DURABILITY OF POST-TENSIONING TENDONS · durability of post-tensioning tendons in the past: (1)...

COST 534, Final Workshop, Nov 07 Peter Matt / 1

DURABILITYOF

POST-TENSIONING TENDONS

Peter MattCivil Engineer ETHZ/SIA/USIC

1 Introduction

2 Design concepts for durable pt-tendons

3 Materials and construction

4 Maintenance, assessment and rehabilitation

5 Conclusions

Contents

1 Introduction

� Beneficial effect of prestressing known since long (e.g. barrel hoops, wooden ships)

� Many impulses by E. Freyssinet, F. Dischingerand many others from the thirties onwards

� First commercially applied pt-system patented 1939

� Prestressing enabled a technical revolution

1 Introduction

Bridge at Oelde, Germany (1938)

1 Introduction

Marne Bridge at Lucancy, France (1941-45)

1 Introduction

First Railway Bridge worldwide at Freiburg, Switzerland (1943)

1 Introduction

Fib Recommendation "Durability of post-tensioning tendons"

Published as bulletin 33 in December 2005

J.-Ph. Fuzier, H.-R. Ganz, P. Matt

1 Introduction

PT systems have performed very well, in general. However, some problems have occurred with the durability of post-tensioning tendons in the past:

(1) Quality of prestressing steels (brittle failure)

(2) Grout voids and grout segregation (bleed, corrosion)

(3) Water/Chlorides penetrating tendon (corrosion)

Above problems (1) and (2) have been addressed by material and construction specifications. However, protection for problem (3) needs further improvements.

1 Introduction

The main reason for durability problems of prestressing tendons is the access of water and chlorides (or other aggressive media) from the outside into the tendon. Quality grout protects the tendon against water and humidity (alkalinity) but not against chlorides (>>>> 0.6%).

Problem: Problem: Access of Access of water to water to tendontendon

� Pt-tendons are structural elements decisive for the functioning of pc-structures

� Multi Layer protectionConcrete cover:Thickness and quality verified, e.g. permeability measured by methods such as the Torrent Test:

"Birth certificate" at commissioning (reference condition)

Source: E. Brühwiler Source: E. Brühwiler

� Multi Layer protectionOther protection systems such as waterproofing membranes and surface protections:

� Protection levels for pt-tendons based on aggressivity/exposure vs. structural protection layers

PL1Low

eLowMediumHighAction

Structural protection layersStructuralProtection

� Examples of recommended pt-tendons (internal):

PL1: A duct with a filling material providing durable corrosion protection, e.g. traditional non-encapsulated tendons (for benign to moderately aggressive environments)�

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PL2: PL1 plus an envelope, enclosing the tensile element over its full length and providing a permanent leak tight barrier, e.g. tendons fully encapsulated in a robust plastic duct (for aggressive environments)�

P�l�a�s�t�i�c� �d�u�c�t�

>�6�0�m�m�

Protection cap�

PL3: PL2 plus integrity of tendon or encapsulation to be monitorable or inspectable at any time, e.g. tendons fully encapsulated in a robust plastic duct and monitorable (for aggressive environments including stray currents and where monitoring is required)�

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Example: EIT (Electrically Isolated Tendons)

• Requirements for materials- prestressing steels- anchorages- ducts- accessories- corrosion protection materials

• Requirements for construction- packaging, transportation, storage, handling- installation equipment- installation- stressing- injection/filling of tendons

• Certification of suppliers and installers

Prestressing steels:

7-wire prestressing strand

Hot rolled prestressing bar

Greased and sheathed strand

Above prestressing steels ensure high quality suitable for PL 1-3

Cold drawn prestressing wire

Anchorages and coupling devices are key components for the safety and reliability of pt- systems. Performance shall be in accordance with fib Recommendations orETAG 013. For durability the connections to the duct and the possibility of providing encapsulation are important.

PL 1 PL 2 , PL 3

Ducts for internal pt-tendons:

Corrugated metal duct:Duct testing to standards EN 523, 524Blank steel or galvanisedSleeve and tape connectionsSuitable for benign/moderate environment

Corrugated, robust plastic duct:Duct and system approval to fib Bulletin 7High quality plastic materialsMechanical leak-tight connectionsSuitable for aggressive environment

PL 1 PL 2, PL 3

Ducts for internal pt tendons – Advantages of plastic ducts:

Friction Coefficients Steel - Plastic Duct(112 Tendons)

Friction Coefficient

PT-PLUS/SUPER

PT-PLUS/DYFORM

Steel Duct

Leak tight encapsulation

Improved fatigue resistance

Electrical isolation

Accessories for internal pt-tendons:

Use of temporary or permanent grout caps permits to verify the quality of grouting. Do not accept sealing of anchorages with mortar or concrete before grouting

Temporary steel cap Permanent steel cap Permanent plastic cap

PL 2 PL 2, PL 3

Temporary Corrosion Protection:

• Suitable packaging/wrapping of prestressing steel in factory

• Suitable dry and clean storage of prestressing steel on site

• For installation for extended periods between stressing and grouting : Use water soluble oils (not harmful to steel, select oils which do not need to be removed before grouting of tendon). N.B.: Can also be used for long storage

• Protection of pt-components

Suitable packaging of strand

Unprotected anchor head

Studies in the US have confirmed the availability of water soluble oils which provide good protection and which do not excessively harm the bond properties of the tendon

Swiss study with exposure ofstressed tendons for 6 months during winter period

Temporary Corrosion Protection:

Source: EMPA

Permanent Corrosion Protection:

Most common method is byinjection of cementitiousgrout (only use approved special grouts which comply with recentspecifications such as fib Bulletin 20 and EN445-447). Apply approval, suitability, and quality control testing regimes systematically PL 1, PL 2, PL 3

Use recent stringent bleed tests (fib Bulletin 20 and EN445-447)

Poor performance

Good performance

Permanent Corrosion Protection:

1m Wick-Induced bleed test5m Inclined Tube bleed test

Pt Systems installation / Certification:

• Installation is done by specialist contractor who has either developed the specific pt-system, or who has been properly trained by the pt-systemdeveloper.

• Personnel on site is specifically trained and certified as “PT craftsman” for the specific system, e.g. CMP in France (Chargé de Mise en Précontrainte).

Reference:CEN Workshop Agreement

CWA 14646

Quality installation of pt-systems is best achieved if:

� New structures:

Structures and pt-tendons designed and executed according to Eurocodes and fib recommendations

To establish :

• Maintenance plan (required measures for maintenance, intervals etc.)

• Surveillance or Monitoring plan (required surveillance or monitoring activities, intervals, tolerances etc.)

Internal tendons:

• PL1 and PL2:No particular maintenance needed, except in cases with exposed anchorages

• PL3:Maintenance of the monitoring system (EIT)

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Protection cap�

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Maintenance:

Revised Guideline 2007 first published in 2001.

Available in German and French on Website:

www.astra.admin.ch

English version under preparation

Tendons of category PL3 (e.g. EIT):

Monitoring of the leak-tightness of the encapsulation

Instrument for measuring the electrical resistance of tendons PL3 (EIT)

End caps and electrical junction box for monitoring of tendons PL3 (EIT)

Lake Bridge at Lucerne, Switzerland (1997)

"Weak points" in typical box girder bridge

� Existing structures:

Inspection methods (NDT):

• Georadar

• Covermeter

• Fiberscopy

• Potential Mapping

• Impact-Echo Method

• Remanent Magnetism Method

• Radiography

• Reflectometrical Impulse Measurement

• Ultrasonic Methods

• Acoustic Monitoring

• Thermography

• Tomography

Acoustic monitoring (SoundPrint):

Ponte Moesa, Roveredo, Switzerland, built 1952

2001 2004 2005

Sensor and cables

Wire breakage Hammer drill Pneumatic hammer

Signals:

Artificially induced wire breakage

Potential measurement and wire breakages

Original bridge with auxiliary piers

• None of the mentioned NDT-Methods can provide a complete assessment of PT tendons in existing structures

• Some methods however provide at least partial results

Assessment:

The method that overall still rates best in terms of informationand interpretation:To provide access to the tendons by carefully drilling or chiselling in critical areas determined by a desk study and visual inspection.

• Cleaning of prestressing steel as needed• Grouting of existing voids (internal and external

tendons):

• Replacement of the tendon pipe (external tendons)

− large voids �� grouting (generally vacuum grouting)− small voids �� patching with suitable mortar

• Replacement of the prestressing steel (internal, greased and sheathed strands and external tendons injected with grease or wax)

Rehabilitation:

5 Conclusions

� Prestressing is a proven technology with good past record

Summary and conclusions

5 Conclusions

• High costs for rehabilitation or replacement:

• Collapse:

� Post-tensioning tendons are important structural elements

� Consequences of inadequate design, detailing and execution of PT systems:

Source: E. Proverbio

5 Conclusions

� New concept minimized maintenance costs and increased service life

DURABILITY OF POST-TENSIONING TENDONS · durability of post-tensioning tendons in the past: (1)...

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