Cathodic Protection Cathodic Protection of of

Steel in ConcreteSteel in Concrete

Steel in concreteSteel in concrete• Steel passivation in an alkaline environment with pH ≥ 9,5• Portland cement hydration > CSH + Ca(OH)2 with pH = 12 – 13• Similar coefficient of linear expansion (13 vs. 14.5 um/m K )• Reasonable good adhesion of cement on steel surface

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

acid alkaline

passivation

Pourbaix diagram for Iron at 25 degr. CPourbaix diagram for Iron at 25 degr. C

CORROSION

PASSIVATION

Question : If steel and concrete is a good combination Question : If steel and concrete is a good combination then how come steel corrodes in many occasions ?then how come steel corrodes in many occasions ?

• Concrete contains pores.• too much water added will enhance pore forming.

– keep w/c factor strictly conform manufactures spec.

• cement type has an effect on pore forming during curing :

– Blast furnace slag and Fly Ashes are better then Portland cements for marine environment (denser concrete + and lower temperatures during curing )

– Lower temperature during curing will produce less cracks due to internal and external temperature differences

• Never dismantle the concrete forms too early• water needed for curing will evaporize in c. cover• use temporary covers like plastic foil.

Main causes of concrete cracks and spallingMain causes of concrete cracks and spalling

• Carbonation Acidification of the concrete (lowering pH) due

to CO2 with Ca(OH)2 into CaCO3 (calcium carbonate pH=8-9)

Ca(OH)2 + CO2 CaCO3 + H2O

• Chlorides (salts) additive (CaCl2) accelerator Sand contaminated with salt, Salt penetration, seawater, salt-spray, etc

Effect of anion on the inhibition of pittingEffect of anion on the inhibition of pitting

Chloride content in concrete

pH1

pH2

carbonation

Examples of damageExamples of damage

Old patchwork with damage Old patchwork with damage

-450 mV CSE -275 mV -285 mV CSE

cracking and spalling

Conventional patching

-275 mV - 100 mV CSE -285 mV CSE

new cracking and spalling new cracking and spalling

corrosion cell

corrosion cell

Traditional PatchingTraditional Patching

• Within a short period of time new cracks will form just beside the patched area.

• Not known how far the steel within the concrete is corroded.

• Symptoms control

• Non-durable rehabilitation technique.

ChloridesChlorides

• Corrosion is possible in an alkaline environment,

• Criteria for pitting: Active/Passive behavior,

• Chlorides forms acids within the pit during pitting proces

• Chlorides are hygroscopic en pulls moist into the concrete,

• Anode/cathode surface ratio of pitting is very small = increased corrosion velocity

Examples of chloride pittingExamples of chloride pitting

Why CP of concrete?Why CP of concrete?

• Traditional patching and repair will re-introduce cracks, spalling and damage

• Due to pitting : Risc and safety regarding the integrity of the construction

• Costs : Properly performed traditional patching needs to replace all with chloride infested concrete by fresh mortar.

• Proved and effective system

Why can CP applied on concrete be so effective ?Why can CP applied on concrete be so effective ?

• Due to the concrete’s inhomogenity chloride and oxygen concentrations differ strongly from location to location

• These concentration cells cause corrosion due to potential differences

CP levelize the potential differences.

potential difference = 275 mV -250 mV CSE -450 mV CSE -175 mV CSE

corrosion cell

zinc/hydrogel adhesiveelectric field

current

potential : appr. -600 mV potential : appr. -600 mV

CP system

anode cathode

electric field

rebar

current

Dechlorination

Cl-

Cl-

Cl-

Cl2

OH-

OH-

OH-

anode cathode

electric field

current

electrochemical cathodic reaction :2H2O + O2 + e (current) => 4OH-

Realkalization

OH-

OH-

OH-

Concrete repair requirement for CP Concrete repair requirement for CP

• polymer or epoxy modified mortars should be avoided as much as possible.

• use mineral based mortars for repair.

• much less repair work is required by removing loose concrete only

• for surface applied anodes clean and coating free concrete surface (sand-blasting or high Temp steam cleaning).

Types of CP of concreteTypes of CP of concrete

• Impressed current systems with discrete anodes, titanium wire or conductive coatings.

• Galvanic systems with surface applied or descrete applied anodes.

Design and system criteriaDesign and system criteria

• Determine amount of steel surface within concrete construction design & field-inspection,

• Minimal design current density of 20 mA/m2

steel surface in concrete (min 100 mV depolarization by help of reference cells -EN12696),

• Reinforcement must be electric continuous,• If required for monitoring : determine amount

and location of Reference cells within the concrete.

Steel to concrete surface area ratio Steel to concrete surface area ratio [m[m22/m/m22]]

• Generally : – building, apartments : 0.6 – 0.8– civil engineering :

• bridge decks• concrete jetties, parking garages 0.8 – 1• columns, pillars, beams 1.2 – 1.5

CP praxis – discrete anodesCP praxis – discrete anodes

CP praxis – conductive coatingCP praxis – conductive coating

1 2 3 4

5

Wire installationWire installation

Kathodecontact

Rectifiers (powersource)Rectifiers (powersource)

Galvanic system Galvanic system

Advantages :

• 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

Points of attention :

• Clean coating free concrete surface.

• Avoid use of polymer based repair-mortars.

• Zinc can be overcoated with acrylic based topcoat.

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

• Sacrificial system, limited durability but can be replaced.

Galvanic systemGalvanic system

• Reinforcement directly connected to the zinc anode,

• Principle based on sacrificial anode (zinc) system.

electrons

anode cathode

- +

electrolyte ions

galvanic cell

zinc

anode cathode

electrolyte (adhesive gel + concrete )

concrete

steel

ZLA - ZLA - ZincLayerAnode ZincLayerAnode

• This product is specifically developed for cathodic protection of steel reinforced concrete. ZLA is applied up on the concrete surface

• As soon as the electrical connection is set, the current will flow and the Zinc-Layer-Anode will sacrifice itself. The electrical connection can be opened and used for monitoring purposes. Additional data loggers can be installed to store data like current-consumption and reference cell potentials for depolarization measurements if required.

• A cathodic protection system which utilizes the ZLA is a galvanic system. Zinc has a lower (more negative) potential than steel, so the ZLA provides the current to inhibit the corrosion of steel in concrete..

Applications in civil-worksApplications in civil-works

Applications in civil-worksApplications in civil-works

ApartmentsApartments

NEW GALVANIC ANODES FOR PROTECTION NEW GALVANIC ANODES FOR PROTECTION

OF STEEL IN CONCRETEOF STEEL IN CONCRETE

Galvanic protection systems utilize sacrificial anodes that naturally generate an electrical current to mitigate corrosion of the reinforcing steel. In concrete structures, zinc anodes are typically used. We manufacture a wide range of zinc anodes specifically developed for concrete applications.

Our Engineering ActivitiesOur Engineering Activities

• Inspections of structures

• Corrosion Survey and Monitoring

• Corrosion Consultancy

• Product Development

Chloride analysis and a chloride Chloride analysis and a chloride penetrationpenetration

0 - 25 1,225 - 50 150 - 80 0,8

0

0,2

0,4

0,6

0,8

1

1,2

1,4

0 - 25 25 - 50 50 - 80

mm chloride penetration

% C

l- [w

/w c

em

en

t]

critical Cl-content

dia 30mm x 150mm concrete core

Corrosion Survey Corrosion Survey Potential mapping Potential mapping

Color cardsColor cards

Pre-inspection activitiesPre-inspection activities•Corrosion detection of pre-stressed or post-tensioned cables •Magnetic Interference•Fractographic Analysis •Metallurgic studies•Research of hystorical case studies