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A cement and concrete industry publication

Technical Re

Cathodic Protection of Stee Concrete Including Mode l Specif icat ion

8

n T I T U T I O l C O R R O S I O N

cpa 1, T I T U T I O l C O R R O S I O N

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Acknowledgements The Concrete Society is grateful to the following companies and individuals for providing photographs and diagrams for inclusion in the Report:

Andrew Arnold (Cathodic Protection Company Limited) Chris Atkins BAC Corrosion Control Ltd BASF Mash Biagioli (Telpro Limited) John Broomfield Corrosion Control Services Ltd/Freyssinet Kevin Davies Fosroc International Tony Cerrard Careth Class Mott MacDonald Mouchel Adrian Roberts George Sergi Kevin Woodland Brian W y a t t

Published by The Concrete Society

CCIP-054 Published August 2011 ISBN 978-1-904482-65-9 The Concrete Society

The Concrete Society Riverside House, 4 Meadows Business Park, Station Approach, Blackwater, Camberley, Surrey CU17 9AB Tel: +44 (0)1276 607140 Fax: +44 (0)1276 607141 www.concrete.org.uk

CCIP publications are produced by The Concrete Society (www.concrete.org.uk) on behalf of the Cement and Concrete Industry Publications Forum - an industry initiative to publish technical guidance in support of concrete design and construction.

CCIP publications are available from the Concrete Bookshop at www.concretebookshop.com Tel: +44 (0)7004 607777

All advice or information from The Concrete Society is intended for those who will evaluate the significance and limitations of its contents and take responsibility for its use and application. No liability (including that for negligence) for any loss resulting from such advice or information is accepted by The Concrete Society or its subcontractors, suppliers or advisors. Readers should note that publications are subject to revision from time to time and should therefore ensure that they are in possession of the latest version.

Printed by Information Press Ltd. Eynsham, UK.

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Cathodic Protection of Steel in Concrete Including Model specification

Contents

Members of the Working Party v[ List of figures vii List of tables vii[ Introduction ix

1. Background 1 1.1 Introduction to cathodic protection 1_ 1.2 History of cathodic protection of reinforced concrete 4

2. Corrosion of steel in concrete: an overview 8 2.1 Carbonation 8 2.2 Chloride-induced corrosion 9 2.3 Effect of corrosion 10 2.4 Concrete quality 12

3. Survey, investigation, diagnosis and concrete repairs 13 3.1 Survey and investigation 13_ 3.2 Inspection procedures 15_

3.2.1 Visual inspection 15 3.2.2 Delamination survey ]6_ 3.2.3 Covermeter survey 16_ 3.2.4 Depths of carbonation 16 3.2.5 Testing for chloride 17 3.2.6 Assessment of concrete patch repairs 18 3.2.7 Evaluation of reinforcement corrosion 18 3.2.8 Reinforcement electrical continuity 20 3.2.9 Quantification of concrete repairs 21

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3.2.10 Other considerations Z\_ 3.3 Interpretation of survey and investigation results 22 3.4 Repairs to structures 22

3.4.1 Breaking out defective concrete 23 3.4.2 Repair materials 25 3.4.3 Repair methods 26

4. Choice of remedial action for reinforcement corrosion 28 4.1 Minimal intervention 28 4.2 Concrete repair and replacement 28 4.3 Surface treatments 29

4.3.1 Barrier coatings 29 4.3.2 Hydrophobic impregnations 30 4.3.3 Penetrating (migrating) corrosion inhibitors 30

4.4 Electrochemical techniques 31_ 4.5 Design 3J 4.6 Sustainability 32 4.7 Selection of suitable remedial action 32

5. Cathodic protection 33 5.1 Cathodic protection for steel in concrete 33 5.2 Design criteria for cathodic protection for steel in concrete 35

5.2.1 Protection current density 35 5.2.2 Protection criteria 35

5.3 Cathodic protection of prestressed concrete structures 38 5.4 Cathodic protection of buried and immersed structures 39

6. Cathodic protection of new construction 41

7. Cathodic protection anodes 45 7.1 Anodes for atmospherically exposed steel in concrete 45

7.1.1 Impressed current anodes for atmospherically exposed steel in concrete 45 7.1.2 Galvanic anodes for atmospherically exposed steel in concrete 46 7.1.3 Further information 49

7.2 Anodes for reinforcement in buried and submerged concrete 49 7.2.1 Impressed current anodes for buried and submerged structures 49 7.2.2 Galvanic anodes for buried and submerged structures 51_

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8. Cathodic protection equipment and instrumentation 54 8.1 Power supplies 54 8.2 Connections 55 8.3 Monitoring sensors and instrumentation 56_

8.3.1 Reference electrodes 56_ 8.3.2 Macro-cell probes 58_ 8.3.3 Electrical resistance (ER) probes 59_ 8.3.4 Current density assessment 59_ 8.3.5 Monitoring locations 60

8.4 Measurement and monitoring instrumentation 60_ 8.4.1 Portable meters 61_ 8.4.2 Communications and remote control 61_

8.5 Cables 63 8.5.1 Low-voltage DC supply 63_ 8.5.2 Monitoring equipment cable 64 8.5.3 Communications cabling 65_ 8.5.4 Mains AC cabling 65_ 8.5.5 Cable management -trunking 65_ 8.5.6 Junction boxes 66

8.6 Operation and maintenance 67

9. Cathodic protection system design 70 9.1 Design and procurement routes 70 9.2 Personnel 71_ 9.3 Design process 71_ 9.4 Documentation deliverables 75

9.4.1 Design document 75 9.4.2 Design calculation package 75 9.4.3 Construction drawings 76 9.4.4 Material and installation specification or method statements 76

9.5 Design checks 76

10. Installation 77 10.1 Installation contractors, personnel, qualifications and experience 77 10.2 Installation quality management 78 10.3 Concrete repair 78

10.3.1 Removal of damaged concrete and substandard repairs 78

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10.3.2 Low cover repairs 79 10.3.3 Removal of extraneous tie wire/embedded items 79 10.3.4 Electrical continuity of steel reinforcement 79 10.3.5 Methods of bonding 80 10.3.6 Reinstatement of concrete 80

10.4 Installation of system and test negatives 80 10.5 Connection and testing of ancillary steel work 81_ 10.6 Installation of performance monitoring devices 82 10.7 Anode system installation 82 10.8 Electrical installation 83

10.8.1 Installation of cabling and cable management 83 10.8.2 Installation of transformer-rectifier and monitoring equipment 83 10.8.3 Installation of communication services 84 10.8.4 AC installation requirements 84

10.9 Records and documentation 84 10.9.1 Installation and commissioning report 85 10.9.2 Operation and maintenance manual 85 10.9.3 As-built drawings 86

11. Cathodic protection system testing, performance verification and commissioning 87 11.1 Electrical continuity testing: negative and test connections 87 11.2 Anode-to-cathode (steel) isolation and anode system inspection 88

11.2.1 Embedded anodes 88 11.2.2 Surface-mounted anodes 89

11.3 Monitoring system function checks 90 11.4 Commissioning 91_ 11.5 Operation 92

References 93 Further reading Glossary of terms

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ON THE ACCOMPANYING CD

Appendix A - Review of anode materials and systems A1 Conductive organic coatings A2 Sprayed zinc coating A3 Mixed metal oxide coated titanium mesh with electro-catalytic coating A4 Mixed metal oxide coated titanium ribbon A5 Conductive cementitious overlays A6 Probe or discrete anodes

Appendix B - Review of galvanic anode materials and systems B1 Thermal sprayed zinc B2 Thermal sprayed Al-Zn-ln B3 Adhesive zinc sheet B4 Zinc-based jackets B5 Discrete anode arrays in cored holes B6 Discrete anodes in patch repairs B7 Hybrid anodes

Appendix C - Review of impressed current anode materials and systems for buried and submerged reinforced concrete structures C1 General requirements for all anode systems C2 Mixed metal oxide coated titanium C3 High-silicon iron C4 Carbonaceous backfill C5 Magnetite C6 Platinum on titanium, niobium or tantalum substrates

Appendix D - Review of galvanic anode materials and systems for buried and submerged reinforced concrete structures D1 General requirements and features common to all anode systems D2 Magnesium D3 Zinc D4 Aluminium

Appendix E - Model specification for cathodic protection of steel in concrete

Appendix F - Typical items to be included in Bills of Quantities or Activity Schedules for works which include cathodic protection

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Members of the Working Party Full members Chris Atkins John Broomfield Andy Came John Clarke Terry Davies Richard Edwards Tony Gerrard Gareth Class Gareth John Peter McCloskey Vitalis Ngala Jim Preston Paul Segers George Sergi Ali Sharifi Ian Spring Kevin Woodland Brian Wyatt

Mott MacDonald Broomfield Consultants (Chairman from July 2010) Concrete Repairs Ltd The Concrete Society (Secretary) VolkerLaser Corrosion Control Services BAC Corrosion Control Ltd Concrete Preservation Technologies Intertek-CAPCIS Ltd Fosroc Ltd Mouchel Corrosion Prevention Ltd Halcrow Vector Corrosion Technologies Amey Corrosion Prevention Ltd Penspen Limited Corrosion Control (Chairman tojuly 2010)

Corresponding members Paul Chess Chris Clear Kevin Davies Robert Walker

Cathodic Protection International Mineral Products Association CorroCiv Limited URS/Scott Wilson

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List of figures Figure 1 Anode and cathode formation on a corroding reinforcing bar in concrete. Figure 2 Schematic of an impressed current cathodic protection system on an

atmospherically exposed reinforced concrete structure. Figure 3 Schematic of a galvanic anode system. Figure 4 Buried or submerged anodes protect larger areas of steel. Figure 5 Example of completed cathodic protection system on a beam supporting a

motorway viaduct. Figure 6 Risk of steel corrosion with increasing chloride concentration in uncarbonated

concrete in the vicinity of the steel. Figure 7 Corrosion risk for steel in concrete related to chloride content, extent of

carbonation and relative humidity. Figure 8 Expansive corrosion products lead to spalling of the cover concrete. Figure 9 Incipient anode effect - enhanced corrosion adjacent to previous repair. Figure 10 Carbonation testing on freshly broken concrete. Figure 11 Examples of surface coatings applied to repaired structures. Figure 12 Impressed current system. Figure 13 Galvanic anode system. Figure 14 Standard and non-standard decay curves. Figure 15 Cathodic protection using MMO/Ti ribbon anodes cast into the cover

concrete of a new marine structure. Figure 16 Zinc sheet lined with an adhesive containing an activating agent attached to

the soffit of a concrete beam. Figure 17 Compact discrete galvanic anodes installed in a white backfill in the parent

concrete on beams exposed to a marine splash zone after the soffit had been repaired.

Figure 18 A concrete beam coated with a black conductive coating anode with a smaller anode segment used in corrosion rate measurement.

Figure 19 The corrosion rate plotted as a function of potential shift and current density together with an example of its interpretation.

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List of tables Table 1 Relationship between different reference electrodes (mV). Table 2 Characteristics of principal anode types. Table 3 Typical information for sacrificial anode materials.

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Introduction The original Technical Report, Cathodic protection of reinforced concrete^, was prepared by a Working Party of the Corrosion Engineering Association (CEA) in conjunction with The Concrete Society. The CEA was formed between the Institution of Corrosion Science and Technology (now the Institute of Corrosion) with headquarters in Leighton Buzzard, England, and the National Association of Corrosion Engineers (now NACE International) with headquarters in Houston, USA. The Technical Report was published in 1989 at a time when there were no International Standards for cathodic protection of steel in concrete. There were emerging Recommended Practices from NACE, and British Standard BS 7361: Part 1, Cathodic protection - Code of practice for land and marine applications, which was drafted concurrently with the work of the Working Party, included a brief section on this subject.

This edition of the Technical Report exists within a significantly changed framework of International Standards and other technical advice documents, notably BS EN 12696, Cathodic protection of steel in concrete, first published in 2000. This is a performance Standard and offers only superficial design advice; it was not intended as a design manual. There is a nearly identical Australian Standard (AS 2832.5

The report has been considerably revised from its original scope of impressed current cathodic protection of atmospherically exposed concrete to cover both impressed current and galvanic cathodic protection of reinforced concrete that is atmospherically exposed, buried and submerged.

A Model Specification, Concrete Society Technical Report 37

Background 1

1. Background This chapter introduces the principles of cathodic protection and briefly reviews the history of its application in the protection of steel against corrosion in concrete.

1.1 Introduction to cathodic protection

Corrosion occurs by the formation of anodes and cathodes on the reinforcement surface, as shown in Figure 1. Corrosion occurs at the 'anode' while a generally harmless reduction reaction occurs at the 'cathode'.

Figure 1 Anode and cathode formation on a corroding

reinforcing bar in concrete.

Ionic current

JR. ftp . Anode Cathode

There are two types of cathodic protection; impressed current and galvanic (also known as sacrificial) and the principles are relatively straightforward.

By introducing a separate anode and an applied DC electrical current, the electrically continuous steel reinforcement can be forced to become electrically charged (made to become more negatively charged). This application process promotes the 'cathodic' reaction and reduces the 'anodic' reaction, hence the term 'cathodic protection'. In practice, corrosion may not be actually stopped completely, but can be reduced to insignificant levels.

Impressed current cathodic protection (ICCP) comprises an anode system, a DC power supply, monitoring devices, DC wiring and control circuitry. Galvanic anode cathodic protection systems comprise an anode system of more reactive metals (usually aluminium, zinc or magnesium alloy), and in some cases a DC electrical installation and/or a monitoring system.

The choice of anode is one of the key decisions in designing cathodic protection systems.

For existing atmospherically exposed reinforced concrete, the anodes are usually fixed to the concrete surface or embedded in the concrete in order to distribute the protective current evenly to the reinforcing steel. Anodes include: conductive coatings, conductive overlays, metallic spray applied coatings, mixed metal oxide coated titanium (MMO/Ti) mesh or ribbons in a concrete overlay, MMO/Ti ribbons in grouted slots generally applied to the concrete surface, or various discrete or probe anode arrays grouted into predrilled or cored holes in the concrete. For new structures the anodes may be cast into the concrete at the time of construction.

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1 Background

Figure 2 Schematic of an impressed current cathodic

protection system on an atmospherically exposed reinforced concrete structure.

Monitoring system

DC power supply

Anode Ionic current flow from anode

- Reference electrodes

A schematic of an impressed current system is shown in Figure 2.

The principles of galvanic cathodic protection are the same as for impressed current cathodic protection, except that the anode is a more reactive metal (i.e. one that corrodes more readily) than the steel to be protected. When connected electrically to steel, the more reactive metal is consumed preferentially in a corrosive environment. This generates the cathodic protection current flow due to the electrical potential difference between the anode and cathode. The current flow is a function of difference in the potentials of the anode and cathode materials and the circuit resistance which in turn is dependent on environmental conditions.

Figure 3 illustrates a galvanic cathodic protection system.

Figure 3 Schematic of a galvanic anode system.

Galvanic Anode M M n + + ne'

Ionic current flow from anode

t t X t H 2 0 + a0 2 +2e '^20H" f > - - - >

~\i i * ^Ti , . . Concrete

Galvanic anodes are well proven for applications to buried or submerged steel and reinforced concrete structures. In some conditions they can be applied to atmospherically exposed reinforced concrete structures, particularly in marine exposure conditions where the moisture and chloride levels help to keep the anodes active and the circuit resistance low.

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Background 1

Figure 4 Buried or submerged anodes protect larger

areas of steel. Current flow Buried or submerged anode

If the concrete is in a conductive medium such as damp soil or seawater, it is possible to use a 'remote' anode to protect a large area of reinforcement as illustrated in Figure 4. This can either be an impressed current or galvanic system.

The wide range of anodes for cathodic protection of steel in concrete is discussed in more detail in Chapter 7 and the appendices.

As a consequence of the current applied during cathodic protection, the potential of the steel is changed or 'polarised', usually made to become more negatively charged. The potential of the steel at the concrete interface can be measured with respect to a stable independent reference. Reference electrodes are used for this purpose. These monitoring devices define a reference point, which is unaffected by the application of the cathodic protection, against which the potential of the steel can be measured. When cathodic protection is applied to the steel, the resultant potential changes can be measured and used to determine the effectiveness of the cathodic protection using known parameters.

There are many types of reference electrodes. Some are designed for surveys or inspections where they are placed temporarily on the concrete surface in grids; the measurements are used to grid equipotential contour plots. Others are embedded within the concrete permanently and are used for long-term monitoring of the steel potentials at fixed locations.

Details of reference electrodes are given in Section 8.3.1 and in the Glossary of terms. The most common types used are silver/silver chloride/potassium chloride (Ag/AgCl/0.5M KCl), manganese/manganese dioxide/sodium hydroxide (Mn/Mn02/NaOH) and copper/copper sulfate (Cu/CuS04).

For Ag/AgCl/0.5M KCl electrodes their potential is dependent on the concentration of chloride at the electrode element. A range of different concentrations has been used, typically 0.5,1.0 or 3.5M.The difference between the different electrode types is shown in Table 1.

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1 Background

Table 1 Relationship between different reference

electrodes (mV).

In this report all potentials will be referred to the most commonly used Ag/AgCl/0.5M KCl reference, in line with the requirements of BS EN 12696, unless otherwise stated. The use of other electrolyte concentrations of other reference electrodes requires conversion using the offsets shown in Table 1.

Ag/AgCl/seawater electrodes, very widely used for structures immersed in seawater, have a potential very similar to that of Ag/AgCl/0.5M KCl (5mV); the potential varies with salinity. Ag/AgCl/0.5M KCl electrodes are sometimes used immersed in estuarine waters.

Due to the significant risk of CuS0 4 leakage, through the normal wooden or ceramic porous plugs onto the concrete and the resultant significant errors in measured steel potential, the use of simple single-junction Cu/CuS04 (sat) electrodes is not recommended for either permanent installation into concrete or portable surveying of steel in concrete even though this is still sometimes specified.

1.2 History of cathodic protection of reinforced

concrete

Cathodic protection of metals in seawater has been practised since 1824 (see Davy'9'), and during the past 70 years it has been used extensively and successfully for the protection of steel in water and soil environments. The earliest applications of cathodic protection to reinforced concrete were to prestressed concrete water pipelines - see for example Unz ( 10 ) and Heuze(11) - with reported applications before 1955 to buried reinforced concrete water tanks, steel reinforcement and linings of nuclear reactor containment vessels and concrete-coated piling (see Vrable (12)). Most of the early applications relate to reinforced concrete buried in soils. Such applications allowed the use of conventional buried pipeline cathodic protection design principles and anode systems.

The first major step towards cathodic protection of atmospherically exposed reinforced concrete occurred in the USA as early as 1959 when Stratfull'13' applied a trial impressed current cathodic protection system to bridge beams and pile caps on the seven mile long San Mateo-Hayward Bridge in San Francisco Bay.

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Background 1

Between the first full installation in 1973 (see Stratfull) and 1989 a total of Z87 systems were installed on US interstate highway bridges (see Broomfield115!), predominantly on bridge decks suffering from de-icing salt attack. Many more systems were applied to other structures as well as to other bridges.

The early systems used simple high-silicon cast iron anodes in an asphalt overlay made conductive by the addition of carbon particles. In the period 1973-1980 some 35 of these systems were installed and many were reported as still operating satisfactorily in 1983-1985 (see Stratfull116'). A variation of these conductive overlay systems, with added sand and stone aggregates to improve the mechanical properties, became a standard repair option for the Canadian Province of Ontario which had installed some 40 systems by 1987, as reported by Schell ef a/.(17).

One of the problems with this particular cathodic protection anode system was that North American bridge decks were not originally designed for overlays. There was therefore frequently a preference that the anode system did not change the profile of the bridge or increase the dead load. To overcome this requirement an anode system was developed in which the anodes were placed into slots cut into the deck. More modern anode systems are now lightweight titanium based with mixed metal oxide coatings, either as ribbon in slots or as mesh under overlays.

From the initial use of heavy, awkward to handle and install, high-silicon cast iron anodes in an asphalt overlay for bridge decks, there was a rapid proliferation of impressed current anodes suitable for decks, substructures and buildings as described in Section 7.1.

Millions of square metres of cathodic protection systems have been applied to North American (see Broomfield and Wyatt ( 1 8 )), European and Middle Eastern structures and buildings, including: bridge decks bridge substructures car parking structures wharves, etc. buildings (particularly on the Florida coast).

The first trial and full-scale cathodic protection systems in the UK and Australia were undertaken in the mid- to late 1980s (see Broomfield ef a/.(19)), on buildings suffering from the deliberate addition of calcium chloride as a set accelerator, on a jetty subject to marine exposure, a cement works subject to sea salt contamination and on highway bridge substructures suffering from road de-icing salt contamination (see Irvine and Wyattt2 0'). The predominant anode systems in the late 1980s to early 1990s in the UK were conductive organic coatings.

Figure 5 shows a completed mesh and overlay cathodic protection system on a beam supporting a six-lane motorway viaduct in the UK. Corrosion control was necessary due to salt contamination of the beam due to de-icing works on the carriageways above.

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1 Background

Figure 5 Example of completed cathodic protection system on a beam supporting a motorway

viaduct.

In 1989, Rasheeduzzafar eta/.(21) estimated that up to 74% of reinforced concrete structures in the Middle East were showing significant corrosion damage within 10-15 years from construction, due to the prevalence of salt in the soil, air and water and cast into the concrete. Many structures were faced with having to be rebuilt after as little as ten years unless extensive rehabilitation or repair was carried out.

There are several individual systems with anode areas over 20,000m2, particularly in the Middle East where the total area of anode installed is believed to be between 1 and 2 million m 2 of concrete surface with approximately 200,000m2 per year being installed.

A concern with the application of cathodic protection to prestressed concrete structures has been that if the steel potential was made to exceed the hydrogen evolution potential, then hydrogen embrittlement could theoretically ensue. This is a major concern for certain susceptible high-strength steels, particularly those used in prestressing where tendon failure could lead to catastrophic failure. The problem is that monatomic hydrogen is generated at the steel if the potential exceeds the hydrogen evolution potential. The hydrogen can diffuse into the steel and become trapped at grain boundaries in certain types of high-tensile pre- and post-tensioning steels. This can lead to weakening of the steel and failure.

One of the earliest applications of cathodic protection to concrete was to prestressed concrete water pipes in the early 1950s (see Unz ( , 0 ) and Heuze(11)). Significant work was done in the 1990s in the USA on the application of impressed current cathodic protection to prestressed concrete structures, both pre-tensioned and post-tensioned.This led to the NACE State-of-the-art report: Criteria for cathodic protection of prestressed concrete structures^ which gives guidance on how to 'qualify' a prestressed concrete structure according to the susceptibility of the steel to hydrogen embrittlement.

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Background 1

The technique known as 'cathodic prevention' was developed in Italy to apply cathodic protection in new prestressed concrete bridges (see Pedeferri'23'). The application of cathodic protection to uncorroded prestressing steel before chlorides have initiated corrosion has a lower risk than applying it to tensioned steel that may be pitted. Missouri Department of Transport applied impressed current cathodic protection to dozens of segmental prestressed bridges but only the conventional reinforced concrete top of the box section was protected (see Girard'24').

It is now reasonably well established that cathodic protection of normal reinforcing steels, designed and operated to the International Standards now published and as described later in this report, presents little risk of hydrogen embrittlement. Cathodic protection of prestressed elements requires particular care and rigour in design and operation.

Also the risk of reduction in the steel-concrete bond strength was raised in early research (see Locke (25)). It has been established that this is not a risk at normal cathodic protection current densities.

A further concern was the risk of inducing alkali-silica reaction (ASR) (see Sergi and Page ( 2 6 )). However, this problem has not been observed on any of the structures treated with ICCP and known to have susceptible or potentially susceptible aggregates.

In recent years, very significant projects of new construction, notably of marine bridges, basement car parks, industrial plant and water conveyances, have incorporated cathodic prevention as a corrosion preventative technique. This is discussed further in Chapter 6.

As well as being applied to virtually every type of reinforced concrete structure susceptible to reinforcement corrosion, more recently, ICCP has also been applied to early 20th century steel-framed buildings (see Cibbs'27'), although this application is not covered in this report.

Galvanic cathodic protection was first applied to atmospherically exposed concrete in the form of thermal sprayed zinc. This was initially developed as an impressed current anode by Stratfull's successors at California Department of Transportation (see Apostolos ef a/. (28)). It was taken up by Florida Department of Transportation to treat marine bridge piles in the splash and tidal zone. Florida Department of Transportation also developed an anode system consisting of an expanded zinc mesh grouted into a permanent formwork installed on columns in marine splash zones (see Leng ef a/. (29)). In the UK, Page and Sergi, then at Aston University, developed the embedded galvanic anode for concrete repair'30'. There are other galvanic systems, such as surface-applied zinc foil with conductive adhesive gel, metallised thermal applied zinc and aluminium alloys and discrete tubular anodes, which can be operated as 'hybrid' cathodic protection systems. Such systems, which are galvanic in nature but can be initially and periodically energised throughout the service life as an impressed current anode, have been developed and are currently undergoing trials and initial installations (see Glass eta/.'31' and Segers and Gerrard (32)).

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2 Corrosion of steel in concrete: an overview

2. Corrosion of steel in concrete: an overview Normally steel embedded in sound, uncontaminated concrete is protected against corrosion. This is because of the formation of a stable, protective, iron oxide film on the steel surface due to the high alkalinity (typically pH > 12) of the concrete. This protective (passive) film ensures that the corrosion rate is essentially negligible (at less than 0.1um/year).

Unfortunately, this condition can be, and often is, disrupted. In practice, the two most common causes of corrosion of steel in reinforced concrete are carbonation and contami-nation by chlorides.

2.1 Carbonation As well as being alkaline, concrete is also a microporous material and the hydrated cement matrix is open to gaseous diffusion through its continuous pore structure. Carbon dioxide in the air can react with small amounts of moisture, resulting in the formation of a weak carbonic acid. This allows acid-base reactions to occur within these micropores, leading to the neutralisation of the alkaline phases in the concrete. This process is termed carbonation.

When the pH in the concrete at the reinforcing steel depth drops below about pH 10, the protective passive oxide film is no longer stable, thus creating conditions for corrosion to initiate. The extent and rate of subsequent corrosion will depend on the ease with which the anodic and cathodic reactions can progress (see Section 1.1). The presence of moisture in the carbonated zone is essential for corrosion of the steel to occur - see for example Sergi and Dunster'33'.

The rate of carbonation is dependent on the rate of gaseous diffusion (within the concrete pores) and the quantity of alkali in the cement paste matrix. Thus a more permeable microstructure will be more vulnerable to carbonation than one which is essentially impermeable and devoid of liquid water and/or where the pores are water filled. Carbonation will proceed most rapidly where the atmospheric relative humidity is 60-70%.

For any concrete, provided the exposure conditions are generally constant, the rate of carbonation, as measured by the depth (x) with time (t) follows an approximate relationship of:

x = k\lt

where k is a constant dependent on the concrete quality and the exposure conditions.

The depth of carbonation is determined by spraying phenolphthalein solution on a freshly fractured piece of concrete. A pink/purple colour shows the concrete is still alkaline (pH > 9.5), with the carbonated area colourless - see BS EN 14630'34'. Hence, provided that the depth of carbonation is measured after a known exposure period (see Section 3.2.4 and Figure 10), the time for the carbonation to reach the depth of the reinforcing steel can be estimated.

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Carbonation has been a major problem of corrosion damage to reinforced concrete structures (mainly buildings) constructed in the 1960s and 1970s, where low depth of cover (~25mm) and high water/cement (w/c) ratios (which led to high porosity) were used.

Durability requirements in modern design codes are intended to ensure that reinforced concrete components are designed and constructed so that the process of carbonation should not extend to the full depth of cover within the anticipated life of the structure. However, misplacement of the reinforcement relative to the concrete surface, inappropriate grading of the aggregates, low cement/aggregate ratio, high w/c ratio, construction defects and inadequate compaction or curing can all lead to situations where the finished concrete is more vulnerable to carbonation than anticipated.

There are several sources from which, and ways in which, chlorides can get into concrete. In older concrete structures, chlorides were sometimes cast into the mix through constituents such as calcium chloride based set accelerators. This practice was stopped in the UK by around the mid-1970s. The use of poorly processed marine aggregates has also contributed to chloride being cast into the concrete and it is likely that many such chloride-containing structures still remain in use. Chloride levels for concrete mixes are normally strictly controlled, for example in the UK by BS 8500.

More commonly now, chloride ingress into concrete arises from exposure to saline ground-water, seawater or from de-icing salts applied to highway structures during the winter months. Chloride ingress from an external source into the cover concrete can be in the form of: absorption, particularly following a dry period diffusion through the water-filled pores wick action if the concrete is partially immersed in chloride-contaminated water and

drying of the concrete occurs at a higher level capillary action through capillary pore networks.

The actual role of chlorides in corrosion of steel in concrete is complex and is still subject to research, but its effect is undisputed and widespread. The chloride in the pore water within the concrete destabilises the passive iron oxide film, allowing corrosion to proceed which is often localised in nature.

The concentration of chloride required to initiate corrosion is a subject of much debate, and is, among other parameters, dependent on the concrete pH/cement content and is normally expressed (in Europe) as percentage with respect to mass of cement within the concrete. Concrete Society Technical Report 60, Electrochemical tests for reinforcement corrosion^, presents regions of risk associated with chloride contamination. At low levels (

2 Corrosion of steel in concrete: an overview

Chloride Ion Concentration (% by weight of cement)

Another feature of chloride-induced corrosion is that acid is produced at the site of corrosion initiation - see Sergi and Glass'38'. Local pH values below 5 have been reported at corroding areas (areas of pitting corrosion) in what is otherwise a very alkaline concrete environment - see Bertolini eta/.'39'. The effect of acidification is not clear in the literature on corrosion of steel in concrete because chloride-induced corrosion is distinguished from carbonation-induced corrosion with the observation that chloride-induced corrosion occurs despite the high pH of the concrete cover. However, a local pH reduction at a site of pitting corrosion is regarded as an essential requirement for chloride-induced corrosion damage - see Szklarska-Smialowska'40'.

Reinforced concrete components are now more likely to be designed and constructed to codes intended to ensure that the diffusion rate of chlorides through the concrete is slow. Such durable concrete can be predicted from laboratory determinations to have decades of predicted life before initiation of corrosion. However, as noted above in the case of carbonation, misplacement of the reinforcement relative to the concrete surface, inappro-priate grading of the aggregates, low cement/aggregate ratio, high w/c ratio, construction defects and inadequate compaction or curing can all lead to situations where the finished concrete is more vulnerable to chloride-related corrosion than anticipated and the time to corrosion damage can be significantly less than that predicted by laboratory-determined diffusion characteristics.

2.3 Effect of Corrosion The interrelationship between chloride content, concrete quality, concrete pH (i.e. carbonated or uncarbonated), relative humidity and overall corrosion risk to reinforcing steel, is shown in Figure 7, which is based on Figure 12.4 of Durable concrete structures^.

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Figure 7 Corrosion risk for steel in concrete related to chloride content, extent of carbonation and

relative humidity.

l i i 50 85 100

(Low corrosion (High corrosion risk) (Low corrosion risk; electrolytic risk; lack of oxygen) process impeded)

RH: %

As reinforcing steel corrodes, the corrosion products (rust) continue to react with oxygen to form hydrated oxides with many times the volume of the steel consumed. This applies an expansive bursting force on the cover concrete, leading to the cracking and spalling of concrete usually associated with atmospherically exposed reinforced concrete structures (see Figure 8). However, in some conditions, the corrosion products can stay in solution and bars can corrode away with no visible evidence at the concrete surface. This is sometimes known as 'black rust', though in practice these corrosion products can also be green.

Figure 8 Expansive corrosion products lead to spalling

of the cover concrete.

Where concrete is fully submerged in water, oxygen transport through water-saturated pores is restricted, resulting in a lower rate of corrosion that will eventually cease. This is typically accompanied by the lowering of the potential of the steel to more negative values.

In cases where parts of the reinforced concrete structure are water saturated and parts are exposed to air, the different oxygen levels create a galvanic cell where the available oxygen (cathode) drives corrosion at the low oxygen (anode) area, leading to 'macro-cell' conditions. Where the cathodic (oxygen reduction) reaction occurs over a much larger surface area than the anodic (metal oxidation) reaction, increased localised corrosion can occur.

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Figure 9 Incipient anode effect - enhanced corrosion

adjacent to previous repair.

The repair of only those sites which are actively corroding in a chloride-contaminated structure is likely to stimulate corrosion at sites adjacent to the repair. This phenomenon is known as the incipient anode, ring anode or halo effect (see Figure 9). Therefore, the repair of chloride-induced corrosion damaged concrete poses a complex electrochemical problem and although repairs require physical intervention, ongoing corrosion can be resolved by the control of the potential of the reinforcement by cathodic protection.

size. These are present at the interface between the steel surface and the concrete. A reduction of the level of this natural voidage within the concrete reduces the probability of corrosion initiation significantly, at a particular level of chloride contamination. An increased level of voids from poorly compacted concrete, honeycombing, low cement content, porous aggregates, etc., leads to a condition where the corrosion process can be initiated readily and the corrosion rate is generally higher.

For the UK there is comprehensive general guidance in BS 8500 ( 3 5 ) on the quality of concrete and nominal cover recommended to resist corrosion induced by carbonation and chlorides, for intended working lives of at least 50 or 100 years. Additional guidance is available in BS 6349

Survey, investigation, diagnosis and concrete repairs 3

3. Survey, investigation, diagnosis and concrete repairs Before undertaking any repair and refurbishment it is important to clarify and define: the structure's current condition and history the cause, or causes, of deterioration and distress the extent of deterioration and distress any limiting parameters in terms of repair and future maintenance, defined by the

client, the client's representatives or others (e.g. intended residual life of the structure, expected condition at the end of the period)

a select list of repair options, appropriate for the specific structure, client, client's representatives or others

any specific requirements for a chosen repair option any possible side effects of a chosen repair option.

Although the following procedures are relatively simple to carry out, their importance, in terms of ensuring the successful completion of a concrete repair and rehabilitation project, including its completion to programme and to budget, cannot be underestimated. These tasks should not therefore be delegated to inexperienced personnel; they should be undertaken by personnel experienced both in the survey and investigation techniques concerned and the requirements of the concrete repair and rehabilitation strategies to be employed. The survey and investigation are only one aspect of the overall process. While it is difficult to give guidance on how much money and other resources should be spent on investigation, it should be noted that access to the structure is usually a major factor in the costs of investigation, so ensuring that all relevant information is gathered at one time is usually of vital importance. If key information is missing from the information provided to bidding contractors, then they will price their perceived risk into their bids or limit their risk in the terms they are willing to agree with the structure owner.

3.1 Survey and investigation Most elements of the infrastructure are inspected routinely. In some cases, such as highway and rail structures, this is mandated by the structures' owners, who are typically government backed. In the nuclear industry this is mandated in the site licensing conditions, and in the case of reservoirs this is mandated by law. In other cases, such as buildings and car park structures, routine inspections are desirable, but often are aimed at checking that items such as drainage and lighting are adequately functioning rather than assessing the condition of the structural elements. This may be further aggravated by changes of owner-ship throughout the life of the structure. Adequate records are often not handed over.

The basic procedure most commonly adopted is to use routine visual inspection by experienced personnel. This is often an efficient method of gathering data but may be complicated by the types of structures involved. Transport structures are normally large and span difficult terrain. Access for a visual inspection is therefore often difficult. In the water industry the structures may contain sewage or strategic services and will only be available for inspection during outages. There can be similar problems in the process industries.

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Often access will be required in order to carry out survey and investigation works to the structure prior to carrying out the detailed design. The type of access required will vary considerably and will largely depend on the type of structure. For example, access requirements for a multi-storey car park may be limited to lightweight aluminium scaffold towers, whereas large bridges may require abseiling techniques or the use of mobile elevating work platforms and, almost certainly, lane closures. It may not be necessary at the initial stage to provide 100% coverage to all of the structure.

AH responsible structure owners should undertake routine inspections and should store the information in an accessible place and format.

Reinforced concrete structures can deteriorate due to any number of reasons, but it is important to understand that cathodic protection is only appropriate for addressing deterioration caused by corrosion of the reinforcement (or other embedded metallic objects).

Therefore, the techniques employed for the inspection and diagnosis should generally be confined to those designed to evaluate the extent and possible causes of that corrosion, together with collecting information that will be beneficial when deciding upon future remedial actions and maintenance strategies.

However, it is important always to approach any structure with an open mind. Therefore, the concrete, together with any other associated materials, should also be closely inspected and the exposure conditions assessed in order to identify any distress not consistent with corrosion of the reinforcement and, therefore, requiring further investigation and additional testing.

Cracking and surface spalling of the concrete, associated with rust staining and/or pitting of the reinforcement, are all obvious indications of corrosion. However, it should also be noted that not all corrosion leads to the formation of expansive 'rust' products and that corrosion can take place (even to an advanced stage) without the usual symptoms of cracking and surface spalling. This is particularly so with chloride-induced corrosion in an anaerobic environment, so visual inspection alone may not identify the full extent of the problem.

Having established that reinforcement corrosion is the primary cause of deterioration and distress, the investigations detailed in the following sections should be considered in order to evaluate both the form and extent of work required before a cathodic protection system can be applied and also the scale and type of cathodic protection system required. Any such list is inherently generic. Specific structures may have specific requirements and as new techniques, for both evaluating and repairing concrete structures, become available, the list should be revised. However, at present, the following guidance should generally be considered as a minimum necessary for any structure.

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Guidance on the selection of particular procedures, the interpretation of test results and the selection of remedial works can be found in Concrete Society Technical Report 60, Electrochemical tests for reinforcement corrosion6*, BRE Digest 444 ( 3 7 ), Concrete Society Technical Report 54, Diagnosis of deterioration in concrete structures^, Concrete Bridge Development Group Technical Guide 2 ( 4 4 ) , and other publications as follows. a) For assessing concrete material condition, diagnosis and the extent of concrete repairs:

visual inspection delamination survey covermeter survey testing for carbonation, see BS EN 14630 analysing for chloride, see BS EN 14629 petrographic analysis, see Concrete Society Technical Report 71, Concrete

petrography^^.

b) For evaluating the extent of reinforcement corrosion: potential survey, at least of the area(s) to be protected corrosion rate measurements concrete resistivity measurements exploratory breaking out and inspection of the reinforcement.

c) For aiding the assessment of cathodic protection system requirements: surface-exposed metallic items, e.g. tie wires reinforcement, and other steel embedment, continuity measurements identification of previous patch repairs.

In addition to these investigations, the following factors should also be considered: loss of reinforcement cross-section which will require small breakouts load-bearing capacity measurement of concrete surface area calculation of steel surface area to be protected possible stray current interference power availability and communications access requirements.

3.2 Inspection procedures General information may be found in Concrete Bridge Development Group Technical Guide 2, Guide to testing and monitoring the durability of concrete structures^ and in Concrete Society Technical Report 60, Electrochemical tests for reinforcement corrosion^.

3.2.1 Visual inspecti On The structure, and particularly those areas to be protected, should be subjected to full, close quarters, 'tactile', visual inspection, with all defects and deterioration identified. The location, form and detail of any previously installed repairs should be noted and included within any subsequent investigations. Although this is the simplest of the procedures, its importance should not be underestimated.

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3.2.2 Delamination Survey In its simplest form the concrete surfaces should be subjected fully to sounding using a hammer drawn over the concrete surfaces and used to lightly tap the concrete in order to identify loose, hollow, delaminated and/or spalling areas (including latent or incipient spalling). However, other techniques including chain dragging, radar, thermography, ultrasound and impact echo may be appropriate in some circumstances.

It should be noted that all delamination should be identified and would normally be repaired prior to installation of cathodic protection.

3.2.3 Coverrneter Survey The concrete surfaces should be subjected to covermeter surveying using a proprietary instrument, in accordance with the manufacturer's instructions and in general accordance with Part 204 of BS 1881

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Figure 10 Carbonation testing on freshly broken

concrete. Note: Concrete is carbonated to approximately

6mm in this example.

3.2.5 Testing for Chloride At selected locations drilled concrete dust samples should be prepared using a rotary-percussive drill and masonry bit in general accordance with recommendations detailed in BS EN 14629

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3.2.6 Assessment of concrete patch repairs

In circumstances where previously installed patch repairs are noted during the visual inspection, it is important not only to assess their condition but also to assess the causes of deterioration resulting in the distress that was repaired and the materials and practice employed. Such investigations should enable a diagnosis of the original defect.

The sufficiency of the existing repairs should be assessed along with its compatibility with cathodic protection. The effect of varying electrical resistivity between the repair material and the original concrete should be considered as discussed in Section 3.4.2.

The absence or presence of insulating coatings on the repair and their effect on the cathodic protection system should be considered by the designer.

In the absence of such investigations, existing repairs may require removal and reinstatement with cathodic protection compatible material, as the original repairs may have been carried out using incompatible materials.

3.2.7 Evaluation of reinforcement corrosion

General Prior to removing any concrete, the structural impact of the removal should be assessed.

The risk and rate of reinforcement corrosion can be assessed using the measurement of potential, resistivity and polarisation resistance, in general accordance with Concrete Society Technical Report 60, Electrochemical tests for reinforcement corrosion6* and ASTM C876(48>.

The general condition of the reinforcement should be assessed by direct visual inspection of the steel at selected cut-out and/or spalled locations. The selection of inspection locations should be aided by observations made during visual inspection and hammer testing and also from the results of the potential and resistivity measurements.

Potential measurements (half-cell) Measurement of the corrosion potential of the reinforcement can provide information as to whether the steel is passive or corroding. This is also referred to as half-cell potential measurement.

All measurements should be carried out using a calibrated reference electrode, typically a silver/silver chloride/potassium chloride (Ag/AgCl/0.5M KCl) reference electrode; the potassium chloride concentration will be stated with the reference electrode and must be recorded. Copper/copper sulfate (Cu/CuS04 (sat)) reference electrodes should not be used for surveying steel potentials as significant errors can arise if copper sulfate leaches through the porous membrane (often a wooden plug) and contacts the concrete.

The electrical continuity of the reinforcement across the area to be surveyed should be checked and electrical resistance measured. Connections to the reinforcement, suitable for the areas of continuity, should then be made. This is discussed in the Section 3.2.8.

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Measurement node points, arranged in a nominal 300-500mm grid, should be treated with an appropriate wetting agent and the potential values then be recorded. (The size of the grid could be modified depending upon the dimensions of the elements to be surveyed.)

The potential values should be tabulated and can then be used to plot colour-coded, iso-potential, contour maps.

The interpretation of the results, in terms of prescribing levels of corrosion with recorded potential values, in isolation, needs very careful consideration, especially on surfaces where there may be water saturation or restricted oxygen access. On such surfaces the measurement of negative potentials, which according to the various standards would suggest significant corrosion, would be quite normal, but with no significant corrosion necessarily occurring. The magnitude of potential gradients is probably as important as the level of measured potential. It is therefore sometimes more appropriate to use a reference electrode in a fixed position rather than a connection to the reinforcement, in conjunction with a reference electrode being moved across the surface of the concrete. This is known as the dual half-cell technique - see Concrete Society Technical Report 60, Electrochemical tests for reinforcement corrosion6*.

For obtaining potential readings across the surface of a structure, in addition to 'single' reference electrodes, proprietary systems incorporating either a number of reference electrodes connected together or a 'wheel' system all connected to a suitable data logger are also available. These allow a large amount of data to be obtained in a relatively short period but do require generally free access to the concrete surface. Generally this approach is not suitable for use in monitoring the performance of cathodic protection systems but is of considerable value in assessing the extent and causes of corrosion prior to the installation of cathodic protection. The measurement of steel potentials using portable electrodes may not be possible with surface-applied anode systems unless particular provisions have been provided to enable such measurements.

Electrical resistivity Bulk concrete electrical resistivity determinations are not commonly used. Where they are required they should be carried out using either a Wenner-type, four-electrode resistivity meter or a two-probe meter, the latter generally with the electrodes inserted into drilled holes in the concrete surfaces.

To ensure no distortion of readings due to the proximity of embedded steel, measurements should not be made directly above reinforcing bars. To minimise errors due to varying aggregate distribution, the array spacing should be greater than the maximum aggregate size, and readings should be taken at a number of points in a given area. Concretes coated with low-permeability surface treatments, or containing additives such as fly ash, ground granulated blastfurnace slag or silica fume, may give very high electrical resistivity values.

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Corrosion rate The corrosion rate of the steel can be estimated using the polarisation resistance or linear polarisation technique (LPR). This technique uses a reference electrode and an auxiliary electrode. The steel potential is measured and then a small current is passed from the auxiliary electrode to the steel, shifting the potential. The corrosion rate is proportional to the current applied divided by the potential shift.

The measurement and interpretation of corrosion rates by the LPR technique is relatively slow compared to the potential and other surface measurements. It requires specialist equipment and is usually done by specialists. It can be useful in distinguishing between high corrosion rates and saturated concrete which can be indistinguishable in potential measurements alone and can be used to estimate steel section loss rates and time to cracking and spalling, if the time of corrosion initiation can be adequately calculated - see Concrete Society Technical Report 60, Electrochemical tests for reinforcement corrosion6*.

Site-specific validation As described above, the reinforcement within selected 'active' or anodic and 'passive' or cathodic areas, as determined using steel potential and possibly concrete resistivity measurements, should be exposed and inspected for evidence of deterioration and the extent of any corrosion.

3.2.8 Reinforcement electrical continuity

In order for a cathodic protection system to work efficiently, the reinforcement must be electrically continuous. Testing must be carried out in areas where the reinforcement has been exposed previously by spalling concrete or in areas of steel purposely exposed for testing. The leads must be attached to clean bright steel to identify the level of electrical continuity.

Electrical continuity can be checked by a number of methods: measuring the resistance between bars using a resistance meter measuring the potential difference between bars using a high-impedance voltmeter measuring the steel potential at a remote location using the steel connection at the

two test points.

For the electrical resistance testing method, bar-to-bar resistances of 1Q or less are generally considered to indicate adequate electrical continuity. The polarity of the measurement should be reversed and the two measurements averaged, as otherwise the likely potential difference between the locations of the two connections to the reinforcement will result in errors in most DC measurement circuits.

For the potential difference testing, a potential difference less than ImV generally indicates electrical continuity - see BS EN 12696

Survey, investigation, diagnosis and concrete repairs 3

Major issues can occur in the case of fusion-bonded epoxy-coated reinforcement where every bar should be checked and made continuous, particularly where impressed current cathodic protection is being applied.

3.2.9 Quantification Of The extent and depth of the concrete repairs should be assessed, based on the delamination, Concrete repairs visual and cover surveys and exposure at representative spalled or cracked locations. It

should be noted that surveys often underestimate the actual quantity of repair required, and this can be increased further if there is a significant delay between the survey and the start of repair work.

3.2.10 Other considerations Loss of reinforcement section The advice of a structural engineer should be sought to confirm the integrity of the structure if there are any indications of significant loss of reinforcement cross-section. Cathodic protection will not restore lost metal. In general, structural repairs should be carried out prior to the installation of cathodic protection. It is important to ensure electrical continuity between 'old' and any 'new' reinforcement.

Measurement of concrete surface area For the accurate quantification and costing of cathodic protection system components, the form and area of the concrete surfaces to be protected should be ascertained.

The age of the structure and any history of repairs and modification should be recorded.

Calculation of steel surface area to be protected The actual 'as-built' rather than 'designed' surface area of steel should be determined, together with the position of the reinforcement and its size. This could be undertaken as a part, or extension, of the covermeter survey described in Section 3.2.3. The surface area of the steel generally forms the basis for the cathodic protection design calculations.

Possible stray current interference The presence of other electrical systems in the vicinity of a structure, which may affect the structure or a future cathodic protection system, should be noted, e.g. electrical power transmission lines, DC or AC traction systems, or other impressed current cathodic protection systems. The presence of other foreign metallic structures in the vicinity of a structure which may affect the structure or a future cathodic protection system should be noted, for example buried metallic pipelines and embedded metallic utility fixings. Electrical earthing systems that are connected either directly or indirectly to the steel should be considered.

Power and communication availability A stable power source is required to run an impressed current cathodic protection system. A suitable electrical power supply point should be identified.

Communication systems will be required if the system is to be monitored remotely and possible systems should be identified on site, e.g. mobile coverage or landline.

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3.3 Interpretation Of Survey Once the raw test data are received from site and the laboratory it will need to be inter-and investigation results preted by a suitably experienced and qualified engineer. This is a vital step in ensuring

that the correct repair and maintenance strategy is developed for the structure to ensure that it can continue to function for the remainder of its design life. Whoever carries out this function needs clear instructions from the client as to the future use of the structure, the likely loads it will be expected to carry and the expected design life. The various available options can then be considered and the most appropriate strategy determined.

3.4 Repairs tO Structures Repairs to damaged concrete will be required prior to the application of cathodic protection or any of the other remedial systems discussed in Section 4. Providing the loss of reinforce-ment cross-section due to corrosion has not impaired structural integrity and additional loading is not expected, then making good areas of cracked, honeycombed, spalled or delaminated concrete is usually the only concrete repair required before the installation of a cathodic protection system. The use of cathodic protection avoids problems associated with potential incipient anode effects at the boundaries of repairs. There is no need to remove physically sound, but chloride-contaminated or carbonated concrete and this often results in significant time and cost savings.

There are a number of merits of cathodic protection: Undamaged chloride-contaminated or carbonated concrete does not require replace-

ment and hence concrete repair costs are minimised. Since concrete breakout is minimised, it is likely that temporary works such as structural

propping during repair will also be minimised. For highway structures, concrete repair work and structural propping frequently require

lane closures, pedestrian and traffic control. These costs are consequently minimised. Minimising concrete breakout reduces uncertainties over structural behaviour due to

redistribution of stresses. Cathodic protection controls corrosion for all the targeted steel regardless of present

or future chloride levels or carbonation. Cathodic protection can be applied to specific elements, parts of elements (e.g. cross-

heads, columns, part columns) or to entire structures. The cathodic protection current controls corrosion in areas adjacent to concrete

repairs that would normally require removal if only patch repairing was carried out. The requirement for regular monitoring of a cathodic protection system is usually

regarded as an argument against cathodic protection. However, its use means that a continuous assessment of the corrosion condition is instigated.

The integration of continuous corrosion condition monitoring can benefit critical structures, particularly structures in severe exposure conditions. Costs of continuous monitoring, inspection and control are low (typically less than five man-days per year).

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Where there has been a significant loss of section and/or extensive pitting of the steel reinforcement, additional reinforcement may be required in order that the structure can carry its full design load. Reinforcement can be added by placing addition reinforcement alongside the existing bars. These can be tied to the existing steelwork or chemically grouted in place, but must be made electrically continuous with the existing steel so that they become part of the cathodic protection scheme. Where the additional reinforcement is chemically grouted using an electrically isolating material such as epoxy, the designer of the system needs to consider the risk of the grout preventing current flow to areas behind the grouted bars. An alternative is to weld additional reinforcement in place - see BS EN ISO 17660-1 and BS EN ISO 17660-2. However, the heat treatment involved can locally reduce the strength of high-yield steel reinforcement. The extent of this reduction depends on how much of the bar has been heated during the welding process. As with all aspects of the repair process, all welding should be carried out by competent personnel using pre-qualified procedures - see UK CARES Guide Welding of reinforcing sree/s(51>.

As a cathodic protection current cannot be passed through an air gap, all delaminated areas need to be identified and removed or repaired. Delaminations can often be detected simply by hammer sounding. Care needs to be taken to ensure that the repairs are well bonded and that no voids are left between the repair material and the concrete substrate. For repairs to the soffits of members it is recommended that a proportion of the area of each repair should be taken back behind the reinforcement to provide the repair with a mechanical key. Exposed reinforcement should have any loose scale removed to ensure good contact between the steel and the repair material, but there is no need to clean reinforcement to bright metal.

Generally for civil engineering structures, such as bridges, jetties, retaining walls, etc., the most appropriate way to break out defective concrete is by using high-pressure water jetting equipment, commonly known as hydrodemolition. There are two systems in common use today: high pressure, which operates at around 1000 bar and 44 litres/minute; and ultra-high, operating at 2000 bar and 15 litres/minute. Both are effective at breaking concrete, the former being used to remove large volumes, with outputs of around 1m3

per machine per eight-hour shift being achieved. The latter is normally used when more delicate cutting is required - such as cutting pockets for the installation of anodes - as it is far more controllable, although outputs are considerably reduced. Removal of concrete from occupied buildings by hydrodemolition is more problematic, generally because of the potential damage caused by the wastewater, which makes the use of ultra-high-pressure/ low flow more attractive in these situations.

The machinery and techniques need specialist operators and it is recommended that these works are carried out by companies that are members of the Water Jetting Association (WJA).

3.4.1 Breaking out defective concrete

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The advantages of using hydrodemolition over conventional techniques of mechanical breakers include the following: reduction in operator fatigue and hand-arm vibration (HAV) issues reduction in dust generation effective removal of all corrosion products from the reinforcing steel washing away of chlorides from areas of pitting on the reinforcement, provided

potable water is used no risk of damage to embedded steel reduced noise and vibration to building occupants.

However, there are some issues that need to be considered before using this technique, including the following: How to dispose of the wastewater. Generally this can be collected, filtered and disposed

of into stormwater drains, though consent may be required from the Environment Agency or highway authority.

Isolation of the working area to prevent injury to public and to workers due to flying debris, etc.

Risk assessments and safety measures that need to be introduced to prevent damage to human limbs.

When delaminated areas are broken out, the opportunity should be taken to carry out checks for reinforcement continuity and to measure any loss of section from the reinforcement so that structural integrity can be assessed.

To avoid short-circuiting the cathodic protection system, any tie wire, nails, etc. visible on the surface of the concrete that might be in contact with the reinforcement should be removed to a depth that ensures no possible anode/cathode short circuit. Any steel tie wires or other metallic items that can be identified at, or close to, the concrete surface and that are electrically discontinuous should also be removed. These may corrode at an accelerated rate which could result in staining or sometimes localised spalling. Often a survey using a holiday detector developed for such an application is undertaken to locate such objects.

Prior to the concrete repair the exposed reinforcement should be cleaned of all significant corrosion product and contaminants. An appropriate visual standard of preparation is Sa 2, in accordance with BS EN 8501-1'52', if cathodic protection is being applied (some documents advise Sa 2Vz for 'conventional' repairs without cathodic protection). Some light surface rerusting can be permitted before concrete placement but if significant corrosion occurs during a long period between concrete removal and repair, as it can with pitted steel with retained chlorides in the pits, additional cleaning of the steel with wet or dry grit-blasting or high-pressure water jetting may be necessary.

For cathodic protection, materials used for concrete repairs should have a similar resistivity to that of the parent concrete so that a reasonably uniform current distribution can be achieved. However, there will be wide variations in the resistivity of any reinforced concrete member; areas that are wet and contaminated with chloride will have a much lower resistivity than dry, uncontaminated areas.

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Survey, investigation, diagnosis and concrete repairs 3

3.4.2 Repair materials Experience has shown that resin-based repair mortars are not suitable repair materials for corroding reinforced concrete in conjunction with electrochemical treatments. Nor are those incorporating electrical conductors including steel and carbon fibres, since they prevent the electrochemical reactions from occurring on the steel surface that control the corrosion. They may also form electrical short circuits between the anode system and the reinforcement. However, it is also considered by many engineers that a good-quality concrete repair (in accordance with BS EN 1504'53') is the overriding issue rather than its electrical resistivity as long as resin-based materials and conductive fibres are avoided.

Resin-based repair materials have a resin binder with sand, aggregate and cement filler. A cementitious repair will have hydraulic cement binder but may be modified with polymers to improve performance.

The general recommendation in BS EN 12696:2000(3) is that any repair concrete should have a resistivity in the range 50-200% of the parent concrete (Section 5.10.4 of the standard) and that old repairs exceeding these values should be replaced (Section 5.10.3 of the standard). However, polymer modified cement-based materials with resistivities as high as 133 kohm.cm (laboratory tested) have been successfully used both as repairs and as cementitious overlays to impressed current cathodic protection system on concrete that was actively corroding and therefore of low resistivity (5-10 kohm.cm). These are therefore well beyond the BS EN 12696:2000(3) recommended maximum of 200% of parent concrete - see Atkins eta/.'54'.

It is important to recognise that resistivity measurements made on concrete prisms in the laboratory in controlled conditions such as when 100% vacuum saturated, are difficult to compare with those made in the field on actual structures. Results can be affected by the presence of reinforcement, the moisture content and the temperature of the concrete which cannot be accurately measured or controlled. All these factors may vary with time, with location on the structure and with depth into the concrete cover.

It should be noted that low-resistivity repair mortars will also affect the current distribution in the system and repair mortars specifically stated as suitable for cathodic protection by repair material manufacturers may in fact have a low resistivity (

3 Survey, investigation, diagnosis and concrete repairs

For London, the average climatic conditions show the average relative humidity to be 72%, varying +15% with a standard deviation of 10%. Therefore measuring both the laboratory specimen and the structure concrete when the relative humidity is controlled to around 70% should be representative of typical field conditions. Field measurements should also be taken at similar temperatures or variations compensated for. A method for doing this is given in RILEM TC 154 (57 ). Alternatively, cores from the structure can be vacuum saturated in the laboratory and the resistivity compared directly with a vacuum saturated laboratory specimen of the repair concrete. Another alternative is to install a repair, allow it to cure and compare the resistivity of the repair with the adjacent parent concrete, ideally at around the typical atmospheric relative humidity value of 72%.

3.

4.

In the absence of better data, the recommendations below are to be considered general guidance. 1. The cathodic protection designer should be consulted on how the repair system should

be specified and designed to deal with issues that may include varying resistivities due to different exposure conditions, concretes or repairs and the requirement for current to reach steel below repairs.

2. The repair material should be cementitious with no electrically conducting admixtures or fibres and it shall not be resin based. The quality of the repair is of overriding importance rather than an arbitrary measure-ment or comparison of resistivities, particularly for impressed current cathodic protection systems. If a repair material can be shown to have performed well in comparable cathodic protection systems under similar conditions then its resistivity value should not be the overriding determinant in its use.

5. When comparing laboratory measurements with resistivity values from the field there will typically be a coefficient of variation of 30% for field measurements as well as further errors and variations due to temperature and relative humidity differences.

6. For impressed current cathodic protection systems, the laboratory-tested resistivity (vacuum saturated and tested in compliance with RILEM TC 154(57') should not exceed 150 kohm.cm or should be within 50-200% of the resistivity of the parent concrete measured, as far as possible in a comparable manner. See item 4 above.

7. It should be noted that low-resistivity repair mortars will also affect the current distribution in the system.

8. For galvanic anodes the repair material resistivity should be within 50-200% of the resistivity of the parent concrete measured in a comparable manner and/or should not exceed 15 kohm.cm or any other limit specified by the anode supplier.

3.4.3 Repair methods Cast repairs use conventional concrete or flowable (sometimes referred to as micro-pourable) concrete placed into temporary shutters. Historically these have been used to good effect on many structures with cathodic protection systems installed, including the support structures of the M5 and M6 motorways around Birmingham.

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Survey, investigation, diagnosis and concrete repairs 3

Machine-placed mortars and concretes are generally referred to as sprayed concretes. When applied correctly they can provide robust and long-lasting repairs. It is essential that trained and experienced operatives are used when considering this technique. In particular, the skill of the nozzleman is crucial to ensure that the sprayed concrete is well compacted, with no voiding behind the reinforcing bars. Most proprietary sprayed concrete materials can be finished with a trowel, but this can cause surface crazing and more importantly disturb the bond with the parent concrete. Therefore it is preferable to leave an 'as-shot finish' or a 'cut-and-flash' finish, i.e. trimmed to true lines and after initial set sprayed with an overwetted flash coat, to produce a textured finish.

If aesthetics are a primary concern, it may be necessary to apply a fairing coat over the cured sprayed concrete but this introduces a possible failure point at the interface between the concrete and fairing coat.

It is important to ensure that the repairs are cured properly. In order to facilitate this, the concrete substrate should be pre-wetted to a saturated surface dry condition and cured with wet hessian under polythene. Some proprietary curing membranes have been known to cause problems as they can have an adverse effect on the electrical properties of repairs with overlaid cathodic protection systems.

The last process in the repair procedure will be to prepare the concrete surface to accept the cathodic protection anode if a surface-mounted anode is to be used - see CPA Technical Note 13 ( 5 8 ). The type of preparation will depend on the anode system and can vary from a light grit blast to a heavy mechanical or high-pressure water-jetted surface.

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4 Choice of remedial action for reinforcement corrosion

4. Choice of remedial action for reinforcement corrosion A number of options are available for deteriorated structures. These include: simply monitoring the deterioration of the structure, applying patch repairs, replacing the concrete, applying surface coatings and treatments, through to applying electrochemical methods such as cathodic protection, chloride removal and realkalisation. Some of these options take active steps to improve the environment within the concrete by removing aggressive species, a number of them aim to minimise further contamination from the environment and others aim to minimise the effects of further deterioration.

4.1 Minimal intervention In some cases the intention may be to 'do nothing' or to do the minimum necessary to maintain the function of the structure until the end of its useful life which is determined by other factors. It is true that nearly all structures will undergo periodic inspections and routine and preventive maintenance. For highway and railway structures, routine monitoring will fall into two categories: a general inspection every two years or a principal inspection every six years. Safety inspections and special inspections may also be carried out as and when is necessary. Nuclear structures are also routinely monitored as a condition of their site licence. Reservoirs are also routinely monitored. There is no requirement on the owners of the bulk of remaining structures to carry out routine monitoring. The risks associated with doing nothing or doing the minimum all need to be assessed and may be too great for this to be a viable option.

Other interventions that may then be considered under this heading include simply monitoring the structure and containing any falling debris. Monitoring may be used to assess the risks and the likelihood of any failure. This will ensure that the integrity of the structure is maintained and that the safety of the public is not compromised. Before using this option, a thorough understanding of the structure is required and intervention points must be developed, e.g. how big does the crack need to get before something is done?

4.2 Concrete repair and replacement

Conventional patch repairs to delaminated and spalled concrete may be used to restore the concrete profile. There is no further corrosion risk to the steel in contact with the repair materi