Cathodic Protection of Reinforced Concrete Structures

www.vc-austria.com

The economic Alternative

Cathodic corrosion protection as a preservation

as well as preventive corrosion protection method

is used in a wide fi eld of civil engineering.

The imagination of maintenance-free building ma-

terial does no longer correspond with the expe-

rience of the last centuries. Concrete is exposed

to several external factors that lead to disintegrati-

on of material. Construction errors, for example a

not suffi ciently dense concrete cover, result in an

enormous maintenance and repair work. It espe-

cially concerns transport facilities like bridges or

parking garages, which are eminently exposed to

chloride pollution.

The positive economic aspects of cathodic corro-

sion protection become apparent when the dama-

ges at the reinforcement and concrete are not too

severe and the building elements are in a good

structural order.

The Method

The idea of using cathodic corrosion protection

for the rehabilitation of steel reinforced concrete

constructions was established in the mid-1970s in

the USA. Background for this decision has been

enormous damages on the concrete surfaces at

the bridges of the Interstate Highway Systems.

Conventional rehabilitation techniques had all

failed. In Europe it has been Great Britain which

did the fi rst attempts with cathodic protection sy-

stems at reinfored concrete constructions. V&C

has been carrying out cathodic corrosion protec-

tion at concrete structures since 1996 and is pi-

oneer in the Central European region.

Cathodic corrosion protection as an active protec-

tion method attacks - in contrast to other systems

- the problem at its roots. The success of the me-

thod rests on the connection between the potential

of steel and the corrosion rate. The potential of

the reinforcement is brought to a stable passive

state through installation of a negative protective

current. Through the formation of hydroxide ions

on the reinforcement the protective passive layer

is restored.

Proven Performance

Different repair and rehabilitation measures such

as patching, sealers, inhibitors or membranes

have only partially proven themselves. Howe-

ver, with constant patching the maintenance of

constructions is neither useful nor cost-effective.

The only possibility to stop the long-term advance

of corrosion is the installation of a cathodic corro-

sion protection system.

A cathodic corrosion protection system offers a

life-time of at least 20 up to more than 40 years.

This makes it an attractive alternative and/or sup-

plement to conventional repair works.

Loss of Durability

Why does the durability of bridges, multi-level car

parks, supporting walls, tunnels, trash burning fa-

cilities and sea water structures decrease? Can-

not the concrete protect the rebar?

The main problem in Central Europe is de-icing

salt on the streets. These salts contain chlorides

which penetrate into the constructions and de-

stroy the protective layer of the rebar - the conse-

quence: corrosion.

But also aggressive substances precipitated by

the exhaust of burning facilities or even chloride

containing groundwater are further sources of cor-

rosion.

These factors together with a too thin concrete co-

ver and too low density as well as changing wea-

ther conditions and humidity lead to an increased

risk of corrosion.

Corrosion of the rebar reduces the steel cross-

section and as a consequence the support safety.

Furthermore it causes cracks due to the increased

volume of the rust.

Fig. 2: Local-cell corrosion

CATHODE

CI-

(OH)-

Fe (OH)2

Fe++

O2

H2O

1/2O2+H

2O+2e- → 2OH-

ANODE

Fig. 1: Factors of infl uence on the corrosion risk of the rebar

Corrosion risk

Corrosion potential U

Korr

Pitting potential U L

pH o

f t

he pore water CI-/OH - ratio around the steel

O2 content Temperature

CI- co

ntent

on

stee

l

Qu

alit

y of

con

cret

e, im

perm

eability of concrete cover Concrete cover Concrete m

oistu

re

Exposure conditions Time

Electric

al re

sistiv

ity

The Principle of Corrosion and Catho-dic Corrosion Protection

The reinforcement in the concrete is protected

against corrosion by a very thin protective fi lm as

long as the concrete remains chloride free and the

alkalinity is suffi cient (pH > 12). This protective

fi lm can be destroyed by chloride containing gra-

vel or sand, concrete additivs or the penetration

of de-icing salt that forces its way through pores

and cracks in the concrete until it exceeds a criti-

cal concentration.

The focus of corrosion is the anode - the still pas-

sive steel - the cathode. A corrosion current fl ows.

The metal loss represents the anodic reaction, the

oxygen reaction the cathodic reaction (see fi g. 2).

The electrolyte is given through the reinforcement

as well as the pore solution in concrete.

Very often carbonation is responsible for the star-

ting of corrosion. CO2 reduces the alkalinity of the

concrete which is the basis for the passive layer.

Furthermore the density of the concrete decre-

ases.

Fig. 3: The principle of cathodic corrosion protection

Cathodic corrosion protection intervenes into the

electrochemical corrosion process. Through appli-

cation of an anode system on the concrete surface

a protective current is opposed to the corrosion

current. This protective current polarises the rein-

forcing steel in a way that the steel cannot corrode

anymore.

anode

concrete

+ _

steel rebar

cathode cathode

rectifi er

protective current

The reinforcement which is bared in places is con-

nected to the minus pole and the anode to the plus

pole of a rectifi er which serves as current source.

The reinforcement is polarised through electron

fl ow in a way that it is re-passivated.

Anode Systems

Depending on the case of application there are dif-

ferent kinds of anode systems that can be deplo-

yed. A very cost-effective solution is a conductive

coating which reaches - relative to the required

protective current - a life-time of up to 20 years.

However, titanium anode meshes or titanium an-

ode ribbon meshes guarantee a life-time of at

least 40 years.

Conductive Coating

The method of conductive coating has been used

since the 1980s in the USA and Great Britain. It

is especially recommanded in cases where an

increase of weight is - due to statical reasons -

not possible, concrete is showing only marginal

damages and the building elements which should

be protected are consisting of smaller centers of

corrosion.

Titanium Anode Mesh

Cathodic corrosion protection with activated tita-

nium mesh is the most commonly used system

worldwide. It is mainly designed for the protection

of existing buildings and can be adjusted to any

structure.

The anode material consists of activated titani-

um in the form of a mesh which is embedded in

shotcrete. The consumption ot the anode material

and the durability of the embedding material deter-

mine the life-time of the system. In practice a life-

time of up to more than 40 years can be assumed.

By sand and high pressure water blasting loose

and deteriorated concrete is removed in order to

ensure a good bond between the concrete sur-

face and the anode mesh. The anode material is

Most of the conductive coatings are produced

water- or polymer-based. The production of the

conductve fi ller is based on acrylic resin in which

fi bres with high conductivitiy are placed or on car-

bon or graphite basis.

To reach an optimum effectiveness of the catho-

dic corrosion protection system the right prepara-

Fig. 4: System design conductive coating

polymer-coating

secondary anode

primary anode(Pt/Nb/Cu-wire)

rectifi er

protective current

reinforced concrete

tion of the background is essential. It has to be

ensured that the surface is clean, dry and prune of

loose concrete.

The contact for the current fl ow is carried out by a

copper- or titanium wire and alternatively ribbons

that are placed in the coating. It is called primary

anode. The conductive coating is applied in two

easy work steps by rolling, brushing or spraying.

A strength of coating of about 0,5 to 10 mm is ge-

nerally enough.

A conductive coating can generelly cover a pro-

tective current of about 20 mA/m² concrete sur-

face. It has to have a low electrical resistance and

ensure a homogeneous current fl ow.

Following past experience the life-time of this sy-

stem is about 20 years. However, in the case of

local defects the conductive coating can be rene-

wed subsequently.

Titanium Anode Ribbon Mesh

Titanium anode ribbon mesh is primarly used for

preventive cathodic protection at new buildings or

in cases where due to statical reasons an increase

in weight of the building is not allowed.

In the case of rehabilitation or repair the anode

ribbons are installed similar to the anode mesh

directly on the concrete surface. In the case of

preventive corrosion protection the anode ribbons

are fi xed to the reinforcement by keeping a certain

distance from it by plastic bar clips.

The optimum distance between the neighbouring

anode ribbons is determined by the reinforcement

density and by the desired current distribution.

Generally the anode ribbons are installed in inter-

vals of 20 to 40 cm.

applicated directly on the concrete surface. The

minimum distance to the reinforcement must not

be lower than 1,5 cm.

For the monitoring of the plant reference elec-

trodes on a silver-silver chloride basis (Ag/AgCl)

are embedded. In the fi eld of the reference elec-

trodes a rebar connection is established. In the

course of this a piece of rebar is welded on the exi-

sting reinforcement and isolated with epoxy resin.

A cable connection leads from the welded rebar to

the cathodic protection rectifi er.

The titanium anode mesh is supplied with current

by a titanium conductor which is spot-welded at

regular intervals. The maximum current density

is 110 mA/m²/titanium surface. Generally a maxi-

mum of 20 mA/m²/concrete surface must be suf-

fi cient.

The anode system is embedded in mortar or

shotcrete in a way that the original appearance is

retained.

Irrespective of the anode system, before, during

and after commissioning control measurements

and tests from the EN-Norm 12696-1 prewritten

protection criteria provicde optimal operation.

Fig. 5: System design activated titanium anode mesh

reinforced concrete

anode

rectifi er

protective current

Fig. 6: System design activated titanium anode ribbon mesh

rectifi er

protective current

reinforced concrete

ribbon mesh(embedded in trenches)

repair mortar

The V&C-CCP-Controller

For the control and monitoring of the effective-

ness of a cathodic corrosion protection system the

V&C-CCP-Controller is used.

Due to the experiences and fi ndings about deve-

lopments on the fi eld of cathodic corrosion protec-

tion of steel reinforced concrete structures V&C

has developed a special control system for the

protection of reinforced concrete constructions.

The V&C-CCP-Controller delivers not only the re-

quired protective current but is also responsible

for a current and voltage constant operation, auto-

matically measuring routines, regular data recor-

ding as well as remote control and wireless data

transmission.

Advantages that convince

Instead of carrying out repetitive repair and main-

tenance work on the construction a cathodic pro-

tection system offers durable protection of the

whole building. The list of advantages is long:

• Chloride-contaminated concrete must not be

removed.

• Further chloride exposures do not cause any

further corrosion damages.

• The corrosion process is stopped.

• Cathodic corrosion protection allows rehabilitat-

ion while the building is in full operation and red-

uces down-times to a minimum.

• Traffi c is hardly infl uenced (no jams, no diver-

sions, etc.).

• The status of the building and the effectiveness of

the CCP-system is permanently monitored.

• Corrective interventions in the system can be

done at any time.

• No necessity for rehabilitation over the next 30

to 40 years.

• The required current to operate a CCP-

system is an insignifi cant factor. The po-

wer of a light bulb (100 Watt) can protect

about 5,000 m² cathodically.

The V&C Service Package

• Inspection of Building Structures in cooperation

with Concrete Specialists

• Evaluation of the CCP-System

Data Recording and Remote Control of a CCP-Plant

Provider of Protective Current

In each V&C-CCP-Controller an individual amount

of voltage modules can be integrated. The voltage

modules deliver the required protective current.

The modules are mounted in a control cabinet in

19“ modular system and are connected to the con-

trol module.

Current and voltage constant operation

The control module is responsible for the control

of the plant. Each voltage module can be individu-

ally operated current or voltage constant. Depen-

ding on the mode of operation a current or voltage

limitation can additionally be adjusted in order to

guarantee safeness in the case of an error.

Automatic Measuring Routine for the Control

of Effectiveness of the CCP-System

The control modul runs periodically measuring

routines. Depending on the adjustment an auto-

matic depolarisation of 4 or 24 hours is executed

in a 1 to 8 week cycle. The measuring results are

saved and shown in a control chart at the end of

the measuring routine. In this way the plant can be

easily checked for proper operation.

Data Recording

The control module also controls the whole mea-

suring data recording. The current, voltage and

potential data are recorded and saved in a data-

base. The measuring data are even kept in the

case of power breakdown.

For registration of the measuring data a recording

rate of up to 100 ms in a daily rhythm can be ad-

• Design and Installation

• Commissioning

• Monitoring and Maintenance

justed. The collection of the fi rst measuring data

after leading-in an automatic depolarisation can

also be adjusted accurate to 100 ms.

Remote Control and Data Transmission

Due to an user-friendly software the measuring

data are transmitted via modem or GSM network

and can be easily evaluated in the offi ce. Further-

more eventual readjustments can be done from

any place.

Pict. 7: The V&C-CCP-Controller: Automatic regulated rectifi er with remote control

Lückenlose Kontrolle und Überwa-chung

Die Wirksamkeit eines Kathodischen Korrosions-schutzes wird durch die Einhaltung der in den Nor-men und Richtlinien festgelegten Kriterien sicher-gestellt. Darüber hinaus ist auch eine regelmäßige Überwachung der Korrosionsschutzanlagen erfor-derlich. Ein Überwachungssystem setzt sich da-bei aus verschiedenen Elementen zusammen. So kommen Referenzelektroden, Messgeräte, Mess-stellen oder eine Datenfernübertragung via GSM-Netz zum Einsatz.

V&C bietet seinen Kunden die Überwachung der Schutzanlagen über ein drahtloses Fernübertra-gungssystem an. Die Messdaten werden dabei über ein GSM-Netz automatisch übertragen und können bequem vom Büro aus ausgewertet wer-den. Auf diese Weise ist eine permanente Über-wachung der Schutzanlagen möglich und eventu-ell auftretende Störungen können sofort behoben werden. V&C verfügt außerdem über einen bestens ge-schulten und langjährig erfahrenen Mitarbei-terstamm an Messtechnikern, welcher vor Ort

Contact:

V&C Kathodischer Korrosionsschutz Ges.m.b.H.Josef Perger-Str. 2/A-05A-3031 Pressbaum

Phone: +43 (0)2233 57 771Fax: +43 (0)2233 57 771 -15

offi ce.engineering@vc-austria.comwww.vc-austria.com