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