METHODS OF PRESTRESSING
PRESTENSIONING & POST
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 .
Placing of concrete
around reinforcing tendons that have been
stressed to the desired degree.
are stretched by jacks whilst keeping them in
serted in voids left pre
hand during curing of
These spaces are then pumped full of grout to
bond steel tightly to the concrete.
STEEL BARS BEING
STRETCHED BY JACKS
WHAT IS POST
Concrete is very strong in compression but weak
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.
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 .
tensioned structures can be designed to
have minimal deflection and cracking, even
under full load.
tensioning allows longer clear spans, thinner
slabs, fewer beams and more slender, dramatic
Thinner slabs mean less concrete is required. It
means a lower overall building height for the same
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
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
Reduces occurrence of cracks .
Freezing & thawing durability is higher than non
This innovative form is result of
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
ground and mat
foundations reduce problems with cracking and differential
tensioning allows bridges to be built to very
demanding geometry requirements, including complex
curves, and significant grade changes.
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
tensioning can also be used to produce virtually crack
concrete for water
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
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
strength, usually between 3000 and 3500 psi
square inch”), the tendons are stressed and
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
(elongated) state causes a compressive force to act
The compression that results from the post
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
of the concrete.
tensioned concrete is cast in place at the
job site, there is almost no limit to the shapes that can
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
Method of post
Wedges tensioned by
, invented by Eugene
in 1928 is a method for overcoming
natural weakness in tension . It can
be used to produce beams , floors or bridges with
a longer span than is practical with ordinary
It can be accomplished in three ways: pre
tensioned concrete, and bonded or
tensioned concrete is cast around already tensioned
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.
concrete elements are
in a factory and must be transported to the
construction site, which limits their size.
tensioned elements may be balcony elements, lintels
, floor slabs, beams or foundation piles.
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
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
is also used in the construction of various bridges.
The advantages of this system over unbonded post
DECK STEEL LAYING
Large reduction in traditional reinforcement
requirements as tendons cannot
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.
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
itself and burst out of the slab if
damaged (such as during repair on the slab).
The advantages of this system over bonded
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.
also carried out on the use of fibre
(FRP) reinforcement as external tendons in both simply
supported and continuous beams.
Fallingwater is comprised of a series of concrete cantilever
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
solution for its durability, aesthetics and structural
Construction plans called for strengthening of three support
girders spanning in the north
south direction with multistrand
tensioning tendons consisting of multiple 0.5” diameter
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
The monostrand tendons were stressed in the east
direction and then the multistrand tendons were stressed in the
south direction and grouted with a high quality, low
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
Cline Avenue Bridge
The Cline Avenue Bridge (SR 912) is a predominately cast
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
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,
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
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
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
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 accomplished
Grouting of void using VSL’s specialized vacuum grouting equipment
85th Street Bridge
The 85th Street North Bridge is a seven span post
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
Floodway. VSL post
systems utilized for this project include 5
tendons as well as 6
4 transverse tendons.
slab bridges are noted for ease of
construction. Once the geometry of the bridge
been obtained, prefabricated spacer frames are set into
place. The spacer frames serve as templates for profiling the
tensioning tendons and aid in the placement
of the remaining conventional reinforcement. Transverse
tendons maintain mid
depth placement along the geometry of
slab and provide the minimum pre
compression over the length of the structure.
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
tensioning, added compression improves the longevity
of the structure by adding resistance to de
such as salt and magnesium chloride. Post
slab bridges allow for a larger span to depth ratio
than that of conventionally reinforced
and material savings on mild reinforcement is
another clear advantage to using post
tensioning for this
Overlooking the 85th Street Bridge prior to concrete placement
The Colorado Convention
project is a 1.4 million square foot expansion of
the existing facility. This
was a multi
project, which included a 1,000
The garage above the street was constructed
precast tees and columns with a cast
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,
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.
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
bonded system was installed
The Seward Silo project involved the post
tensioning of three
interconnected ash silos that are part of the Seward Re
Project in Seward, Pennsylvania. The overall project involved
construction of a new, state
art 208 MW power plant designed to
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’
line silos; two 82’
4” diameter fly ash silos and one
8” diameter bed ash silo. The silos were built using the slip
method of construction and are believed to be the first interconnected
silos in the world built using post
tensioning as the primary
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
The strand installation, stressing and grouting operations were
completed successfully, with cold
weather grouting made possible
through a variety of heating methods.
THE BICYCLE WHEEL
Bicycle wheel as we know it today
associated with an application of prestressing to
a structural system.
The first and most obvious is the tensioned
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
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
08 Air Powered Grout Pump
Pumps cement grout only, no sand. 32 Gallon Mixing
Tank. Mixes up to 2 sacks of material at once and allows
for grout to be pumped during mixing or mixed without
8 gallons per minute
at 150 psi
Colloidal Grout Plant
The heavy duty, high volume Colloidal Grout Plant is favored for precision
tension grouting. The unit features a high speed shear mixer that
thoroughly wets each particle and discharges the mixed material into a 13
cubic foot capacity agitating holding tank. A direct coupled progressing
cavity pump delivers slurries at a rate of up to 20 gpm and pressures of up
to 261 psi. The unit easily mixes and pumps slurries of Portland cement,
fly ash, bentonite, and lime flour. All controls are conveniently located on
the operator platform for easy one
variable up to 20
, 261 psi
13.0 CF with
bottom clean out
2 x 3 x 6 diffuser
300 CFM, 100 psi
96" L x 60" W x 63"
T7Z Hydraulic Jacks
Used for testing and pre
stressing anchor bolts. Available
with up to 5
1/8" center hole. Unit comes with ram, pump,
gauge, hoses, jack stand, high strength coupling, high
strength test rod, plate, hex nut and knocker wrench.
Calibrations are available upon request.
Jack pull rods should have a higher capacity than the
With the T80 series the enclosed bearing
housing contains a geared socket drive to
tighten the bolt hex nut during tensioning. Test
jack housing will accommodate up to a 9”
18 Hydraulic Torque Wrench
The hydraulic torque wrench is used for tensioning
anchors in tight fitting locations where it would be difficult
to use an hydraulic jack. The wrench is also
recommended for use when setting the large diameter
Lock anchors. The torque wrenches are light weight
and can achieve a maximum of 8,000 ft
Tensioning charts Williams products can be found here.
T8Z Torque Wrench
For applying torque to the anchor bolt
when setting the anchor.
Tensioning charts Williams products can
be found here.
04 Torque Multiplier (4:1)
For use with T8Z Torque Wrench. Other sizes
T1Z & T2Z Long Fitting Tool Adapters
For driving hex nuts and setting tools,
typically with our Spin
systems. Works with torque wrench or
Available with 1" or 1
1/2" square drive.
Please specify square drive for
with your equipment.
2Z Regular Socket
T1Z Deep Socket
26 Long Fitting Wrench Adapter
For applying torque to recessed anchor nuts that are under tension
when using hydraulic jacks. Available in all anchor sizes.
2", 3" or 4"
Methods of Corrosion Protection
Methods of Corrosion Protection
Fusion bonded epoxy coating of steel bars to help prevent
corrosion has been successfully employed in many applications
because of the chemical stability of epoxy resins. Epoxy coated
bars and fasteners should be done in accordance with ASTM A
775 or ASTM 934. Coating thickness is generally specified
between 7 to 12 mils. Epoxy coated bars and components are
subject to damage if dragged on the ground or mishandled.
Heavy plates and nuts are often galvanized even though the bar
may be epoxy coated since they are difficult to protect against
abrasion in the field. Epoxy coating patch kits are often used in
the field for repairing nicked or scratched epoxy surfaces.
Cement Grout filled corrugated polyethylene tubing is often used to
provide an additional barrier against corrosion attack in highly
aggressive soils. These anchors are often referred to as MCP or
Multiple Corrosion Protection anchors. The steel bars are wrapped
with an internal centralizer then placed inside of the polyethylene tube
where they are then factory pre
grouted. When specifying couplings
with MCP ground anchors, verify coupling locations with a Williams
Hot Dip Galvanizing
Zinc serves as a sacrificial metal corroding preferentially
to the steel. Galvanized bars have excellent bond
characteristics to grout or concrete and do not require as
much care in handling as epoxy coated bars. However,
galvanization of anchor rods is more expensive than
epoxy coating and often has greater lead time. Hot dip
galvanizing bars and fasteners should be done in
accordance with ASTM A
153. Typical galvanized coating
thickness for steel bars and components is between 3
and 4 mils.
150 KSI high strength steel bars should
always be mechanically cleaned (never acid washed)
to avoid problems associated with hydrogen
Williams strand tendons contain an extruded high density
polyethylene sheathing around each individual strand in the
stressing portion of the anchorage. The sheathing is
minimum 60 mils thick and applied once the 7
wire strand has
been coated with a corrosion inhibiting compound. Extruded
polyethylene sheathing provides a moisture tight barrier for
corrosion protection and allows the strand to elongate freely
throughout the free
stressing length during the prestressing
Williams corrosion inhibiting compounds can be placed in the
free stressing sleeves, in the end caps, or in the trumpet
areas. Often bars are greased/waxed and PVC is slipped
over the greased/waxed bar prior to shipping. Each are of
an organic compound with either a grease or wax base.
They provide the appropriate polar moisture displacement
and have corrosion inhibiting additives with self
properties. They can be pumped or applied manually.
Corrosion inhibiting compounds stay permanently
viscous, chemically stable and non
reactive with the
prestressing steel, duct materials or grout. Both
compounds meet PTI standards for Corrosion Inhibiting
Corrosion Inhibiting Wax or Grease with Sheath
Coal Tar Epoxy
Coal tar epoxy has shown to be abrasion resistant,
economical and durable. This product when specified
should meet or exceed the requirements of (a) Corp of
200, C200a and (b) AWWA C
exterior. Typically the thickness is between 8 and 24 mils.
Make sure that the surfaces of the bar are clean and dry
Heat Shrink Tubing
Heat Shrink Tubing provides a corrosion
protected seal when connecting smooth or
Epoxy Coating Patch Kits are available upon request.
Epoxy Coating Patch Kits
Anchor Head Protection
The most important section of a ground anchor that needs adequate
corrosion protection is the portion of the anchor exposed to air/oxygen. This
is typically defined as the "anchor head", which generally consists of a steel
bearing plate, a hex nut and washer for a bar system, or a wedge plate and
wedges for a strand system. For permanent ground anchors it is best to
galvanize the hex nut and plates even if the bar is epoxy coated.
Galvanized components, if scratched during shipping, are less likely to
cause corrosion concerns than scratched epoxy coated components. The
end of the steel bar protruding out from the hex nut is often protected by the
use of a plastic or steel end cap packed with grease or cement grout.
Williams offers several different types of PVC and metal end caps to
provide corrosion protection at otherwise exposed anchor ends.
Steel Tube Welded on Flange with
Threaded Screw Connections
Field Splice for Bars
Continuous corrosion protection can even be
accomplished for the MCP
manufactured from Williams Form Engineering. To
achieve the equivalent levels of corrosion protection
the coupled sections of bar anchors can be wrapped
in a grease impregnated tape that is further protected
with heat shrink
. This scheme is acceptable
by most governing agencies and is specified in the
PTI Recommendations for
Rock and Soil