Tailor Made Concrete Structures – Walraven & Stoelhorst (eds)
© 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8
Monitoring of electrically isolated post-tensioning tendons
B. Elsener
ETH Zurich, Institute for Building Materials, Zurich, Switzerland
ABSTRACT: Electrically Isolated Tendons (EIT) have been introduced as one possible solution to reach the
highest protection level (PL3) in the framework of fib recommendation for grouted post-tensioned tendons. This
approach allows to check the integrity of the plastic duct during and after construction and to monitor the corrosion
protection of the high-strength steel during the whole service life with electrical impedance measurements. The
paper presents results on PC structures with EIT regarding quality control, long term monitoring and location
of defects. Practical experience in Switzerland over the last six years was included in the revision of the Swiss
Guideline “Measures to ensure the durability of post-tensioning tendons in bridges”.
1
INTRODUCTION
Post-tensioning tendons contribute decisively to the
serviceability, safety and durability of pre-stressed
concrete (PC) bridges. In order to reach the goals
of durability (optimum corrosion protection) and of
monitoring requested by the Italian Railways (Italian Standard, 1997) and the Swiss Federal Roads and
RailwayAuthorities, the new system of electrically isolated tendons according to the Swiss Guideline (Swiss
Guideline, 2001) has been adopted. In Switzerland
about 80 bridges of different length have been constructed since 1995 with thick-walled corrugated plastic ducts and electrically isolated anchorages (Elsener
et al. 2002; Ayats et al. 2002) Similar systems have
been massively applied for the first time in Italy for
the design and construction of several bridges and
viaducts of the new high-speed lines (Della Vedova
& Elsener 2006; Prevedini et al. 2004; Bonasso et al.
2006). In Italy, traditional choice for railway bridges
is the use of simply supported spans and about 90%
of the viaducts of the new lines are realised with partial or total pre-casting of PC decks; this allowed to
carry out test programmes on the construction site and
extensive quality control during construction – a point
that has been recognized to be more difficult on continuous span bridges as those usually constructed in
Switzerland.
Electrically isolated tendons have been introduced
as one possible solution to reach the highest protection
level (PL3) in the framework of fib recommendations (fib 2005, Elsener (ed.) 2004). Using electrically
isolated tendons allows to check the electrical isolation of the tendons and the integrity of the plastic
duct during and after construction (Ayats et al. 2002,
Della Vedova et al. 2004) and to monitor the corrosion protection of the tendons during service life with
impedance measurements (Della Vedova et al. 2004,
Elsener 2005).
To achieve the required durability and safety, new
post-tensioning tendons are currently designed and
executed following a multi-layer protection approach.
The single tendons should easily be monitored over
time and damage should be detected at a very early
time. Industry in the last 10 years has developed such
new post-tensioning tendons, based on a complete
encapsulation of the sensitive high-strength steel into
a grouted polymeric duct. Combined with electrical
isolation of the anchorages a simple, non-destructive
monitoring of the individual tendons is possible.
2
2.1
QUALITY CONTROL OF EIT TENDONS
Measuring principle
The impedance measurements are performed between
the steel strands in the grouted ducts and the normal
reinforcement in concrete (Fig. 1). The measuring system thus includes the grout in the duct, the duct (with
pores and defects) and the concrete surrounding the
duct. Grout and concrete are (at least in the range
of measuring frequencies between 100 and 1000 Hz)
pure resistances, the polymer duct instead is essentially
a capacitance in parallel with a very high resistance
(Fig. 1). Any system related imperfections (e.g. not
fully closed grout vents) and/or defect in the duct are
represented by an ohmic resistance in parallel.
231
Table 1. Calculated specific values of the ohmic resistance R, the capacitance C and the loss factor D from
the flyover “P.S. du Milieu” (length 100 m). Measurements
performed 28 days after grouting.
Tendon Nr.
R (km)
C (nF/m)
D (–)
1
2
3
4
5
6
723
1370
2087
1781
2825
short circuit
2.34
2.33
2.35
2.37
2.35
0.093
0.048
0.032
0.037
0.023
Figure 1. Principle of measuring the electrical impedance
of a tendon with an LCR meter. Recommended measuring
frequency is 1 kHz.
2.2
Results from laboratory measurements
Measurements on 1 m long grouted plastic ducts
(ø 59 mm) in concrete blocks (Elsener et al. 2002) have
shown that intact (reference), welded or coupled ducts
have a very high resistance value (>2.3 M at 1 kHz)
and very low loss factors D (<0.034), thus they behave
essentially as capacitance. Ducts with a 2 mm hole
show comparably low resistance values (<100 k)
and the resistance drops to less than 1 k for 40 mm
holes. The loss factor is very high, thus at 1 kHz these
systems behave as resistance. An open grout vent that
ends in the concrete (thus a very small electrolytic
contact is possible) has a resistance of 573 k and a
loss factor of 0.098. As a not perfectly closed grout
vent represents a “defect” for the impedance measurement but by no means a loss in durability, this situation
was chosen as borderline between acceptable and nonacceptable defect. The acceptance criteria in the Swiss
Guideline (2001) was defined as R = 500 km, the
control value D < 0.1.
The capacitance values C of the 1 m long segments
of plastic ducts were measured to 2.34 ± 0.04 nF/m
irrespective of the presence of holes.
2.3
Results from field applications – Switzerland
Both resistance and capacitance values measured
depend on the length L of the tendon: the resistance
values R decrease proportional to the length L, the
capacitance value C increase. For quality control the
specific values R in km and C in nF/m have to be
used.
The first structures constructed with EIT posttensioning tendons were built in Switzerland (Ayats
et al. 2002). In these pilot projects the acceptance
Figure 2. Capacitance C of the individual EIT tendons in a
flyover near Basel as a function of the tendon length. Diameter
of plastic duct 59 mm.
criteria of the Swiss Guideline (2001) could be reached
(Elsener et al. 2002). In one of the flyovers a 100 m long
tendon (Nr. 5) even reached the maximum theoretical
value of the resistance R, indicating a perfect isolation
at the anchorages and execution on site (table 1).
The increasing number of PC structures constructed
with EIT tendons in Switzerland allowed to gain more
practical experience. Often contractors and owners
complained about the difficulty or even impossibility to reach the acceptance criteria fixed in the Swiss
guideline (2001). Some case studies are documented
in a report by Büchler et al. (2005), other examples are
given in this paper.
A flyover constructed for a highway link with
about 60 EIT tendons of different length showed
that the capacitance values of the individual tendons
are proportional to the tendon length L as expected
(Fig. 2). From the slope of the diagram a value of
C = 2.35 nF/m was obtained in good agreement with
laboratory results.
The results of the resistance values R (multiplied by
the length of the tendons) showed values from 10 to
232
Table 2. Experimentally measured capacitance C (mean
value and standard deviation) of 71 decks of the Piacenza
viaduct. Tendon length 32.1 m. Measurements performed 28
days after grouting.
Type
C (nF)
std dev (nF)
C spec (nF/m)
ø 76 mm
ø 100 mm
70.3
73.5
2.34
2.33
2.2
2.3
Figure 3. Cumulative probability plot of the resistance
values measured for EIT tendons in a flyover near Basel.
Diameter of plastic duct 59 mm.
1800 km. In the cumulative probability plot (Fig. 3)
clearly two distributions (lines) could be observed: the
one at high R values was associated to “good” tendons
(acceptance criteria fulfilled). However, ca. 50% of the
tendons did not reach the acceptance criteria.
Overall the percentage of success in different PC
structures showed big differences: bridge structures
with EIT tendons where 100% were considered as
“good”, others where only 30% of the tendons fulfilled the acceptance criteria. Many reasons, e.g. the
beginning of the transfer of the EIT technology to practice, the length of the cables, the presence of couplers,
design or execution problems were discussed.
2.4
Results from field application – Italy
In the Italian high speed network the Piacenza viaduct
on the Milano-Bologna line is an example for fullspan pre-casting of 151 simply supported pre-cast prestressed concrete decks composed by a monolithic box
girder with two cells, spanning 33.1 m and weighting
about 1000 tons. The design of the elements and of the
viaduct has been reported previously (Prevedini et al.
2004, Bonasso et al. 2006). Data have been collected
from the first 71 decks of the Piacenza viaduct (Della
Vedova & Elsener 2006), each deck containing 9 cables
with 12 wires, duct ø 76 mm (in the lower slab) and 15
cables with 19 wires, duct ø 100 mm (in the webs).
The values of the capacitance C (table 2) allow a
first control on the execution quality. The values of the
capacitance are Gaussian distributed and show a very
small standard deviation, indicating the good reproducibility. The mean value is higher for ducts with
higher diameter, the specific capacitance (per meter
length) is well below the control values specified in
the Swiss Guideline (2001).
The statistical analysis of the measured resistance R
on more then 1000 tendons is more complicated
Figure 4. Cumulative probability plot of the resistance values measured for EIT tendons in 71 segments of the Piacenza
viaduct. Diameter of plastic duct 100 mm. The numbers correspond to individual tendons position in the web. Note the
logarithmic x-axis. (Della Vedova & Elsener 2006).
because the values – despite the constant length of the
tendons – do not show a Gaussian distribution. The
analysis is thus performed with the cumulative probability plot (Fig. 4, 5). For the tendons with ø 100 mm
from the segments of the Piacenza viaduct (Fig. 4), less
then 1% of all values are below 10 Ohm, thus cables
with a short circuit (electrical contact between tendon and normal rebars). From the limiting value of the
specific resistance R (300 km in the Swiss Guideline 2001) a limiting resistance value R of 9 k can be
calculated for the tendon with length 32.1 m. As can
be seen from figure 4 the limiting value is not reached
by only 9% of the tendons (Della Vedova & Elsener
2006).
The cumulative probability plot shows further that
there is no distinct tendon position that makes more
difficulties then others. In addition, it can be noted
233
Table 3. Limiting values (acceptance criteria) for the electrical resistance measured 28 days after grouting the tendon
for the three main criteria “monitoring”, “fatigue” and “stray
current” according to the new Swiss guideline (2007).
Figure 5. Evolution of the resistance measured for the 6
tendons in the flyover “Pré du Mariage” with time. Diameter
of plastic duct 76 mm.
that for each tendon position about 5% of all segments were produced with perfect isolation (reaching
the theoretical value of a completely tight plastic duct).
For the tendons in the lower slab with diameter
76 mm the situation is similar (Della Vedova & Elsener
2006). About 20% of all the tendons did not reach the
acceptance criteria (resistance R 12 k). For each tendon position about 5% of all segments showed a perfect
electrical isolation (Della Vedova & Elsener 2006).
The broad distribution of resistance values measured on EIT tendons in prefabricated decks shows that
there is a strong influence of the human factor, as written procedures, approved material and components,
deck formwork, reinforcement and pre-stressing were
always the same.
3
REVISED SWISS GUIDELINE
Practical experience reported in part above, in a report
by Büchler et al. (2006) and from a great number of PC
structures with EIT not published lead to the conclusion that the acceptance criteria in the Swiss Guideline
(2001) were in part too severe and not sufficiently
related to engineers and owner needs.
Tendons that do not reach the old acceptance criteria at 28 days but are without short circuit cannot be considered “defective” for several reasons: 1)
these tendons with plastic ducts have a better protection against fatigue and chloride ingress compared to
metallic ducts, 2) the electrical impedance can still be
measured and followed over time (see chapter 4) and 3)
the resistance will increase with time and might reach
the criteria.
For this reason the new Swiss guideline (2007)
defines limiting values for the three main causes to
Diameter
(mm)
Monitoring
R (km)
Fatigue
R ()
Stray current
R (km)
60
75
100
130
Maximum
Failure
50
50
50
50
10%
20
20
20
20
0%
250
200
150
125
20%
apply electrically isolated tendons in PC structures:
a limiting value of the resistance for monitoring, for
fatigue and for stray current (table 3). In addition a
simple formula to account for the influence of time is
given, too.
The guideline establishes also what actions have to
be taken is the acceptance criteria are not fulfilled:
For the main criteria “fatigue” a resistance value
R < 20 indicates a metallic contact between the
rebars and the high-strength steel in the duct. This
might lead to fretting corrosion (Oertle 1988). The
position of the short-circuit has to be located (see chapter 5). If the contact is in an area critical for fatigue the
consequences for a failure of this tendon have to be
checked.
For the main criteria “stray current” all the tendons
should have a very high resistance in order to prevent
stray current on the high-strength steel. Tendons that
do not fulfill the acceptance criteria have to be electrically connected to the normal reinforcement. These
connections can be opened for monitoring purpose.
For the main criteria “monitoring” the acceptance
criteria is 50 km (table 3). If this criteria is not
reached the tendon is not necessarily less durable,
but the detection of the ingress of water (and chlorides) at defects is less sensitive. The defect should
be located and if it is found in an area critical for the
ingress of water additional protection measures can be
applied.
4
LONG TERM MONITORING
One of the major concerns regarding internal bonded
post-tensioned tendons is the inability to inspect the
tendons visually and the absence of established nondestructive techniques to monitor corrosion of the steel
strands (Matt 2000). Using electrically isolated tendons with plastic ducts, the evolution of the resistance
values over time can be used to control the integrity of
the corrosion protection system.
234
Figure 7. Experimental setup of magnetic flux measurements to locate short circuits in a electrically isolated tendon
(Büchler et al. 2005).
One tendon clearly shows a decrease in the resistance
value, indicating the ingress of water at a defect in
the duct.
Figure 6. Evolution of the normalized electrical resistance
measured for four tendons in a small bridge (tendon length
22.9 m) with time. Diameter of plastic duct 76 mm.
4.1
First example
The flyover “Pré du Mariage” is a relatively simple,
short box girder structure with only one column in
the centre of the span. Six electrically isolated tendons of ø 76 mm and length 49.3 m were used. At “Pré
du Mariage” electrical impedance measurements have
been performed at frequent intervals since the time of
grouting (Elsener et al. 2002). The evolution of the
electrical resistance with time is shown in figure 6.
As can be noted, the values for the six individual tendons show a certain scatter, but the overall trend is an
increase of the electrical resistance with time over a
period of nearly 8 years.
In the log R vs log t plot (Fig. 5) a straight line with
slope 0.5 is found (annotated “trend”). This increase
of the electrical resistance with time is due to the progressive hydration and drying out of grout and concrete
(Bürchler et al. 1996). The results can be interpreted in
the way that none of the six tendons shows water (and
chloride) ingress so far and the high strength steel is
protected against corrosion.
4.2
Second example
In a second example, a small 22.9 m long bridge,
impedance measurements have been performed over
time, too. The resistance values of the individual
tendons differed strongly, so the resistance was normalized at 28 d (giving all tendons the resistance value
1 at this time). This allows to follow the resistance over
time more easily and without being influenced by the
initial amount of defects present.
As can be seen from figure 6, the trend line corresponding to an asymptotic increase of the electrical
resistance of the tendons hold also for this example.
5
DEFECT LOCATION
When short circuits (resistance R < 10 ) or very low
resistance values are measured at the time of quality
control (acceptance), the question arises whether the
defect can be located in order to estimate its consequences for the durability, to improve the system in
upcoming applications, or to repair the defect. Techniques for detecting these defects were developed and
tested (Büchler et al. 2005)
5.1
Locating short circuits
Imposing an AC electric field (frequency 500–
1000 Hz) between the high strength steel and the
reinforcement (using the electrical connections provided for the impedance measurements), a current is
flowing through the tendon. Measuring the magnetic
flux B of the resultingAC-current allows determine the
areas with current flow and, as a consequence, to locate
the preferred sites (short circuits) where the current is
leaving the tendon. A schematic representation of the
experimental setup is shown in figure 7. As instrument
a commercial cable locater CL20 (company BAUR
Prüf- und Messtechnik, Sulz, Austria) was used.
Figure 8 shows the result of short-circuit location
on a 100 m long bridge deck with 16 tendons. The
electrical connection to the tendon was made at the left
end (position 100 m). The magnetic flux B varies along
the tendons due to the different distance from the deck
surface (high points at 30 m and 70 m). Tendon Nr. 1
shows a short circuit at 32.5 m, tendon 16 two defects at
27.6 and 31.6 m, tendon 14 a short circuit at 71.6 m and
tendon 3 might have a defect in the anchorage zone.
The technique primarily locates the defect with the
lowest resistivity (e.g. metallic contacts between tendon and reinforcement). Such low resistive defects
might mask other defects (e.g. small holes in the duct).
235
Figure 8. Experimental setup of magnetic flux measurements to locate short circuits in a electrically isolated tendon
(Büchler et al. 2005).
The most reliable location is possible if the tendon is
electrically connected from both ends.
6
CONCLUSIONS
Electrically isolated tendons (EIT) are a new system to enhance the durability of structures with
post-tensioned tendons to the protection level PL3.
Measurements of the electrical impedance on electrically isolated tendons have shown to be an efficient
way for quality control of the tendons.
Monitoring over time allows detecting the penetration of (chloride containing) water at defects in the
ducts. Thus for the first time, a simple, cost-effective
early warning system for post-tensioned tendons is
available.
Magnetic flux measurements allow locating defects
(short circuits and holes) in the tendons. For optimum
success the tendons should be electrically connected
at both ends.
ACKNOWLEDGMENTS
The colleagues of WG2 “New Systems” of COST 534
“New materials and systems for pre-stressed concrete
structures” – M. Della Vedova, M. Büchler, A. Gnägi –
are greatfully acknowledged for their collaboration.
REFERENCES
Ayats J., Gnägi A., Elsener B. (2002). Electrical Isolation
as Enhanced Protection for Post-tensioning Tendons in
Concrete Structures, Proc. Int. fib Congress 2002, October
13–19, Osaka, Japan.
Bonasso R., Traini G., Della Vedova M. (2006). High speed
railway prestressed concrete bridges, Proc. 2nd International fib congress, 5/8 July 2006, Naples.
Büchler M., Schiegg, Y. and Voûte C.-H. (2005). Electrically
isolated tendons: Use in regions with stray currents and
localization of short circuits to the reinforcement and of
defects. Report 585, VSS, Zürich. In German.
Bürchler D., Elsener B. and Böhni H. (1996). Electrical resistivity and dielectric porperties of hardened cement paste
and mortar., Electrically based Microstructural Characterization, ed. R.A. Gerhardt, S.R. Taylor and E.J.
Garboczi, Mat. Res. Soc. Symp. Proc. Vol. 411 p. 407.
Della Vedova M., Elsener B., Evangelista L. (2004). Corrosion protection and monitoring of electrically isolated
post-tensioning tendons, Schriftenreihe der Technischen
Universität Wien, Proc. Third European Conference on
Structural Control, 3ESCC, Vienna July 2004, ed. R.
Fleisch, H. Irschik and M. Krommer, Vol. II pp. S5-47–
S5-51.
Della Vedova M. & Elsener B. (2006). Enhanced Durability,
Quality Control and Monitoring of Electrically isolated
tendons, Proc. 2nd International fib congress, 5/8 July
2006, Naples.
Elsener B. (ed.) (2004). Proceedings of the second international workshop on durability of post-tensioning tendons,
Zurich 10.–13. October 2004. ETHZ Institute of Building
Materials.
Elsener B. (2005). Long-term monitoring of electrically
isolated post-tensioning tendons, fib Journal Structural
Control 6: 101–106
Elsener B., Toller L., Voute C., Böhni H. (2002). Ueberprüfen des Korosionsschutzes von Spanngliedern mit
Kunststoffhüllrohren, Report Nr. 564 (VSS) Zürich
fib recommendation (2005). “Durability of post-tensioning
tendons”, fib bulletin Nr. 33
Ganz H.R. (1997). Plastic ducts for enhanced performance
of post-tensioning tendons, FIP Notes 1997/2, The Institution of Structural Engineers, London, pp. 15–18.
Italian Standard (1997), Istruzione F.S. n. I/SC/PS-OM/2298
del 2.6.1995 “Sovraccarichi per il calcolo dei ponti ferroviari – Istruzioni per la progettazione, l’esecuzione e il
collaudo”, Final review 1997.
Matt P. (200). Non-destructive evaluation and monitoring of
post-tensioning tendons, fib bulletin 15 Durability of posttensioning tendons pp. 103–108.
Oertle J. (1988). Reibermüdung einbetonierter Spannkabel,
Bericht No. 166, Institut für Baustatik und Konstruktion
ETHZ, Birkhäuser (Basel).
Prevedini C., Averardi Ripari F., Della Vedova M. (2004).
“Il Viadotto Piacenza per la Linea ad Alta Velocità
Milano-Bologna, una soluzione costruttiva tecnologicamente avanzata con un sistema di precompressione
innovativo”, Giornate Aicap, 27/29 Maggio 2004, Verona.
Swiss Guideline (2001), “Measures to ensure the durability of
post-tensioning tendons in bridges”, Swiss Federal Roads
Authority and Swiss Federal Railways, edition 2001. PDFDownload under www.astra.admin.ch.
Swiss Guideline (2007). “Measures to ensure the durability of
post-tensioning tendons in bridges”, Swiss Federal Roads
Authority and Swiss Federal Railways, edition 2007. PDFDownload under www.astra.admin.ch.
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