Cathodic Protection (CP) of Steel in Concrete

Why CP of concrete

• Traditional patching and repair will re-

introduce cracks, spalling and damage,

• Risc and safety regarding the integrity of

the construction, and such as falling

pieces of concrete,

• Costs.

Examples of damage

Old patchwork with damage

Steel in concrete• Steelpassivation in an alkaline environment with pH ≥ 9,5

• Portland cement hydration > CSH + Ca(OH)2 with pH = 12 - 13

1 2 3 4 5 6 7 8 9 10 11 12 13

acid alkaline

passivation

Main causes of concrete

cracks and spalling

• Carbonation Acidification of the concrete (lowering pH) due

to CO2 with Ca(OH)2 into CaCO3

(calciumcarbonate pH=8-9)

• Chlorides (salts) additive (CaCl2) accelerator

Sand contaminated with salt,

Salt penetration, seawater, salt-spray, etc

Carbonationprocess

CO2

Carbon Dioxyde in

the air penetrates

the concrete.

Lowering the

alkalinity

CO2

CO2

The

carbonationprocess

continues slowly

Until reaches

the steel

reinforcement CO2

CO2

Reinforcement

starts to corrode

CO2

CO2

Cracking

and spalling

continues.

Pieces of

concrete

comes off

Chlorides

• Corrosion is possible in an alkaline

environment,

• Chlorides forms acids during pitting proces

• Chlorides are hygroscopic en pulls moist

into the concrete,

• Anode/cathode surface ratio of pitting is

very small = increased corrosion velocity

Potentialmapping

• If no visual damage can be seen, but reinforcement corrosion might be present.

• Selective repair method for economic reasons.

Potentialmapping

V

beton Referentie Cel

oppervlak

- 431 mV - 86 mV

Wapening

V

Potentialmapping praxis

Color cards

Repair methods in case of

chloride initiated corrosion

• Reinforcement has to be completely

recovered with fresh (chloride-free)

concrete or mortar.

• Chloride-extraction,

• Cathodic Protection (CP).

Basics of cathodic protection

• By making the potential (in Voltage) of the

reinforcement more negative then the

most negative corrosion potential :

cathodic reaction.

• NACE and EN standard : < -850 mV CSE.

• Cathodic electrochemical reaction :

O2 + 2H2O + 4e => 4OH- (high pH).

Types of CP of concrete

• Galvanic systems.

• Impressed current systems with discrete

anodes, titaniumwire and conductive

coatings.

Galvanic system

• Reinforcement directly connected to the zinc

anode,

• Principle based on sacrificial anode (zinc)

system.

electrons

anode cathode

- +

electrolyte

ions

galvanic cell

• Anode reaction : Zn => Zn2+ + 2e

• Cathodic reaction : 2e + O2 + H2O => OH-

Galvanic system

galvanic system

• No wiring installation or external powersource needed.

• Simple and easy installation.

• Cost-effective

• Offers sufficient current in dry environment (acc. European standard EN 12696).

• Repair mortars : mineral based sand–cement mixtures

• Clean coating free concrete surface.

• No use of polymere based repair-mortars.

• Zinc can be overcoated with acrylic based topcoat.

• Use sealant or avoid situations when running water is present.

• Longevity appr. 21 years.

Zinc Layer Anode (ZLA) principle

Applications in civil-works

Applications in housing and

public works

Engineering

• Determine amount of steel surface within

concrete constructiondesign & field-inspection,

• Minimal currentdensity of 20 mA/m2 steelsurface

in concrete (min 100 mV depolarization by help

of reference cells -EN12696),

• Reinforcement must be electric continuous,

• Determine amount and location of Reference

cells within the concrete.

CP praxis – discrete anodes (impressed current)

CP praxis – conductive coating (impressed current)

1 2 3 4

5

Wire installation

Kathodecontact

Rectifiers (powersource)