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Cathodic Protection

Marine and Civil Maintenance is a leader in the repair and cathodic protection of large concrete strucutures in Australia. We are able to design and install a wide variety of systems including: impressed-current, galvanic and hybrid anodes.

For examples, click on the titles below


Repair and Maintenance
of Marine Structures by Cathodic Protection

Alan R. Bird, Michael P Cope† and Anna Rix*
† Solomon Corrosion Control Services Pty Ltd,
22 Eskay Road, Oakleigh South, VIC 3167
*Monash University, Department of Civil Engineering,
Melbourne, VIC 3000

Abstract

Marine structures such as bridges, wharves, jetties and mooring dolphins are required to function in an aggressive environment for many years. Maintenance expenditure is sometimes sporadic and the required service life may be increased as the structure ages, with the result that many marine substructures deteriorate to the extent that major repairs are needed.

The technology available for repair and maintenance of reinforced concrete structures has grown in recent years and electrochemical treatments, such as cathodic protection, are now becoming common for marine structures. This paper indicates the range of cathodic protection systems and hardware in current practice, with reference to the treatments applied to port structures in Victoria, Queensland, New South Wales and Tasmania. Their different methods of contract delivery are also indicated.

1.
REINFORCED CONCRETE IN MARINE ENVIRONMENTS

Steel and concrete have been used effectively together in civil construction for over 100 years. Their combination has been commonly used because of its versatility, durability, fire resistance and low cost. However embedded steel reinforcement frequently corrodes and this corrosion can cause significant deterioration of the structure.

1.1.
The Corrosion Process

During construction a protective oxide film forms on the surface of reinforcing steel as the cement hydrates. The film is formed due to the high alkalinity (pH 12.6-15.5) of the hydrating cement and it will continue to protect the steel against corrosion while the high alkalinity is retained.

Concrete is a microporous material and therefore susceptible to the permeation of gases and diffusion of ions, such as chlorides, in solution. Penetration of chloride ions in the concrete microstructure will occur where elements are exposed to seawater and salt spray.

In most cases the laws of diffusion can approximately predict the rate of chloride ingress. Salt water is rapidly absorbed by dry concrete through the initial mechanism of suction. Chloride ions then move through the concrete pores through diffusion. When in sufficient concentration, the presence of chlorides at the steel surface can initiate corrosion. Once the layer is destroyed the chlorides are not consumed in the reaction but act as a catalyst in the ongoing corrosion process.

Corrosion is an expansive process because rust products occupy a much larger volume than the original steel. This leads to cracking, delamination and finally spalling of the concrete cover to the steel. Once exposed to the atmosphere, the steel reinforcing will then continue to corrode at an accelerated rate.

In low oxygen conditions, such as in buried or submerged areas, the corrosion rate is reduced due to the limited availability of oxygen. Corrosion under these conditions is slower and usually results in a partially soluble and less expansive 'black rust'.

Chloride ingress is the principal cause of extensive and severe corrosion of embedded steel reinforcing in concrete structures at coastal locations. The implementation of remediation measures is usually left until the presence of chlorides is widespread with high concentrations of chlorides at or beyond the level of steel reinforcement. At this stage, cathodic protection is often the most appropriate technique to prevent further deterioration.

1.2
Cathodic Protection of Reinforced Concrete

Cathodic protection of metals against corrosion was first demonstrated in the early nineteenth century and it is now used in a wide variety of applications to slow corrosion of metals including pipelines, ships' hulls, underground storage tanks and offshore structures.

Since the 1970s this technology has been applied to effectively stop corrosion of embedded steel reinforcing within concrete structures. The anodes used in cathodic protection systems for buried and immersed concrete structures are well established but the technology of the anodes for atmospherically exposed concrete structures is still evolving. Over the past ten years cathodic protection is being used more frequently as a cost-effective solution to control corrosion of steel reinforcing in atmospherically exposed concrete structures.

Cathodic protection controls the corrosion process by altering the thermodynamics and kinetics of the affected steel and is achieved by forming an electrical circuit with an introduced anode using the concrete as an electrolyte and the protected steel as the cathode. Cathodic protection can be driven either by galvanic means or an impressed current.

2.0
CATHODIC PROTECTION SYSTEMS

Galvanic and impressed current cathodic protection systems can both be used to protect embedded steel reinforcing in atmospherically exposed concrete structures from corroding.

2.1
Galvanic Cathodic Protection

Galvanic (also known as sacrificial) anodes are the oldest form of cathodic protection with their use dating back to the early 1800s. They are based on the principle of dissimilar metal corrosion and use metals that are further down the electrochemical series than iron (such as zinc, aluminium and magnesium) to protect the steel by preferential consumption of the anode.

In submerged or buried areas discrete zinc anodes are positioned adjacent to the structure where the low resistivity of the soil or water allows the current to pass readily from the anode to the steel reinforcing. In atmospherically exposed concrete it is necessary to distribute the zinc anode more closely in the areas to be protected, as the resistivity of concrete is substantially higher than soil or water.

Spraying zinc in atmospherically exposed areas has been common practice in the US over the past ten years. In Australia, VicRoads installed similar systems on four coastal bridges along the South Gippsland Highway some five years ago. The zinc was applied by heating and then spraying it onto exposed surfaces. The thickness of the zinc after application was approximately 0.5mm. Provision was made for interrupting the circuit between the anode and steel reinforcing and measuring permanent reference electrodes within an enclosure adjacent to each structure. These cathodic protection systems have been regularly monitored since energising and are still performing satisfactorily.

More recently, proprietary discrete zinc anodes have been developed for extending the life of patch repairs in chloride-saturated concrete. Encased in porous mortar plugs and fixed to the reinforcing steel at intervals around the perimeter of each patch, the units are designed to prevent the formation of incipient anodes in the adjacent unrepaired concrete. The zone of effectiveness of each unit is about 500mm and their expected lifespan is up to ten years. Trials of these sacrificial anodes have been conducted within the last year on two wharves in Queensland.

The service life of galvanic cathodic protection systems is governed by the quantity of anode used. In buried/submerged situations it is common to design for a 30-year service life and a 10 to 15-year service life in exposed atmospheric situations.

2.2
Impressed Current Cathodic Protection

Impressed current cathodic protection systems have been used to protect steel reinforcing in atmospherically exposed concrete structures for more than 25 years. These systems consist of the following fundamental components:

  • A direct current power supply
  • An anode system
  • Cables to create a circuit between the anode and reinforcing steel
There are a number of different types of anode that are available in varying geometric forms, but activated titanium or conductive ceramics anodes are most commonly used in marine applications.

2.2.1
Activated Titanium Anodes

The anode comprises a titanium product with an electro-catalytic coating containing oxides of rare earth elements (such as platinum, iridium, etc). Activated titanium anodes may be operated at current densities of up to 400mA/m2 of anode surface area for short periods, but at this density acid attack of the surrounding concrete can occur.

For this reason it is recommended that the anode current density is limited to a long-term maximum of 110mA/m2 of anode surface, but for short-term periods (ie months) current densities of up to 220mA/m2 may be permitted. At these operating levels the anodes have an effective service life of at least 75 years. The anodes are commonly available as open expanded mesh (in wide rolls), narrow strip ribbons (which may also be in expanded mesh form), or small-diameter rods.

The open mesh is usually fixed to the concrete surface and covered by a sprayed low-resistivity, cementitious overlay about 25mm thick. In some cases, the mesh can be fixed to the inside face of formwork and incorporated in a poured repair mortar (this can also be used for 'cathodic prevention' in new concrete structures). It should be noted that an overlay increases the dead weight of the structure by 50-80kg/m2, and careful quality control must be implemented to prevent it from debonding from the substrate.

Ribbon anodes are usually recessed into the cover concrete in sawn grooves and backfilled with a cementitious grout of moderately low resistivity. The ribbon is usually between 12mm to 25mm in width and approximately 1mm thick. The anode slots are typically spaced 150 to 300mm apart and the depth of cover to the reinforcing steel dictates their depth and orientation. In some applications, ribbon anodes can be fixed to the reinforcing steel with insulating clips before pouring the repair mortar or new concrete.

Activated titanium rods have been used as an internal anode system when installed in holes drilled into the concrete to depths of around 100 to 200mm, or even deeper if remote layers of reinforcing are also to be protected. The spacing of the anodes is determined by the distribution of reinforcing steel and the location of corrosion activity. One such internal system, which has been used on a number of Australian bridges since 1990, uses 3mm diameter mixed metal oxide coated titanium rods, backfilled with a conductive graphite-based paste or gel. The graphite is consumable due to gas generation at the anode and is estimated to have an effective service life of 10 to 15 years.

2.2.2
Conductive Ceramic Anodes

Conductive ceramic anodes comprise a ceramic tube that is electrically conductive and corrosion resistant that encapsulates a titanium core. Ceramic anodes can be operated at up to 900mA/m2 of anode surface however it is more usual for the anode to operate at around 400mA/m2. At these operating levels there is the possibility of anodic gases being generated and therefore the anodes are supplied with a dedicated gas venting system to allow any gases to dissipate.

They are installed via holes drilled from the exposed surface, then backfilled with a cementitious material. Unlike their titanium counterparts they are not required to be backfilled with a conductive paste or gel and at normal operating levels are expected to have a service life in excess of 50 years.

Conductive ceramic anodes were developed in the UK some eight years ago and have been used widely in Australia over the past five years. The discrete anodes are available in different lengths and diameters but are usually from 18mm to 28mm in diameter and 150mm to 270mm in length. They are typically installed between 300mm and 600mm apart.

2.2.3
Choice of Impressed Current System

The extended service life required for marine structures is often greater than 30 years and in these situations impressed current cathodic protection systems are usually more appropriate than the shorter-lived sacrificial ones.

Within the variety of systems outlined above, there are a number of different proprietary anodes presently available. Selection is usually dictated by the geometry of the structure and the distribution and corrosion activity of its embedded reinforcing steel, as well as cost and logistical issues, such as access for installation.

Experience suggests that activated titanium ribbon strips are usually chosen where protection of reinforcing steel is required in the surface of wide elements such as deck slab soffits and walls. Where protection of the steel reinforcing in more than one face is required (for example columns and beams, as well as thick slabs and walls) the use of discrete anodes may be more cost-effective by permitting installation from one face only.

Ribbon anodes are often used in overhead applications such as the soffits of beams. This may be the only face accessible if it is necessary for the wharf to continue operating during repair work. Crane beams at container berths in Brisbane and Newcastle, for example, are protected by ribbon anodes for the length and width of the beam soffit. Anode spacings vary from 140mm to about 300mm, depending on the density of reinforcement. The methods used on these structures to fix the anode include embedding in a sprayed overlay, grouting in sawn slots and embedding in the sprayed repair.

A mixture of anode types is often used to optimise the performance and cost of a cathodic protection system. An example is the rehabilitation of a large wharf in Portland, Victoria. The main beams, typically 840mm wide by 690mm deep, were protected on their sides and soffit by seven ribbon anodes. The 600mm thick, heavily reinforced fender wall was protected on both sides by a pattern of discrete anodes, installed from one side at 300 to 450mm spacings.

A further example of anode variety is in the repair and protection of two wharves at Townsville, Queensland. Patch repairs were made to isolated areas of deck slab and ribbon anodes were fixed to the steel in each patch in order to prevent the formation of incipient anodes beyond it. Repairs were also made to some of the large, circular columns; these were protected by discrete anodes installed in their centres.

Discrete anodes are often utilized where access to one face of a concrete structure is not possible. An example of this is the protection of reinforcing steel in the soffit of berthing dolphins at Port Giles in South Australia. In this case the clearance between the water level and the soffit of the dolphin was too small to allow installation of anodes from below. As a result, discrete anodes were installed in holes up to 1400mm long drilled from the top and sides of each dolphin.

3.0
CATHODIC PROTECTION FEASIBILITY AND DESIGN

Prior to the implementation of remediation measures the available technology together with life cycle costing should be evaluated to ensure the most effective solution is selected. In order to assess the feasibility of cathodic protection the condition of the structure and deterioration mechanisms should be determined as well as establishing the following:

The reinforcement is continuous. For cathodic protection to be effective the reinforcement must be electrically continuous.

The current requirement, which is usually based on 20mA/m2 steel surface area.

The design life of the cathodic protection system, which is usually between 10 and 50 years.

3.1
Remediation Strategies

For smaller concrete structures such as mooring and berthing dolphins, the use of a single remediation technique is often appropriate. However, for larger structures, such as wharves and jetties, the extent and rate of deterioration observed could vary with the different micro-environments that can exist. In these cases, a combination of treatment techniques may be used so that the overall cost, both initially and in future years, best meet the needs of the owner. A common solution is to apply cathodic protection in areas where the chloride concentration of the concrete at the level of steel reinforcement is high, and protective coatings (eg silane) where chloride ingress has not yet penetrated to the level of steel reinforcement and corrosion damage has not started. The silane requires replacement every ten years or so.

If re-casting of concrete is necessary, mesh, ribbon or discrete anodes can be installed prior to placing of the concrete. Where chloride ion ingress is localized the incorporation of discrete galvanic anodes within the patch repairs can prevent the occurrence of incipient corrosion to provide a more durable solution. Hot spots of local corrosion can be similarly treated with isolated anodes.

3.2
Cathodic Protection Design and Standards

The design of a cathodic protection requires identification of different anode zones for control purposes and to ensure provision of uniform protection levels. These zones are usually based on areas with similar characteristics, such as concrete resistivity and environment. Other issues that should be considered include monitoring requirements, cable routing and protection, positioning of the control and monitoring unit as well as the power source. Guidance on the design, installation and monitoring of cathodic protection systems for atmospherically exposed structures is outlined in the draft Australian Standard 'Cathodic Protection of Metals : Part 5 Steel in Concrete Structures' (to be AS 2832.5).

3.3
Performance Monitoring

Performance monitoring of cathodic protection systems is carried out periodically to ensure adequate protection is being achieved over the whole structure. Relevant cathodic protection criteria for steel in atmospherically exposed concrete structures are detailed in the draft Australian Standard (to be AS 2832.5). In summary the cathodic protection is usually demonstrated by achieving at least one of a number of criteria. These are based on changes in electrochemical potentials of the reinforcement during operation of the cathodic protection system and immediately following its temporary disconnection. The criteria are designed to ensure that the steel is adequately but not excessively protected.

Galvanic cathodic protection systems are self-regulating. Monitoring usually consists of a hammer-tapping and/or half-cell potential survey. With impressed current cathodic protection systems there is the risk of over-protection causing hydrogen generation at the reinforcing steel surface and the possibility of embrittlement particularly in pre-stressed steel. To eliminate this risk permanent monitoring devices are installed together with the cathodic protection system.

Silver/silver chloride/0.5M potassium chloride gel (Ag/AgCl/0.5M KCL) reference electrodes embedded in the cover concrete are commonly used for this purpose. Other sensors including potential decay electrodes have also been used in conjunction with reference electrodes. It is also usual practice to monitor current and voltage outputs to the various zones to ensure the operating limits of the anode are not exceeded.

Permanent reference electrodes should be installed to take into account areas that have the following characteristics:

  1. particular sensitivity to under-protection (at least 500mm away from any anodes),
  2. particular sensitivity to over-protection (less than 250 mm away from an anode);
  3. high corrosion risk or activity (area with a steel/concrete potential more negative than -350 mV Ag/AgCl); and,
  4. low corrosion risk or activity (area with a steel/concrete potential more positive than -150 mV Ag/AgCl).
Recent developments in computer and communication technology have introduced the option for remote monitoring and control of cathodic protection systems. The incorporation of a remote system can be particularly cost effective where access to the structure is difficult, or it is distant from major urban areas. It is also appropriate for owners of a number of structures protected by impressed-current systems.

4.0
CONCLUSIONS

Marine and coastal concrete structures are exposed to one of the most aggressive environments, with older structures often suffering extensive and severe deterioration. It is common practice to extend the original service life of these structures and this has been successfully achieved by applying cathodic protection.

Recent developments in anode technology for use in atmospherically exposed concrete structures have resulted in a wide selection of products available. This offers added versatility to the technology in the implementation of both short- and long-term solutions to control steel reinforcing corrosion in deteriorating concrete structures.

The application of enhanced communication facilities as well as more widespread use of computers can also be harnessed so that asset managers can remotely monitor and control the protection of their structures.

Cathodic Protection of Reinforced Concrete Structures - A Practical Method of Arresting Rebar Corrosion

Alan R Bird Bsc CPEng MIEAust RPEQ

INTRODUCTION

The major factors leading to the deterioration of reinforced concrete are poor construction practice and the environment. These factors are interrelated, for example: inadequate depth of cover to rebars, or excessively porous concrete, may allow penetration of atmospheric contaminants such as carbon dioxide or chlorides to reach the steel. In the presence of oxygen and moisture, this will cause it to corrode. Sooner or later, cracking and spalling of the concrete cover will occur as the expanding rust products build up bursting stresses around the rebars.

It is therefore of great importance in aggressive environments to provide a sufficient cover of high quality concrete to all embedded steel. This is particularly so in coastal and marine structures, where chloride contamination from wind-borne salt spray can be severe.

It remains doubtful, however, whether concrete conforming to AS 3600 and placed by the best practices will, in such severe environments, avoid significant deterioration during the lifespan typically required of such structures.

There are three primary methods commonly used to treat concrete which is suffering, or threatened by, corrosion damage to embedded steel:

  • Patch repairs or partial rebuild
  • Protective coatings
  • Cathodic protection
There are other methods, such as chloride extraction, which are not yet proven commercially in Australia.

In the case of a chloride-contaminated structure, patch repairs are likely to provide only a limited cure to the problem because new corrosion sites are often formed adjacent to the repair.

Coatings will restrict the ingress of chlorides or moisture into the concrete and thus reduce the development and rate of corrosion in mildly contaminated structures, but they are unable to arrest active rusting as a result of severe contamination. It is also necessary to replace the coatings at regular intervals to give continuing protection.

Cathodic protection is designed to halt all the active corrosion and prevent new sites from developing. Although it has been used for immersed and buried structures for well over 100 years, Cathodic Protection of reinforced concrete by means of an impressed current has been widely used only since the early 1980s. The relatively high electrical resistivity of concrete, compared with soil or water, and the difficulty in obtaining a uniform current spread to all the embedded steel components, have required the development of special materials and procedures.

Cathodic Protection is an economical alternative to patch repairs in chloride-damaged structures, not only because it provides a long-term solution but also because it obviates the need for massive removal and replacement of contaminated concrete. It is also cost-effective in severely carbonated structures. It is now used extensively as a means of corrosion control in concrete and has been applied to a wide variety of structures in coastal regions in Australia.

Cathodic Protection is an economical alternative to patch repairs in chloride-damaged structures, not only because it provides a long-term solution but also because it obviates the need for massive removal and replacement of contaminated concrete.

THE PROCESS OF CORROSION OF STEEL REINFORCEMENT IN CONCRETE

Reinforcing steel in concrete initially protected from corrosion by the high alkalinity provided by the cement, which stabilises the passive oxide layer on the surface of the steel. The passive layer can be destroyed by a reduction in alkalinity to below about pH 10, such as may be caused by carbonation from the atmosphere or by the presence of aggressive chloride ions. These ions may be present as a result of chloride contamination of the concrete materials at the time of placing, or by ingress from external sources such as a marine environment or de-icing salts. The mechanism by which chloride ions disrupt the passive layer is unclear but empirical testing has allowed a threshold contamination level to be defined with a high degree of confidence. It may be represented approximately as 0.4% of total (acid soluble) chlorides by weight of cement in a standard OPC concrete sample.

Once the passive layer on the steel has been disrupted, an electrochemical cell can be formed in the presence of oxygen and moisture. The concrete provides the electrolyte in the cell, with the steel rebar completing the circuit and transmitting electrons from anode to cathode.

The moisture content of the concrete has a significant impact on the efficiency of the cell, since it affects both the electrical resistance to the circuit and the chemical reactions.

While oxygen is consumed at the cathode to release hydroxyl ions and thus to increase the local alkalinity, rust (iron oxides) and acid are formed at the anodic site. Rust occupies a far greater volume than its parent metal and the process leads to a gradual buildup of bursting stresses within the cover concrete and, eventually, to spalling of parts of the concrete surface.

It is important to note the potentials set up by the corrosion process. These are typically of the order of - 500 mV at the anodes, compared with - 100 mV at the cathodes. If the concrete at the most actively corroding anodes is removed and replaced with a high quality repair mortar, ie by traditional patch repair methods, the induced protection will be removed from the untreated cathodes and the formation of new anodic sites will be encouraged. These incipient anodes will grow and repeat the cycle of corrosion damage, sometimes in a very short time if the environment is sufficiently aggressive.

CATHODIC PROTECTION OF STEEL REINFORCEMENT IN CONCRETE

Cathodic protection involves the establishment of a small DC current from an external anode, through the concrete to the rebar. The current charges the steel negatively and it becomes cathodic, ie not corroding. By passing this very small current from a supplemental anode to embedded reinforcement, corrosion can be halted for an indefinite period.

The supplemental anode transmits electrons which are consumed at the reinforcing steel. Other benefits of this process include the production of hydroxyl ions at the steel surface (thus reverting the pore water in the concrete to an alkaline state) and the gradual migration of the negatively charged chloride ions towards the new anode and away from the steel.

When a cathodic system is energized, the rebars are polarised to the 'protection potential', which is the electrochemical potential at which the corrosion rate becomes negligible. This requires a negative potential shift, from the natural (as found) potential, of the order of 100-300 mV and it is generally defined as that which will give a potential decay of at least 100 mV in the 4 to 24 hours following complete disruption of the DC current.

Before designing a Cathodic Protection system for a reinforced concrete structure, various parameters need to be established, mainly by non-destructive testing. The electrical continuity of the reinforcement must be established, to ensure that all bars will be protected. The electrical resistivity of the concrete is measured in order to determine operating voltages. The location and extent of the corrosion damage (anodic areas) are determined by means of a half-cell potential survey. The applied current densities are calculated on the basis of steel surface area and extent of corrosion damage.

Typical initial Cathodic Protection current densities in chloride contaminated stucutres are in the range of 1 to 20 mA/m2 of steel surface.

During the operation of the Cathodic Protection system, the initial current density can be reduced as chloride migration away from the rebar proceeds and the pH level increases.

Reactions at the anode surface in a Cathodic Protection system generate acidity and the anode current density must therefore be kept within certain limits in order to prevent excessive production of acidity. A maximum anodic current density of 110 mA/m2 is recommended by NACE. It is also necessary to ensure that no steel is polarized to a more negative potential than - 1150 mV versus a copper/copper sulphate reference electrode (CSE) to avoid the possibility of hydrogen embrittlement of the steel surface.

The most recent development in cathodic protection of concrete structures is the internal anode system, where probe anodes, usually made from titanium mesh ribbon, are placed in drilled holes in the concrete surface and embedded in grout.

OPERATING A CATHODIC PROTECTION SYSTEM

Cathodic Protection systems are generally powered by mains electricity converted to DC at the required voltage by a transformer/rectifier. The positive terminal is connected to the anode and the negative to the reinforcing steel. Solar power can be used at remote sites.

Various criteria have been proposed to establish the effectiveness of a Cathodic Protection system once it has been energised. These included parameters such as:
  • potential shift;
  • potential decay;
  • operating potential; and
  • current-potential relationship.
The criterion generally accepted today is that the system is performing effectively if the 100mV potential decay has been achieved.

After commissioning, this performance is monitored by means of reference cells embedded in the concrete close to the reinforcement. The half cells may be composed of silver/silver chloride, activated titanium, zinc or other materials, the primary requirements being stability, accuracy and longevity. The operating voltage and current are also monitored and adjusted as necessary; this can be done manually at the control cabinet, through a local computer or by modem connection to a remote computer.

It is normal for a Cathodic Protection system to be tuned over the first two years or so of operation as potentials stabilize and current demands reduce. Monitoring of protection levels and trends is therefore required at intervals of three or six months in the initial stages and annually thereafter until the system has balanced and needs only regular checking of its operational status.

DEVELOPMENT OF ANODE SYSTEMS

The high resistivity of concrete as an electrolyte has been an obstacle to the use of cathodic protection of reinforcement until recent years. Developments in anode technology since the 1980's have seen the emergence of a wide variety of anode systems for various applications:

High silicon cast iron anodes within a conductive asphalt layer were one of the first developments for flat slabs, particularly bridge decks. The additional weight of the asphalt and its short lifespan were disadvantages which led to the development of alternative systems such as slotted anodes, where the conductive string anode was placed in a sawcut in the surface of the concrete. Various anode types were tried, platinum-clad niobium wire proving quite successful, but the high current density required gave rise to excessive generation of acidity at the anode. Conductive coatings, using carbon-laden acrylic paint as the conductive anode, have also proved popular in situations where additional weight must be minimized, but there lifespan is limited. Flame sprayed zinc is a new development of this approach.

Conductive polymer wire was popular in the early 80s but was superseded by the introduction of expanded titanium mesh anodes. These systems comprise an activated titanium mesh coated with mixed metal oxides, encased in a conductive cementitious overlay, often of gunite. The mesh configuration minimizes and spreads the current distribution over each protection zone of the concrete surface and thus reduces the risk of acid generation at the anode. This method remains widely used because of its effectiveness with very small current densities and the longevity of titanium. It is particularly suited to large concrete surfaces such as slabs and beams.

A more recent development in cathodic protection of concrete structures is the internal anode system, where probe anodes of activated titanium mesh are placed in drilled holes in the concrete surface and embedded in cementitious grout. This method has the advantages of negligible added weight, relative cheapness and the ability to protect distant rebars. It is particularly suited to massive elements such as beams and columns but not to thin slabs.

SOME EARLY APPLICATIONS OF CATHODIC PROTECTION

Both the titanium mesh and internal anode systems are becoming widely used on concrete structures around the world. Some early examples of remedial projects are:

A floor slab in a Sydney apartment building was found to have suffered extensive corrosion of top mat reinforcement arising from the use of a chloride-rich magnesite topping. Conventional patch repairs would have required the removal of all the top 100 mm of concrete and would have promoted corrosion of the bottom mat steel; cathodic protection was therefore considered the only practical method of rehabilitation. A titanium mesh system was installed within a 25 mm concrete topping and the protection levels have been found to be satisfactory since its energising in 1989.

An eight-storey apartment building in Auckland, New Zealand, had been seriously damaged by wind-blown chloride contamination and carbonation in its structural concrete frame since construction 56 years ago. Conventional patch repairs had been attempted during a major renovation in 1982 and had subsequently failed. A titanium mesh anode system in four zones was placed on all external beams and columns in 1990 and successful polarization of external face steel was achieved within three months. Over two years of operation, protection has developed gradually in the internal-face reinforcing steel as far as 800 mm from the anode mesh.

A bridge abutment at Frankston, Victoria, was found to be severely contaminated by water-borne chlorides. Corrosion levels were highest in the tidal zone at the base of the abutment and lowest in the atmospheric zone. A titanium mesh anode divided into three horizontal zones was installed and encased in a layer of gunite in 1991. Protection levels were achieved in each zone with markedly different current densities.

A housing estate in Copenhagen, Denmark, comprising 1943 apartments in 17 blocks, was found to have extensive corrosion of reinforcement in most of the beams and columns in the access balconies. Each beam/column set required 13 internal anodes and approximately 40,000 in total were installed between 1990 and 1992. All elements were connected to a central personal computer which monitors and controls the system and each anode.

A wharf at King Island, Tasmania, was contaminated with chlorides to the extent that much of the cast insitu beams and precast deck soffit was cracked or delaminated. A titanium mesh system encased in gunite was chosen for 1300 m2 of concrete requiring treatment; the installation was completed and energised in mid 1992.

A road and rail bridge at Weipa, Queensland, across 1.1 km of trial estuary, was found to have significant chloride contamination in its precast concrete piles and insitu concrete headstocks. Trials of different Cathodic Protection systems were undertaken on four piers. On the tidal and splash zones of the piles, mesh anodes were installed in modular foam-lined FRP shells, cement-grouted FRP shells and fabric-formed concrete jackets. Underwater, titanium rod anodes were installed at each pier. In the atmospheric zone of the piles and headstocks, internal anodes were used. The final design chosen was for internal anodes on headstocks and upper piles, combined with immersed rod anodes for the underwater sections of pile. Installation was completed in 1995.

Cathodic protection has become accepted and widely used as a means of halting corrosion of steel in deteriorating reinforced and prestressed concrete structures.

CONCLUSION

Cathodic protection has become accepted and widely used as a means of halting corrosion of steel in deteriorating reinforced and prestressed concrete structures. The advantages of Cathodic Protection over other rehabilitation methods can be summarized as follows:

  • Cathodic Protection has the ability to stop the corrosion process for the extended life of the structure.
  • Cathodic Protection is a long-term solution (in excess of 25 years), with minimal maintenance requirements.
  • Cathodic Protection exhibits long-term economical advantages when discounted over the design life of the system. In many cases, the first cost may be less than a conventional patch repair, with a life four to five times longer.
It is also recognized as a means of prevention of corrosion damage in new structures, where for a small percentage of the capital cost the design life expectations can be met without repeated and expensive repairs.

BIBLIOGRAPHY

Cathodic Protection of Reinforced Concrete - State of the Art and Case Studies, 1992, Remedial Concrete Engineering Pty Ltd, Melbourne.
'Cathodic Protection of Reinforced Concrete Structures', in NACE Technical Report No. 36, 1989, The Concrete Society, London.
Mussinelli, G L, Tettamanti, M, Irwin, R W and Lawson, M J 1988, Corrosion of Reinforcing Steel in Concrete and its Prevention by Cathodic Protection, Pacific Concrete Conference, Auckland.
Irwin, R W 1990, Cathodic Protection of Chloride Saturated Concrete Structures, Construction Techniques Group Ltd, Auckland.
Gronvold, F O and Nielsen-Dharmarante, K 1991, Internal Anodes in Concrete Structures, NACE Corrosion '91 Conference, Cincinnati.
Tettamanti, M, Irwin, R W and Smith, C V 1992, Cathodic Protection of Reinforced Concrete Buildings: Two Years of Experience at Westminster Court Apartment Building, NACE Conference.
Gaggin, I A Booth, G L, Kirby, T A and Collins, F G 1991, Grassy Wharf - a Case Study in Concrete Deterioration, Austroads Conference, Brisbane.

REPAIR AND CATHODIC PROTECTION OF SUBURBAN CONCRETE BRIDGES

  • Case study of rehabilitating two concrete bridges over tidal estuaries in NSW
  • Design aspects - what to repair, what to protect, and how to do it
  • Construction aspects - access, condition of structures, methodologies, environment
Alan Bird, BSc, CPEng, MIEAust Director, Marine & Civil Maintenance Pty Ltd, Sydney

An increasing number of suburban concrete bridges are affected by chloride-induced corrosion of their reinforcing steel. Traditional patch repair of the damaged areas of concrete is recognised to be a short-term solution because it does not prevent further damage. Cathodic protection is now widely used to extend the life of such structures by arresting the corrosion in all areas that are at risk, in conjunction with repairing whatever has already been damaged. This paper reviews two similar bridges in the Illawarra, and demonstrates how different environmental and structural conditions affect the design and construction methods required for the repair and protection of each.

1.
INTRODUCTION

Brooks Creek Bridge
Fairy Creek Bridge


Brooks Creek Bridge is a three-span, two-lane suburban concrete bridge built in the late 1960's in Kanahooka, south of Wollongong. It spans a tidal creek that empties into Lake Illawarra a short distance away.

The piers and abutments consist of headstocks, 600mm deep and 900mm high, each supported by seven precast piles (300mm x 300mm). The upstream pile in each pier has a protective reinforced concrete jacket extending 1.5m below the headstock. The deck consists of precast, prestressed girder beams with an in-situ RC topping.

Fairy Creek Bridge is a five-span, two-lane suburban concrete bridge built in the early 1960's in Fairy Meadow, to the north of Wollongong. It spans a tidal creek that empties, via a large lagoon and a sand bar, into the sea about 1km away.

The structural elements of the Fairy Creek Bridge are very similar, in type and dimension, to those of the later Brooks Creek Bridge, although there are some significant differences that will be discussed further.
Both bridges showed evidence of reinforcement corrosion damage in the form of spalling and cracking to the cover concrete in exposed parts of the substructure. An investigation by CTI Consultants in 2004 concluded that the main cause of the deterioration was chloride ingress, and an impressed-current cathodic protection ('CP') system was recommended as a solution.

In 2008 Wollongong City Council invited tenders for a design-and-construct contract for concrete repairs and installation of a Cathodic Protection system, augmented by silane treatment, to the two bridges. The performance specification was provided by Connell Wagner. The installation contract was awarded to Marine and Civil Maintenance.

The contract programme required Brooks Creek to be completed before work started on Fairy Creek.

2.
DESIGN

Marine & Civil Maintenance engaged Corrosion Control Engineering to carry out the detailed design of the Cathodic Protection system, and monitor and adjust its performance over the two-year maintenance period required by the contract.

2.1
Design Requirements

The performance specification required the repair and protection of all piles above and below water, and all headstocks, including exposed surfaces of the abutments. Four independently operated and controlled zones were required for the Cathodic Protection system:
  • Submerged or buried
  • Tidal
  • Splash
  • Atmospheric
The Cathodic Protection system design life was required to be at least 30 years, and the specified minimum current density to be applied to the reinforcing steel in all zones was 20mA/m2. Design was to be based on AS2832.5 (Cathodic Protection of Metals - Steel in Concrete) as well as other international standards. Protection criteria were as defined in the Standard.

Other aspects of the Cathodic Protection performance specification included the required number of reference electrodes for monitoring each zone, the maximum current density for each likely anode type, the maximum driving voltage for the titanium components (conductor bar), and the minimum acceptable experience of the Cathodic Protection designer.

While the tidal, splash and atmospheric zones were required to be protected by an impressed-current Cathodic Protection system, the submerged and buried zone was permitted to be protected by sacrificial or impressed-current anodes.

Multiple connections were required, in order to provide redundancy.

Concrete repairs were specified to be made with a proprietary repair mortar and the procedures for breaking out the concrete, treating the exposed steel and curing the repairs were defined.

A silane impregnation or protective coating was specified for all exposed surfaces of the substructure that were less contaminated by chlorides and were not protected by the Cathodic Protection. This included the precast deck soffit and the vertical sides of the bridge.

2.2
Design Concept

Based on the performance requirements outlined in Section 2.1, the Cathodic Protection design concept was that all anode zones were to be driven by impressed current, and the optimum configuration was
  • Ribbon mesh anodes grouted into slots cut in the atmospheric zone surfaces
  • Discrete anodes (manufactured from ribbon mesh) grouted into holes drilled into the splash and upper tidal zones
  • MMO water anodes in submerged and buried zones
Design considerations included:
  • Achieving uniform current distribution from the anode layout in each zone
  • The requirement to ensure that the maximum driving voltage is not exceeded
  • The possibility of current dumping and localised over-protection in the tidal zone
  • Redundancy and spare capacity in all connections and reference electrodes
  • Ease of installation
A detailed system design was carried out, with various differences between the two bridges arising from the differing environmental and structural factors at each, as noted in the following sections 2.3 and 2.4.

The final design layout is indicated in the drawings in the Appendix to this paper.

2.3
Environmental factors

Water Levels
The primary environmental factor which influenced the design of the Cathodic Protection was the water level and how it could be expected to fluctuate.

At Brooks Creek, the water flowed into Lake Illawarra, which is relatively stable in level. Flood levels were forecast on the basis of historical records, but in normal conditions the zones could be defined with some confidence.

At Fairy Creek, the level of the lagoon downstream of the bridge was controlled entirely by the ebb and flow of sand on the sand bar at the beach outlet. Generally the sand bar was high enough to dam the lagoon up to near, and sometimes above, the headstock levels. On occasions the actions of wind and tide would cut a deeper channel in the sand bar, and the water level at the bridge would drop significantly as a result. Based on records of prior floods, however, the predicted flood levels were up to one metre above the roadway.

These factors influenced both the zoning of each structure and the choice of anode types. At Brooks Creek, the stable and relatively low water level allowed for well-defined zones:
Atmospheric = Headstock beams full height (760mm)
Splash = 1000mm below headstock soffits
Tidal = 400mm
Submerged & Buried = 6500mm These dimensions permitted the use of optimum anode configurations of
  • ribbon mesh anodes in the headstocks, grouted into slots cut in the sides and soffit of the beams;
  • five discrete anodes in the splash and upper tidal zone of each pile, grouted into holes drilled horizontally into each pile at spacings calculated from the amount of steel;
  • MMO water anodes for the submerged and buried elements.
At Fairy Creek, the water level was generally too close to the headstocks, and too dependent on the vagaries of the sand bar, to permit four zones to be identified. The design was therefore based on the following zones:
Atmospheric = Headstock beams full height (760mm)
Splash and Tidal = 600mm below headstock soffits
Submerged & Buried = 7500mm

This led to a significant departure from the atmospheric zone design used on the other bridge, as it was determined that there was insufficient clearance to install anodes on the headstock soffits. The congestion of stirrups in the beans made discrete anodes inappropriate for the soffit reinforcing, so additional ribbon mesh anodes were specified to be grouted into slots cut at the bottom corners of each beam. Furthermore, there was only space for two discrete anodes to be grouted in each pile above low water.

The submerged and buried anode design concept remained the same for both bridges.

Siltation and Scour
At Fairy Creek Bridge, it was observed that the south abutment was silted up but the north abutment was scoured out by the water flow. About 600mm of rock fill was exposed below the abutment. These factors meant that the soffit of the south abutment could be protected by the buried/submerged anodes, but the exposed heights of piles on the north side required the same ribbon anode protection as on the pier piles. This was only possible on the three accessible sides of each pile.

Protective RC Jackets at top of Piles
At both bridges, there is a protective reinforced concrete jacket extending 1.5m below the headstock on the upstream piles at each pier. These jackets were all specified to be protected by horizontal ribbon anodes, as there was too much congestion of steel to permit discrete anodes to be installed. At Brooks Creek, the predictability of the water levels allowed the five anodes to be separated into two zones, so that current flows in the splash zone could be controlled in detail, and none was in the tidal zone.

At Fairy Creek, there was only room for two anodes but they were treated similarly.

2.4
Structural aspects

There were a number of structural aspects which affected the Cathodic Protection design, and some of them were different at the two sites.

Precast Piles
The precast piles at both bridges were the same size, and a pile offcut was found at the Brooks Creek site, so this was cut open to confirm that it was reinforced without any prestressing. The reinforcement consisted of four vertical corner bars within a cage of stirrups; this suited protection by discrete anodes grouted into holes drilled horizontally into each pile at spacings calculated from the amount of steel.

At Fairy Creek, however, it was found on opening up the cover concrete that the piles were prestressed with 12 strands contained within a spiral stirrup, with a reinforcing cage in the top section of each pile (presumably to resist extra stresses during pile driving). This congestion of steel made it impractical to use discrete anodes as designed for Brooks Creek, and it was decided to use two horizontal ribbon anodes fixed in slots in the cover concrete.

Because there was insufficient cover to the stirrups, and because of the probability of immersion, the ribbon anodes were provided with a minimum of 30mm of cover by building up the grout thickness beyond the original concrete surface, and two coats of a cement-based waterproofing compound were applied to the surface of the grout.

At both bridges, there is a protective reinforced concrete jacket extending 1.5m below the headstock on the upstream piles at each pier. These jackets were all specified to be protected by horizontal ribbon anodes, as there was too much congestion of steel to permit discrete anodes to be installed.

Beam Locating Dowels
Each of the precast deck beams was located on the headstocks by vertical dowels, grouted into the headstock and fitting into the gap between each beam as it was positioned. These dowels were electrically isolated from the rest of the reinforcing steel cage.

At Brooks Creek, these dowels were set in the middle of the rebar cage. Establishing electrical continuity between the dowels and the cage would have caused structural issues, and it was decided that they were sufficiently remote from the anodes and the external chlorides that they could remain not bonded into the Cathodic Protection system.

At Fairy Creek, the dowel detail was different in that one side of each headstock had a cast-in railway iron that supported the deck beam on that side, and on the other was a dowel, with less cover than at Brooks Creek. It was therefore decided that both the railway iron, which had little cover, and the dowels would require bonding into the Cathodic Protection system. This decision was facilitated by the fact that the majority of the beam sides needed repair, which exposed many of the dowels and all of the railway irons.

Extent of Damage to Concrete Elements
At Brooks Creek, there was relatively little repair needed, and no structural implications from that which was required.

At Fairy Creek, however, there was extensive damage to the headstocks and piles, and previous repairs consisted of a mixture of epoxy and cementitious products, many of which had failed. Wollongong City carried out a review of the structural implications, and it was determined that the headstocks could be repaired without restrictions. This was facilitated by the shallow nature of the repairs; a traditional patch repair requires removal of all contaminated concrete around the steel, including otherwise sound concrete from behind the bars, whereas repairs prior to Cathodic Protection only need whatever has spalled or become loose to be removed and replaced. This typically means that a Cathodic Protection repair exposes a part only of the outer bars, and the structure is less invasively threatened.

Some piles at Fairy Creek were badly damaged, including loss of reinforcing steel, and structural repairs were required, as well as restrictions on the number of piles that could be repaired at once. This was particularly so in the northern abutment, where extensive movement of the rock fill had occurred behind the headstock and structural cracks were evident. A detail was developed with Wollongong City to stabilise the rock fill with concrete and stitch the cracks with rebars epoxied into the concrete.

New reinforcing bars were welded in place where the existing steel had lost more than a specified amount of cross-section.

Presence of Previous Concrete Repairs
The Brooks Creek Bridge had no previous concrete repairs, while there was evidence of many repairs to the headstocks at Fairy Creek. Closer inspection revealed most of these to be epoxy resin mortar repairs, and most were delaminated, cracked or otherwise failed. All epoxy mortar repairs had to be removed, irrespective of their condition, because their electrical resistivity was too high for the cathodic protection.

Configuration of Reinforcing Steel
It was found during the construction phase that the existing reinforcing bars, particularly the beam stirrups, were often very variable in both position and cover. This required design checks to ensure that the anode distribution was adequate in all locations. The low cover in some places necessitated the repairs on one beam to be built out beyond the original surface in order to ensure sufficient cover.

2.5
Concrete Repair

In all areas where corrosion damage was evident but no Cathodic Protection was to be installed, the performance specification called for repairs to be excavated at least 25mm behind the reinforcing steel, and the repair was to be extended in plan sufficiently to expose steel that was not actively corroding. The steel was to be cleaned of all corrosion products and treated with a primer before backfilling with high-performance cementitious repair mortar.

In all areas to be protected by Cathodic Protection, the repair specification was to remove all loose, drummy or otherwise unsound concrete, clean the exposed reinforcing steel, add supplementary steel where excessive corrosion loss was identified, and reinstate the repair with the same mortar. No primer was permitted on the steel, as this would interfere with the performance of the Cathodic Protection.

The primary method of concrete removal was to be either by jack-hammers (for small repairs) or by hydrodemolition (for large repairs). Hydrodemolition uses water at very high pressures (in the order of 15,000psi) and significant volumes (around 60 litres per minute) to break up the concrete matrix. This is a standard repair procedure which is both efficient and creates a good surface for bond of the repair mortar.

The methods chosen for reinstating the repairs included forming and pouring, and hand patching. Two different products were used for the two methods.

Curing products were chosen that would not interfere with the ability of the Cathodic Protection system to breathe through the concrete surface.

2.6
Protective Silane

The performance specification called for application of a penetrating silane or suitable coating on areas of the substructures exposed to the atmosphere. As this type of product interferes with the ability of the Cathodic Protection anodes to breathe, the application was restricted to the soffit of the precast deck planks and the sides of the bridges.

A thixotropic cream was selected for environmental reasons, as a liquid silane requires flood coating of the concrete and it was important to prevent pollution of the waterway. This consisted of a water dispersed emulsion of iso-octyltriethoxysilane which is formulated to limit the ingress of chlorides and water into the concrete, and is applied as a gel coat.

3.
CONSTRUCTION

3.1
Environmental aspects

The main environmental consideration for the work at both bridges was the water level.

At Brooks Creek, the level was stable, predictable and at a suitable depth below the bridge for the access platforms to be suspended above the water. This provided a dry work environment and allowed the use of electric power tools under the bridge.

At Fairy Creek, the water was always near the soffit of the beams and arrangements were made with Council to ensure the seaward outlet of the downstream lagoon was kept open during the work. This made the water level predictable but still high and the technicians were obliged to wear waders at all times. Compressed air was required to power all equipment under the bridge, including breakers, concrete saws and drills. It was also necessary to have various environmental precautions such as a screen and boom to catch any spillages, and additional safety precautions such as life rings.

The water level at Fairy Creek also prevented any Cathodic Protection work to be done to the soffit.

The predicted flood water levels at both bridges dictated the positioning of the control and monitoring boxes. Both were fixed to the wing walls end posts, but the flood level at Fairy Creek required the cabinet to be set well off the ground.

3.2
Structural aspects

Continuity of Reinforcing Steel
It is essential to be confident that all the reinforcing to be protected by a Cathodic Protection system is electrically continuous, as stray current corrosion may affect any bars that are not bonded into the system. For this reason, the steel exposed in all repair patches is tested for continuity, and any bars that are found to be discontinuous are connected by welding small bars between them. Where there are no repairs, the element must have holes broken in the surface to expose two remote bars for testing.

The normal situation in reinforced concrete is that there are sufficient points of contact between the bars in a cage for all bars to be continuous. In the case of these two bridges, however, initial testing indicated that the stirrups in the beams in both were not reliably continuous. At Brooks Creek, it was decided to cut a slot along the faces of each beam to expose all the stirrups in order to weld a 6mm continuity bar to the bars for the length of the beam. It was necessary to do this on both faces because the stirrups are in pairs, overlapping in the middle of the beam, and not all were continuous from one side of the beam to the other.

The piles were found to be continuous both internally and from pile to pile via the main longitudinal bars.

At Fairy Creek the extensive repairs made testing simpler but confirmed the lack of continuity more emphatically. It was also found that the piles were not only discontinuous with each other; the individual strands were also discontinuous in each pile. As noted in 2.4, the precast dowels required bonding in, and this was achieved in most cases after they were exposed during the extensive repairs.

A programme was developed for cutting continuity chases in each element to ensure all discontinuities were removed from this structure.

Extent of Concrete Repair Work
Although Brooks Creek Bridge was in relatively good condition, extensive concrete repair was needed in the beams at Fairy Creek Bridge. This necessitated checks by Council to ensure that the beams would not be overstressed during the work.

In places the exposed reinforcing steel was corroded to the extent that additional bars were required. These were welded alongside the original steel.

Two piles at the south abutment at Fairy Creek were extensively damaged by corrosion of the prestressing, and a detail was provided by Council for reinstating the structural capacity of these piles by epoxy grouting additional bars into the concrete. In addition, cracks were identified in the south abutment beam and these were repaired by stitching epoxy-grouted bars across the cracks.

The small amount of repair work at Brooks Creek indicated that the defective concrete was most economically broken out by jackhammers. At Fairy Creek, however, the extensive repairs led to the selection of hydrodemolition as the preferred method, on the grounds of its higher productivity and excellent surface preparation. This method is also more suited to the semi-immersed conditions, as the hydrodemolition operator wears a full dry suit.

Loss of Rockfill behind South Abutment at Fairy Creek
The scour noted in 2.3 above had a structural aspect as well as a Cathodic Protection aspect. It was decided to reinstate the scoured material by rebuilding the rock fill, and then pumping mass concrete behind the abutment. This precaution was made both timely and justified by the opening of a sink hole in the footpath at one end of the abutment.

3.3
Installation and Commissioning

Brooks Creek Bridge
The concrete repair and Cathodic Protection installation work at Brooks Creek was started at the end of March 2009. The Cathodic Protection system was energised on 29 June 2009 and it has been monitored regularly since. All zones have been found to be operating satisfactorily and full protection has been achieved.

Fairy Creek Bridge
The work at Fairy Creek began at the end of June 2009 and the Cathodic Protection system was energised on 17 December 2009. Regular monitoring since then has shown that all zones are operating satisfactorily, with full protection being achieved.

4.
CONCLUSIONS

The Paper describes two bridges, of very similar design and age, which were in very different condition when rehabilitated.

Relatively minor environmental and structural differences between the two had a major impact on the methodologies of the rehabilitation at each site.

Differences in the costs of the work at the two sites were exacerbated by apparent differences in the quality of construction at the more exposed site.

In the author's opinion, there are some conclusions that can be drawn from this, and other similar, experiences:

  • The Design and Construct contract format was effective for these projects, and its value is evident when conditions change from those expected, as the process of changing design details and construction methods can be integrated, costed and optimised relatively quickly.
  • Irrespective of the amount of investigation work that goes into it, the extent of concrete repair work can easily be under-estimated.
  • Continuity of reinforcing steel cannot be assumed and must be thoroughly verified in all elements of a structure that is to receive cathodic protection.
  • Cathodic protection is a long-term rehabilitation strategy that is sufficiently flexible to cope with significant changes in installation requirements.

5.
ACKNOWLEDGEMENTS

This paper was published at the 4th Small Bridges Conference, Melbourne, May 2011.
This technical article has been reprinted from "Corrosion & Materials" Journal, Vol 37 No1, February, 2012, with permission from The Australasian Corrosion Association Inc and the Authors, Infracorr Consulting Pty Ltd

Concrete Mine Thickener Tank Repaired with Hybrid Cathodic Protection

Introduction

In November 2010 Infracorr Consulting (then Ian Godson & Associates) was consulted to provide a repair specification to a 80m diameter coal thickener tank suffering from significant concrete deterioration issues. The 3m high tank at Curragh Coal Mine, located near Blackwater Queensland, was suffering from corrosion of the reinforcement due to chloride contamination from the treatment water in the tank, with severe spalling and delamination especially in the external launder wall. Corrosion surveys also indicated that sections of the main tank walls and the service tunnel beneath the tank were also corroding, with less visible spalling.

Corroded Reinforcement causing concrete spalling in the external launder wall of the tank


Design of Repair

The repair of the tank was to extend the life of the structure for a minimum of 25 years with the main client requirement to minimize downtime of the tank during repair works to a maximum of the programmed shutdown of 2 weeks for other works inside the tank

The corrosion mitigation system chosen was the DuoGuard Hybrid Anode system which uses zinc alloy internal anodes in an impressed current phase (usually 1 -2 weeks) to effectively passivate the steel. At the achievement of set criteria, the power supply and all cables are removed and the anodes are connected in "galvanic mode" at a series of junction boxes, with the galvanic current flowing to maintain the long life corrosion protection. This allows the advantages of Cathodic Protection type protection levels without the high costs of permanent cables and transformer rectifiers with vastly reduced monitoring and maintenance costs.

The largest challenge was the anode design to protect the very thin launder wall section which was only 150mm thick and reinforced with a single matt of reinforcement. To minimize the risk of drilling through the launder tank wall, the short Duoguard 175 anodes, 44mm long , were designed to be installed into 60mm deep holes of 30mm diameter, with controlled depth drilling to eliminate the risk of penetrating to the operating (full ) tank. A dry process gunite overlay of 30mm was then to be placed over the outer surface of the launder wall, providing cover to the anode system.

The works commenced in late January 2011, with hydro-demolition utilized around the clock to maximize the productivity of defective concrete removal which was required to but not behind the bar. The exposed reinforcement was checked for electrical continuity with isolated problems resulting in a 6mm continuity bar being required, tack welded to all vertical bars around the tank. Dry process gunite closely followed the hydro-demolition and continuity work, restoring the concrete to the original profile, with cover to the reinforcement of approximately 20mm. All hydro-demolition and gunite repair was completed during the tank shutdown of two weeks, with the tank re-filled and restored to full operating condition by mid February 2011.

Coal Thickener Tank during 2 week shutdown. Hydro-demolition & dry process gunite progressing around the tank.
Launder wall after hydro-demolition has removed the delaminated and spalling concrete.


The anode installation was completed progressively around the tank. Permanent reference electrodes (MnMnO2) are installed initially with 36 installed in monitoring areas around the tank. The 30mm diameter holes were percussively drilled initially and soaked with water for 24 hours to maximize the moisture in the concrete. The anodes are installed in the holes in the alkaline paste backfill in batches (sub-zones), checked for isolation from the reinforcement and immediately powered by the temporary transformer rectifiers at 12 volts. The impressed current is maintained at 12 Volt for a minimum of 7 days, with the current recorded daily in each sub-zone. A minimum current criterion of 50 KC/m2 reinforcement area is maintained to ensure the reinforcement is completely passivated. Once the impressed current criteria are met, the system is converted into galvanic mode with the reinforcement negative connection connected to the titanium anode connection in the distributed junction boxes (24 No) around the tank. The galvanic current achieved averaged between 2 to 3 mA/m2 reinforcement. Following the installation of the anode system, the 25mm layer of dry process gunite is applied over the entire outer launder wall.

Evaluation of the system performance is assessed against a corrosion rate criterion based on the Stern Geary Equation, with the targeted corrosion rate to be less than 2mA/m2 steel, approximately equivalent to a 2mm section loss in 1000 years. (This criterion is in the latest European Cathodic Protection standard) This is measured by recording the galvanic current of the sub-zone and completing an instant off and depolarization test (similar to impressed current Cathodic Protection) and utilizing these figures to calculate the actual corrosion rate. The corrosion rates calculated by this means ranged from 0.3 to 0.8 mA/m2 steel, well below our criterion.

The overall system was finalized with the installation of a remote monitoring system that was designed to take galvanic current and depolarization readings and update those to an internet database, allowing considerable travel cost and time savings for the monitoring process.

Anodes Installed in Proprietary Paste backfill ad powered at 12V from the temporary transformer rectifiers
Part drawing of hybrid anode arrangement. The launder wall was treated with short (D175) hybrid anodes with an overlay of 25mm gunite to reduce risk of drilling through the wall.


The installation was complete by mid April 2007, a project duration of approximately 10 weeks. The finalized concrete repair incorporated approximately 500m2 of hydro-demolition/surface preparation and 40 tonne of dry process gunite. The hybrid Cathodic Protection system incorporated approximately 9000 anodes (D175 & D350), 36 reference electrodes and the remote monitoring system. The repair works and Hybrid installation was completed by Freyssinet Australia in joint venture with Marine & Civil Maintenance PL. The design, supervision, commissioning and monitoring of the works was completed by Infracorr Consulting.

Completed Tank Concrete Surface showing monitoring and junctionboxe.


Ian Godson & Luke Thompson

Infracorr Consulting Pty Ltd