CATHODIC PROTECTION CATHODIC PROTECTION OF REINFORCED OF REINFORCED

CONCRETE STRUCTURESCONCRETE STRUCTURES

Center for Engineering ResearchResearch Institute, KFUPM

REINFORCEMENT CORROSION

Passivity• High pH leading to formation of passive layer• Chemical binding of chlorides• Dense and relatively impermeable structure of

concrete Depassivation

• Chloride ingress• Carbonation

Chloride-induced Reinforcement Corrosion

• Due to external chlorides

• Due to chloride contamination from the mix constituents

Chloride Limits

ACI 318 (0.1 – 0.15%; water soluble) ACI 224 (0.2%; acid soluble) BS 8110 (0.4%; total)

CARBONATION

• Ca(OH)2 + CO2 CaCO3 + H2O

• Reduction in pH (up to 8.5)

FACTORS AFFECTING CARBONATION

• Concrete quality• Concrete cover• Moisture condition• Temperature• Chloride-sulfate contamination

PREVENTION THROUGH DESIGN

Cover over reinforcing steel Water-cement ratio Cement content Cement type

PROTECTION OF STEEL

• Metallic coatings• Non-Metallic coatings• Inhibitors• Electrochemical protection

• Cathodic protection• Chloride removal• Realkalization

CATHODIC PROTECTION

Sacrificial anode Impressed current

Corrosion cell in concrete

Corrosion cell in a repaired concrete

Corrosion cell after CP

Comparison of CP systems (Merits)Impressed current system Sacrificial anode system

Longer anode life Inherently simple

Current can be controlled No monitoring and maintenanceNo requirement for electrical isolationRisk of hydrogen embrittlement on high strength steel is minimalSaw cutting and concrete encapsulation for anodes is not required

Comparison of CP systems (Demerits)

Impressed current system Sacrificial anode system

Requires monitoring and maintenance

Short anode life

Electrical isolation required between anode and steel

Anode current delivery is dependent on anode chemistry and surrounding environment

Conduit and wiring required Current cannot be adjusted or controlled

Sacrifical Anode System

Anode material• Alloys of Mg, Al, Zn

Anode shapes• Zinc foil (Zinc hydrogel)• Metallized zinc (thermal spray)• Jackets• Zinc capsules or discs encased in mortar

Corroded pile in sea

Pier with SACP

Discrete anode for repaired structure

Discrete anode

Placement of discrete anodes

Mesh anode

Placement of anodes

Specimens with discrete anodes

Specimens with jacket anodes

Specimens with mesh anodes

Specimens and measurement boxes

Sacrificial Anode

Impressed Current CP

ICCP Anodes Conductive coating

• Sparyed, rolled or brushed over a nibium wire (300 um thickness) Arc sprayed zinc

• Spraying metal on concrete with compressed air (300 -400 um) Titanium anode mesh Titanium anode mesh jackets Titanium ribbon Discrete anodes Thermally sprayed titanium

Current requirements

Atmospheric zones• 5 to 15 mA/m2

Under water• 0.2 to 2 mA/m2

Decreases with time

ICCP – Power System

Transformers/Rectifiers Constant voltage/current system Stable over long period of time Solar- or wind-powered units are also used

ICCP - Connections

Durable and corrosion resistant Low-voltage wires Diameter for minimum voltage drop Color coded Multiple connections and cables

recommended

ICCP Monitoring System

Potential measurements Embeddable reference electrodes

• Ag/AgCl• Mn/MnO2

Anode mesh fixed on concrete

Conductor bar welded to anode mesh

Negative connection to the steel bar

Connection of embedded items

Reference electrode

Installing overlay

Finishing operation

Anode and steel cage

Details of anode

Connection of anode ribbon to the titanium conductor bar

Steel connection

Placement of electrodes and connections to the steel bar

Pouring of concrete

Chloride Removal

CE - Details Anode

• Titanium or steel mesh Electrolyte

• Water or saturated calcium hydroxide solution Current

• 1 to 2 A/m2

Time • 6 to 10 weeks

Not advisable for prestressed concrete Impressed/sacrificial anode cathodic protection systems

may be used Side effects – Alkali-silica reaction

Re-Alkalization

Realkalization - details Anode

• Steel or titanium mesh Electrolyte

• Sodium carbonate solution (1 mole/L) Current

• 1 to 2 A/m2

Time• Days to weeks

Not advisable for prestressed concrete Impressed/sacrificial anode cathodic protection systems

may be used Side effects – Alkali-silica reaction

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