CHAPTER 1
INTRODUCTION Cathodic protection is a method to reduce corrosion
by minimizing the difference in potential between anode and
cathode. This is achieved by applying a current to the structure to
be protected (such as a pipeline) from some outside source. When
enough current is applied, the whole structure will be at one
potential; thus, anode and cathode sites will not exist. Cathodic
protection is commonly used on many types of structures, such as
pipelines, underground storage tanks, locks, and ship hulls.
1.1 The Principle of Corrosion and Cathodic Corrosion
Protection
The normal situation of steel reinforcement in concrete is
passivity. This is a state of almost negligible corrosion rate,
caused by an atomically thin oxide film on the steel surface, which
is stabilized by the high pH in concrete (about 13). This
passivation may be lost by two mechanisms: either carbon dioxide
ingress, which reduces the pH to values about 9 (carbonation),
causing a more or less uniform loss of passivation, or the presence
of chloride ions, which locally break down the passive film
starting pitting corrosion. Chloride may be either cast in as a set
accelerator or penetrate from de-icing salts or sea water.
Corrosion is an electrochemical phenomenon, in which the
potential of the steel and the exchange of electrical current
between steel and concrete pore solution play important roles. In
the passive state, the potential of the steel is relatively
positive, due to a reaction of oxygen at the steel surface,
consuming electrons (termed the cathodic reaction). When
passivation is lost, iron passes into solution as ferrous ions
(Fe2+), leaving excess electrons in the steel, which make the
potential more negative; this reaction is termed anodic. Potential
differences between cathodic and anodic sites cause currents to
flow in the concrete pore liquid, accelerating the steel
dissolution reaction. The overall reaction rate (in atmospheric
concrete structures) is thought to be limited by the electrolytic
resistance of the concrete. The ferrous ions react with hydroxide
ions formed at the cathodes and with more oxygen to form various
solid hydrated ferric oxides, commonly called rust. These corrosion
products are more voluminous than the original steel. The net
effect is expansion, causing tensile stresses in the surrounding
concrete cover. After relatively small amounts of steel have been
transformed into corrosion products, concrete cracks and spalling
or delamination occurs. Cracking and spalling have to be taken as a
warning of further decay: when left to corrode, the steel bar
diameter may decrease below structurally acceptable values.
Normally, spalled concrete is repaired using new, alkaline and
chloride-free concrete. However, if chloride ions remain, corrosion
can start again, which may relatively soon cause new damage to the
concrete.
Fig 1: Local cell corrosion
Cathodic protection is based on changing the potential of the
steel to more negative values, reducing potential differences
between anodic and cathodic sites and so reducing the corrosion
current to negligible values. The reduction of potential is called
polarization.ln practice, this is realized by mounting an external
electrode, the anode on the concrete surface, connecting it with
the positive terminal of a low voltage direct current source, while
connecting the negative terminal to the reinforcement cage, as
illustrated in Figure 1. Through the reinforcement cage, electrons
flow to the steel / concrete interface, increasing the cathodic
reaction, which produces hydroxide ions from oxygen and water. The
hydroxide ions migrate through the concrete cover to the anode
where they are oxidized to oxygen and electrons. The electrons flow
to the current source, which closes the electrical circuit. As a
result of this current circulation, cathodic reactions at the steel
are favoured and anodic reactions are suppressed.
Fig 2: Principle of cathodic corrosion protection Relatively
moderate current densities are able to restore passivation and have
various beneficial chemical effects: hydroxide ion production at
the steel increases the pH; migration of chloride ions to the
anode, away from the negatively charged steel. The required
polarization makes CP a permanent method: the current must flow
during the remaining service life of the structure. Due to the
chemical changes (increase of hydroxide and reduction of chloride
at the steel), the protection improves in the course of time and
theoretically the current may be reduced. If the current is
interrupted, the protection will remain intact for some time. For a
uniform distribution of the protection current, the steel must be
electrically continuous and the concrete must have a reasonably
homogeneous conduction. At any time and place, short circuits
between the anode and the steel must be avoided. Possible negative
effects of CP are: degradation of concrete around the anode, which
is only significant at high current densities; and too strong
negative polarization of stressed high strength steel (evolution of
hydrogen may cause embrittlement of high strength steel). When
overprotection is avoided, these negative effects are
negligible.
The quality of protection offered by a CP system is tested
regularly (normally a few times per year).Because of the complexity
of the causes of corrosion (chloride, pH, moisture) it is not
possible to predict a fixed value for the potential or the current.
As mentioned before, overprotection should be avoided. As a general
measure of the quality of cathodic protection, the amount of
polarization that actually takes place in the structure is
measured: as long as CP causes a certain minimum amount of
polarization, it may be assumed that the polarization is strong
enough to suppress corrosion to an insignificant level. This is
tested for by interrupting the protection current and monitoring
the subsequent change of the steel potential over periods up to 24
hours at several representative points in the concrete structure
using embedded sensors (called reference electrodes). With the
current switched off, the steel potential relaxes from polarized to
non (or less) polarized; this test is called depolarization.
Empirically, a minimum depolarization of 100 mV is considered
indicative of sufficient protection for atmospheric concrete
structures. For submerged or buried structures other criteria are
applied.
One of the main advantages of CP is that only spalls and
detached parts need to be repaired. Structurally sound but chloride
contaminated concrete can remain in place, because CP takes over
the protection. Compared to conventional repair, the cost of repair
may be reduced considerably. The added cost of the CP system is
justified because of the increased reliability of the protection to
the steel.
CHAPTER 2
TYPES OF CATHODIC PROTECTION SYSTEMSThere are two main types of
cathodic protection systems: galvanic and impressed current. Figure
3 shows these two types. Note that both types have anodes (from
which current flows into the electrolyte), a continuous electrolyte
from the anode to the protected structure, and an external metallic
connection (wire). These items are essential for all cathodic
protection systems.
Galvanic system.
Fig 3: Types of cathodic protection2.1 Galvanic anode system
A sacrificial anode is a form of cathodic protection, it is made
from a metal alloy from the galvanic series which has a more
negative electrochemical potential than the steel reinforcement of
the structure. This works because the difference in potential
between the anode and steel causes a positive current to flow in
the electrolyte, making the steel more negatively charged, thus
becoming the cathode. The difference in potential between the steel
reinforcement and the sacrificial anode, indicated by their
relative positions in the galvanic series, means that the galvanic
anode corrodes (sacrificed) in preference to the steel. The
sacrificial anodes are directly electrically connected to the steel
to be protected. Metals that are commonly used as sacrificial
anodes are aluminum, zinc and magnesium. These metals are also
alloyed to improve the long-term performance and dissolution
characteristics.
2.1.1 Advantages of SACP
Unlike ICCP, an external power source is not required to install
SACP. This greatly reduces the start up costs as no provision has
to be made to connect to a power supply. Also, the SACP system is
easier to maintain and this leads to significantly less minimal
running costs throughout the life of the system. In addition, the
SACP system voltages and current outputs are lower compared to the
ICCP system, leading to a low risk of cathodic interference in
adjacent structures.
Sacrificial anodes are relatively easy to install, as sound but
chloride contaminated or carbonated concrete does not require
replacement, only specific areas require concrete breakout. Repairs
can be targeted; focusing on specific areas of deterioration or
elements of the structure, preventing inefficient protection of the
steel and therefore keeping costs down. The anode also controls
corrosion in areas adjacent to concrete repairs that would normally
require removal if only conventional concrete patch repair was
carried out. Since concrete breakout is minimised, it is unlikely
that temporary works such as structural propping, which is
expensive, will be required during repair .Also with minimal
breakouts, uncertainties over structural behaviour due to
redistribution of stresses are reduced. These all leads to less
traffic disruption as he remedial works can be completed in a
shorter timeframe.
A SACP system is easier to design and specify as it has fewer
critical components, with the main critical component being the
anode itself. The system is considered to be a sustainable option
as it is making the most of the structure in its current form and
extending its life through relatively minor repair work. There is
also less waste going to landfill as often relatively little
concrete is broken out and repaired.
Overall the SACP system is much cheaper than the ICCP system, in
the short and medium term, is easier to install, no monitoring is
required and it causes less disruption as less time is required on
site.2.1.2 Disadvantages of SACP
The main disadvantage is the uncertain lifespan of the anodes;
the life expectancy of the system is dependent upon the average
current output of the anodes. The anodes only have a finite amount
of material available for sacrifice and a higher current uses up
that material at a higher rate. Changing conditions can affect the
current output of the anode. Factors which are known to affect the
current output are chloride content, temperature, oxygen content
and humidity.
There is no way of knowing when all of the material in the
anodes have been used up and the anode has stopped working, this is
a predicament, as new deterioration is likely to be the first sign
that the anodes are spent .
Compared to ICCP, the current output of the SACP system is
limited and this means that the current output cannot be altered
over time to compensate for changing conditions. There is no way of
adjusting the SACP system other than adding or taking away anodes
and because the system is not monitored in the same way as ICCP, it
is difficult to know when adjustments are required; this may lead
to a failure to arrest active corrosion.
Monitoring of an SACP system takes the form of survey at set
intervals to monitor for signs of deterioration. Although there are
no running costs associated with the system itself, the structure
requires a regular visual and delamination survey to monitor its
condition; however, this can be done during the structures regular
inspection schedule.
As a design consideration, the resistivity of the concrete must
be taken into account as the lower driving voltage of the anodes
means they may not work in high resistivity environments. If there
is significant loss of section to the steel reinforcement, steel
replacement needs to be carried out at the same time anodes are
installed as no cathodic protection system can restore lost
metal.2.2 Impressed current cathodic protection (ICCP)
The majority of cathodic protection systems applied to
reinforced concrete structures internationally, and particularly in
the UK, are impressed current cathodic protection (ICCP) systems.
ICCP systems arrest steel reinforcement corrosion activity by
supplying electrical current from an external source to overcome
the ongoing corrosion current in the structure. ICCP involves the
permanent installation of a low voltage, controlled electrical
system which passes direct current to the steel so that all of the
steel is made into a cathode, thus preventing the steel from
corroding. The anode can be applied on the surface of or drilled
into small holes in the structure. It is the main electrochemical
treatment that provides protection that can be effectively
monitored and controlled in the long term. The main components of a
typical ICCP system include the anode system, reinforcing steel,
electrolyte (in the concrete), cabling, monitoring devices, e.g.
reference electrodes and a direct current (dc) power supply.
Protection is provided by connecting the impressed current anode to
the positive terminal and the reinforcing steel to the negative
terminal of a dc power supply. The direct current is normally
provided by an ac powered transformer rectified or equivalent power
supply. Typical dc power supply outputs are in the region of 15 A
and 224 V to each independently controlled anode zone. The main
benefit of ICCP is its flexibility and durability. The current
output of the power supply can be adjusted to optimize the
protection delivered. ICCP systems can be controlled to accommodate
variations in exposure conditions and future chloride
contamination. The durability of ICCP systems is largely determined
by the choice of anode. This is because the damaging reactions are
moved from the steel to the installed anode. There are a number of
impressed current anode systems for reinforced concrete on the
market. These include conductive coatings, titanium based mesh in
cementitious overlay, conductive overlay incorporating carbon
fibres, flame-sprayed zinc and various discrete anode systems.
There are a range of factors which influence the selection of
impressed current anodes for ICCP systems for particular
applications. These include environmental conditions, anode zoning,
accessibility, maintenance requirements, performance requirements
and operating characteristics, life expectancy, weight
restrictions, track record and costs.2.2.1 Anode Systems
Depending on the case of application there are different kinds
of anode systems that can be deployed. 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 anode 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 recommended 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. Most of the conductive coatings are
produced water- or polymer-based. The production of the conductive
filler is based on acrylic resin in which fibres with high
conductivitiy are placed or on carbon or graphite basis. To reach
an optimum effectiveness of the cathodic corrosion protection
system the right preparation of the background is essential. It has
to be ensured that the surface is clean, dry and prune of loose
concrete.
Fig 4: System design conductive coating
The contact for the current flow 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.
Strength of coating of about 5 to 10 mm is generally enough. A
conductive coating can generelly cover a protective current of
about 20 mA/m concrete surface. It has to have a low electrical
resistance and ensure a homogeneous current flow. Following past
experience the life-time of this system is about 20 years. However,
in the case of local defects the conductive coating can be renewed
subsequently.
Titanium Anode Mesh
Cathodic corrosion protection with activated titanium 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 titanium in the
form of a mesh which is embedded in shotcrete. The consumption of
the anode material and the durability of the embedding material
determine the life-time of the system. In practice a lifetime 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 surface and the anode
mesh. The anode material is 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 electrodes on a
silver-silver chloride basis (Ag/AgCl) are embedded. In the field
of the reference electrodes a rebar connection is established. In
the course of this a piece of rebar is welded on the existing
reinforcement and isolated with epoxy resin. A cable connection
leads from the welded rebar to the cathodic protection rectifier.
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/m/titanium surface. Generally a maximum
of 20 mA/m/concrete surface must be sufficient. The anode system is
embedded in mortar or shotcrete in a way that the original
appearance is retained.
Fig 5: System design activated Titanium Anode Mesh
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 fixed 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 intervals of 20 to 40 cm.
Irrespective of the anode system, before, during and after
commissioning control measurements and tests from the EN-Norm
12696-1 prewritten protection criteria provide optimal
operation.
Fig 6: System design activated Titanium Anode Ribbon Mesh
2.2.2 Advantages of impressed current CP (ICCP)The application
of ICCP systems means that significant cost savings are possible
due to minimal concrete removal (limited physical repair) as ICCP
requires that only physically unsound concrete i.e. delaminated,
honeycombed, cracked concrete be removed while
chloride-contaminated but sound concrete is left in place. As a
result, ICCP retains more of the original structure with less
effect on aesthetics. Consequently, the installation of ICCP
systems eliminates the need for removing chloride-contaminated but
sound concrete with associated reduction of noise, dust, disruption
and propping. Installation of ICCP also limits the need to cut
behind the reinforcement.
ICCP controls corrosion at any chloride level regardless of
present or future chloride levels or carbonation. It controls
pitting and general corrosion and prevents accelerated corrosion
around repairs. ICCP can be applied to specific elements, e.g.
crossheads or to entire structures and can be used to protect any
buried or submerged metallic items. CP has lead to its wide
application on reinforced concrete structures including bridges
(bridge decks and substructures), car parks, tunnels, ports and
harbour facilities (jetties/wharves), industrial and residential
buildings and marine structures. There are good specifications and
standards have been developed over time and are now available to
assist with the design, installation and performance monitoring of
ICCP systems, which can be designed with up to 30 years design life
subject to the quality of the existing concrete. However, an
impressed current CP system could in theory have a life expectancy
of between 10 and 120 years depending on the type of anode system
selected and the monitoring and maintenance regimes put in place.
Any electrical components and cabling would be expected to be
renewed after about 20 years but with proper design, monitoring and
maintenance, the period to first maintenance can be well in excess
of this time frame.
Impressed current CP systems can be divided into zones to
account for different levels of reinforcement, different
environments or different elements of the structure. It can also be
utilised to provide protection to critical reinforcement at great
depths i.e. along the length of half-joints and deep bearing
shelves. With ICCP systems, various remote monitoring and control
options are available to enable selective and continuous monitoring
to be undertaken for each anode zone.
2.2.3 Disadvantages of impressed current CP (ICCP)The
application of ICCP mandates the structures owner to undertake
regular monitoring in order to assess the levels of cathodic
protection being afforded to the structure. There is, therefore, an
ongoing cost of electrical power (usually insignificant) and cost
of specialist monitoring, control and assessment. Competent, highly
trained & specialised persons are required in order to monitor
ICCP system performance for the service life of ICCP systems. There
is an initial high cost outlay to install ICCP systems and future
regular maintenance/controlling costs are approximately 2,500/annum
to ensure effectiveness of system.
ICCP requires a constant electrical power (permanent power)
supply and where none is locally available arrangements must be
made and allowed for in the costing. In the case of the impressed
current CP systems utilising discrete anodes extensive drilling is
required as part of the installation process. The drilled holes and
chases have an impact on the appearance of the structure and there
is also concern about Health and Safety issues due to the risk of
vibration white finger through the use of extensive drilling. In
addition, there are installation problems associated with the use
of certain impressed current anode systems such as discrete anodes
in areas of congested steel and the application of discrete anodes
to the soffits of structural elements. Also, discrete anodes
occasionally have problems associated with achieving sufficient
current distribution when compared with surface applied impressed
current anode systems.
The interface between cementitious overlay and bearing shelves
in the case of the MMO/Ti impressed current anode system acts a
potential point of weakness as ponding/excess seepage can
potentially cause freeze/thaw action. ICCP system power supplies,
monitoring systems and their enclosures are often vulnerable to
environmental damage, in particular vandalism and to atmospheric
corrosion. Cabling and control boxes associated with ICCP systems
are required to be strategically placed in order to avoid the risk
vandalism.
Certain impressed current anode systems such as conductive
coating anode systems cannot tolerate water during installation or
prolonged wetting during operation. They also do not tolerate
traffic or abrasion. Bulky equipment is required for the
installation of certain impressed current anode systems, e.g. the
Thermally Sprayed Zinc anode system.
The cementitious overlay for the MMO/Ti meshes and overlay anode
system changes the profile, loading, appearance and clearances of a
structure. Clearance may be an issue, e.g. on the soffit of
overbridges, around bridge bearings or in car parks. When an as
shot appearance is unacceptable then a flash coat would need to be
applied in order to achieve the desired finish.
Due to the risk of hydrogen evolution and possible occurrence of
hydrogen embrittlement on high strength steels ICCP is not
routinely applied to any prestressing or post-tensioned elements
without specific consideration for suitable safeguard criteria.
Provided the tendons are in good condition with no corrosion then
the use of ICCP is usually considered with suitable safeguard
criteria involving the minimisation of overprotection and the use
of appropriately placed monitoring probes at carefully selected
locations, together with appropriately screened cables.
The use of impressed current CP systems in the presence of
Network Rail lines and equipment or other electrical systems needs
to be strictly controlled in order to prevent incidents of stray
current interfering with associated overhead line/equipment and
track signaling equipment. In addition, any isolated reinforcement
steel or adjacent surface mounted steelwork must be made continuous
with the ICCP system in order to prevent stray current corrosion.
CHAPTER 3CATHODIC PROTECTION DESIGN 3.1 Required information Before
deciding which type, galvanic or impressed current, cathodic
protection system will be used and before the system is designed,
certain preliminary data must be gathered. 3.2 Physical dimensions
of structure to be protected One important element in designing a
cathodic protection system is the structure's physical dimensions
(for example, length, width, height, and diameter). These data are
used to calculate the surface area to be protected. 3.3 Drawing of
structure to be protected The installation drawings must include
sizes, shapes, material type, and locations of parts of the
structure to be protected. 3.4 Electrical isolation If a structure
is to be protected by the cathodic system, it must be electrically
connected to the anode. Sometimes parts of a structure or system
are electrically isolated from each other by insulators. For
example, in a gas pipeline distribution system, the inlet pipe to
each building might contain an electric insulator to isolate
in-house piping from the pipeline. Also, an electrical insulator
might be used at a valve along the pipeline to electrically isolate
one section of the system from another. Since each electrically
isolated part of a structure would need its own cathodic
protection, the locations of these insulators must be determined.
3.5 Short circuits All short circuits must be eliminated from
existing and new cathodic protection systems. A short circuit can
occur when one pipe system contacts another, causing interference
with the cathodic protection system. When updating existing
systems, eliminating short circuits would be a necessary first
step.3.6 Corrosion history of structures in the area Studying the
corrosion history in the area can prove very helpful when designing
a cathodic protection system. The study should reinforce
predictions for corrosivity of a given structure and its
environment; in addition, it may reveal abnormal conditions not
otherwise suspected. Facilities personnel can be a good source of
information for corrosion history.
3.7 Electrolyte resistivity survey A structure's corrosion rate
is proportional to the electrolyte resistivity. Without cathodic
protection, as electrolyte resistivity decreases, more current is
allowed to flow from the structure into the electrolyte; thus, the
structure corrodes more rapidly. As electrolyte resistivity
increases, the corrosion rate decreases (Table 1). Resistivity can
be measured either in a laboratory or at the site with the proper
instruments. The resistivity data will be used to calculate the
sizes of anodes and rectifier required in designing the cathodic
protection system. Table 1: Corrosivity of soils on steel based on
soil resistivitySoil resistivity range(ohm-cm)Corrosivity
0 to 2000Severe
2000 to 10000Moderate to severe
10000 to 30000Mild
Above 30000Not likely
3.8 Electrolyte pH surveyCorrosion is also proportional to
electrolyte pH. In general, steel's corrosion rate increases as pH
decreases when soil resistivity remains constant. 3.9 Structure
versus electrolyte potential surveyFor existing structures, the
potential between the structure and the electrolyte will give a
direct indication of the corrosivity. According to NACE Standard
No. RP-01, the potential requirement for cathodic protection is a
negative (cathodic) potential of at least 0.85 volt as measured
between the structure and a saturated copper-copper sulfate
reference electrode in contact with the electrolyte. A potential
which is less negative than -0.85 volt would probably be corrosive,
with corrosivity increasing as the negative value decreases
(becomes more positive).Table 2: Potential required for cathodic
protectionMetalPotential
(Cu/CuSO4)
Steel-850 mV
Steel (sulphate reducing bacteria)-950 mV
Copper alloys-500 to 650 mV
Lead -600 mV
Aluminium
-950 to 1200 mV
Some potential values for protection of other metals are shown
in Table 2. Values for lead and aluminium must be carefully
controlled to avoid damage by excess alkali which could build up at
the surface of the metals if the protection potentials are too
negative.
3.10 Current requirement
A critical part of design calculations for cathodic protection
systems on existing structures is the amount of current required
per square foot (called current density) to change the structures
potential to -0.85 volt. The current density required to shift the
potential indicates the structure's surface condition. A well
coated structure (for example, a pipeline well coated with coal-tar
epoxy) will require a very low current density (about 0.05
milliampere per square foot); an uncoated structure would require
high current density (about 10 milliamperes per square foot). The
average current density required for cathodic protection is 2
milliamperes per square foot of bare area. The amount of current
required for complete cathodic protection can be determined three
ways:
An actual test on existing structures using a temporary cathodic
protection setup.
A theoretical calculation based on coating efficiency.
An estimate of current requirements using tables based on field
experience.
Some typical values of current density for steel are shown in
Table 3.Having decided on the appropriate current density, the
total anode current can be determined from the area of the
structure. The second and third methods above can be used on both
existing and new structures. Current requirements can be calculated
based on coating efficiency and current density (current per square
foot) desired. The efficiency of the coating as supplied will have
a direct effect on the total current requirement, as Equation 1
shows:
I = (A) (I) (1.0-CE) (Equation 1)
Where I is total protective current, A is total structure
surface area in square feet, I is required current density, and CE
is coating efficiency. Equation 1 may be used when a current
requirement test is not possible, as on new structures, or as a
check of the current requirement test on existing structures.
Table 3: Current densities required to protect
steelEnvironmentCurrent density
A /m2
Acidic solutions350 500
Saline solutions0.3 10
Sea water0.05 0.15
Saline mud0.025 0.05
Coating efficiency is directly affected by the type of coating
used and by quality control during coating application. The
importance of coating efficiency is evident in the fact that a bare
structure may require 100,000 times as much current as would the
same structure if it were well coated. 3.11 Coating resistance A
coating's resistance decreases greatly with age and directly
affects structure to electrolyte resistance for design
calculations. The coating manufacturers supply coating resistance
values.3.12 Protective current required By knowing the physical
dimensions of the structure to be protected, the surface area can
be calculated. The product of the surface area multiplied by
current density obtained previously in I above gives the total
current required.3.13 The need for cathodic protection For existing
structures, the current requirement survey (above) will verify the
need for a cathodic protection system. For new systems, standard
practice is to assume a current density of at least 2 milliamperes
per square foot of bare area will be needed to protect the
structure. (However, local corrosion history may demand a different
current density.) In addition, cathodic protection is mandatory for
underground gas distribution lines(Department of Transportation
regulationsTitle 49, Code of Federal Regulations, Oct 1979) and for
water storage tanks with a 250,000-gallon capacity or greater.
Cathodic protection also is required for underground piping systems
located within 10 feet of steel reinforced concrete because
galvanic corrosion will occur between the steel rebar and the
pipeline. 3.14 Determining type and design of cathodic protection
system When all preliminary data have been gathered and the
protective current has been estimated, the design sequence can
begin. The first question to ask is: which type (galvanic or
impressed current) cathodic protection system is needed? Conditions
at the site sometimes dictate the choice. However, when this is not
clear, the criterion used most widely is based on current density
required and soil resistivity. If the soil resistivity is low (less
than 5000 ohm-centimeters) and the current density requirement is
low (less than 1 milliampere per square foot), a galvanic system
can be used. However, if the soil resistivity and/or current
density requirement exceed the above values, an impressed current
system should be used. 3.15 Cathodic Protection ControllerFor the
control and monitoring of the effectiveness of a cathodic corrosion
protection system the CP Controller is used. Due to the experiences
and findings about developments on the field of cathodic corrosion
protection of steel reinforced concrete structures has developed a
special control system for the protection of reinforced concrete
constructions. The CP Controller delivers not only the required
protective current but is also responsible for a current and
voltage constant operation, automatically measuring routines,
regular data recording as well as remote control and wireless data
transmission. In each 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
19modular system and are connected to the control module.
Fig 7 Remote Monitoring Cathodic Protection SystemThe control
module is responsible for the control of the plant. Each voltage
module can be individually operated current or voltage constant.
Depending 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. The control module runs periodically measuring
routines. 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.3.16 Data RecordingThe
control module also controls the whole measuring data recording.
The current, voltage and potential data are recorded and saved in a
database. The measuring data are even kept in the case of power
breakdown. Due to user-friendly software the measuring data are
transmitted via modem or GSM network and can be easily evaluated in
the office. Furthermore eventual readjustments can be done from any
place.
CHAPTER 4CASE HISTORY4.1 Bridge piersMouchel were commissioned
to carry out a special inspection of a 68 span, pre-stressed,
pre-tensioned concrete beam and slab bridge carrying a single lane
carriageway in a tidal estuary that was experiencing concrete
deterioration due to reinforcement corrosion. The bridge comprises
of two abutments and 67 piers, with each pier comprising of
transverse reinforced concrete crossheads or capping beams with
inverted T construction. From the special inspection, it was
concluded that the major cause of concrete deterioration was
chloride induced corrosion of the steel within the piers, in
particular the inter-tidal zone. This was a consequence of de-icing
salts being sprayed onto the road in poor weather conditions, along
with the salt spray from the sea itself. The corrosion of the
reinforcement had caused cracking and disruption and spalling of
concrete cover.
It was deemed that the most practical way to address the
corrosion problems without significantly altering the geometry,
aesthetics and structural integrity of the bridge was to
incorporate an electrochemical repair solution alongside concrete
repairs. Various electrochemical repair options were considered
including concrete replacement, chloride extraction and cathodic
protection and the impressed current CP was chosen as the most cost
effective repair option that is likely to meet the clients
requirements which includes a minimum life expectancy of 30 years.
The ICCP was designed and installed for four of the piers as part
of the major trial repairs carried out to the bridge, with other
piers to follow pending the outcome of the trial installations.
The design of the ICCP system was based on two anode types;Mixed
metal oxide coated expanded titanium mesh in a cementitious overlay
(approx. 730 m2 of anode coverage) and Discrete anodes - installed
at two depths (Ebonex CP10/300 installed horizontally at 450 mm
depth, and Ebonex CP10/1300 installed vertically at 1450 depth).
The two anodes were utilised in 5 zones distributed on each bridge
pier as shown below:
Zone 1 Stem wall and top surface of capping beam.
Zone 2 Diaphragm walls.
Zone 3 Capping beam soffit, sides and ends.
Zone 4 Atmospherically exposed part of columns to mid-tide
level.
Zone 5 Submerged part of columns from mid-tide to bed level.
Fig 7: Typical sectional elevation of pier showing anode
zonesThe zoning arrangement is shown in figure .The pier was
divided into five zones to give targeted and controlled protection
to the various elements of the pier and the clients requirement was
to include remote monitoring and control in order enable monitoring
offsite and thus less need for site visits and traffic management.
The number of reference electrodes per zone ranged from 4 to 8 and
these numbers were chosen to enable close monitoring and control in
the presence of the prestressing steel.
Some of the risks, hazards and challenges that had to be dealt
with in the design and installation of the CP system were the
presence of Macalloy bars, prestressing beams, working over water,
hazardous materials, working at height and the presence of
services.CHAPTER 5
CONCLUSION
Cathodic protection is now a widely used and accepted repair
method for arresting corrosion of reinforcement in concrete
structures.
It is normally used alongside concrete repairs to minimize the
extent of conrete breakout, thereby maintaining structural
intergrity and aesthetics.
The decision whether to use an impressed current cp system or a
sacrificial anode cp system is influenced by a number of factors
including but not limited to the condition (the level and extent of
deterioration) of the structure, the clients budget and the
anticipated life expectancy of the structure following the
repairs.
Both the sacrificial anode cp system and the impressed current
cp system are appropriate repair options that can be used in the
right circumstances.
SACP is most suitable includes small and targeted repairs,
repairs where budget costs are limited and repairs where the life
expectancy is anticipated to be around 10 years.
On the other hand, ICCP is generally used to address significant
corrosion problems to large structures and surface areas, where
life expectancy is expected to be more than 25 years.REFERENCES[1]
Keir Wilson, Mohammed Jawed, Vitalis Ngala, The selection and use
of cathodic protection systems for the repair of reinforced
concrete structures, www.elsevier.com,Construction and Building
Materials 39, 2013, pp1925
[2] J. Paul Guyer, P.E., R.A., Introduction to Cathodic
Protection, Fellow ASCE, Fellow AEI [3] www.vc-austria.com,
Cathodic Protection of Reinforced Concrete Structures[4] Rob B.
Polder, Cathodic Protection of Reinforced Concrete Structures in
The Netherlands - Experience and Developments, TNO Building and
Construction Research, Rijswijk, the Netherlands 23