precasting of concrete are inter-related features of the modern
industry. Through the application of imaginative design and quality
they have, since the
1930’s, had an increasing impact on architectural and
construction procedures. Prestressing of concrete is the application of
compressive force to concrete members and may be achieved by either
pretensioning high tensile steel strands before the concrete has set,
post-tensioning the strands after the concrete has set. Although these
techniques are commonplace, misunderstanding of the principles, and the
they are applied, still exists. This paper is aimed at providing a
outline of the basic factors differentiating each technique and has
prepared to encourage understanding amongst those seeking to broaden
knowledge of structural systems.
2.1 Prestressed Concrete: Prestressing
of concrete is defined
as the application of compressive stresses to concrete members.
zones of the member ultimately required to carry tensile stresses under
load conditions are given an initial compressive stress before the
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
2.2 Pre-tensioning: Pre-tensioning
casting, of a tensile
force to high tensile steel tendons
around which the concrete is to be cast. When the placed concrete has
sufficient compressive strength a compressive force is imparted to it
releasing the tendons, so that the concrete member is in a permanent
2.3 Post-tensioning: Post-tensioning
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
locking the stressed tendons with mechanical anchors. The tendons are
grouted in place.
|| 3. Advanages
3.1 General Advantages: The
use of prestressed concrete offers distinct advantages over ordinary
concrete. These advantages can be briefly listed as follows:
the effect of cracks in concrete elements by holding the
reduced beam depths to be achieved for equivalent design
concrete is resilient and will recover from the effects of
a greater degree of
overload than any other structural material.
the member is subject to overload, cracks,
which may develop, will close
removal of the overload.
enables both entire structural elements and structures to be formed
number of precast units, e.g. Segmented
and Modular Construction.
elements permit the use of longer spanning members with a
high strength to
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
can provide significant
cost advantages over structural steel sections or
ordinary reinforced concrete.
of Prestressing: The
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
runs of limited quantity of small numbers of non-standard units.
5.1 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
unreinforced concrete as being too unreliable. Cracks in the
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
design for service loadings. In ordinary reinforced concrete, steel
introduced to overcome this low tensile strength. They resist tensile
and limit the width of cracks that will develop under design
Reinforced concrete is thus designed assuming the concrete to be
unable to carry any tensile force. Prestressing gives crack-free
by placing the concrete in compression before the application of
5.2 The Basic Idea: A
prestressing will best explain the basic idea. Consider a row of books
blocks set up as a beam. See Fig.2
This "beam" is able to resist
compression at the top but is unable to resist any tension forces at
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
applied. The application of the external downward load merely reduces
proportion of precompression acting in the tensile zone of the beam.
5.3 The Position of the
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
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
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
at 1/3 the beam depth above the bottom face. As well as the overall
we now have a further compressive stress acting on the bottom fibre due
moment of the eccentric prestress force about the neutral axis of the
We then find it is possible to achieve the same compression at the
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
is again negated whilst the final compressive stress in the top fibre
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
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:
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
deflection is upwards producing a hogging profile. See Fig.4(b).
the loads of Fig.4(a) to our prestressed beam, the final deflection
produced is a sum of Figs.4(a) and 4(b) as shown in Fig.4(c).
though shown exaggerated in the Fig.4(c), is controlled within limits
code and bylaw requirements. Such control of deflection is not possible
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
5.5 Prestress Losses:
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
return completely to its original shape. There has been an irreversible
deformation or permanent set. This is known as "creep"
concrete and "creep" of concrete and of steel reinforcement are
potential sources of prestress loss and are provided for in the design
The magnitude of shrinkage may be in the range
of 0.02% depending on the environmental conditions and type of
pre-tensioning, shrinkage starts as soon as the concrete is poured
post-tensioned concrete there is an opportunity for the member to
part of its shrinkage prior to tensioning of the tendon, thus
loss from concrete shrinkage is less.
¶ Creep: With
concrete the effect is to compress and shorten the concrete. This
must be added to that of concrete shrinkage. In the tensioned steel
effect of "creep’’ is to lengthen the tendon
causing further stress loss.
Allowance must be made in the design process for these losses. Various
prestressing systems employing wedge type gripping devices, some degree
pull-in at either or both ends of a pre-tensioning bed or
can be expected. In normal operation, for most devices in common use,
pull-in is between 3 mm and 13 mm and allowance is made when tensioning
tendons to accommodate this.
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
shortening due to creep and shrinkage. it is necessary to
use the high tensile
steels which were developed to produce a large elongation when
This ensures that there is sufficient elongation reserve to maintain
desired pre-compression. The relationship between the amount of load,
stress, in a material and the stretch, or strain, which the material
while it is being loaded is depicted by a stress-strain curve. At any
stress there is a corresponding strain. Strain is defined as the
a member divided by the length of the member. The stress-strain
some grades of steel commonly encountered in construction are shown in Fig.5.
It is apparent from these relationships that considerable variation
between the properties of these steels. All grades of steel have one
common: the relationship between stress and strain is a straight line
certain stress. The stress at which this relationship departs from the
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
structural sections and reinforcing bars, show a deviation from this
relationship at a much lower stress than high strength prestressing
strength steels cannot be used for reinforced concrete as the width of
under loading would be unacceptably large. These high strength steels
their strength largely through the use of special compositions in
with cold working. Smaller diameter wires gain strength by being cold
through a number of dies. The high strength of alloy bars is derived by
of special alloys and some working.
5.6.2 Concrete: To accommodate the degree
of compression imposed by the tensioning tendons and to minimise
losses, a high strength concrete with low shrinkage properties is
Units employing high strength concrete are most successfully cast under
controlled factory conditions.
6.1 General: Methods
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
tendons around which the concrete is cast. . ." and para 2.3
"Post-tensioning is the application of a compressive force to the
at some point in time after casting. When the concrete has hardened a
prestress is induced by tensioning steel tendons passing through ducts
into the Concrete".
6.2 Types of Tendon: There
are three basic
types of tendon used in the prestressing of concrete:
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
steel from which they are made has a yield stress (fy Fig.5) in the
620 MPa. This gives bar tendons a lower strength to weight ratio than
wires or strands, but when employed with threaded anchorages has the
of eliminating the possibility of pull-in at the anchorages as
para. 5.5, and of reducing anchorage costs.
mainly used in post-tensioning systems for prestressing concrete, is
and stress relieved with a yield stress of about 1300 MPa. Wire
commonly used in New
are 5mm, 7mm, and 8mm.
which is used in
both pre and post-tensioning is made by winding seven cold drawn wires
on a stranding machine. Six wires are wound in a helix around a centre
which remains straight. Strands of 19 or 37 wires are formed by adding
subsequent layers of wire. Most pre-tensioning systems in New Zealand
based on the use of standard seven wire stress relieved strands
BS3617:"Seven Wire steel strand for Prestressed concrete." With wire
tendons and strands, it may be desirable to form a cable to cope with
stressing requirements of large post-tensioning applications. Cables
by arranging wires or strands in bundles with the wires or strands
each other. In use the cable is placed in a preformed duct in the
member to be stressed and tensioned by a suitable posttensioning
bars, wires, strands, or made up cables may be used either
straight or curved.
steel tendons are still by far the most commonly used tendons in
curved tendons are used primarily in post-tensioning
applications. Cast-in ducts are positioned in the concrete unit to a
curve chosen to suit the varying bending moment distribution along the
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
and hardened around it. The tensile force in the stressing steel is
one of three methods:
method - an anchor block cast in the ground.
method - the bed is designed to act as a strut without
tensioning forces are applied.
method - tensioning forces are resisted by strong steel
is usual in pretensioning factories to locate the abutments of the
bed a considerable distance apart so that a number of similar units can
stressed at the same time, end to end using the same tendon. This
is called the "Long Line
Process". After pouring, the concrete is
cured so that the necessary strength and bond between the steel and
has developed in 8 to 20 hours. When the strength has been achieved
be released and the units cut to length and removed from the
systems are based on the direct longitudinal tensioning of a steel
one or both ends of the concrete member. The most usual method of
post-tensioning is by cables threaded through ducts in cured concrete.
cables are stressed by hydraulic jacks, designed for the system in use
ducts thoroughly grouted up with cement grout after stressing has
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
anchorages, especially where moisture or corrosion is present.
To bond the tendon to the structural concrete thus limiting crack
width under overload.
of Prestressed Concrete to Fire:
incombustible. In a fire, failure of concrete members usually occurs
due to the
progressive loss of strength of the reinforcing steel or tendons at
temperatures. Also the physical properties of some aggregates used in
can change when heated to high temperatures. Experience and tests have
however that ordinary reinforced concrete has greater fire resistance
steel or timber. Current fire codes recognise this by their reference
concrete has been shown to have at least the same fire
resistance as ordinary reinforced concrete. Greater cover to the
tendons is necessary however, as the reduction in strength of high
steel at high temperatures is greater than that of ordinary mild steel.
of Prestressing: The
construction possibilities of prestressed concrete are as vast as those
ordinary reinforced concrete. Typical applications of prestressing in
and construction are:
components for integration with ordinary reinforced concrete
floor slabs, columns, beams.
components for bridges.
tanks and reservoirs where water tightness (i.e. the absence of cracks)
components e.g. piles, wall panels, frames, window mullions, power
construction of relatively slender structural frames.
bridges and other structures.
concrete design and construction is precise. The high stresses imposed
prestressing really do occur. The following points should be carefully
adequately protect against losses of prestress and to use the materials
economically requires that the initial stresses at prestressing be at
allowable upper limits of the material. This imposes high stresses,
member is unlikely to experience again during its working life.
the construction system is designed to utilise the optimum stress
both the concrete and steel, it is necessary to ensure that these
meet the design requirements. This requires control and responsibility
everyone involved in prestressed concrete work - from the designer
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.
the design eccentricities are varied only slightly, variation from
stresses could be such as to affect the performance of a shallow unit
full working load. The responsibility associated with prestressing work
that the design and construction should only be undertaken by engineers
manufacturers who are experienced in this field.