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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,
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(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
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All advice or information from The Concrete Society is intended
for those who will evaluate the significance and limitations of its
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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
98 98
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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
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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.
3
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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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Corrosion of steel in concrete: an overview 2
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 (
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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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Corrosion of steel in concrete: an overview 2
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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2 Corrosion of steel in concrete: an overview
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
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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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3 Survey, investigation, diagnosis and concrete repairs
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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Survey, investigation, diagnosis and concrete repairs 3
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 Survey, investigation, diagnosis and concrete repairs
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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Survey, investigation, diagnosis and concrete repairs 3
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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Survey, investigation, diagnosis and concrete repairs
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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Survey, investigation, diagnosis and concrete repairs 3
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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3 Survey, investigation, diagnosis and concrete repairs
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
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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 Survey, investigation, diagnosis and concrete repairs
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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Survey, investigation, diagnosis and concrete repairs 3
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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3 Survey, investigation, diagnosis and concrete repairs
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 (
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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