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COLLEGE OF ARCHITECTURE AND PLANNING
Department of Architecture and Building Sciences
Dr. Mohammed Ghonim
4. PRESTRESSED CONCRETE
ARCH 436Contem porary Building Construction Met hods
Lecture Objectives
Upon completion of this lecture, the student will be able to:
1. Describe the concept of prestressed concrete construction.
2. Discuss the advantages and disadvantages of prestressed concrete.
3. Compare between pretensioned and posttensioned concrete.
4. List and explain the types of prestressed concrete.
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Lecture Content
• Introduction.
• Historical Brief.
• Advantages of Prestressed Concrete.
• Disadvantages of Prestressed Concrete.
• Applications of Prestressed Concrete.
• Structural Background.
• Understanding Prestressed Concrete.
• Prestressing Process.
• Types of Prestressed Concrete.
• Pretensioned & Posttensioned Prestressed
Concrete.
• Pretensioning.
• Posttensioning.
• Posttensioned Concrete: Tools & Materials.
• External & Internal Prestressing.
• Linear & Circular Prestressing.
• Full, Limited & Partial Prestressing.
Introduction
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.
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Historical Brief
The concept of prestressed concrete appeared in USA in 1888 when P.H. Jackson was
granted the first patent in the United States for prestressed concrete design. Jacksons idea
was perfect, but the technology of high strength steel that exhibited low relaxation
characteristics was not yet available.
It was not until the French engineer Eugene Freyssinet defined the need for these materials
that prestressed concrete could be used as a structural building material. Unfortunately,
although Freyssinet, a brilliant structural designer and bridge builder, lacked the teaching
qualities necessary to communicate his ideas to other engineers. In 1935, he usedprestressing to strengthen the maritime station of Le Havre which was threatening
to settle beyond repair. Freyssinet introduced prestressed concrete beams, and jacked up the
shipyard buildings. Following this success, he joined the firm of Campenon-Bernard and
went on to design several prestressed bridges.
Le Havre maritime station
Historical Brief
Simultaneously, Urlich Finsterwalder, the German bridge engineer, was revolutionizing the
construction methods for prestressed concrete bridges. HE invented the free-cantilever
construction method of prestressed concrete bridges. Freyssinets major prestressed works
came after the reinforced-concrete Plougastel Bridge and included a the Luzancy Bridge
(1946), with a span of 54 meters.
Later, the Roebling family developed the first stress-relieved wire followed by the first stress-
relieved strand. T.Y. Lin once again brought prestressed concrete back into the spotlight
when he organized the “First Prestressed Concrete World Conference” in 1957. Shortly after
this conference, Lin published a technical paper in the “Prestressed Concrete Institute - PCI”
Journal that introduced a new Load Balancing technique which allowed most structural
engineers to design prestressed concrete very easily.
Luzancy Bridge, France.
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The Advantages of Prestressed Concrete
The advantages of prestressed concrete include crack control and thinner slabs that are
important in high rise buildings in which floor thickness savings can translate into additional
floors, since the distance that can be spanned by post-tensioned slabs exceeds that of
reinforced constructions with the same thickness. Increasing span lengths increases theusable floor space in buildings, diminishing the number of joints leads to lower maintenance
costs during the life of a building, since joints are the major focus of weakness in precast
concrete buildings.
• It takes the full advantages of high strength concrete and high strength steel
• It needs less materials, so it is a smaller and lighter structure.
• It usually has no cracks, because of using the entire section to resist the load.
• It has better corrosion resistance
• It isvery effective for deflection control.
• It has better shear resistance
The Disadvantages of Prestressed Concrete
• It is more expensive.
• It needs higher quality materials.
• It is more technically complex, and requires high skilled labour.
• Concrete casting needs superior quality and accurate supervision.
• Special tools are needed to provide the prestressing.
• It is more risky, especially if not stressed properly.
• It is difficult to be recycled.
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Applications of Prestressed Concrete
Prestressed concrete is the main material for floors in high-rise buildings and the entire
containment vessels of nuclear reactors. Also, it is commonly used in bridges and parking
garages, and 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 permanentrepairs can be made.
Prestressed concrete is commonly used in parking garages
because of the relative high loads and height limitations as well as
wide span requirements.
Structural Background
Bending in a beam.
Beam failure due to overloads
Bending in a beam causes compression stresses
in the upper part of the beam and tension stresses
in the lower part.
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Understanding Prestressed Concrete
Ordinary concrete beam
structural behavior
Prestressed concrete beam
structural behavior
The idea of prestressed concrete isto produce a compressive stress
that balances the tensile stress that
the concrete compression member
would otherwise experience due to
a bending loaded.Concrete beam test.
Prestressing Process
Prestressed concrete is reinforced by high-strength steel tendons within their elasticlimit to actively resist a service load. Thetensile stresses in the tendons aretransferred to the concrete, placing the entirecross section of the member in compression.The resulting compression counteract thetensile bending stresses from the appliedload, enabling the prestressed member todeflect less, carry a greater load, or span agreater distance than a reinforced member of the same size, proportion, and weight.
There are two types of prestressingtechniques. Pretensioning isaccomplished in a precasting plant,while posttensioning Is usuallyperformed at the building site,especially when the structural unitsare too large to transport fromfactory to site.
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Types of Prestressed Concrete
Types According to
1. Pretensioned & Posttensioned According to the sequence of casting the concrete and applying
tension to the tendons.
2. External & Internal Based on the location of the prestressing tendons.
3. Linear & Circular According to the shape of the member prestressed.
4. Full, Limited & PartialBased on the amount of prestressing force.
Posttensioned External Circular Full
Pretensioned & Posttensioned Prestressed Concrete
In Pretensioned concrete, the steel is first tensioned in a frame or between anchorages
external to the member. The concrete is then cast around it. After the concrete has developed
sufficient strength the tension is slowly released from the frame or anchorage to transfer the
stress to the concrete to which the tendons have by that time become bonded. The force is
transmitted to the concrete over a certain distance from each end of a member known as the
transfer length.
Post-tensioned concrete is made by casting concrete that contains ducts through which
tendons can be threaded. An alternative is to cast the concrete around tendons that are
greased or encased in a plastic sleeve. When the concrete has sufficient strength the
tendons are tensioned by means of portable jacks. The load is transmitted to the concrete
through permanent anchorages embedded in the concrete at the ends of the tendons. Ducts
are usually grouted later to protect the tendons against corrosion.
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Pretensioning
Anchorage Abutments
Pretensioning prestresses a concrete
member by stretching the reinforcing
tendons before the concrete is cast.
Posttensioning
Posttensioning is the prestressing of a
concretemember by tensioning the reinforcing
tendons after the concrete has set. These
members tend to shorten over time due to
elastic compression, shrinkage, and creep.
Attached elements that would be affected by
this movement should be constructed after the
posttensioning process is completed and be
isolated from the posttensioned members with
expansion joints.
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Posttensioned Concrete: Tools & Materials
Stressing Jack
Tendons
Anchors
External or Internal Prestressing
External Prestressing
Internal PrestressingWhen external steel wires known as tendons are placed onthe outer section of the region being stressed, then thetechnique is called external prestressing. The tendons lieoutside or inside the hollow space of a box girder and forcesare only transferred at the deviations or anchorage blocks.External prestressing requires a great deal of accuracy inplanning, executing and maintenance; therefore, it is not verycommon. The technique is suited for strengthening buildingsand constructing bridges. On the other hand, internalprestressing is achieved by placing the tendons within astructure. Prestressing applications use internal techniquebecause it iseasy to achievea greater degree ofaccuracy.
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Linear or Circular Prestressing
Circular PrestressingLinear Prestressing
Linear prestressing is a type of prestressing used in
straight or flat structures. The technique can even be
used on curved structures as long as the tendons don’t
go round in circles because they are linearly prestressed.
For example, prestressing of slabs and beams can be
done using the linear prestressing technique.
In contrast, curved or circular structures employ the
circular prestressing technique, the tendons are wound
in circles, for example, this include pipes, silos and
tanks.
Full, Limited & Partial Prestressing
The technique where no restrictions are placed on concretetensile stresses under service conditions is called partialprestressing. The part under tension is reinforced usingadditional mild steel bars to control the cracks and hence thecrack width is within allowable limit. Deviations from the setprestressing can cause deformation, cracking and fatigue onthe structure; therefore, when using this technique duringconstruction, youmustalwaysmonitor theprestressing forcecarefully. On the other hand, full prestressing is a techniquewhere no tensile stress is allowed in concrete under serviceloads. The area of prestressing steel is more than the areaneeded for safety at ultimate state, therefore leading toinefficient useof prestressing.
Full Prestressing
When the level of prestressing issuch thatno tensilestress isallowedin concreteunder serviceloads.
LimitedPrestressing
When the level of prestressing is such that the tensile stressunderservice loads iswithinthe cracking stress of concrete.
Partial Prestressing
When the level of prestressing is such that under tensilestresses due to service loads. the crack width is within theallowablelimit.
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Summary
• 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.• The advantages of prestressed concrete structures are
as follows: they take less materials, so they are smaller
and lighter structures with no cracks and have better
corrosion resistance, this is very effective for
deflection control and better shear resistance.
• Prestressed concrete is the main material for floors
in high-rise buildings and the entire containment
vessels of nuclear reactors. Also, it is commonly used
in bridges and parking garages.
• Prestressed concrete is classified into four categories
as follows; 1. Pretensioned or posttensioned; 2.
External or internal; 3. Linear or circular; 4. Full, limited
or partial.
• In pretensioned concrete, the steel is first tensioned in
a frame or between anchorages external to themember. The concrete is then cast around it.
• Post-tensioned concrete is made by casting concrete
that contains ducts through which tendons can be
threaded.
Lecture Activity
Each student is required to prepare a brief but informative internet-based research about one of the topicsrelated to this lecture (Prestressed Concrete); i.e., new developments, successive and local case studies or
important details. The research should be presented in only one A4 sheet.
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References
• Allen, EA. (2009) Fundamentals of building construction materials and methods, John Wiley & Sons.
• Ching, Francis (2008) Building Construction Illustrated, John Wiley & Sons.
• Emmitt, S.& Gorse, C. (2006) “Barrys Advanced Construction of Buildings”, Blackwell Publishing.
• Mehta, Madan et. al. (2013) “Building Construction”, Pearson, USA.
• http://en.wikipedia.org.
• http://nptel.ac.in/courses/105106117/pdf/1_Introduction/1.2_Advantages_Types_of_Prestressing.pdf
• http://vimeo.com
• http://www.britannica.com
• http://youtube.com
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