CATHODIC PROTECTION OF REINFORCED CONCRETE STRUCTURES Center for Engineering Research Research 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 maintenance No requirement for electrical isolation Risk of hydrogen embrittlement on high strength steel is minimal Saw 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 Arc sprayed zinc Titanium anode mesh 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 Under water Decreases with time 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 Electrolyte Current Time 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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