4- Prestressed Concrete

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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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