Weiher and Zilch - Lehrstuhl für Massivbau

Weiher and Zilch 2008 CBC
Unbonded Post-Tensioning – Concepts and Applications in Germany
Hermann Weiher, Dr.-Ing., Technische Universität München, Munich, Germany
Konrad Zilch, Prof. Dr.-Ing., Technische Universität München, Munich, Germany
ABSTRACT
With unbonded post-tensioning it is possible to design various structures or to
strengthen them. In Germany, not before the 90ies of the last century this type
of post-tensioning was able to succeed against the “Freyssinet”-method of
bonded post-tensioning - about 60 years after the first post-tensioned bridge
worldwide was built in Aue/Saxony. This bridge also was post-tensioned with
unbonded and restressable bars. This paper focuses on the main features, the
pros and cons of unbonded post-tensioning for design, construction,
maintenance and safety and finally presents some applications in Germany.
Keywords: Post-Tensioning, unbonded, tendons, projects.
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Weiher and Zilch 2008 CBC
INTRODUCTION
Post-tensioned concrete structures combine the pros of steel and concrete. So, these
structures or members are stiffer and show less cracking than comparable RC members. As a
consequence, they can be designed more slender. Post-tensioning allowed in the past
bridging of very big spans with concrete structures. After WWII, post-tensioning was applied
to the majority of road and railway bridges 1,2,3 . Although the first pt bridge had unbonded
tendons, the huge stock of concrete brides was built using bonded post-tensioning according
to the ideas of Eugène Freyssinet. This did not change before the 90ies of last century, when
first pilot project have been realized using external cables. Another reason for that
development might have been the prohibition of building concrete bridges using segments in
Germany.
The first experiences in applying only external post-tensioning for road and railway bridges
have been published in a workshop at University of Karlsruhe in 1998 4,5,6,7,8,9 . Since 1999
post-tensioned concrete bridges with box-section must be built using external tendons (ARS
17/1999 10,6 ). They have to be arranged outside the concrete cross section and inside the box.
Moreover, the arrangement of tendons in the webs was forbidden for boxes because the
compact arrangement of tendons often lead to consolidation problems and poor corrosion
protection quality 11 . Using only external tendons with a restricted maximum force causes a
high number of tendons in the box. In addition to design problems when arranging concrete
deviators the concrete surfaces are difficult to reach for regular inspection (e.g. viaduct
Rumbeck, figure 1). That’s why combined post-tensioning was invented. Some tendons (e.g.
the primary tendons) are arranged internally with bond. During the last years some pilot
projects were realized using unbonded tendons instead of bond tendons as internal tendons.
Unbonded tendons transfer normal forces only at the anchorages. This also applies to
unbonded tendons that are arranged within the concrete cross section. By deflecting the tendons
it is possible to compensate for external stresses due to bending („load balancing“).
Anchor forces as well as deviation forces can be seen as loads to the structure.
Fig. 1 Rumbeck Viaduct: External tendons during construction and at final state (courtesy of Schäfer)
Fig. 2 RC T-Beam with external tendons 12
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TENDONS FOR UNBONDED POST-TENSIONING
Following, presently used tendon systems for unbonded post-tension will be shown.
Unbonded tendons systems consist of prestressing steel (wire, strand, compact strand or bar).
The steel is protected from external influences by polyethylene sheathing (e.g. HDPE-pipe).
For durable protection of the steel from corrosion the sheathings are filled with grout, grease
or wax.
EXTERNAL TENDONS
External tendons can be distinguished by their cross section. Presently applied systems either
are rectangular shaped or circular (Fig. 3). Deviation of tendons has to be done at the
concrete deviator. To reduce its dimensions the radius of deviation has to be small which
leads to high transverse loads. These loads might lead to significant imprints of the steel into
the soft polyethylene, which has to be taken into account when designing for durability 13 .
There are some opportunities to reduce the loading on the ducts, e.g. increasing the radius or
using a different duct shape.
For band tendons the transverse loading can be determined exactly in contrast to bundle
tendons. That’s why design of ducts can be done with a high reliability. Additionally, a lot of
testing has be done in the past to investigate the imprint behaviour at concrete
deviators 14,15,16 . To get an European technical approval ETA for an external tendon system it
is necessary not only to show the suitability of steel and anchorage by load transfer, static
and fatigue tests but also to show the suitability of the ducts with a test of a deviated tendon
according to the European guideline ETAG 013, Ed. 2002 17 .
a) b) c)
Fig. 3 External tendon types, cross section at high points of the curvature 13
INTERNAL TENDONS
Internal unbonded tendons can be deflected continuously just like bonded tendons. In
contrast to those the unbonded tendons can be restressed or even exchanged. The smallest
tendon for unbonded pt is the monostrand (Fig. 4a). Arranging second polyethylene
sheathing allows exchangeability of the monostrand (Fig. 4b). Monostrands can be combined
in different ways to form larger tendons. For example four monostrands can be arranged in a
line. Internal unbonded tendons are forming a void in the concrete section and thus may
reduce the transverse force bearing capacity, especially in the bridge deck. The dimension of
the created void shall be minimized and be very compact. A tendon design that accomplishes
the requirement to the cross section very well is shown in Fig. 4c. Four rows of monostrands
(bands) are stacked staggered to minimize the remaining spaces. The steel ratio of the tendon
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Weiher and Zilch 2008 CBC
is at 45% for the not exchangeable type (Fig. 4c, Table 1). Presently used external tendons
can also be arranged inside of the concrete section but they are clearly less compact. For pt of
slabs the deviation radii usually are quite large and the tendons small – transverse loading of
the steel to the ducts is of less importance. This is quite different when using loop-anchors
with small radii of curvature. There you also have to proof the durability respectively the
resistance of the duct against the loading by the steel members.
a)
b)
c) d)
Fig. 4 Internal tendon types, cross section at high points of the curvature 13
Table 1 Filling ratio of steel for tendons with a prestressing force of about 3 MN
___________________________________________________________________________
Post-tensioning system (F ≈ 3 MN) Ap / Atotal
SUSPA CD-66 (Fig. 3b) 27 %
VBF-CMM D (Fig. 3a) 28 %
VBF compact, one PE-duct (Fig. 4c) 45 %
VBF compact, double PE-duct (Fig 4d) 29 %
VT-CMM D (similar to Fig. 3a) 23 %
VT-CMM KD (similar to Fig. 3a) 25 %
Features of unbonded pt and consequences for design and construction
Unbonded tendon systems basically differ from bonded systems by their corrosion protection
of the steel. Unbonded tendons are only protected by the components of the tendon:
corrosion protection mass, PE-HD sheathing(s). Bonded tendons using corrugated steel ducts
need the surrounding concrete cover to protect the steel. Mainly, thin concrete cover (e.g. RC
structures built before implementation of the national guideline ZTV-K 80 in 1980 19 ) caused
corrosion of the prestressing steel in the past 1,11 .
BOND / REINFORCEMENT
Because the steel is not bonded to the concrete there is a higher need of reinforcing steel, as
an increase of loading just leads to a small increase of steel stress – but over the total length
of the tendon. In addition to that, the prestressing steel cannot be counted as steel that is
necessary for limitation of crack width. Generally, more reinforcement makes the structure
robust but also leads to higher costs.
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BOND / FATIGUE
Unbonded tendons show very small stress ranges when the structure is loaded dynamically
and due not tend to fatigue fracture (e.g. at coupling joints 1 ). The stress due to live loads
dispenses all over the length of the tendons. At multi-span structures these stresses can even
compensate each other. Concrete bridges with bonded pt that are subjected to fatigue danger
can be strengthened very effectively 20 .
FRICTION / FRETTING FATIGUE
The immediate stress losses of a tendon depend on the elastic concrete contraction when
stressing other tendons, on the slip of the wedges as well as on the friction when deviated.
The friction coefficient of prefabricated tendons with waxlike corrosion protection mass and
polyethylene duct (e.g. monostrand) is significantly smaller than the one of bonded tendons
with metal ducts. This is of special interest if large deflections have to be done, e.g. for tanks
or multi-span bridges. There, a smaller amount of prestressing steel can be used for the same
prestressing force necessary at certain points.
Fretting occurs only at bond tendons between two metal friction partners. It depends on the
pressure between those metal partners (e.g. strand-duct or strand-strand). For unbonded
tendons fretting is not a problem because of the small stress ranges. That’s why the deviation
radii can be chosen smaller with regard to this task 21 . However, it has to be taken into
account that transverse pressure when deviating the tendon with a small radius might lead to
deep imprints of the steel into the polyethylene and therefore might reduce the corrosion
protection 13,23,24 .
DESIGN AS RC MEMBER
For concrete structures with only unbonded post-tensioning the concrete cover is not needed
for the protection of the prestressing steel. The tendons are protected separately by wax and
PE-HD ducts. In contrast to bonded post-tensioning with metal ducts it is clearly less critical
whether the structural concrete member is cracked which would allow the permeation of
corrosion-promoting substances. Unbonded post-tensioned structural concrete members have
to be designed only as RC members for serviceability limit state (SLS). A documented
evidence of conformity has not to be accomplished. Larger maximum crack widths are
allowed and decompression has not to be avoided. Thus, the required amount of reinforcing
steel is smaller (see table 18 and 19 of DIN 1045-1 25 or Eurocode 2.
RESTRESSABILITY, INSPECTION, EXCHANGABILITY
One of the main aspects for the application of unbonded post-tensioning is its ability of
restressing after completion of the structure. By using a hydraulic jack it is possible to
restress the steel because there is no bond with the surrounding concrete. For example timedependant
losses of the force like creep, shrinkage or relaxation of the steel can be
compensated at a later time. When measuring the force of the jack with the oil pressure
during lifting of the anchor it is also possible to check the prestressing force of the tendon.
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Weiher and Zilch 2008 CBC
Here it is commendable to use jacks for monostrands. At abutments enough space for
handling has to be kept free or the backfill has to be removed. Both are very complex and its
application mainly for short-span pt bridges is not economical. At multispan bridges most
tendons are anchored at the diaphragms and thus are easy to access.
If restressing is done, one has to bear in mind that the additional stressing length is large
enough so that the “new” imprints of the wedges into the steel are not at the same place than
the “old” imprints. Therefore, depending on the stress increase, a certain minimum length of
the tendon is required - otherwise the tendon has to be exchanged. An exchange also is
required if the tendon is damaged (e.g. by fire or vandalism).
PRODUCT QUALITY
Unbonded tendons are often fabricated completely in a factory. Therefore, grouting on site
can be avoided. The quality of works and the schedule is not influenced by weather
conditions (e.g. frost). With unbonded tendons the installation and stressing of the tendons
can be performed almost entirely independent from the reinforcing and pouring of the
concrete. When using internal tendons the quick assembly can be assured by preparing few
supports. Between these supports the tendon finds its final vertical position due to its
stiffness (“free tendon curvature” 29 ).
In Germany, the arrangement of tendons in the webs of boxes has been forbidden in 1999.
For T-beams this is not valid. Compact shaped cross sections are advantageous because they
weaken the cross section of the RC less. The fabrication of unbonded tendons off-site not
only guarantees a high quality of the products but moreover accelerates the construction
progress. The tendons can be shipped to the site briefly before it will be installed. With mass
products (e.g. automobile industry) this Just-in-Sequence-concept (JIS) led to remarkable
time and cost savings 30 . To prevent damages of the soft PE-ducts there are higher demands
on the accuracy of installation and handling on-site. Deviation along sharp edges must be
avoided which might lead to cutting of the steel through the polyethylene.
INDICATION OF FAILURE
Simple indication of tendon failures can be achieved by using unbonded tendons. They
consist of prestressing steel, a wax-like corrosion protection mass and polyethylene
sheathing. Fig. 5 (left) shows a two span girder with a bonded tendon (top) and a girder with
an unbonded tendon (bottom), both with failed tendons close to the left support, e.g. due to
stress corrosion. For the girder with the unbonded tendon, a local tendon failure is equivalent
to a total failure along the total tendon length since there is no reintroduction of the force due
to bonding. Therefore, the prestressing is also reduced in the areas of maximum bending
moments (mid-support, mid-span) so that the girder will crack in these areas. Concrete
cracking can be observed by many monitoring systems and can be verified numerically as an
indication of tendon failure. The cracks are indicating the failure of the tendon; see Fig. 5
(left). Zilch, Hennecke and Gläser have shown in their research, that cracking occurs far from
the ultimate limit state of the structure 26 . In Fig. 5 (right) they illustrate their results in which
a rather long part of a span will show cracks at rates of prestressing losses that do not harm
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the load capacity (e.g. 25% loss). Hence, a sudden collapse of the structure (e.g. a bridge) can
be avoided by using unbonded tendons.
x
L L
7
100%
Fig. 5 Early indication of failure for unbonded post-tensioning
EXAMPLES / PILOT PROJECTS
length of cracked zone / span (x/L)
80%
60%
40%
20%
fctk,0.05
fctm
permanent
0%
0,0% 12,5% 25,0% 37,5% 50,0%
loss of prestressing
Already since the 90ies of the last century external tendons have been widespread in
Germany. For road bridges with box girders the combination with internal bonded tendons
(only in deck and bottom slab) has shown to be better. The pros of unbonded post-tensioning
(product quality, structural safety, restressability and exchangability etc.) can be combined
with the pros of bonded post-tensioning (larger lever arm, bigger steel strength for ULS,
continuous tendon curvature). However, the structure cannot be seen as an RC member any
more because of the bonded tendons and thus has to fulfil more strict design criteria in SLS.
Since few years unbonded tendons have been uses as internal tendons. Pilot projectrs have
been realized, like Mühlenberg Viaduct (North Rhine-Westphalia) or Roßriether Graben
(Bavaria). For tank construction the advantages of internal guided unbonded post-tensioning
can be extensively used, e.g. for the four digestion tanks of the sewage water treatment plant
“Gut Großlappen” near Munich, Germany. One example of the vast application opportunities
of unbonded post-tensioning is demonstrated at a pedestrian bridge with a granite slab.
MÜHLENBERG VIADUCT
The Mühlenberg Viaduct at Arnsberg (B 229n) spans with six fields a distance of 255m 31 .
The box is 2,45m in height (see Fig. 6a). The structure was built stepwise. According to
Eisler 31 twelve external tendons à 3 MN and two precaution tendons would have been
necessary if not using internal tendons. This would have led to problems of space and
uneconomic diaphragms. For internal tendons unbonded ones have been chosen. Fig. 6b
shows the pt concept. The internal tendons are arranged as additional tendons spanwise in the
bottom slab for the pier regions and in the deck slab for mid-span. The external tendons are
deviated at all diaphragms (pier, approaching span and twice in the inner spans). These
diaphragms or concrete deviator are densely reinforced concrete members serve both for
deviation and for anchoring (see Fig. 6c). Despite the higher prices for unbonded tendons in

Weiher and Zilch 2008 CBC
comparison to bonded tendons and a higher demand of reinforcing steel the savings of prestressing
in this project did not lead to higher costs 31 . With this concept it is possible to
realise the pros pointed out before, without higher efforts. A durable and safe product can be
offered the owner. One has to bear in mind that the unbonded pt systems for internal use
differ in quality and suitability.
a)
c)
Fig. 6 a: cross section, arrangement of internal and external tendons at diaphragm
Fig. 6 b: longitudinal section, pt span 1 to 3
Fig. 6 b: detail of diaphragm, deviation and anchoring of external (red) and internal (blue) tendons
ROSSRIETHER GRABEN
Near Mellrichstadt (Bavaria) another pilot project with combined unbonded pt with external
and internal tendons has been realized (three span structure with single box - approaches 40m
each, inner span 50 m). As internal tendons a bundle tendon with wires in a duct has been
used 32,33 . They are arranged in the slabs like at the Mühlenberg Viaduct. In each span just one
concrete deviator has been placed. Also for this project there was no difference in the initial
costs compared to the use of bonded internal tendons 32 . The design has been done as to the
national code DIN 1045-1 25 and DIN Fachbericht 102 34 which are based on the Eurocodes 1
and 2. Within this project the exchange of the steel of an internal tendon has been shown to
check the feasibility 33 . The PE duct remained inside the structure. Transverse pressure
created friction and clamping between the wires also led to a high force necessary for
removal. Thus, the length and the deflections will be limited if the duct shall not be stressed
to seriously. After re-installation of the prestressing steel, the duct has been refilled with wax.
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b)

Weiher and Zilch 2008 CBC
Fig. 7 Roßriether Graben
GUT GROSSLAPPEN
North of Munich the construction of four digestion tanks has been finished in 2007. The
complex post-tensioning of the digestion tanks has been realized by internal monostrand
bands (four monostrands in a row):
- Ring post-tensioning: horizontal tendons are distributed over the height of the tanks
and string around the tanks.
- Meridian post-tensioning (Fig. 8a, Fig. 8c): Vertical tendons are arranged all around
the tanks, they are anchored with loops in the bottom of the tanks.
- Conic post-tensioning (Fig. 8b): The bottom cone is hung up, the support is realized
at a ring founded on bore piles which is over the bottom cone.
a)
b)
Fig. 8 digestion tanks a: meridian pt, b: conic pt, c: top view during construction
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c)

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The use of numerous small unbonded tendons was in a specific proposal preferred to less big
tendons with bonded post-tensioning. Thereby decompression would have occurred – when
using unbonded tendons there are fewer requirements to fulfil and consequently there is the
potential to save a lot of prestressing 39 . On the other hand there is a high effort for planning,
design and ensuring the accurate installation of the post-tensioning. The use of small tendon
units for the tank construction also has to be assessed positively because of easier installation
between the rebars and easier pouring of the self-consolidating concrete.
POST-TENSIONED GRANITE
Unbonded tendons are suitable for external application because of their corrosion protection
system. A pedestrian bridge out of granite delivered by the company Kusser, Aicha v. Wald,
has been prestressed by Monostrands without any additional reinforcement (see Fig. 9). To
proof the suitability of granite load transfer to the structure testing has been done according
to the ETAG 013 at Technische Universität München.
a)
b)
c)
Fig. 9 Single span granite bridge post-tensioned with monostrands
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