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. ElsenerETH Zurich, Institute for Building Materials, Zurich, Switzerland

ABSTRACT: Electrically Isolated Tendons (EIT) have been introduced as one possible solution to reach thehighest protection level (PL3) in the framework of fib recommendation for grouted post-tensioned tendons. Thisapproach allows to check the integrity of the plastic duct during and after construction and to monitor the corrosionprotection of the high-strength steel during the whole service life with electrical impedance measurements. Thepaper presents results on PC structures with EIT regarding quality control, long term monitoring and locationof defects. Practical experience in Switzerland over the last six years was included in the revision of the SwissGuideline “Measures to ensure the durability of post-tensioning tendons in bridges”.

1 INTRODUCTION

Post-tensioning tendons contribute decisively to theserviceability, safety and durability of pre-stressedconcrete (PC) bridges. In order to reach the goalsof durability (optimum corrosion protection) and ofmonitoring requested by the Italian Railways (Ital-ian Standard, 1997) and the Swiss Federal Roads andRailwayAuthorities, the new system of electrically iso-lated tendons according to the Swiss Guideline (SwissGuideline, 2001) has been adopted. In Switzerlandabout 80 bridges of different length have been con-structed since 1995 with thick-walled corrugated plas-tic ducts and electrically isolated anchorages (Elseneret al. 2002; Ayats et al. 2002) Similar systems havebeen massively applied for the first time in Italy forthe design and construction of several bridges andviaducts of the new high-speed lines (Della Vedova& Elsener 2006; Prevedini et al. 2004; Bonasso et al.2006). In Italy, traditional choice for railway bridgesis the use of simply supported spans and about 90%of the viaducts of the new lines are realised with par-tial or total pre-casting of PC decks; this allowed tocarry out test programmes on the construction site andextensive quality control during construction – a pointthat has been recognized to be more difficult on con-tinuous span bridges as those usually constructed inSwitzerland.

Electrically isolated tendons have been introducedas one possible solution to reach the highest protectionlevel (PL3) in the framework of fib recommenda-tions (fib 2005, Elsener (ed.) 2004). Using electricallyisolated tendons allows to check the electrical isola-tion 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 corro-sion protection of the tendons during service life withimpedance measurements (Della Vedova et al. 2004,Elsener 2005).

To achieve the required durability and safety, newpost-tensioning tendons are currently designed andexecuted following a multi-layer protection approach.The single tendons should easily be monitored overtime and damage should be detected at a very earlytime. Industry in the last 10 years has developed suchnew post-tensioning tendons, based on a completeencapsulation of the sensitive high-strength steel intoa grouted polymeric duct. Combined with electricalisolation of the anchorages a simple, non-destructivemonitoring of the individual tendons is possible.

2 QUALITY CONTROL OF EIT TENDONS

2.1 Measuring principle

The impedance measurements are performed betweenthe steel strands in the grouted ducts and the normalreinforcement in concrete (Fig. 1). The measuring sys-tem thus includes the grout in the duct, the duct (withpores and defects) and the concrete surrounding theduct. Grout and concrete are (at least in the rangeof measuring frequencies between 100 and 1000 Hz)pure resistances, the polymer duct instead is essentiallya capacitance in parallel with a very high resistance(Fig. 1). Any system related imperfections (e.g. notfully closed grout vents) and/or defect in the duct arerepresented by an ohmic resistance in parallel.

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Figure 1. Principle of measuring the electrical impedanceof a tendon with an LCR meter. Recommended measuringfrequency 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) haveshown that intact (reference), welded or coupled ductshave a very high resistance value (>2.3 M� at 1 kHz)and very low loss factors D (<0.034), thus they behaveessentially as capacitance. Ducts with a 2 mm holeshow comparably low resistance values (<100 k�)and the resistance drops to less than 1 k� for 40 mmholes. The loss factor is very high, thus at 1 kHz thesesystems behave as resistance. An open grout vent thatends in the concrete (thus a very small electrolyticcontact is possible) has a resistance of 573 k� and aloss factor of 0.098. As a not perfectly closed groutvent represents a “defect” for the impedance measure-ment but by no means a loss in durability, this situationwas chosen as borderline between acceptable and non-acceptable defect. The acceptance criteria in the SwissGuideline (2001) was defined as R = 500 k�m, thecontrol value D < 0.1.

The capacitance values C of the 1 m long segmentsof plastic ducts were measured to 2.34 ± 0.04 nF/mirrespective of the presence of holes.

2.3 Results from field applications – Switzerland

Both resistance and capacitance values measureddepend on the length L of the tendon: the resistancevalues R decrease proportional to the length L, thecapacitance value C increase. For quality control thespecific values R in k�m and C in nF/m have to beused.

The first structures constructed with EIT post-tensioning tendons were built in Switzerland (Ayatset al. 2002). In these pilot projects the acceptance

Table 1. Calculated specific values of the ohmic resis-tance R, the capacitance C and the loss factor D fromthe flyover “P.S. du Milieu” (length 100 m). Measurementsperformed 28 days after grouting.

Tendon Nr. R (k�m) C (nF/m) D (–)

1 723 2.34 0.0932 1370 2.33 0.0483 2087 2.35 0.0324 1781 2.37 0.0375 2825 2.35 0.0236 short circuit

Figure 2. Capacitance C of the individual EIT tendons in aflyover near Basel as a function of the tendon length. Diameterof 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 longtendon (Nr. 5) even reached the maximum theoreticalvalue of the resistance R, indicating a perfect isolationat the anchorages and execution on site (table 1).

The increasing number of PC structures constructedwith EIT tendons in Switzerland allowed to gain morepractical experience. Often contractors and ownerscomplained about the difficulty or even impossibil-ity to reach the acceptance criteria fixed in the Swissguideline (2001). Some case studies are documentedin a report by Büchler et al. (2005), other examples aregiven in this paper.

A flyover constructed for a highway link withabout 60 EIT tendons of different length showedthat the capacitance values of the individual tendonsare proportional to the tendon length L as expected(Fig. 2). From the slope of the diagram a value ofC = 2.35 nF/m was obtained in good agreement withlaboratory results.

The results of the resistance values R (multiplied bythe length of the tendons) showed values from 10 to

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Figure 3. Cumulative probability plot of the resistancevalues measured for EIT tendons in a flyover near Basel.Diameter of plastic duct 59 mm.

1800 k�m. In the cumulative probability plot (Fig. 3)clearly two distributions (lines) could be observed: theone at high R values was associated to “good” tendons(acceptance criteria fulfilled). However, ca. 50% of thetendons did not reach the acceptance criteria.

Overall the percentage of success in different PCstructures showed big differences: bridge structureswith EIT tendons where 100% were considered as“good”, others where only 30% of the tendons ful-filled the acceptance criteria. Many reasons, e.g. thebeginning of the transfer of the EIT technology to prac-tice, 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 viaducton the Milano-Bologna line is an example for full-span pre-casting of 151 simply supported pre-cast pre-stressed concrete decks composed by a monolithic boxgirder with two cells, spanning 33.1 m and weightingabout 1000 tons. The design of the elements and of theviaduct has been reported previously (Prevedini et al.2004, Bonasso et al. 2006). Data have been collectedfrom the first 71 decks of the Piacenza viaduct (DellaVedova & Elsener 2006), each deck containing 9 cableswith 12 wires, duct ø 76 mm (in the lower slab) and 15cables with 19 wires, duct ø 100 mm (in the webs).

The values of the capacitance C (table 2) allow afirst control on the execution quality. The values of thecapacitance are Gaussian distributed and show a verysmall standard deviation, indicating the good repro-ducibility. The mean value is higher for ducts withhigher diameter, the specific capacitance (per meterlength) is well below the control values specified inthe Swiss Guideline (2001).

The statistical analysis of the measured resistance Ron more then 1000 tendons is more complicated

Table 2. Experimentally measured capacitance C (meanvalue and standard deviation) of 71 decks of the Piacenzaviaduct. Tendon length 32.1 m. Measurements performed 28days after grouting.

Type C (nF) std dev (nF) C spec (nF/m)

ø 76 mm 70.3 2.34 2.2ø 100 mm 73.5 2.33 2.3

Figure 4. Cumulative probability plot of the resistance val-ues measured for EIT tendons in 71 segments of the Piacenzaviaduct. Diameter of plastic duct 100 mm. The numbers cor-respond to individual tendons position in the web. Note thelogarithmic x-axis. (Della Vedova & Elsener 2006).

because the values – despite the constant length of thetendons – do not show a Gaussian distribution. Theanalysis is thus performed with the cumulative proba-bility plot (Fig. 4, 5). For the tendons with ø 100 mmfrom the segments of the Piacenza viaduct (Fig. 4), lessthen 1% of all values are below 10 Ohm, thus cableswith a short circuit (electrical contact between ten-don and normal rebars). From the limiting value of thespecific resistance R (300 k�m in the Swiss Guide-line 2001) a limiting resistance value R of 9 k� can becalculated for the tendon with length 32.1 m. As canbe seen from figure 4 the limiting value is not reachedby only 9% of the tendons (Della Vedova & Elsener2006).

The cumulative probability plot shows further thatthere is no distinct tendon position that makes moredifficulties then others. In addition, it can be noted

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Figure 5. Evolution of the resistance measured for the 6tendons in the flyover “Pré du Mariage” with time. Diameterof plastic duct 76 mm.

that for each tendon position about 5% of all seg-ments were produced with perfect isolation (reachingthe theoretical value of a completely tight plastic duct).

For the tendons in the lower slab with diameter76 mm the situation is similar (DellaVedova & Elsener2006). About 20% of all the tendons did not reach theacceptance criteria (resistance R 12 k�). For each ten-don position about 5% of all segments showed a perfectelectrical isolation (Della Vedova & Elsener 2006).

The broad distribution of resistance values mea-sured on EIT tendons in prefabricated decks shows thatthere is a strong influence of the human factor, as writ-ten procedures, approved material and components,deck formwork, reinforcement and pre-stressing werealways the same.

3 REVISED SWISS GUIDELINE

Practical experience reported in part above, in a reportby Büchler et al. (2006) and from a great number of PCstructures with EIT not published lead to the conclu-sion that the acceptance criteria in the Swiss Guideline(2001) were in part too severe and not sufficientlyrelated to engineers and owner needs.

Tendons that do not reach the old acceptance cri-teria at 28 days but are without short circuit can-not be considered “defective” for several reasons: 1)these tendons with plastic ducts have a better protec-tion against fatigue and chloride ingress compared tometallic ducts, 2) the electrical impedance can still bemeasured and followed over time (see chapter 4) and 3)the resistance will increase with time and might reachthe criteria.

For this reason the new Swiss guideline (2007)defines limiting values for the three main causes to

Table 3. Limiting values (acceptance criteria) for the elec-trical resistance measured 28 days after grouting the tendonfor the three main criteria “monitoring”, “fatigue” and “straycurrent” according to the new Swiss guideline (2007).

Diameter Monitoring Fatigue Stray current(mm) R (k�m) R (�) R (k�m)

60 50 20 25075 50 20 200

100 50 20 150130 50 20 125

Maximum 10% 0% 20%Failure

apply electrically isolated tendons in PC structures:a limiting value of the resistance for monitoring, forfatigue and for stray current (table 3). In addition asimple formula to account for the influence of time isgiven, too.

The guideline establishes also what actions have tobe taken is the acceptance criteria are not fulfilled:

For the main criteria “fatigue” a resistance valueR < 20 � indicates a metallic contact between therebars and the high-strength steel in the duct. Thismight lead to fretting corrosion (Oertle 1988). Theposition of the short-circuit has to be located (see chap-ter 5). If the contact is in an area critical for fatigue theconsequences for a failure of this tendon have to bechecked.

For the main criteria “stray current” all the tendonsshould have a very high resistance in order to preventstray current on the high-strength steel. Tendons thatdo not fulfill the acceptance criteria have to be elec-trically connected to the normal reinforcement. Theseconnections can be opened for monitoring purpose.

For the main criteria “monitoring” the acceptancecriteria is 50 k�m (table 3). If this criteria is notreached the tendon is not necessarily less durable,but the detection of the ingress of water (and chlo-rides) at defects is less sensitive. The defect shouldbe located and if it is found in an area critical for theingress of water additional protection measures can beapplied.

4 LONG TERM MONITORING

One of the major concerns regarding internal bondedpost-tensioned tendons is the inability to inspect thetendons visually and the absence of established non-destructive techniques to monitor corrosion of the steelstrands (Matt 2000). Using electrically isolated ten-dons with plastic ducts, the evolution of the resistancevalues over time can be used to control the integrity ofthe corrosion protection system.

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Figure 6. Evolution of the normalized electrical resistancemeasured for four tendons in a small bridge (tendon length22.9 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 inthe centre of the span. Six electrically isolated ten-dons of ø 76 mm and length 49.3 m were used. At “Prédu Mariage” electrical impedance measurements havebeen performed at frequent intervals since the time ofgrouting (Elsener et al. 2002). The evolution of theelectrical resistance with time is shown in figure 6.As can be noted, the values for the six individual ten-dons show a certain scatter, but the overall trend is anincrease of the electrical resistance with time over aperiod of nearly 8 years.

In the log R vs log t plot (Fig. 5) a straight line withslope 0.5 is found (annotated “trend”). This increaseof the electrical resistance with time is due to the pro-gressive hydration and drying out of grout and concrete(Bürchler et al. 1996). The results can be interpreted inthe way that none of the six tendons shows water (andchloride) ingress so far and the high strength steel isprotected against corrosion.

4.2 Second example

In a second example, a small 22.9 m long bridge,impedance measurements have been performed overtime, too. The resistance values of the individualtendons differed strongly, so the resistance was nor-malized at 28 d (giving all tendons the resistance value1 at this time). This allows to follow the resistance overtime more easily and without being influenced by theinitial amount of defects present.

As can be seen from figure 6, the trend line cor-responding to an asymptotic increase of the electricalresistance of the tendons hold also for this example.

Figure 7. Experimental setup of magnetic flux measure-ments to locate short circuits in a electrically isolated tendon(Büchler et al. 2005).

One tendon clearly shows a decrease in the resistancevalue, indicating the ingress of water at a defect inthe duct.

5 DEFECT LOCATION

When short circuits (resistance R < 10 �) or very lowresistance values are measured at the time of qualitycontrol (acceptance), the question arises whether thedefect can be located in order to estimate its conse-quences for the durability, to improve the system inupcoming applications, or to repair the defect. Tech-niques for detecting these defects were developed andtested (Büchler et al. 2005)

5.1 Locating short circuits

Imposing an AC electric field (frequency 500–1000 Hz) between the high strength steel and thereinforcement (using the electrical connections pro-vided for the impedance measurements), a current isflowing through the tendon. Measuring the magneticflux B of the resultingAC-current allows determine theareas with current flow and, as a consequence, to locatethe preferred sites (short circuits) where the current isleaving the tendon. A schematic representation of theexperimental setup is shown in figure 7. As instrumenta commercial cable locater CL20 (company BAURPrüf- und Messtechnik, Sulz, Austria) was used.

Figure 8 shows the result of short-circuit locationon a 100 m long bridge deck with 16 tendons. Theelectrical connection to the tendon was made at the leftend (position 100 m).The magnetic flux B varies alongthe tendons due to the different distance from the decksurface (high points at 30 m and 70 m). Tendon Nr. 1shows a short circuit at 32.5 m, tendon 16 two defects at27.6 and 31.6 m, tendon 14 a short circuit at 71.6 m andtendon 3 might have a defect in the anchorage zone.

The technique primarily locates the defect with thelowest resistivity (e.g. metallic contacts between ten-don and reinforcement). Such low resistive defectsmight mask other defects (e.g. small holes in the duct).

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Figure 8. Experimental setup of magnetic flux measure-ments to locate short circuits in a electrically isolated tendon(Büchler et al. 2005).

The most reliable location is possible if the tendon iselectrically connected from both ends.

6 CONCLUSIONS

Electrically isolated tendons (EIT) are a new sys-tem to enhance the durability of structures withpost-tensioned tendons to the protection level PL3.

Measurements of the electrical impedance on elec-trically isolated tendons have shown to be an efficientway for quality control of the tendons.

Monitoring over time allows detecting the penetra-tion of (chloride containing) water at defects in theducts. Thus for the first time, a simple, cost-effectiveearly warning system for post-tensioned tendons isavailable.

Magnetic flux measurements allow locating defects(short circuits and holes) in the tendons. For optimumsuccess the tendons should be electrically connectedat both ends.

ACKNOWLEDGMENTS

The colleagues of WG2 “New Systems” of COST 534“New materials and systems for pre-stressed concretestructures” – M. Della Vedova, M. Büchler, A. Gnägi –are greatfully acknowledged for their collaboration.

REFERENCES

Ayats J., Gnägi A., Elsener B. (2002). Electrical Isolationas Enhanced Protection for Post-tensioning Tendons inConcrete Structures, Proc. Int. fib Congress 2002, October13–19, Osaka, Japan.

Bonasso R., Traini G., Della Vedova M. (2006). High speedrailway prestressed concrete bridges, Proc. 2nd Interna-tional fib congress, 5/8 July 2006, Naples.

Büchler M., Schiegg, Y. and Voûte C.-H. (2005). Electricallyisolated tendons: Use in regions with stray currents andlocalization of short circuits to the reinforcement and ofdefects. Report 585, VSS, Zürich. In German.

Bürchler D., Elsener B. and Böhni H. (1996). Electrical resis-tivity and dielectric porperties of hardened cement pasteand mortar., Electrically based Microstructural Char-acterization, 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). Cor-rosion protection and monitoring of electrically isolatedpost-tensioning tendons, Schriftenreihe der TechnischenUniversität Wien, Proc. Third European Conference onStructural 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 isolatedtendons, Proc. 2nd International fib congress, 5/8 July2006, Naples.

Elsener B. (ed.) (2004). Proceedings of the second interna-tional workshop on durability of post-tensioning tendons,Zurich 10.–13. October 2004. ETHZ Institute of BuildingMaterials.

Elsener B. (2005). Long-term monitoring of electricallyisolated post-tensioning tendons, fib Journal StructuralControl 6: 101–106

Elsener B., Toller L., Voute C., Böhni H. (2002). Ueber-prüfen des Korosionsschutzes von Spanngliedern mitKunststoffhüllrohren, Report Nr. 564 (VSS) Zürich

fib recommendation (2005). “Durability of post-tensioningtendons”, fib bulletin Nr. 33

Ganz H.R. (1997). Plastic ducts for enhanced performanceof post-tensioning tendons, FIP Notes 1997/2, The Insti-tution of Structural Engineers, London, pp. 15–18.

Italian Standard (1997), Istruzione F.S. n. I/SC/PS-OM/2298del 2.6.1995 “Sovraccarichi per il calcolo dei ponti fer-roviari – Istruzioni per la progettazione, l’esecuzione e ilcollaudo”, Final review 1997.

Matt P. (200). Non-destructive evaluation and monitoring ofpost-tensioning tendons, fib bulletin 15 Durability of post-tensioning tendons pp. 103–108.

Oertle J. (1988). Reibermüdung einbetonierter Spannkabel,Bericht No. 166, Institut für Baustatik und KonstruktionETHZ, Birkhäuser (Basel).

Prevedini C., Averardi Ripari F., Della Vedova M. (2004).“Il Viadotto Piacenza per la Linea ad Alta VelocitàMilano-Bologna, una soluzione costruttiva tecnologi-camente avanzata con un sistema di precompressioneinnovativo”, Giornate Aicap, 27/29 Maggio 2004, Verona.

Swiss Guideline (2001), “Measures to ensure the durability ofpost-tensioning tendons in bridges”, Swiss Federal RoadsAuthority and Swiss Federal Railways, edition 2001. PDF-Download under www.astra.admin.ch.

Swiss Guideline (2007). “Measures to ensure the durability ofpost-tensioning tendons in bridges”, Swiss Federal RoadsAuthority and Swiss Federal Railways, edition 2007. PDF-Download under www.astra.admin.ch.

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