COST 534 WORKING GROUP 4 - INSA Toulouse · PDF fileCOST 534 WORKING GROUP 4 ... corrosion protection of steel in concrete ... • EN 12696 and EN 14038 (Cathodic protection and Realkalisation)

COST534 WG 4 Coordinator Øystein Vennesland1

Workshop Toulouse November 26-28 2007

COST 534 WORKING GROUP 4ELECTROCHEMICAL MAINTENANCE

METHODS FOR PRESTRESSED CONCRETE STRUCTURES

Øystein Vennesland, NTNU



Content

• Projects in WG 4 Electrochemical techniques• Introduction• Corrosion of the embedded prestressed steels• Description of the electrochemical methods• Examples - briefly• Conclusions



Projects• TNO and CPM Systems: Service life of concrete CP systems

– Project leader: Rob Polder– Partner: Jan Leggedoor

• Politecnico di Milano: Cathodic protection of reinforced and prestressed concrete structures subjected to carbonation– Project leader: Luca Bertolini

• KWH Bautechnologien AG: Modelling of cathodic protection (CP) and electrochemical chloride extraction (CE) applied to pre-tensioned and post-tensioned concrete structures– Project leader: Henryk Wojtas

• LMDC Toulouse: Electrochemical maintenance of prestressed concrete elements– Project leader: Gilles Escadeillas



Projects, continued• Carinthian Technical Institute: Non-destructive methods for

corrosion protection of steel in concrete– Project leader: Erwin Baumgartner

• LCPC: Applications of electrochemical treatments to prestressed concrete structures– Project leader: Andre Raharanaivo

• ETH: Electrochemical chloride removal on prestressed concrete structures - mechanisms– Project leader: Bernhard Elsener

• NTNU: Electrochemical treatment of prestressed concrete structures– Project leader: Øystein Vennesland



Introduction The project and the individual part projects have the main reasons:– Lack of information (or search for more precise

information) on the transport processes in the concrete during application of electrochemical methods

– A strong polarization of a high strength prestressed steel may induce hydrogen embrittlement

– Lack of information on service life and failures of practical cathodic protection projects



Introduction continued

• Electrochemical techniques for corrosion protection achieve their purpose by means of direct current applied to the embedded steel

• These techniques can be considered mature, both to protect reinforcement against corrosion or to prevent corrosion initiation – Advantages and side effect related to their application to

reinforced concrete structures have been clarified • COST 509 and COST 521

– The methods have been subjected to standardization• EN 12696 and EN 14038 (Cathodic protection and Realkalisation)



Introduction continued

• The application of electrochemical techniques to prestressed or post-tensioned structures is much less frequent

• Mainly due to the risks related to possible development of hydrogen at the steel surface and the lack of knowledge regarding real risks of promoting brittle failure on prestressing steel



On the corrosion of prestressing steel

• Corrosion may have serious consequences on prestressed concrete members

• When concrete is carbonated or polluted with chloride, steel in contact with the concrete may corrode

• Such steel is either reinforcement or tendons for prestressing by pre-tensioning

• In the case of post-tensioned structures, tendons are in ducts filled with cement grout or an organic product (wax, grease) and possible corrosion is due to incorrect filling of the ducts



Different types of prestressing

Types of prestressed concrete

A: pre-tensioning, tendon in direct (electrolytic) contact with concrete

B: post-tensioning with internal tendon inside duct

C: post-tensioning with external tendons (unbonded)

A B C



Morphologies of corrosion attack

Different types of attacks of steel in concrete:

a) general corrosion

b) pitting corrosion

c) stress corrosion cracking

d) stress corrosion cracking originating from a pit



Localized attack

• Localized attack is of particular concern

• It may promote brittle failure when pit size and sharpness are such to allow fast propagation

(Failure without warning)



Broken prestressing steels

A Surface free from rusting

B Fracture surfaces with cracks covered with magnetite

C The same as B

A B C



Propagation of cracks• Once a crack has initiated, hydrogen concentrates in the

zones of the metal lattice where the tensile stress is highest. Most important is that hydrogen accumulates at the tip of the crack, leading to crack propagation

• The crack propagation rate might be several mm/year– The steel characteristics– The environment – The crack tip properties

Hydrogen induced SCC can affect high strength steel with ultimate tensile strength higher than 700-900 MPa



Application of electrochemical treatment

• When a concrete structure is carbonated or contaminated with chloride, electrochemical treatments may be applied to its ordinary reinforcement

• When the tendons of prestressed structures are in isolating ducts, only their ends are in contact with concrete and therefore influenced by the polarisation

• In the case of metallic ducts, the potential of an enclosed tendon is not known because of the possible random contacts between tendon and duct

• FOR PRESTRESSED CONCRETE: NO HYDROGEN SHOULD BE FORMED ON THE TENDON SURFACE



Electrochemical methods• Methods for maintenance of buildings and structures

subject to reinforcement corrosion– Cathodic protection

• In soil or submerged• In chloride contaminated structures• In carbonated structures

– Cathodic prevention – Chloride removal (also named extraction)– Electrochemical realkalisation

• All electrochemical methods means negative polarisation of the steels and possible risk of hydrogen evolution

• The polarisation is different for the different methods



Summarizing the effects

• During cathodic protection and prevention the reinforcement gets a potential causing that it no longer corrodes

• During electrochemical realkalisation and chloride removal the environment within the concrete is changed from corroding to non-corroding



Set-up for electrochemical methods

Reinforcement (cathode)

Anode

Direct currentsource

+ -

Concrete



The main differences

1 mA/m2PermanentCathodic prevention

1 A/m2Weeks to monthsChloride removal

1 A/m2Days to weeksRealkalisation

10 mA/m2PermanentCathodic protection

Typical c. d. DurationMethod

• The special electrolyte (N2CO3 or K2CO3) forced into the concrete during electrochemical realkalisation

• The strength of the current forced through the concrete and taken up by the reinforcement

• The duration of the electrochemical treatment



Monitoring • The most important tool for control of an

installation of e.g. cathodic protection is embedded reference electrodes

• An overview of relevant reference electrodes used for monitoring and control for cathodic protection of concrete structures is presented in the following



Reference electrodes for concrete

A much used reference electrode for embedding inconcrete

A graphite rod in a paste ofmanganese dioxide surrounded by a solution of saturated or 0.5 molar potassium hydroxide

Manganese dioxide(ERE 10 or ERE 20)

A classical reference electrode much used for concrete structures

A silver rod in a paste of silver chloride surrounded by a solution of saturatedor 0.5 molar potassium chloride

Silver/silver chloride

CommentConstructionElectrode



Reference electrodes for concrete

A corroding electrodeMade by isolating all sidesexcept the tip of small lengths of lead

Lead electrode

The potential value depends on the oxygen content and the pH of theconcrete. Should only be used for short term measurements, e.g. control of cathodic protection

Made by isolating all sides except the tip of small lengths of the material

Oxygen electrodes- Graphite- Activated titanium - Stainless steel

CommentConstructionElectrode



Two examples• Pre-tensioned jetty in New Zealand

– The Calliope Access jetty was constructed in 1982– The structure consists of 28 prestressed beams of 16 m length,

configured with 4 transverse beams and 7 spans to form the jetty of approximate 115 m in length and 10 m wide

– The beams are relatively slender: 1.2 m high, 0.45 m wide at the base and only 0.13 m wide in the web (outer wideness)

– The 22 prestressing strands are 12.5 mm in diameter and are located mainly in the lower section of the beam

• Pre-tensioned bridge in Belgium– The North-South viaduct (road bridge) is situated over the main roads

towards Brussels Airport Zaventem– The road bridge is composed of prefabricated pre-tensioned deck

elements, supported by reinforced tabular column heads and columns

Reference is made to the next presentation



• The cathodic protection of the prestressed beams of Calliope Access Jetty was completed successfully by using a conservative design approach

• The main issues considered in the design and installation were– Good understanding of the construction of the element– Providing continuity of strands and conventional reinforcement– Conservative design regarding steel densities and current densities– Close spacing of anodes at 150 mm maximum– Close matching of anode output to required current density by using two

different ribbon anode outputs– Large number of small anode zones and sub zones, 14 and 56 respectively, in

order to achieve better control of the current distribution– High number of reference electrodes, 70 for 650 m2 of concrete– Location of reference electrodes in areas prone to underprotection and

overprotection

Conclusions: New Zealand



Conclusions: Belgium• The example shows that with

– application of the usual principles of CP– careful design of anode layout– system for monitoring the potentials in critical areas (near the prestressing

steel)– CP can be used for all types of concrete structures including prestressed

structures• To safeguard the prestressing steel against hydrogen embrittlement, the

true potential should be measured

Reference is made to the next presentation



Pre-tensioned steel (no ducts)

Monitor prestressing steel potentials

Cathodic protection Prestressing steel corrodes


Cathodic protection Reinforcement corrodesPrestressing steel does not corrode


Cathodic preventionNo corrosion

RemarksRecommended option

Condition



Post-tensioned steel (in ducts)

RemarksRecommended option

Condition

__________Not possibleTendons corrode inside ducts

Monitor duct potentialsCathodic protectionDucts corrode

Monitor duct potentials

Monitor prestressingpotentialsPerform trial

Cathodic protection

Chloride removal

Reinforcement corrodes; ducts do not corrode

Monitor duct potentialsCathodic preventionNo corrosion



Conclusions 1• Electrochemical corrosion protection techniques developed

for reinforced concrete structures may be advantageously applied to prestressed concrete structures

• The probability of inducing hydrogen induced stress corrosion cracking on high strength prestressing steel must be evaluated

• The probability of damage to prestressing steel depends on its susceptibility to this type of attack and on the potential-pH conditions at the steel-concrete interface

• Cathodic prevention and cathodic protection apply relatively low levels of current density and steel polarisation

• Provided that they are properly designed and monitored, these methods will protect the steel and will not promote hydrogen evolution



Conclusions 2• Temporarily applied electrochemical protection techniques

with high current densities, such as electrochemical realkalisation and chloride removal have shown to be effective for reinforced structures

• These techniques polarise the steel to very negative values and hydrogen evolution takes place during the treatment

• For pre-tensioned structures these techniques are not recommended

• For post-tensioned structures a careful examination of the possibilities of hydrogen evolution at the prestressing steels inside the ducts are recommended

Documents

COST 534 WORKING GROUP 4 - INSA Toulouse · PDF fileCOST 534 WORKING GROUP 4 ... corrosion protection of steel in concrete ... • EN 12696 and EN 14038 (Cathodic protection and Realkalisation)