1.Introduction: The
prestressing and
precasting of concrete are inter-related features of the modern
building
industry. Through the application of imaginative design and quality
control,
they 
have, since the
1930’s, had an increasing impact on architectural and
construction procedures. Prestressing of concrete is the application of
a
compressive force to concrete members and may be achieved by either
pretensioning high tensile steel strands before the concrete has set,
or by
post-tensioning the strands after the concrete has set. Although these
techniques are commonplace, misunderstanding of the principles, and the
way
they are applied, still exists. This paper is aimed at providing a
clear
outline of the basic factors differentiating each technique and has
been
prepared to encourage understanding amongst those seeking to broaden
their
knowledge of structural systems.
2.Definitions:
2.1 Prestressed Concrete: Prestressing
of concrete is defined
as the application of compressive stresses to concrete members.
Those
zones of the member ultimately required to carry tensile stresses under
working
load conditions are given an initial compressive stress before the
application
of working loads so that the tensile
stresses developed by these working loads
are balanced by induced compressive strength. Prestress
can be applied in two
ways - Pre-tensioning
or Post-tensioning.
2.2 Pre-tensioning: Pre-tensioning
is the
application, before
casting, of a tensile
force to high tensile steel tendons
around which the concrete is to be cast. When the placed concrete has
developed
sufficient compressive strength a compressive force is imparted to it
by
releasing the tendons, so that the concrete member is in a permanent
state of
prestress.
2.3 Post-tensioning: Post-tensioning
is the
application of a compressive
force to the concrete at some point in time after
casting. When the concrete has gained strength a state of
prestress is induced
by tensioning steel tendons passed through ducts cast into the
concrete, and
locking the stressed tendons with mechanical anchors. The tendons are
then normally
grouted in place.
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3. Advanages
of Prestressing:
3.1 General Advantages: The
use of prestressed concrete offers distinct advantages over ordinary
reinforced
concrete. These advantages can be briefly listed as follows:
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¶
Prestressing minimises
the effect of cracks in concrete elements by holding the
concrete in
compression.
¶
Prestressing allows
reduced beam depths to be achieved for equivalent design
strengths.
¶
Prestressed
concrete is resilient and will recover from the effects of
a greater degree of
overload than any other structural material.
¶
If
the member is subject to overload, cracks,
which may develop, will close
up on
removal of the overload.
¶
Prestressing
enables both entire structural elements and structures to be formed
from a
number of precast units, e.g. Segmented
and Modular Construction.
¶
Lighter
elements permit the use of longer spanning members with a
high strength to
weight characteristic.
¶
The
ability to control deflections in prestressed beams and slabs permits longer
spans to be achieved.
¶
Prestressing permits a more efficient usage of steel and enables the
economic use of high tensile steels and high strength concrete.
3.2 Cost advantages of
Prestressing:
Prestressed concrete
can provide significant
cost advantages over structural steel sections or
ordinary reinforced concrete.
4.Limitations
of Prestressing: The
limitations of
prestressed concrete are few and really depend only upon the
imagination of the
designer and the terms of his brief. The only real limitation where
prestressing is a possible solution may be the cost of providing moulds
for
runs of limited quantity of small numbers of non-standard units.
5. Basics
of Prestressing:
5.1 The Tensile Strength of
Concrete: The
tensile
strength of
unreinforced concrete is equal to about 10% of its compressive strength.
Reinforced concrete design has in the past neglected the tensile
strength of
unreinforced concrete as being too unreliable. Cracks in the
unreinforced
concrete occur for many reasons and destroy the tensile capability. (See
Figure in para 1). With prestressed concrete design
however, the tensile strength of
concrete is not neglected. In certain applications it is used as part
of the
design for service loadings. In ordinary reinforced concrete, steel
bars are
introduced to overcome this low tensile strength. They resist tensile
forces
and limit the width of cracks that will develop under design
loadings.
Reinforced concrete is thus designed assuming the concrete to be
cracked and
unable to carry any tensile force. Prestressing gives crack-free
construction
by placing the concrete in compression before the application of
service loads.
5.2 The Basic Idea: A
simple
analogy to
prestressing will best explain the basic idea. Consider a row of books
or
blocks set up as a beam. See Fig.2
(a).
This "beam" is able to resist
compression at the top but is unable to resist any tension forces at
the bottom
as the "beam" is now like a badly cracked concrete member, i.e. the
discontinuity between the books ensures that the "beam" has no
inherent tension resisting properties.
If it is temporarily supported and a
tensile force is applied, the "beam’’ will fail by
the books dropping out
along the discontinuities. See Fig.2(b).
For the beam then to function properly
a compression force must be applied as in Fig.2(c).
The beam is then
"prestressed" with forces acting in an opposite direction to those
induced by loading.
The effect of the longitudinal prestressing force is thus
to produce pre-compression in the beam before external downward loads
are
applied. The application of the external downward load merely reduces
the
proportion of precompression acting in the tensile zone of the beam.
5.3 The Position of the
Prestressing Force:
Prestressing
can be used 
to
best advantage by varying the position of the prestress force. When the
prestress is applied on the centroid of the cross-section a uniform
compression
is obtained. Consider the stress state of the beam in Fig.3(b).
We can see that
by applying a prestress of the right magnitude we can produce
pre-compression
equal and opposite to the tensile force in Fig.3(b).Then
by adding the stress
blocks we get: i.e. zero stress towards the bottom fibres and twice the
compressive stress towards the top fibres. Now apply the
pre-compression force
at 1/3 the beam depth above the bottom face. As well as the overall
compression
we now have a further compressive stress acting on the bottom fibre due
to the
moment of the eccentric prestress force about the neutral axis of the
section.
We then find it is possible to achieve the same compression at the
bottom fibre
with only half the prestressing force. See Fig.3(d).
Adding now the stress
blocks of Fig.3(b) and 3(d) we find that the tensile stress in the
bottom fibre
is again negated whilst the final compressive stress in the top fibre
is only
half that of Fig.3(c). See Fig.3(e).
Thus by varying the
position of the
compressive force we can reduce the prestress force required, reduce
the
concrete strength required and sometimes reduce the cross sectional
area. Changes in cross sections such as using T
or I or channel sections
rather than rectangular sections can lead to further economies
5.4 The Effect of Prestress
on Beam Deflection:
From
5.3 it is obvious that 
the
designer should, unless there are special circumstances, choose the
eccentrically applied prestress.
Consider again the non-prestressed beam of
Fig.in para 1. Under external loads the beam deflects to a profile
similar to that
exaggerated in Fig.4(a).
By applying prestress eccentrically a deflection is
induced. When the prestress is applied in the lower portion of the
beam, the
deflection is upwards producing a hogging profile. See Fig.4(b).
By applying
the loads of Fig.4(a) to our prestressed beam, the final deflection
shape
produced is a sum of Figs.4(a) and 4(b) as shown in Fig.4(c).
Residual hogging,
though shown exaggerated in the Fig.4(c), is controlled within limits
by design
code and bylaw requirements. Such control of deflection is not possible
with
simple reinforced concrete. Reductions
in deflections under working loads can
then be achieved by suitable eccentric prestressing. In
long span members this
is the controlling factor in the choice of the construction concept an
technique employed.
5.5 Prestress Losses:
Most
materials to varying
degrees are subject to "creep",
i.e. under a sustained permanent load
the material tends to develop some small amount of plasticity and will
not
return completely to its original shape. There has been an irreversible
deformation or permanent set. This is known as "creep"
Shrinkage of
concrete and "creep" of concrete and of steel reinforcement are
potential sources of prestress loss and are provided for in the design
of
prestressed concrete.
¶
Shrinkage:
The magnitude of shrinkage may be in the range
of 0.02% depending on the environmental conditions and type of
concrete.With
pre-tensioning, shrinkage starts as soon as the concrete is poured
whereas with
post-tensioned concrete there is an opportunity for the member to
experience
part of its shrinkage prior to tensioning of the tendon, thus
pre-compression
loss from concrete shrinkage is less.
¶ Creep: With
prestressing of
concrete the effect is to compress and shorten the concrete. This
shortening
must be added to that of concrete shrinkage. In the tensioned steel
tendons the
effect of "creep’’ is to lengthen the tendon
causing further stress loss.
Allowance must be made in the design process for these losses. Various
formulae
are available.
¶
Pull-in:
With all
prestressing systems employing wedge type gripping devices, some degree
of
pull-in at either or both ends of a pre-tensioning bed or
post-tensioned member
can be expected. In normal operation, for most devices in common use,
this
pull-in is between 3 mm and 13 mm and allowance is made when tensioning
the
tendons to accommodate this.
5.6 Materials:
5.6.1 Steel: Early
in the development of
prestressing it was found that because
of its low limit of elasticity ordinary
reinforcing steel could not provide sufficient elongation to counter
concrete
shortening due to creep and shrinkage. it is necessary to
use the high tensile
steels which were developed to produce a large elongation when
tensioned.
This ensures that there is sufficient elongation reserve to maintain
the
desired pre-compression. The relationship between the amount of load,
or
stress, in a material and the stretch, or strain, which the material
undergoes
while it is being loaded is depicted by a stress-strain curve. At any
given
stress there is a corresponding strain. Strain is defined as the
elongation of
a member divided by the length of the member. The stress-strain
properties of
some grades of steel commonly encountered in construction are shown in Fig.5.
It is apparent from these relationships that considerable variation
exists
between the properties of these steels. All grades of steel have one
feature in
common: the relationship between stress and strain is a straight line
below a
certain stress. The stress at which this relationship departs from the
straight
line is called the yield stress, and is denoted by the symbol fy in
Fig. 5. A
conversion factor may be used to 
convert
either stress to strain or strain to
stress in this range. This conversion factor is called the modulus of
elasticity E. Structural grade steels which are commonly used
for rolled
structural sections and reinforcing bars, show a deviation from this
linear
relationship at a much lower stress than high strength prestressing
steel. High
strength steels cannot be used for reinforced concrete as the width of
cracks
under loading would be unacceptably large. These high strength steels
achieve
their strength largely through the use of special compositions in
conjunction
with cold working. Smaller diameter wires gain strength by being cold
drawn
through a number of dies. The high strength of alloy bars is derived by
the use
of special alloys and some working.
5.6.2 Concrete: To accommodate the degree
of compression imposed by the tensioning tendons and to minimise
prestress
losses, a high strength concrete with low shrinkage properties is
required.
Units employing high strength concrete are most successfully cast under
controlled factory conditions.
6.Prestressing
Methods:
6.1 General: Methods
of prestressing
concrete fall into two
broad categories differentiated by the stage at which
the prestress is applied.That is, whether the steel is pre-tensioned or post-tensioned. From
the definitions para 2.2 pre-tensioning is stated as
"the application before casting, of a tensile force to high tensile
steel
tendons around which the concrete is cast. . ." and para 2.3
"Post-tensioning is the application of a compressive force to the
concrete
at some point in time after casting. When the concrete has hardened a
state of
prestress is induced by tensioning steel tendons passing through ducts
cast
into the Concrete".
6.2 Types of Tendon: There
are three basic
types of tendon used in the prestressing of concrete:
¶
Bars
of high strength alloy steel.These bar type tendons are
used in certain types
of post-tensioning systems. Bars up to 40mm diameter are used and the
alloy
steel from which they are made has a yield stress (fy Fig.5) in the
order of
620 MPa. This gives bar tendons a lower strength to weight ratio than
either
wires or strands, but when employed with threaded anchorages has the
advantages
of eliminating the possibility of pull-in at the anchorages as
discussed in
para. 5.5, and of reducing anchorage costs.
¶
Wire,
mainly used in post-tensioning systems for prestressing concrete, is
cold drawn
and stress relieved with a yield stress of about 1300 MPa. Wire
diameters most
commonly used in New
Zealand
are 5mm, 7mm, and 8mm.
¶
Strand,
which is used in
both pre and post-tensioning is made by winding seven cold drawn wires
together
on a stranding machine. Six wires are wound in a helix around a centre
wire
which remains straight. Strands of 19 or 37 wires are formed by adding
subsequent layers of wire. Most pre-tensioning systems in New Zealand
are
based on the use of standard seven wire stress relieved strands
conforming to
BS3617:"Seven Wire steel strand for Prestressed concrete." With wire
tendons and strands, it may be desirable to form a cable to cope with
the
stressing requirements of large post-tensioning applications. Cables
are formed
by arranging wires or strands in bundles with the wires or strands
parallel to
each other. In use the cable is placed in a preformed duct in the
concrete
member to be stressed and tensioned by a suitable posttensioning
method.
Tendons whether
bars, wires, strands, or made up cables may be used either
straight or curved.
Straight
steel tendons are still by far the most commonly used tendons in
pre-tensioned
concrete units.
Continuously
curved tendons are used primarily in post-tensioning
applications. Cast-in ducts are positioned in the concrete unit to a
continuous
curve chosen to suit the varying bending moment distribution along the
members.
6.3 Pre-Tensioning: As
discussed, (para 2.2) pre-tensioning requires the
tensile force to be
maintained in the steel until after the high strength concrete has been
cast
and hardened around it. The tensile force in the stressing steel is
resisted by
one of three methods:
¶
Abutment
method - an anchor block cast in the ground.
¶
Strut
method - the bed is designed to act as a strut without
deformation when
tensioning forces are applied.
¶
Mould
method - tensioning forces are resisted by strong steel
moulds.
It
is usual in pretensioning factories to locate the abutments of the
stressing
bed a considerable distance apart so that a number of similar units can
be
stressed at the same time, end to end using the same tendon. This
arrangement
is called the "Long Line
Process". After pouring, the concrete is
cured so that the necessary strength and bond between the steel and
concrete
has developed in 8 to 20 hours. When the strength has been achieved
tendons can
be released and the units cut to length and removed from the
bed.
Post-tensioning
systems are based on the direct longitudinal tensioning of a steel
tendon from
one or both ends of the concrete member. The most usual method of
post-tensioning is by cables threaded through ducts in cured concrete.
These
cables are stressed by hydraulic jacks, designed for the system in use
and the
ducts thoroughly grouted up with cement grout after stressing has
occurred. Cement
grouting is almost always employed where post-tensioning through ducts
is carried out to:
¶
Protect the tendon against corrosion by preventing ingress of moisture.
¶
Eliminate the danger of loss of prestress due to long term failure of
end
anchorages, especially where moisture or corrosion is present.
¶
To bond the tendon to the structural concrete thus limiting crack
width under overload.
7.Resistance
of Prestressed Concrete to Fire:
All
concrete is
incombustible. In a fire, failure of concrete members usually occurs
due to the
progressive loss of strength of the reinforcing steel or tendons at
high
temperatures. Also the physical properties of some aggregates used in
concrete
can change when heated to high temperatures. Experience and tests have
shown
however that ordinary reinforced concrete has greater fire resistance
than structural
steel or timber. Current fire codes recognise this by their reference
to these
materials. Prestressed
concrete has been shown to have at least the same fire
resistance as ordinary reinforced concrete. Greater cover to the
prestressing
tendons is necessary however, as the reduction in strength of high
tensile
steel at high temperatures is greater than that of ordinary mild steel.
8.Applications
of Prestressing: The
construction possibilities of prestressed concrete are as vast as those
of
ordinary reinforced concrete. Typical applications of prestressing in
building
and construction are:
¶
Structural
components for integration with ordinary reinforced concrete
construction, e.g.
floor slabs, columns, beams.
¶
Structural
components for bridges.
¶
Water
tanks and reservoirs where water tightness (i.e. the absence of cracks)
is of
paramount importance.
¶
Construction
components e.g. piles, wall panels, frames, window mullions, power
poles, fence
posts, etc.
¶
The
construction of relatively slender structural frames.
¶Major
bridges and other structures.
9.Conclusions: Prestressed
concrete design and construction is precise. The high stresses imposed
by
prestressing really do occur. The following points should be carefully
considered:
¶
To
adequately protect against losses of prestress and to use the materials
economically requires that the initial stresses at prestressing be at
the
allowable upper limits of the material. This imposes high stresses,
which the
member is unlikely to experience again during its working life.
¶
Because
the construction system is designed to utilise the optimum stress
capability of
both the concrete and steel, it is necessary to ensure that these
materials
meet the design requirements. This requires control and responsibility
from
everyone involved in prestressed concrete work - from the designer
right
through to the workmen on the site.
¶
We have seen that considerable design and strength economies are
achieved by the eccentric application of the prestressing force.
However, if
the design eccentricities are varied only slightly, variation from
design
stresses could be such as to affect the performance of a shallow unit
under
full working load. The responsibility associated with prestressing work
then is
that the design and construction should only be undertaken by engineers
or
manufacturers who are experienced in this field.
