Electrochemical Methods to Restore Concrete Structures
Transcript of Electrochemical Methods to Restore Concrete Structures
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Electrochemical Methods to
Restore Concrete Structures
Ian CHUNG
Maunsell Consultants Asia Ltd
Maunsell Materials Technology Group
18 February 2008
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Agenda
• Deterioration due to Corrosion • Chloride Extraction (also called Desalination)• Re-alkalisation • Cathodic Protection (CP) • Expelling Water (MPS System)
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In Normal Reinforced Concrete Structures
- Complications/Problems due to:
Aggregates: Soundness, deleterious substances, differential thermal and moisture movements, alkali-carbonate and alkali-silicate reactions
Steel: Corrosion – a Major Problem
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Deterioration due to
Corrosion
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Passivation stables between pH 10-12
So why some corrode and others not ?
Pourbaix Diagram
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Electrochemical Corrosion Cell
� Anodic Reaction 2Fe Æ 2Fe2+ + 4e-
� Cathodic Reaction O2 + 2H2O + 4e- Æ 4OH-
� Overall Reaction 2Fe + 4OH - Æ 2Fe(OH)2
Fe (OH)2 Ferric Hydroxide
(Rust)
Steel Substrate
Fe2+ + 2OH- Æ Fe(OH)2
FeFe
Fe2+
O2
O2
O2
2e- 2e-
OH-OH-
Cathodic Area
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•
Defects Caused by Reinforcement Corrosion
Corrosion product induced bursting force• Cracking, spalling, delamination• Prolonged period of deterioration will
eventually cause structural failure
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Passivation can be destroyed by:
• Reduction of pH, (i.e via carbonation) • Chloride attack • Voltage on the rebar of sufficient
magnitude and duration (Stray Current)
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Carbonation
• Carbonation of Concrete – A process where the dissolution of carbon dioxide (from the atmosphere) into the pore water in concrete to form carbonic acid, which neutralises the local high pH environment and causes the depassivation of steel
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• Penetration of chloride depends on diffusionand absorption by capillary suction
• Can effectively breakdown the passive filmcovering steel reinforcement and initiatecorrosion
Chloride
• Chloride may derive from external or internal sources
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• � 4Cs: concrete, cover, compaction, curing. � Others: e.g. corrosion monitoring and protection,
• coatings
During construction: � compaction, curing, cover
• � workmanship
Maintenance: � Inspection, Monitoring � Repairs: conventional, desalination, re-alkalisation,
cathodic protection, expelling water
What we need to do? Design stage: � Risk assessment / life expectance,
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Chloride Extraction • Draw chloride ions from concrete • Suitable for salt contaminated concrete • Applying an electric field between rebar (cathode) and
temporary anode (mesh and electrolyte)
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Features
• Carry out together with concrete repair to provide electrolyte and restore appearance of structure
• Prolong service life by 5 to 10 years dependent
• Effective process yields 50% to 80% chloride removal
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in HK Examples of Chloride Extraction
• Island Eastern Corridor (trial)• School (trial)• Housing Projects
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Re-alkalisation • Reinstate protective alkalinity (pH 10-12) in concrete • Suitable for carbonated concrete • Applying an electric field between rebar (cathode) and
temporary anode (mesh and alkaline solution)
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•
Features
Carbonated but sound concrete can be retained (common for carbonatedconcrete)
• No major repair and thus less disturbance
• Prolong service life by 10 to 15 years
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•
Examples of Re-Alkalisation in HK
Lion Rock Tunnel (trial)
• Housing Projects
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Cathodic Protection (CP) • Reverse of cathodic
corrosion • Suitable for salt
contaminated, carbonated and stray current affected concrete
• Impressed current passed to layers of reinforcement
• Require a source of direct current (DC) and an auxiliary electrode - Anode
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Example - KCRC Bridge No. 5
• Installed in 1986
• Conductive coating anode
• Effectively operating for over 16 years
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Example - Kwai Chung Terminal 2
• Flexible anode manufactured from conductive polymer
• Anode nailed to concrete surface
• Anode covered with render or spray concrete overlay
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Example – Kwan Chung Terminal 4
• Used on beams at Kwai Chung T4 in1992
• Titanium mesh anode nailed to concrete
• Spray rendercementitious overlay
• Effectivelyoperating for over10 years
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Example - MTRC Sea Channel Bridge
• CP specified, designedand installed at time of construction and tested after construction
• Corrosion monitoring ofstructure ongoing
• CP system will beenergized whenmonitoring results showthat it is necessary.
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Case Study – Eastern Harbour Crossing (EHC)
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Eastern Harbour Case Study –
Crossing (EHC)
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Investigation
• Working conditions• Delamination survey• Visual survey• Chloride samples• Potentials • Resistivity• Cover survey• etc.
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Design • Using information on structure condition from
investigation phase plus structural drawings
• Specify repairs required for delaminations and leaking cracks etc.
• Divide approx 1000m2 of slab into 6 zones to be individually protected
• Specify reinforcement continuity (checked andimplemented during repairs)
• Specify CP system requirements; TR unit, cabling,anode type and spacing, monitoring electrodes, powerrequirements, etc.
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Polyurethane Crack Injection
• Leaking crackssealed with flexible grout
• Allow for seasonal thermal movement
• Cross drill and injectat slab mid section
• Coring after injectionfor QC purposes
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Patch Repairs
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Slab Re-Casting
• Worst area of leakage
• Concrete cracked and delaminated above and below reinforcement
• Rebar cleaned, primed and new concrete placed
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CP Installation - Anode
• Marking anode locations on slab top surface
• Anode laid out in accordance with design spacing
• Groove to be cut at anode locations
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CP Installation - Anode
• Anode placed in groove
• Anode secured with plastic screw
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CP Installation - Anode
• Anode strips welded together
• Anode welded to distribution bar
• Electrical continuity within discreet sections
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CP Installation – Ref. Electrodes • Placed in pairs of
silver/silver chlorideelectrodes
• At upper and lower layersof steel
• RE connection to reinforcement
• Negative return connectionto upper reinforcementlayer
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CP Installation – Negative Return
• Two negative returns per zone
• Upper steel and lower steel
• Cable redundancy on upper steel connection
• Connection epoxy encased to prevent bimetalic corrosion
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CP Installation – Anode Feed
• Positive connection from TR unit to anode
• Two connections to distribution bar per zone
• Connection redundancy
• Connection epoxyencased to preventbimetalic corrosion
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CP Installation – Anode Grouting
• Anode grouted to ensure full bond with concrete
• Proprietary flowable grout used for full anode encapsulation
• Reference electrodes and negative returns also grouted
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CP Installation – Anode Grouting
• Full anode encapsulation critical to system operation
• Core through grout and anode to visually inspect anode encapsulation and grout bond to concrete
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CP Installation – Cabling • Cables protected by conduit
and run to junction boxes for each zone
• Cable in flexible conduit from junction boxes to TR unit
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CP Commissioning
• Checking and energisingthe system
• Taking readings relatedto compliance criteria(instant off, potentialdecay)
• Assessing results• Re-energising system
and adjusting current todesired level
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CP Operation and Monitoring
• System Monitoring � Monthly at first then becoming less
frequent until annual after 4 or 5 years
� General maintenance � Checking system operation � Potential decay � Adjusting current levels to provide
optimum protection
• Operation without significantrepair or replacement expectedfor a period exceeding 20 years
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(CM) system
the full installation and operation of future CP system• Save initial capital costs• Install at critical new structures in potentially hazard
environments (e.g. tunnel, station and otherstructures exposed to drainage channel andaggressive soils)
• The need for energising of CP system is determinedby assessing the data from the corrosion monitoring
CP Provisions in New Structures
• Provide basic and necessary setup so as to facilitate
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Components
• CP system design• Re-bar continuity• Negative returns• Anode system design• CP Monitoring System (CM System)
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Examples of CP Provisions in Hong Kong
• Western Harbour Crossing immersed tube tunnels
• KCRC West Rail tunnels, stations, viaducts, box culverts, nullah structures and ancillary Structures
• KCRC East Rail Extensions viaducts
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Devices
Reinforcement Continuity
• Objective: provide continuity of all reinforcement for future CP
• Device: continuity link reinforcement
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Devices
Negative Returns
• Objective: provide the – ve connections
• Device: mild steel bar fixed to reinforcement
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Devices Reference Electrode • Objective: to reflect the risk of steel
corrosion and stray current influence• • Device: Cu/CuSO4, Ag/AgCl reference
electrode
Multi-depth Sensors• Objective: monitor movement of
chloride ions/carbonation front through concrete
• Device: pre-fabricated device consist of a series of coupled metal rods to beembedded in concrete cover
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Devices
CM System in CP Provisions• Objective: assess corrosion status and the need for
implementation of a full CP System
• Device: reference electrodes and multi-depth sensors
• Monitoring at termination box
• Economical : < 1% of Total construction cost
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Expelling Water by Electrochemical Method
-Multi-Pulse Sequencing (MPS) System
- Multi-function to prevent water ingress as well as slow down re-bar corrosion
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Mechanism • A highly advance form of
electro-osmotic technology that uses low voltage and low current to ionise water and direct those liquid to a specific area
• The system will work under 60 bar of pressure
• Once drying is at the optimum level, the power usage will be less than the original required
• Normal drying time is around 4 to 8 weeks
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Devices • Anode • Cathode in electrolyte (soil/water)• MPS Control System Box• Power Supply
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MPS System at work
Before
After MPS System Installation
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Electrical Cable Tunnel, Zhengzhou PR China
Before the Installation of MPS system
After the Installation of MPS system
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MTRC Hong Kong Central Station Artwork Area
MPS System was installed No MPS installation at the at the Left side of the Artwork Right side of the Artwork
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Examples of MPS in Hong Kong
• Hong Kong International Finance Centre• Hong Kong Central Station, Airport Express• Hong Kong Hang Seng Bank Headquarters • City Plaza Taikoo Shing Centre • Hong Kong XinHua Agency
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Comparison
4
3
2
1
Low to Medium
MediumLowHighLife Cycle Cost
Low to Medium
HighLowMediumCapital Cost
Short to Medium
LongMediumShortExtended Service Life
Water Leakage
AllCarbonatedSalt Contaminated
Application
MPSCPRe-Alkalinsation
Chloride Extraction
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Thank You