Thursday 8 March 2012

Prestressed concrete

Prestressed concrete 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. Prestressing
tendons (generally of high tensile steel cable or
rods ) are used to provide a clamping load which
produces a compressive stress that balances the
tensile stress that the concrete compression
member would otherwise experience due to a
bending load. Traditional reinforced concrete is
based on the use of steel reinforcement bars,
rebars, inside poured concrete .
Prestressing can be accomplished in three ways:
pre-tensioned concrete, and bonded or
unbonded post-tensioned concrete.
Pre-tensioned concrete
Stressed ribbon pedestrian bridge,
Grants Pass , Oregon, USA
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 . An innovative bridge
construction method using pre-stressing is the
stressed ribbon bridge design.
Bonded post-tensioned concrete
Prestress post-tension anchor on
display at Instituto Superior
Técnico 's civil engineering
department
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 that react (push) against the
concrete member itself. When the tendons have
stretched sufficiently, according to the design
specifications (see Hooke's law), 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 (such as adobe clay) 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-tensioning is
also used in the construction of various bridges,
both after concrete is cured after support by
falsework and by the assembly of prefabricated
sections, as in the segmental bridge.
Among the advantages of this system over
unbonded post-tensioning are:
Large reduction in traditional reinforcement
requirements as tendons cannot destress in
accidents.
Tendons can be easily "woven" 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.
History of problems with bonded post-
tensioned bridges
The popularity of this form of prestressing for
bridge construction in Europe increased
significantly around the 1950s and 60s.
However, a history of problems have been
encountered that has cast doubt over the long-
term durability of such structures.
Due to poor workmanship of quality control
during construction, sometimes the ducts
containing the prestressing tendons are not fully
filled, leaving voids in the grout where the steel
is not protected from corrosion. The situation is
exacerbated if water and chloride (from de-
icing salts) from the highway are able to
penetrate into these voids.
Notable events are listed below:
The Ynys-y-Gwas bridge in West Glamorgan,
Wales – a segmental post-tensioned structure,
particularly vulnerable to defects in the post-
tensioning system – collapsed without warning
in 1984.
The Melle bridge, constructed in Belgium
during the 1950s, collapsed in 1992 due to
failure of post-tensioned tie down members
following tendon corrosion.
Following discovery of tendon corrosion in
several bridges in England, the Highways
Agency issued a moratorium on the
construction of new internal grouted post-
tensioned bridges and embarked on a 5-year
programme of inspections on its existing post-
tensioned bridge stock.
In 2000, a large number of people were
injured when a section of a footbridge at the
Charlotte Motor Speedway, USA, gave way
and dropped to the ground. In this case,
corrosion was exacerbated by calcium chloride
that had been used as a concrete admixture,
rather than sodium chloride from de-icing
salts.
In 2011, the Hammersmith Flyover in London,
England, was subject to an emergency closure
after defects in the post-tensioning system
were discovered.
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 embedded 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:
1. The ability to individually adjust cables based
on poor field conditions (For example: shifting
a group of 4 cables around an opening by
placing 2 to either side).
2. The procedure of post-stress grouting is
eliminated.
3. The ability to de-stress the tendons before
attempting repair work.
Picture number one (below) shows rolls of post-
tensioning (PT) cables with the holding end
anchors displayed. The holding end anchors are
fastened to rebar placed above and below the
cable and buried in the concrete locking that
end. Pictures numbered two, three and four
shows a series of black pulling end anchors
from the rear along the floor edge form. Rebar
is placed above and below the cable both in
front and behind the face of the pulling end
anchor. The above and below placement of the
rebar can be seen in picture number three and
the placement of the rebar in front and behind
can be seen in picture number four. The blue
cable seen in picture number four is electrical
conduit. Picture number five shows the plastic
sheathing stripped from the ends of the post-
tensioning cables before placement through the
pulling end anchors. Picture number six shows
the post-tensioning cables in place for concrete
pouring. The plastic sheathing has been
removed from the end of the cable and the
cable has been pushed through the black pulling
end anchor attached to the inside of the
concrete floor side form. The greased cable can
be seen protruding from the concrete floor side
form. Pictures seven and eight show the post-
tensioning cables protruding from the poured
concrete floor. After the concrete floor has been
poured and has set for about a week, the cable
ends will be pulled with a hydraulic jack.
1. Rolls of post-tensioning
cables
2. Pulling anchors for post-
tensioning cables
3. Pulling anchors for post-
tensioning cables
4. Pulling anchors for post-
tensioning cables
5. Post-tensioning cables
stripped for placement in
pulling anchors
6. Positioned post-
tensioning cables
7. Post-tensioning cable
ends extending from
freshly poured concrete
8. Post-tensioning cable
ends extending from
concrete slab
9. Hydraulic jack for
tensioning cables
10. Cable conduits in
formwork
Applications
Prestressed concrete is the main material for
floors in high-rise buildings and the entire
containment vessels of nuclear reactors.
Unbonded post-tensioning tendons are
commonly used in parking garages as barrier
cable. [1] Also, due to its ability to be stressed
and then de-stressed, it can be used to
temporarily repair a damaged building by
holding up a damaged wall or floor until
permanent repairs can be made.
The advantages of prestressed concrete include
crack control and lower construction costs;
thinner slabs - especially important in high rise
buildings in which floor thickness savings can
translate into additional floors for the same (or
lower) cost and fewer joints, since the distance
that can be spanned by post-tensioned slabs
exceeds that of reinforced constructions with
the same thickness. Increasing span lengths
increases the usable unencumbered floorspace
in buildings; diminishing the number of joints
leads to lower maintenance costs over the
design life of a building, since joints are the
major focus of weakness in concrete buildings.
The first prestressed concrete bridge in North
America was the Walnut Lane Memorial Bridge
in Philadelphia, Pennsylvania. It was completed
and opened to traffic in 1951. [2] Prestressing
can also be accomplished on circular concrete
pipes used for water transmission. High tensile
strength steel wire is helically-wrapped around
the outside of the pipe under controlled tension
and spacing which induces a circumferential
compressive stress in the core concrete. This
enables the pipe to handle high internal
pressures and the effects of external earth and
traffic loads.
Design agencies and regulations
In the United States, pre-stressed concrete
design and construction is aided by
organizations such as Post-Tensioning Institute
(PTI) and Precast/Prestressed Concrete Institute
(PCI). In Canada the Canadian Precast/
prestressed concrete Institute assumes this role
for both post-tensioned and pre-tensioned
concrete structures.
Europe also has its own associations and
institutes. It is important to note that these
organizations are not the authorities of building
codes or standards, but rather exist to promote
the understanding and development of pre-
stressed design, codes and best practices. In the
UK, the Post-Tensioning Association fulfills this
role. [3]
Rules for the detailing of reinforcement and
prestressing tendons are provided in Section 8
of the European standard EN 1992 -2:2005 -
Eurocode 2: Design of concrete structures -
Concrete bridges: design and detailing rules.



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