PRESTRESSED

CONCRETE

STRUCTURES

An1lan K. Sengupta, PhD PE
Department of Civil Engineering
Indian Institute of Technology Madras

Module

1: Introduction, PrestressingSystemsand Material Properties

Lecture

1: Introduction

Welcome to PrestressedConcrete Structures.In this course,you will be introducedto
prestressedconcreteand the different applications.In the rst module, we shall cover
introduction,prestressingsystemsand materialproperties.
(Refer Time Slide: 01:35)

In this module, rst we shall learn aboutthe basic conceptsof prestressing.From there,
we shall move on to early attemptsof prestressing.We shall then go throughthe brief
history of the development of prestressedconcrete. Finally, we shall look into the
developmentof building materials- that also comesunderthe brief history in a different
perspective.

(Refer Time Slide: 02:06)

T1

The reference books that we shall follow in this course are: the first one is by Professor
N. Rajagopalan; the title of the book is Prestressed Concrete and the publisher is Narosa
Publishing House. The second book is by Professor N. Krishna Raju and the title is
Prestressed Concrete; the publisher is Tata McGraw-Hill Publishing Company limited.
The third one is an international book; it is written by Professors T. Y. Lin and N. H.
Burns and the title of the book is Design of Prestressed Concrete Structures; the
publisher is John Wiley and Sons. The fourth one is also an international book; it is
written by Professor A. Nilson and the title is Design of Prestressed Concrete; the
publisher is John Wiley and Sons.

(Refer Time Slide: 03:07)

The main code that we shall follow in this course is IS: 1343

1980. The title is Code of

Practice for Prestressed Concrete. It is published by the Bureau of Indian Standards.
Remember, that you are expected to know IS: 456 - 2000 which is the Code of Practice
for Structural Concrete. Some of the provisions of IS: 456 are also applicable for
Prestressed

Concrete.

There are some allied codes which are used for prestressedconcrete structures in various
applications. The rst one is IRC: 18

2000. It is Design Criteria for Prestressed

Concrete Road Bridges, specically for post-tensioned concrete. It is published by The
Indian Road Congress. This code is also in conjunction with the other codes published by
The Indian Road Congress for bridge design. Another code is IRS Concrete Bridge Code:
1997. The title is Indian Railway Standard Code of Practice for Plain, Reinforced and
Prestressed Concretefor General Bridge Construction. This is published by the Ministry
of Railways. This code is also in conjunction with the other codes for bridge design for
the Indian railways.

(Refer Time Slide: 04:56)

Time-to-time, we may refer to some international codes. The two codes which we shall
restrict to ourselves are: the first one is ACI 318M-02, the Building Code Requirements
for Structural Concrete and Commentary. It is published by the American Concrete
Institute. This code covers structural concrete, including prestressedconcrete. The second
international code is the BS 8110: Part 1: 1985; the title is Structural Use of Concrete:
Part 1 Code of Practice for Design and Construction. This is published by the British
Standard

Institution

and this

code

also covers

structural

concrete.

(Refer Time Slide: 05:49)

Next, we are moving on to the basic concept of prestressing. The question that arises in
our mind is what is meant by prestressing? To dene it: Prestressingis the application of
an initial load on a structure, to enable it to counteract the stresses arising from
subsequent loads during its service period. That means the structure initially is loaded
before any external load is applied. Next, when the external load is applied, the previous
stresses

counteract

the stresses

due to the external

loads.

(Refer Time Slide: 06:27)

There are examples of prestressing even before the development of prestressedconcrete.
The concept of prestressing existed before applications in concrete. Two such examples
will be explained in this module.
(Refer Time Slide: 06:50)

The rst one is the force-tting of metal bands on wooden barrels. These types of barrels
were used to transport liquid and oil, and types of grains. Now, what the prestressing does
is that, the metal bands create a hoop compression around the barrel. When this barrel is
lled

with liquid, the liquid tries to push out; it creates a hoop tension. The hoop

compression created by the metal band counteracts the hoop tension that is created by the
liquid while lling the barrel.
(Refer Time Slide: 07:39)

The second example is the pre-tensioning of the spokes in a bicycle wheel. Here you can
see that in a bicycle wheel, each one of these spokes is pulled and tightened. This pretension is to such an extent that there will always be a residual tension in the spoke. Let
me explain these two examples on the black board.

(Refer Time Slide: 08:17)

When the barrel is rst

prestressed, the prestressing causes a hoop compression

throughout the barrel. Next, we are lling

this barrel with liquid; this liquid creates a

hoop tension. The hoop tension that is created by the liquid is counteracted by the hoop
compression that is generateddue to the prestressing.
(Refer Time Slide: 09:45)

The second example is the cycle spokes. Due to the tensioning of the spokes, each of the
spokes is under tension. When the rider rides on the cycle, the weight creates some
compression in the spoke; but since there was an original tension and this tension is

higher than the compression that generates,in each one of the spokes there is a residual
tension. This concept of prestressing is applied in concrete and the denition

of

prestressedconcrete is also similar.
(Refer Time Slide: 11:23)

Concrete in which effective internal stressesare induced (usually, by means of tensioned
steel) before the structure is loaded, to counteract the stressesresulting from the applied
service loads. Here also the concept is similar. That is, after prestressing when the
structure is subjected to the external loads, the stressesdue to prestressing counteracts the
stresses

due to the external

loads.

(Refer Time Slide: 12:27)

Why do we need prestressing for concrete? The main reason is the concretes tensile
strength is only 8 to 14 percent of its compressive strength; this is the weak point of
concrete. For exural members, cracks develop at early stages of loading. Examples of
exural members are beams and slabs. If we gradually load the structure, we shall
observe that cracks are being generatedin the exural members.
To prevent such cracks, compressive forces can be suitably applied in the longitudinal
direction, either concentrically or eccentrically. Now, what this prestressing does is, it
counteracts

these

tensile

stresses

and

reduces

the

chances

of formation

of these

cracks.

The prestressing that is applied in a exural member is called linear prestressing, because
the axis of the prestressing is same as the axis of the exural member.

10

(Refer Time Slide: 13:18)

The prestressing enhancesnot only the bending capacity, but it also enhancesthe shear
and the torsional capacities of the exural members. In the subsequentmodules of this
course, we shall see that both the shear capacity and the torsional capacity are enhanced
due to the pre-compression that is introduced by the prestressing.
There are other examples of prestressed concrete. In cylindrical tanks, the hoop tensile
stress can be effectively counteracted by circular prestressing. This concept is similar to
the barrels that we have seen before. Before the tank is loaded, the prestressing creates a
compression in the tank and then when the tank is filled up with water, the water tries to
expand the tank. The compression due to prestressing counteracts the expansion due to
the hydrostatic force, and thus the chancesof cracking get reduced.

11

(Refer Time Slide: 14:34)

Although it appears to be quite mundane but the early attempts of prestressing were not
completely successful. It was observed that the effect of prestressing reduced with time.
The following sketcheswill explain why this prestressing force has to reduce.
(Refer Time Slide: 14:59)

In the earlier days, the way the prestressing was done was that, initially a rod used to be
pulled with an anchor at one end and then in the mould, the concrete used to be placed.
Once the concrete has hardened, the tension used to be released in the tendon and then

12

this tendon used to be cut. This is the early form of prestressedconcrete by the mild steel
rods.

(Refer Time Slide: 15:39)

What happensis after the hardening of concrete and when the tension is releasedfrom the
rods, the rods will try to regain their original length, but this is prevented by the
surrounding concrete to which the steel is bonded. Thus, the concrete gets effectively
under a state of pre-compression which is capable of counteracting the tensile stresses.

13

(Refer Time Slide: 16:22)

It was observed that, this type of simple beams which were prestressedinitially may have
a good exural capacity. But with time, it was found that the exural resisting capacity
got liIr1ited and if the load is kept sustained, it was observed that sometimes the beams
tend to fail. The question that comes to our mind is why does it happen so? Initially it is
found to be effective, but with time it is found that the effect of prestressing is getting
reduced. Why is this so?

14

(Refer Time Slide: 17:01)

One very important phenomenon of concrete is that concrete during hydration shrinks
with time. That means, it tends to reduce in volume and there is a reduction in length.
Moreover, under sustained load, it undergoes a creep strain. Now, what is meant by
creep? Creep is the strain under a sustained load. Since there is a pre-compression in the
concrete, this concrete over the time tends to get reduced and this is called the effect of
creep. The reduction in length due to creep and shrinkage is also applicable to the
embedded steel. That means when the concrete is shrinking, along with that the
embedded steel is also shrinking. When the length of the embedded steel reduces, it
results in significant loss of the tensile strain; that is, originally it was under a substantial
tensile strain, but with time the length is reducing and the tensile strain in the steel is
getting reduced. After several years, the residual strain and hence, the residual prestress
can be as low as 10 percent of the initial value or even lower than that.

15

(Refer Time Slide: 18:43)

Let us try to understand this by a simple sketch. In this beam, the original length of the
steel rod was L1. It has been tensioned to a length which is equal to the original length of
the concrete beam, that is L2. This is the state of the beam before applying prestress.
(Refer Time Slide: 19:04)

When the rod is cut, since the steel is applying compression in the concrete, the concrete
will tend to compress; this is called the phenomenon of elastic shortening. After the
elastic shortening, the length of the beam reduces to L3 which is lower than the initial

16

length of L2; that means, at transfer of the prestress, the beam gets reduced due to elastic
shortening.
(Refer Time Slide: 19:40)

Then over a period, say a few years, there will be creep and shrinkage in the concrete and
that will lead to further reduction in the length of the beam. Let this length be L4; that
means the length L4 is after the long-term losses of prestress. The long-term losses are
different from the short-term losses. The elastic shortening is the short-term loss which is
happening right at the transfer of prestress, but the long-term losses is over a substantial
period, over a few years, after the prestressing operation has been done.

17

(Refer Time Slide: 20:26)

We have to calculate what the residual strain in the steel is, because it is the residual

strain which will have the residual prestress. The residual strain can be calculated as the
original tensile strain in steel minus the compressive strains corresponding to short-term
and long-term losses. In this expression, we shall consider the tensile strain to be positive
and the compressive strains to be negative; that is why we have placed a subtraction for
the compressive strains.

18

(Refer Time Slide: 21 :07)

For the example that we have shown before, the original tensile deformation in the steel
was L2 minus L1, where L2 was the original length of the beam and L1 was the original
length of the steel rod. That means, when the rod was tensioned, the strain in the steel
was given as L2 minus L1 divided by L1.
Then, there was the elastic shortening which led to some compressive strain. The shortterm loss in prestress can be calculated by the strain which is L2 - that is the original
length of the beam, minus L3, which is the length of the beam after the elastic shortening
divided by L1, which is the original length of the steel rod. This expression L2 minus L3
divided by L1 is the loss in the strain due to the elastic shortening of the beam. This can
be called as the short-term loss in the prestress.
Over the time, there is a compressive strain in the beam due to creep and shrinkage. This
strain can be calculated by the simple expression: L3 minus L4 divided by L1. L3 is the
length after the short-term losses,that means the length after the transfer of prestress,and
L4 is the length after the long-term losses. The expression of L3 minus L4 divided by L1
gives the compressive strain in the concrete and the loss in prestress due to creep and
shrinkage. This loss can be termed as the long-term losses in prestress. If we subtract the
second and the third term from the rst term, then we arrive at the fourth expression,
which is the residual strain in the steel. L4 is the final length of the steel rod and L1 was

19

the original length. L4 minus L1 divided by the original length gives the nal residual
strain in the prestressing steel.
(Refer Time Slide: 23:40)

In the earlier days, when ordinary mild steel was used for prestressing, there was a limit
of the tensile strain that could be applied to the steel. This limit was based on the
allowable tensile stress. The maximum original tensile strain in mild steel can be
calculated as the allowable stress divided by the elastic modulus of steel. The allowable

stress
canbearound140MPa(MegaPascal)
andtheelasticmodulus
is around2 x 105
MPa. When we divide the allowable stressby the elastic modulus, the maximum original
tensile strain is equal to 0.0007. If we observe, the total strain due to the strains of elastic
shortening, creep and shrinkage was almost of the same value; it was almost equal to
0.0007. It means the loss in prestress gets almost equal to the original prestress, because
the values of the corresponding two strains are close. Hence, the residual strain was
negligible and nally, the effective prestress almost becomes nullied over a period of
years.

20

(Refer Time Slide: 25:30)

This problem had led to several research on the type of steel, the type of concrete and
over the decades, this problem was resolved with the development of the material
properties of both steel and the concrete. The question was: what was the solution to
increase the residual strain and hence, the effective prestress? The first solution was to
adopt high strength steel with much higher original strain. This leads to the requirement
of high prestressing force. Compared to mild steel, if we use steel which has much higher
tensile strength, then the allowable stress will also be much higher. The initial tensile
force will also be much higher, if we are using a high strength tensile steel. With the
development of the metallurgical process, high strength tensile steel was developed and
that was applied in the prestressedconcrete. But if we are applying high strength tensile
steel, then we also need to have high prestressing force. We need to have adequatejacks
to apply that high prestressing force.
The second development which helped the growth of prestressed concrete is the
development of high strength concrete. A concrete of 15 to 20 MPa characteristic
strength is not adequate for prestressed concrete. At least, it is recommended to have a
concrete strength of 30 MPa or higher, so as to withstand the high prestressing force. As I
said that with the adoption of high strength steel, the prestressing force became large and
for that, the concrete had to be strong to withstand this high prestressing force. With this,

21

the prestress concrete became successful in the different applications of structural
elements.

Let me now move on to a brief history of the development of prestressedconcrete.
(Refer Time Slide: 28:06)

Before I start the development of prestressed concrete, let us know two signicant
developments of reinforced concrete itself. The two major breakthroughs in the
development of reinforced concrete are: rst, the invention of Portland cement and
second is the introduction of steel in concrete; that means, the whole concept of
reinforcing the concrete by steel was a very innovative concept at that time.
In 1824, Aspdin in England obtained a patent for the manufacture of Portland cement. In
the earlier days, concrete was made by lime and other types of natural cementing
materials. We can say with the development of Portland cement, an industrial product
was developed which can give quite a lot of strength to the concrete. The development of
Portland cement itself is a major breakthrough in the development of reinforced concrete.

22

(Refer Time Slide: 29:25)

In 1857, Monier in France introduced steel wires in concrete to make owerpots, pipes,
arches and slabs. Here we can see that the concept of reinforced concrete has started to
evolve. What Monier did was to place some steel wires to hold the cracked concrete and
like that the application of reinforced concrete started to develop. Although, these
applications of owerpots, pipes and arches may appearto be mundane at present, during
that time, it was quite a significant achievement in the development of the concept of
reinforcing the concrete.

23

(Refer Time Slide: 30:08)

We are moving on to the major events of the development of prestressed concrete. In
1886, Jackson of USA introduced the concept of tightening steel rods in artificial stone
and concrete arches. The arch form was a very popular structural form used in different
types of structures - in buildings and in bridges. What Jackson did was that he introduced
a steel rod within the arch, and increased its capacity by tightening the steel rod. The arch
itself is under compression, but if we introduce a steel rod and tighten it, the compression
is further increased and hence, the capacity of arch also increased.

24

(Refer Time Slide: 31:08)

In 1888, Doehring of Germany manufactured concrete slabs and small beams with
embedded tensioned steel. We can see that in reinforced concrete, the steel was just
introduced, but now the inventors were tightening the steel rods in order to increase the
capacity.
(Refer Time Slide: 31:35)

In 1908, Stainer of USA recognized the losses due to shrinkage and creep. This was a
major breakthrough to understand the problem of reduction of the effective prestress with

25

time. The phenomenon of creep and shrinkage was identified, and Stainer suggested
retightening the rods to recover the loss of prestressafter a certain time.
In 1923, Emperger of Austria developed a method of winding and pre-tensioning high
tensile steel wires around concrete pipes. During the industrial development of the 20th
century, concrete became a very popular construction material. Different types of
innovative techniques were being developed to increase their capacity. The winding of
steel wires around concrete pipes and tensioning the steel wires to check their cracking
was actually an instance of prestressing the concrete pipes with the tensioned steel wires.
(Refer Time Slide: 32:53)

In 1924, Hewett of USA hoop-stressedhorizontal reinforcement around walls of concrete
tanks through the use of turnbuckles. We mentioned at the beginning, the concept of
prestressing a barrel. Here we can see that the concrete tanks were being developed with
prestressing them by some device called turnbuckles. These turnbuckles are a device to
tighten the reinforcement around the concrete tanks. Over the period, thousands of liquid
storage tanks and concrete pipes were built in the decades to follow. As the industrial
production of concrete is increasing, we observe that the application of prestressing is
also increasing with the time.

26

(Refer Time Slide: 33:55)

In 1925, Dill of USA used high strength unbonded steel rods. The concept of unbonded
means, that there was no bond or, there was no physical stress transfer between the steel
and the concrete at the interface. The transfer of stress was only at the ends. The reason
of using an unbonded steel rod was that the rods could be tensioned and anchored after
hardening of the concrete, and they could be again tightened so as to counteract the drop
in the prestress.

27

(Refer Time Slide: 34:42)

In 1926, Eugene Freyssinet of France used high tensile steel wires. We can see that he
used steel which had an ultimate strength as high as 1725 MPa. This is a much higher
value than the reinforcement that is used in conventional concrete. The yield stressof this
type of high tensile strength rods was more than 1200 MPa. From your knowledge of
reinforced concrete, you can understand that this is substantially high yield stress, and
with this type of high strength tensile steel, the development of prestressedconcrete was
possible.
In 1939, he developed conical wedges for end anchorages of post-tensioning and
developed double-acting jacks. The prestressedconcrete is actually an industrial product.
It needs more equipments and devices to apply the prestressing force, and to transfer the
prestressing force from the steel to the concrete. Freyssinet developed and patented
devices like the jacks and the anchorages,which are used in applying prestressing on the
concrete members. Freyssinet is often referred to as the Father of prestressedconcrete.

28

(Refer Time Slide: 36:24)

In 1938, Hoyer of Germany developed the long-line pre-tensioning method. This method
can be compared with the assembly line method in the manufacture of cars. That is, under
one industrial shed and in just one line, several prestressed members were being
produced. This technique was used to produce pre-tensioned transmission poles for
electric

lines.

In 1940, Magnel of Belgium developed an anchoring system for post-tensioning using
at wedges. As I mentioned before that the prestressedconcrete needs more equipments
and devices. The development of the anchoring system which helps in transferring the
prestress from the steel to the concrete was a major breakthrough in the development of
prestressedconcrete, becauseit is the successful behaviour of the anchors which can lead
to the transfer of the prestressto the concrete.

29

(Refer Time Slide: 37:40)

During the Second World War, the applications of prestressed and precast concrete
increased rapidly. What happened is that with mass production of concrete, the concrete
becaIne an industrial product. The concept of precasting concrete, that means casting the
concrete not at the place where it will exist, but at a different place under a much better
control of quality. Precast concrete became popular, and along with that the prestressing
of these precast members also became popular.
In this lecture, the names of the few persons who were involved in developing prestressed
concrete will be mentioned. Guyon of France built numerous prestressedconcrete bridges
in western and central Europe. The application of prestressedconcrete in bridge decks is
a wonderful application. The bridge decks became slender, their spans increased and the
aesthetics of the bridge profile becaIne better. In Europe, the application of prestressing
the concrete bridges became popular during the 1940s and 1950s.
Abeles in England introduced the concept of partial prestressing. Originally, the idea was
that the prestressing force should be such that there will not be any tension in the
prestressed concrete member. But it was found that with this objective, the amount of
prestressing force is also substantially high; due to that, the creep effect was also large.
Sometimes, the camber, that means the upward deection of the prestressed members,
became too high and the sections were not turning out to be economical. That is why a
new concept was introduced, where some tension was allowed in the concrete which is
30

not detrimental to the concrete. When some tension is allowed in the concrete, this is

called partial prestressing. We see a gradual move from the more conservative concept of
fully prestressing the concrete member to partially prestressing the member such that
when the member is under severe loads, there are chances of even cracking; but those
cracks will not be detrimental in the long run becauseunder service loads, those cracks
may not open up at all.
(Refer Time Slide: 40:52)

Leonhardt in Germany, Mikhailor in Russia and Lin in USA are very famous in the eld
of prestressedconcrete. Each one of them contributed to significant developments of the
application of prestressed concrete, whether it is the bridge decks, or the anchoring
systems, or the shell construction, or different types of the applications of prestressed
concrete.

We nd that the professionals who were involved in the prestressed concrete, they
formed the professional organisations. These professional organisations became a forum
for the exchange of ideas and knowledge.
The International Federation for Prestressing, in short FIP, is a professional organization
in Europe which was established in 1952. The Precast/PrestressedConcrete Institute, in
short PCI, was established in USA in 1954. Both these organizations are very well known
in the development and transfer of knowledge of prestressedconcrete.

31

(Refer Time Slide: 42: 13)

Prestressedconcrete was being applied in various types of structures like building frames,
parking structures, stadiums, railway sleepers, transmission line poles, water tanks and
several other types of elements. In India also, the application of prestressed concrete
diversified over the years. One of the classic examples is the Pamban Road Bridge at
Rameshwaram, Tamilnadu, which is a classic example of the use of prestressedconcrete
girders.

32

(Refer Time Slide: 42:52)

This is the picture of the Pamban bridge, and here you can seethat in the central span, the
deck prole is so nice and this was possible because of the prestressing. The span was
long and also you can seethat there is a substantial navigational clearance, becauseof the
reduced depth of the prestressed beam. As I mentioned before, that a bridge is a
wonderful application of prestressed concrete. Later on, time-to-time, we shall mention
some of these applications and we shall show some of the photographs of these
applications of prestressedconcrete. Even in India, there were major steps to change the
railway sleepersfrom the original timber sleepersto prestressedconcrete sleepers.Water
tanks, transmission line poles and other types of applications have become very popular
in India.

33

(Refer Time Slide: 44:20)

Next, let me explain the history of the prestressedconcrete in terms of the development
of the building materials. In the ancient period, the buildings were mostly made of stone
and bricks. These materials were very strong in compression. Depending on the type of
stone and depending on the quality of bricks, massive structures have been made by these
two materials because of their inherent capacity to carry compression. But the
disadvantage of these two materials was that they were weak in tension. Their reduced
capacity in tension, led to the liIr1ited range of spansthat we seein the ancient structures.

34

(Refer Time Slide: 45:06)

If tension was needed, then bamboos and coir ropes were used. For example, in the
suspension bridges of the earlier days, coir ropes were used as suspendersto hang the
deck from the main suspension cable. Subsequently, iron and steel bars were used to
resist tension. But there is a problem with these members also. These members cannot
carry high compression becausethey tend to buckle under compression. In one group, we
have stones and bricks which are strong in compression. In the other group, we have
bamboo, coir, steel and iron bars which are strong in tension. If the steel was moulded
into a form of the structural member like an I-section, then that type of structural steel
and wood were used both in tension and in compression. The wood that we see in the
older buildings and the I-sections, were only the members that were able to carry both
tension and compression under exure.

35

(Refer Time Slide: 46:38)

In reinforced concrete, concrete which is strong in compression and steel which is strong
in tension, are judiciously placed to counteract the stressesthat come due to the external
loads. The concrete resists compression and the steel resists tension. This is a passive
combination of the two materials. Why are we saying passive is that none of these
materials are stressedbefore the load is applied. The stress in these elements come only
after there is some external load, may be just the self-weight. When the self-weight starts
acting and whenever there is an external load, then only these materials get stressed.That
is why we term reinforced concrete as the passive combination of concrete and steel.
Whereas, in prestressed concrete, we see that there is a combination of high strength
concrete and high strength steel to resist the tension and compression effectively, but this
combination is an active combination. What we mean by an active combination is that
even before the external loads start to apply, both these materials are stressed.The steel is
under tension and the concrete is under compression after the prestressing operation.
These

36

stresses

are there

even

before

some

external

loads

come

on to the member.

(Refer Time Slide: 48:30)

Now, this gure summarizes the development of building materials in a nutshell. As we
said before that in the original structures in the olden days, the compression used to be
carried by stones and bricks. They were strong in compression and weak in tension. The
development of concrete, which was originally a lime concrete and then it moved on to
the cement concrete, was termed as the artificial stone. The beauty of the concrete was
that it could be moulded in any shapefor the use for which it is meant.
The tension can be carried by bamboos and ropes which were the original materials for
carrying tension. Gradually they led to the development of the steel. Initially, it was
wrought iron, and then subsequently it became steel. The steel bars and wires are also
very good in carrying tension, but they were weak in carrying compression becausethey
tend to buckle.

There were some materials which were equally capable of carrying exural compression
and tension simultaneously, such as the timber or wood and the structural steel, which
was an industrial product. These two materials were shapedin such a way that they were
able to carry both compression and tension. In reinforced concrete, we see a passive
combination of the concrete and the steel. Why is it passive?Because,there is no stressin
either material before the external loads start to apply.

37

With the development of both concrete and steel, high strength concrete was developed
whose strength can be as high as 100 MPa, and high strength steel whose yield strength
and ultimate strength can be as high as 1200 and 1700 MPa, respectively. These two
materials were combined to develop the prestressedconcrete. The prestressedconcrete is
called

an active

combination.

The

reason

is that

both

these

materials

are stressed

before

the application of the service load. The concrete is compressed so that the tensile stresses
may not generateunder service loads, chancesof cracking get reduced, the members can
have a longer span and the deterioration of the steel is much reduced. That is why
prestressedconcrete has some additional advantagesover the reinforced concrete.

SUMMARY

In todays lecture, what we have covered is: first, we covered the basic concept what is
the denition of prestressed concrete? We have found that prestressed concrete is the
concrete
that

come

where

some

initial

due to the service

stresses

are induced

in the concrete

to counteract

the stresses

loads.

Why do we need prestressedconcrete? Concrete is weak in tension, so to counteract this
deficiency of concrete, the prestressing is done. From the basic concept, we also found
that the concept of prestressing was not just liIr1ited to concrete. The concept was there
even before the application of concrete. We have seen the example of the barrels and the
example of the spokes in the cycle wheels. Next, we moved to the early attempts of
prestressedconcrete and we found that the early attempts were not successful. The reason
was that concrete has a special behavior. It is the change in length or volume over the
time. The rst is the shrinkage, which is a reduction in volume due to the hydration of the
cement and evaporation of water and the second one is the creep, which is the reduction
in length under the sustained load.
In the early attempts of prestressedconcrete, both these phenomenon led to the reduction
of the prestressing force. The original tensile strain got reduced because of the
compressive strain that concrete was undergoing. That is why the residual stress was
subsequently much lower than the original prestress, and the early attempts of
prestressing were not successful becauseof these long-term effects.
38

With time, this problem was identified and concepts of retightening the rods to overcome
the loss in prestress were introduced. Then we find that the development of the high
strength materials, both the concrete and the steel, led to the better development of the
prestressedconcrete. Due to the high strength steel, a much higher original tensile strain
was possible. Even after losses,there will be substantial residual tensile strain in the steel.
Since we were using high tensile steel, the prestressing force also became much larger. It
was only high strength concrete which was able to sustain this high load, and proper
combination of high strength concrete and the high strength steel led to more economical
and effective prestressedmembers.
We also went through the names of several pioneers in the development of prestressed
concrete. We found that initially the prestressedconcrete started as some tightening of the
arches; and then we find its applications in pipes, tanks and other liquid retaining
structures. The applications in bridges were marvelous becausethe bridge decks became
longer, sleeker and aesthetically more appealing.
The development of the industrial products like jacks, the turnbuckles and the anchorage
system also helped in the development of the prestressed concrete. With the industrial
production of concrete, the precast and the prestressed structural members were
becoming popular over the time. During the mid 20th century, there was an exponential
growth of prestressed concrete. In India also, we nd that prestressed concrete became
very popular over the years. Whether it is in bridges or in railway sleepers or in
transmission line poles or in liquid storagetanks, we nd applications in India.
Finally, we discussedthe development of the prestressedconcrete in terms of the building
materials. We found that in one part, we had materials like stone and bricks, which were
strong in compression and weak in tension. On the other group, we had ropes and steel
bars, which were strong in tension, but weak in compression. Only wood and structural
steel were

able to combine

them

both.

In reinforced concrete, we nd a judicious combination of the artificial stone which is the
concrete, and steel. But reinforced concrete is a passive combination, where there is no
stress in either of the two materials. But in prestressed concrete, we find that the high
strength concrete and the high strength steel are judiciously combined to have a member
39

which is more effective in carrying the load. The prestressedconcrete is called an active
combination because both the materials are stressed even before the self-weight of the
member or the external loads are acting.
The concrete is in compression to counteract the tensile stresses and the steel is in
tension. Due to this, the prestressed concrete is much more advantageous than the
reinforced

concrete.

In our next class, we shall move on to the advantagesof prestressedconcrete as compared
to

reinforced

concrete.

We

shall

learn

about

the

different

terms

that

are

used

in

prestressedconcrete structures, and the different types of prestressing. We shall also learn
the prestressing systems and devices that are available at present, with nice illustrations
and sketches.

Thank you.

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