In this chapter, we have defined the basic definitions of prestressed concrete

1. BASIC CONCEPTS
Prestressed Concrete

2. DEFINATION
 Prestressing can be defined in general terms as the
preloading of a structure, before application of the
service loads, so as to improve its performance in
specific ways.
 A prestressed concrete member can also be
defined as one in which, there have been
introduced internal stresses of such magnitude and
distribution that the stresses resulting from the
given external loading are counteracted to a
desired degree.

3. INTRODUCTION
 In Conventional Reinforced Concrete, the high
tensile strength of steel is combined with
concrete's great compressive strength to form a
structural material that is strong in both
compression and tension.
 The principle behind Prestressed Concrete is that
compressive stresses induced by high-strength
steel tendons in a concrete member before loads
are applied will balance the tensile stresses
imposed in the member during service.

4. INTRODUCTION
Prestressing removes a number of design
limitations that conventional concrete places on
span and load and permits the building of roofs,
floors, bridges, and walls with longer
unsupported spans.
This allows architects and engineers to design
and build lighter and shallower concrete
structures without sacrificing strength.

5. Use of precast "double-T" beams carrying a floor with
clear span about 20 ft.

6. Twin box-girder bridge under construction using the
segmentally cast cantilever method

7. Highway crossing in Switzerland, continuous
over three spans

8. Segmentally precast post-tensioned rigid frames for the Olymic
stadium in Montreal

9. INTRODUCTION
Prestressed concrete has experienced
greatest growth in the field of
commercial buildings.
For buildings such as shopping centers,
Prestressed Concrete is an ideal choice
because it provides the span length
necessary for flexibility and alteration of
the internal structure.

13. INTRODUCTION
Prestressed concrete is also used in
school auditoriums, gymnasiums,
and cafeterias because of its
properties and its ability to provide
long, open spaces.
One of the most widespread uses of
prestressed concrete is parking
garages.

17. INTRODUCTION
Although prestressed concrete was patented by
a San Francisco engineer in 1886, it did not
emerge as an accepted building material until a
half-century later.
North America's first prestressed concrete
structure, the Walnut Lane Memorial Bridge in
Philadelphia, Pennsylvania, however, was not
completed until 1951.

18. The Linn Cove Viaduct, North Carolina, U.S. 1997

20. INTRODUCTION
Prestressing means the internal creation of
permanent stresses in a structure or assembly,
for the purpose of improving its behavior and
strength under various service conditions.
The concept of Prestressing Concrete was to
introduce sufficient axial pre-compression in
beams so that all tension in the concrete was
eliminated in the member at service load.

21. The Basic Idea
 The principle behind
prestressing is applied
when a row of books is
moved from place to
place.
 Instead of stacking the
books vertically and
carrying them, the books
may be moved in a
horizontal position by
applying pressure to the
books at the end of the
row.

22. The Basic Idea
When sufficient
pressure is
applied,
compressive
stresses are
induced
throughout the
entire row, and the
whole row can be
lifted and carried
horizontally at
once.

23. METHODS OF PRESTRESSING
Although many methods have been used to
produce the desired state of pre-compression
in concrete members, all prestressed concrete
members can be placed in one of two
categories:
(a) Pre-tensioned concrete
(b) Post-tensioned concrete

24. PRETENSIONED CONCRETE
 In Pretensioning, the steel is stretched before the
concrete is placed.
 High-strength steel tendons are placed between
two abutments and stretched to 70 to 80 percent of
their ultimate strength.
 Concrete is poured into molds around the tendons
and allowed to cure.
 Once the concrete reaches the required strength,
the stretching forces are released.
 As the steel reacts to regain its original length, its
tensile stresses are translated into compressive
stresses in the concrete.

25. PRETENSIONED CONCRETE
Typical products for Pretensioned concrete are
roof slabs, piles, poles, bridge girders, wall
panels, and railroad ties.

26. PRETENSIONED CONCRETE
 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.
 It requires heavy anchoring points between which the
tendon is to be stretched and the tendons are usually
in a straight line.
 Most pretensioned concrete elements are prefabricated
in a factory and must be transported to the
construction site, which limits their size.

27. Prestressing strand, Made of high strength steel
PRETENSIONED CONCRETE

28. The
Prestressing
strand is
stretched
across the
casting bed.
About 30,000
pounds of
tension will
be applied to
the cable
before it's
surrounded
by concrete.
PRETENSIONED CONCRETE

29. The ends are cleaned and the prestressing
strands are sealed with a protective coating.
PRETENSIONED CONCRETE

30. PRETENSIONED CONCRETE
 Pretensioning is well suited to the mass
production of beams using the long-line method of
prestressing.
 In present practice anchorage and jacking
abutments may be as much as 800 ft apart.
 The strands are tensioned over the full length of
the casting bed at one time, after which a number
of individual members are cast along the stressed
tendon.
 When the jacking force is released, the prestress
force is transferred to each member by bond, and
the strands are cut free between members.

32. POST TENSIONING
In Post-tensioning, the steel is stretched after
the concrete hardens.
Concrete is cast around, but not in contact
with unstretched steel.
In many cases, ducts are formed in the
concrete unit using thin walled steel forms.
Once the concrete has hardened to the
required strength, the steel tendons are
inserted and stretched against the ends of the
unit and anchored off externally, placing the
concrete into compression.

33. POST TENSIONING
Post-tensioned concrete is used for cast-in-place
concrete and for bridges, large girders, floor slabs, shells,
roofs, and pavements.
Tendons are normally grouted in their conduits after
they are stressed.
A cement paste grout is forced into the conduit at one
end under high pressure, and pumping is continued until
the grout appears at the far end of the tube.
When it hardens, the grout bonds to the tendon and to
the inner wall of the conduit, permitting transfer of force.

35. POST TENSIONING
Although the anchorage fittings
remain in place to transfer the main
prestressing force to the concrete,
grouting improves the performance
of the member should it be
overloaded and increases its
ultimate flexural strength.

36. Post-tensioned
beam under
construction,
showing
inserted tendon
conduits and
anchorages in
position prior to
placing side
forms and
pouring
concrete
POST TENSIONING

37. Post-
tensioning
a beam
using
multiple-
strand
tendons
POST TENSIONING

38. Two-way
prestressed
slab, using
unbonded
wrapped
tendons,
under
construction.
POST TENSIONING

39. POST TENSIONING
Post Tensioning of a Slab

40. TENDON PROFILES
Different types of tendon profiles are used for
load balancing in prestressed concrete.
Load balancing concept sees prestressed
concrete as primarily an attempt to balance a
portion of the load on the structure.
It has no significant advantage for statically
determinate structure while this method offers
tremendous advantages for statically
indeterminate structures.

41. TENDON PROFILES
Simply supported beams with concentrated
load
Fig illustrates how to balance a concentrated load by
sharply bending the tendon at mid span thus creating
an upward component.
F F
θ θ
L
Parabolic Tendon
Concrete Centroid
P = V
V = 2FSinθ
FSinθ
FCosθ
c.g.s.
c.g.c.

42. TENDON PROFILES
Simply supported beams with UDL
Fig illustrates the balancing of a uniformly
distributed load by means of a parabolic cable with
an upward component v (lb/ft).
F P
θ θ
L
v
h
Parabolic Tendon
Concrete Centroid
Uniform load w

43. TENDON PROFILES
Simply Supported Cantilever Beams
Fig illustrates the balancing of a uniformly
distributed load for a cantilever beam by means of
a parabolic cable.
F
L1
Parabolic Tendon
Concrete Centroid
h
h/4
h1
Uniform load w
.
A C
B
L

44. TENDON PROFILES
Simply Supported Beam with curved centroid.
Fig illustrates the balancing of a uniformly distributed
load for Simply Supported Beam with curved
centroid.
F F
L
h
Parabolic Tendon
Concrete Centroid
Uniform load w
.
.

45. TENDON PROFILES
Continuous Beams
Fig illustrates the balancing of a uniformly
distributed load for Continuous Beam
F F
θ θ
L
Parabolic Tendon
Concrete Centroid
Uniform load w
.
L

46. ADVANTAGES OF PRESTRESSING
Prestressing the steel and anchoring it against
concrete produces desirable strains and
stresses which serve to reduce or eliminate
cracks in concrete.
The entire section of the concrete is effective in
prestressed concrete, whereas only the portion
of section above the neutral axis is supposed to
act in the case of reinforced concrete.

47. ADVANTAGES
The use of curved tendons help to carry some
of the shear in the member. Pre-compression
in the concrete tends to reduce the diagonal
tension. So, it is possible to use smaller
section in prestressed concrete to carry the
same amount of external shear in the beam.
Prestressed concrete is more suitable for
structures of long spans and those carrying
heavy loads.

48. ADVANTAGES
 Prestressed structures are more slender and
hence more adaptable to artistic treatment.
 Under dead load, the deflection is reduced, owing
to the cambering effect of prestress.
 Under live loads, the deflection is smaller because
of the effectiveness of the entire uncracked
concrete section, which has a moment of inertia
two to three times that of the cracked section.

49. ADVANTAGES
Prestressed elements are more adaptable
to precasting because of the lighter
weight.
The resistance to corrosion is better than
that of reinforced concrete for the same
amount of cover, owing to the
nonexistence of cracks.

50. ADVANTAGES
Regarding fire resistance, high-tensile steel is
more sensitive to high temperatures, but for the
same amount of minimum cover, prestressed
tendons can have a greater average cover
because of the spread and curvature of the
individual tendons.
Reduction of weight saves handling and
transportation costs.

51. DISADVANTAGES
Prestressed concrete is not suitable for
structures of shorter spans.
Prestressed concrete cannot be used in such
situations where weight and mass are desired
instead of strength.

52. DISADVANTAGES
Prestressed concrete members require more
care in design, construction and erection than
those of ordinary concrete.
Cost increases due to high strength concrete
and high strength steel.
Cost increases due to highly skilled labour.

53. DISADVANTAGES
Complicated form work is required due to
nonrectangular shapes.
Losses in prestress force due to slip, creep,
friction etc.
Anchorages, conduits & Jacks are required.

54. INSTANTANEOUS LOSSES IN PRESTRESSING
 There is an Instantaneous Stress Loss because of
the elastic shortening of the concrete as the
prestress force is transferred to it.
 In Pretensioned concrete, as the prestress is
transferred to the concrete, the member shortens
and the prestressed steel shortens with it.
 In Post-tensioned concrete member, the concrete
shortens as the tendons are jacked against the
concrete. Hence, there is a loss of prestress force.

55. INSTANTANEOUS LOSSES IN PRESTRESSING
 When the tendon is tensioned to its full value, the jack is
released and the prestress is transferred to anchorage.
The Anchorage Fixtures that are subjected to stress at
the time of transfer, will tend to deform, thus allowing the
tendon to slacken slightly.
 Friction wedges employed to hold the wires will slip a
little distance before the wires can be firmly gripped. The
amount of slippage depends on type of wedge and the
stress in the wires. However, in beams pre-tensioned by
the long-line method, slip loss is apt to be insignificant
because of the great length of tendon over which the slip
is distributed.

56. INSTANTANEOUS LOSSES IN PRESTRESSING
 Change in Prestress force of member also occur, when
member is subjected to bending. There may be a loss
or a gain , depends on the direction of bending and the
location of the tendon.
 Another source of immediate loss of prestress force is
the friction between the steel and the conduit through
which it passes as the tendon is stretched.

57. TIME-DEPENDENT LOSSES IN PRESTRESSING
 The Time-dependent Losses are shrinkage of
concrete and concrete creep under sustained
compressive stress.
 Creep is one of the main sources of loss and a
serious one because the amount of creep ranges
from 1 to 5 times than the elastic shortening, if
the prestress is low and the compression in the
concrete is high.

58. TIME-DEPENDENT LOSSES IN PRESTRESSING
 The percentage of creep increases with
increasing stress, and when steel is under low
stress, the creep is negligible.
 Stress relaxation in steel is the loss of its
stress when it is prestressed and maintained at
a constant strain for a period of time.

59. TOTAL LOSSES IN PRESTRESSING
The sum of all type of losses i.e.,
instantaneous and Time
dependent, may be of the order
of 20 to 35 percent of the
original jacketing force.