durability of post-tensioning tendons · durability of post-tensioning tendons in the past: (1)...
TRANSCRIPT
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COST 534, Final Workshop, Nov 07 Peter Matt / 1
DURABILITYOF
POST-TENSIONING TENDONS
Peter MattCivil Engineer ETHZ/SIA/USIC
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COST 534, Final Workshop, Nov 07 Peter Matt / 2
1 Introduction
2 Design concepts for durable pt-tendons
3 Materials and construction
4 Maintenance, assessment and rehabilitation
5 Conclusions
Contents
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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
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1 Introduction
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1 Introduction
Bridge at Oelde, Germany (1938)
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1 Introduction
Marne Bridge at Lucancy, France (1941-45)
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1 Introduction
First Railway Bridge worldwide at Freiburg, Switzerland (1943)
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1 Introduction
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COST 534, Final Workshop, Nov 07 Peter Matt / 9
1 Introduction
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1 Introduction
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1 Introduction
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1 Introduction
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1 Introduction
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1 Introduction
Fib Recommendation "Durability of post-tensioning tendons"
Published as bulletin 33 in December 2005
J.-Ph. Fuzier, H.-R. Ganz, P. Matt
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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.
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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
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2 Design concepts for durable pt-tendons
� Pt-tendons are structural elements decisive for the functioning of pc-structures
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2 Design concepts for durable pt-tendons
� 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
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2 Design concepts for durable pt-tendons
� Multi Layer protectionOther protection systems such as waterproofing membranes and surface protections:
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2 Design concepts for durable pt-tendons
� Protection levels for pt-tendons based on aggressivity/exposure vs. structural protection layers
PL2
PL3
PL1Low
Med
ium
Hig
h
Agg
ress
ivity
/ Exp
osur
eLowMediumHighAction
Structural protection layersStructuralProtection
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2 Design concepts for durable pt-tendons
� 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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2 Design concepts for durable pt-tendons
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)�
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2 Design concepts for durable pt-tendons
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)
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3 Materials and construction
• 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
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3 Materials and construction
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
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3 Materials and construction
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
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3 Materials and construction
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
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3 Materials and construction
Ducts for internal pt tendons – Advantages of plastic ducts:
Friction Coefficients Steel - Plastic Duct(112 Tendons)
0
5
10
15
0.05
0.08
0.11
0.14
0.17
0.20
0.23
0.26
Friction Coefficient
Num
ber
of T
endo
ns
PT-PLUS/SUPER
PT-PLUS/DYFORM
Steel Duct
Leak tight encapsulation
Improved fatigue resistance
Electrical isolation
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3 Materials and construction
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 1
PL 2 PL 2, PL 3
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3 Materials and construction
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
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3 Materials and construction
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
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3 Materials and construction
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
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3 Materials and construction
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
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3 Materials and construction
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:
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4 Maintenance, assessment and rehabilitation
� 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.)
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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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Maintenance:
4 Maintenance, assessment and rehabilitation
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Revised Guideline 2007 first published in 2001.
Available in German and French on Website:
www.astra.admin.ch
English version under preparation
4 Maintenance, assessment and rehabilitation
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4 Maintenance, assessment and rehabilitation
Tendons of category PL3 (e.g. EIT):
Monitoring of the leak-tightness of the encapsulation
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4 Maintenance, assessment and rehabilitation
Instrument for measuring the electrical resistance of tendons PL3 (EIT)
End caps and electrical junction box for monitoring of tendons PL3 (EIT)
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Lake Bridge at Lucerne, Switzerland (1997)
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"Weak points" in typical box girder bridge
� Existing structures:
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Inspection methods (NDT):
• Georadar
• Covermeter
• Fiberscopy
• Potential Mapping
• Impact-Echo Method
• Remanent Magnetism Method
• Radiography
• Reflectometrical Impulse Measurement
• Ultrasonic Methods
• Acoustic Monitoring
• Thermography
• Tomography
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4 Maintenance, assessment and rehabilitation
Acoustic monitoring (SoundPrint):
Ponte Moesa, Roveredo, Switzerland, built 1952
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2001 2004 2005
�
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Sensor and cables
Wire breakage Hammer drill Pneumatic hammer
Signals:
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Artificially induced wire breakage
Potential measurement and wire breakages
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Original bridge with auxiliary piers
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• 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:
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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.
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• 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:
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5 Conclusions
� Prestressing is a proven technology with good past record
Summary and conclusions
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• 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
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5 Conclusions
� New concept minimized maintenance costs and increased service life