Introduction to Pre-stressed and Precast Concrete Technology

LECTURE# 01
INTRODUCTION
PRESTRESSED AND PRECAST
CONCRETE TECHNOLOGY
7/1/2020
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By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite
https://web.facebook.com/engrshahfarooq
Engr Shah Farooq
Lecturer GCT Swat

 Introduction:
 Definition of Pre-Stress:
Pre-Stress is defined as a method of applying pre-
compression to control the stresses resulting due to
external loads below the neutral axis of the beam
tension developed due to external load which is more
than the permissible limits of the plain concrete. The
pre-compression applied (may be axial or eccentric)
will induce the compressive stress below the neutral
axis or as a whole of the beam c/s. Resulting either no
tension or compression.
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By: Engr Shah Farooq
www.youtube.com/c/CivilEngineersite

Basic Concept
 Pre-Stressed concrete is basically concrete in which
internal stresses of a suitable magnitude and
distribution are introduced so that the stresses
resulting from the external loads are counteracted to
a desired degree.
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 A pre-stressed concrete structure is different from a
conventional reinforced concrete structure due to the
application of an initial load on the structure prior to its
use. The initial load or ‘pre-stress’ is applied to enable
the structure to counteract the stresses arising during its
service period.
 The pre-stressing of a structure is not the only instance of
pre-stressing. The concept of pre-stressing existed before
the applications in concrete. Two examples of pre-
stressing before the development of pre-stressed
concrete are provided.
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 Force-fitting of metal bands on wooden
barrels:
The metal bands induce a state of initial hoop
compression, to counteract the hoop tension caused
by filling of liquid in the barrels.
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 Pre-tensioning the spokes in a bicycle wheel:
The pre-tension of a spoke in a bicycle wheel is
applied to such an extent that there will always be a
residual tension in the spoke.
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Terminology
 1. Tendon: A stretched element used in a concrete
member of structure to impart pre-stress to the concrete.
 2. Anchorage: A device generally used to enable the
tendon to impart and maintain pre-stress in concrete.
 3. Pre-Tensioning: A method of pre-stressing concrete in
which the tendons are tensioned before the concrete is
placed. In this method, the concrete is introduced by
bond between steel & concrete.
 4. Post-Tensioning: A method of pre-stressing concrete
by tensioning the tendons against hardened concrete. In
this method, the pre-stress is imparted to concrete by
bearing.
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Brief History:
 Before the development of pre-stressed concrete, two
significant developments of reinforced concrete are the
invention of Portland cement and introduction of steel in
concrete. These are also mentioned as the part of the
history. The key developments are mentioned next to the
corresponding year.
 1824 Aspdin, J., (England)
Obtained a patent for the manufacture of Portland
cement.
 1857 Monier, J., (France)
Introduced steel wires in concrete to make flower pots,
pipes, arches and slabs.
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 The following events were significant in the
development of pre-stressed concrete.
 1886 Jackson, P. H., (USA)
Introduced the concept of tightening steel tie rods in
artificial stone and concrete arches.
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 1888 Doehring, C. E. W., (Germany):
Manufactured concrete slabs and small beams with
embedded tensioned steel.
 1908 Stainer, C. R., (USA)
Recognised losses due to shrinkage and creep, and
suggested retightening the rods to recover lost prestress.
 1923 Emperger, F., (Austria)
Developed a method of winding and pre- tensioning high
tensile steel wires around concrete pipes.
 1924 Hewett, W. H., (USA)
Introduced hoop-stressed horizontal reinforcement
around walls of concrete tanks through the use of
turnbuckles.
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 1925 Dill, R. H., (USA)
Used high strength un-bonded steel rods. The rods
were tensioned and anchored after hardening of the
concrete.
 1926 Eugene Freyssinet (France)
Used high tensile steel wires, with ultimate strength
as high as 1725 MPa and yield stress over 1240 MPa.
In 1939, he developed conical wedges for end
anchorages for post-tensioning and developed
double-acting jacks. He is often referred to as the
Father of Pres-tressed concrete.
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7/1/2020By: Engr Shah Farooq
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 1938 Hoyer, E., (Germany)
Developed ‘long line’ pre-tensioning method.
 1940 Magnel, G., (Belgium)
Developed an anchoring system for post-tensioning, using flat wedges.
 During the Second World War, applications of pre-stressed and precast
concrete increased rapidly. The names of a few persons involved in
developing pre-stressed concrete are mentioned. Guyon, Y., (France)
built numerous pre-stressed concrete bridges in western and central
Europe. Abeles, P. W., (England) introduced the concept of partial pre-
stressing. Leonhardt, F., (Germany), Mikhailor, V., (Russia) and Lin, T.
Y., (USA) are famous in the field of pre-stressed concrete.
 The International Federation for Pre-stressing (FIP), a professional
organization in Europe was established in 1952. The Precast/Pre-stressed
Concrete Institute (PCI) was established in USA in 1954.
 Pre-stressed concrete was started to be used in building frames, parking
structures, stadiums, railway sleepers, transmission line poles and other
types of structures and elements.

Development of Building Materials:
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 The development of pre-stressed concrete can be studied in the
perspective of traditional building materials. In the ancient period,
stones and bricks were extensively used. These materials are strong
in compression, but weak in tension. For tension, bamboos and coir
ropes were used in bridges. Subsequently iron and steel bars were
used to resist tension. These members tend to buckle under
compression. Wood and structural steel members were effective
both in tension and compression.
 In reinforced concrete, concrete and steel are combined such that
concrete resists compression and steel resists tension. This is a
passive combination of the two materials. In pre-stressed concrete
high strength concrete and high strength steel are combined such
that the full section is effective in resisting tension and compression.
This is an active combination of the two materials. The following
sketch shows the use of the different materials with the progress of
time.

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Advantage of Pre-Stressed Concrete
 1. The use of high strength concrete and steel in pre-stressed
members results in lighter and slender members than is
possible with RC members.
 2. In fully pre-stressed members the member is free from
tensile stresses under working loads, thus whole of the section
is effective.
 3. In pre-stressed members, dead loads may be counter-
balanced by eccentric pre-stressing.
 4. Pre-stressed concrete member posses better resistance to
shear forces due to effect of compressive stresses presence or
eccentric cable profile.
 5. Use of high strength concrete and freedom from cracks,
contribute to improve durability under aggressive
environmental conditions.
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Continued…
 6. Long span structures are possible so that saving in
weight is significant & thus it will be economic.
 7. Factory products are possible.
 8. Pre-stressed members are tested before use.
 9. Pre-stressed concrete structure deflects
appreciably before ultimate failure, thus giving
ample warning before collapse.
 10. Fatigue strength is better due to small variations
in prestressing steel, recommended to dynamically
loaded structures.
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Disadvantages of Pre-Stressed Concrete:
 1. The availability of experienced builders is scanty.
 2. Initial equipment cost is very high.
 3. Availability of experienced engineers is scanty.
 4. Pre-stressed sections are brittle
 5. Pre-stressed concrete sections are less fire
resistant.
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Classifications and Types
 Pre-stressed concrete structures can be classified in a
number of ways depending upon the feature of designs
and constructions.
 1. Pre-tensioning: In which the tendons are tensioned
before the concrete is placed, tendons are temporarily
anchored and tensioned and the pre-stress is transferred
to the concrete after it is hardened.
 2. Post-tensioning: In which the tendon is tensioned after
concrete has hardened. Tendons are placed in sheathing
at suitable places in the member before casting and later
after hardening of concrete.
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Pre-Stressing System
 1. Pre-Tensioning system:
In the pre-tensioning systems, the tendons are first
tensioned between rigid anchor-blocks cast on the
ground or in a column or unit –mould types pre-
tensioning bed, prior to the casting of concrete in the
mould. The tendons comprising individual wires or
strands are stretched with constant eccentricity or a
variable eccentricity with tendon anchorage at one end
and jacks at the other. With the forms in place, the
concrete is cast around the stressed tendon.
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2. Post-tensioned system:
 In post-tensioning the concrete unit are first cast by
incorporating ducts or grooves to house the tendons.
When the concrete attains sufficient strength, the high-
tensile wires are tensioned by means of jack bearing on
the end of the face of the member and anchored by
wedge or nuts. The forces are transmitted to the concrete
by means of end anchorage and, when the cable is
curved, through the radial pressure between the cable
and the duct. The space between the tendons and the
duct is generally grouted after the tensioning operation.
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ooq

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Difference of Pre-Stressed over Reinforced
Cement Concrete
 1. In pre-stress concrete member steel plays active role. The
stress in steel prevails whether external load is there or not.
But in R.C.C., steel plays a passive role. The stress in steel in
R.C.C members depends upon the external loads. i.e., no
external load, no stress in steel.
 2. In pre-stress concrete the stresses in steel is almost constant
where as in R.C.C the stress in steel is variable with the lever
arm.
 3. Pre-stress concrete has more shear resistance, where as
shear resistance of R.C.C is less.
 4. In pre-stress concrete members, deflections are less because
the eccentric pre-stressing force will induce couple which will
cause upward deflections, where as in R.C.C., deflections are
more.
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Continued…
 5. In pre-stress concrete fatigue resistance is more compare to
R.C.C. because in R.C.C. stress in steel is external load
dependent where as in P.S.C member it is load independent.
 6. Pre-stress concrete is more durable as high grade of
concrete is used which are more dense in nature. R.C.C. is less
durable.
 7. In pre-stress concrete dimensions are less because external
stresses are counterbalance by the internal stress induced by
pre-stress. Therefore reactions on column & footing are less as
a whole the quantity of concrete is reduced by 30% and steel
reduced by about 60 to 70%. R.C.C. is uneconomical for long
span because in R.C.C. dimension of sections are large
requiring more concrete & steel. Moreover as self-weight
increases more reactions acted on columns & footings, which
requires higher sizes.
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Introduction to Pre-stressed and Precast Concrete Technology

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