s work was performed from November 2003 through February
2004, during the second coldest winter on record locally. Significant
snowfall and subzero temperatures made progress challenging, yet with
a strong focus on safety, both cold-related and otherwise, the job was
completed with no incidents. The job required close coordination
between the various trades working in close proximity and constant
communication between parties working above and below VSL’s work
locations to phase the work to avoid having personnel under an active
work zone.
 The strand installation, stressing and grouting operations were
completed successfully, with cold-weather grouting made possible
through a variety of heating methods.
Seward Silo

20. THE BICYCLE WHEEL
 Bicycle wheel as we know it today - each is
associated with an application of prestressing to
a structural system.
 The first and most obvious is the tensioned
spokes - the rider's weight is carried from the
forks to the ground not by hanging off the top
spokes, but by reducing the pretension in the
lower spokes - only a couple of spokes are
carrying the load at any one time.
 The second is the pneumatic tyre, where the
compressive load is carried to the ground by
reducing the tension in the sidewall. The air
pressure in the tyre does not change when the
load is applied.
 The final prestressing system is the tyre cord,
which is shorter than the perimeter of the rim.
The cord is thus in tension, holding the tyre on
the rim, which enables the pretension in the
sidewalls to be reacted