corrosion monitoring systems

Corrosion Monitoring Systems for Reinforced Concrete Bridges Background document D5.2-S3

PRIORITY 6

SUSTAINABLE DEVELOPMENT

GLOBAL CHANGE & ECOSYSTEMS

INTEGRATED PROJECT

Sustainable Bridges SB-5.2-S3 2007-11-30 2 (23) This Report is a Part of the Research Project “Sustainable Bridges” which aims to help European railways to use their bridges more efficiently by allowing higher axle loads on freight vehicles and by increasing the maximum permissible speed of passenger trains. This should be possible without causing unnecessary disruption to the carriage of goods and passengers, and without compromising the safety and economy of the working railway.

The Project has developed improved methods for computing the safe carrying capacity of bridges and better engineering solutions that can be used in upgrading bridges that are found to be in need of attention. Other re-sults will help to increase the remaining life of existing bridges by recommending strengthening, monitoring and repair systems.

A consortium, consisting of 32 partners drawn from railway bridge owners, consultants, contractors, research institutes and universities, has carried out the Project, which has a gross budget of more than 10 million Euros. The European Commission’s 6th Framework Programme has provided substantial funding, with the balancing funding coming from the Project partners. Skanska Sverige AB has provided the overall co-ordination of the Pro-ject, whilst Luleå Technical University has undertaken the scientific leadership.

The authors of this report have used their best endeavours to ensure that the information presented here is of the highest quality. However, no liability can be accepted by the authors for any loss caused by its use.

Copyright © Authors 2007.

Figure on the front page: Corrosion sensors mounted on concrete element.

Project acronym: Sustainable Bridges Project full title: Sustainable Bridges – Assessment for Future Traffic Demands and Longer Lives Contract number: TIP3-CT-2003-001653 Project start and end date: 2003-12-01 -- 2007-11-30 Duration 48 months Document number: Deliverable D5.2-S3 Abbreviation SB-5.2-S3 Author/s: Ruth Sørensen & Thomas Frølund, COWI A/S Date of original release: 2007-11-30 Revision date:

Project co-funded by the European Commission within the Sixth Framework Programme (2002-2006)

Dissemination Level

PU Public X PP Restricted to other programme participants (including the Commission Services) RE Restricted to a group specified by the consortium (including the Commission Services) CO Confidential, only for members of the consortium (including the Commission Services)

Sustainable Bridges SB-5.2-S3 2007-11-30 3 (23)

Summary

General This guideline is part of the WP 5 guideline-work. The purpose of the present guideline is to provide guidance on requirements to a corrosion monitoring system for reinforced concrete railway bridges. State of the art within corrosion monitoring systems is still on the theoretical level. No guidance is readily available based on practical application and interpretation of actual data. The work carried out in the Sustainable Bridges project has gather the experi-ence from practical application from the project partners and a guideline has been devel-oped.

WP 5 aims to provide the project with tools for applying monitoring systems for assessing the current condition, among other a corrosion monitoring system. Such a monitoring system is able to quantify the risk of reinforcement corrosion due to carbonation or chloride ingress. The task of the present guideline helps in achieving the objectives of the project by producing a guide which gives recommendations and present practical experiences on how to apply a corrosion monitoring system for the identification of an optimal maintenance plan for rein-forced concrete bridges. By optimal means that it is made possible for the bridge owner to choose a preventive maintenance strategy since a corrosion monitoring system would indi-cate when corrosion can be expected to appear on the reinforcement. A maintenance action like e.g. a cathodic protection system can then be installed in due time. Further it is expected that the guideline will help popularize the application of corrosion monitoring systems and thereby adding new possibilities for railway bridge owners maintenance management.

This guideline includes:

- Description of corrosion monitoring system principles, sensors, design considerations, measurements and equipment.

- Evaluation of measurements - Use of the corrosion monitoring system e.g. for updating of service life models

Two different types of corrosion monitoring systems for reinforced concrete structures are described:

- Post mounted sensors. - Corrosion sensors based on cut off existing reinforcement.

Other means of corrosion monitoring systems are available, among other LPR-probes (Lin-ear Polarisation Resistance). These probes have proven useful in e.g. the Copenhagen Metro, where an automated data acquisition system has been installed. The influence of transients induced by the trains was investigated and no influence was found. The results have not yet been published.

Finally the guideline includes two examples illustrating corrosion monitoring systems in prac-tice.

The examples are for reinforced concrete structures suffering from degradation due to car-bonation and chloride ingress respectively. These structures are not railway bridges, but the principles may be adapted to railway bridges as well. In the European project SMART struc-tures the post mounted corrosion sensors have been used with success on a highway bridge crossing the railway. No influence from transients were found but the up to 150 meter cables picked up AC noise and the signals had to be filtered Ref. [1]


Conclusions and Recommendations The use of Corrosion monitoring systems is still relatively new. The first steps were taken in the late 80'ties where build-in sensors for new concrete structures were developed. These corrosion sensors have been extensively used for the concrete structures of the Great Belt Link, Denmark in the beginning of the 90'ties, and later in the 90'ties for the Oresund Link, Sweden/Denmark and the Copenhagen Metro, Denmark.

In the late 90'ties the principle from the build-in sensors were transferred to corrosion sen-sors for existing structures (post-mounted sensors).

The challenge regarding these sensors are that the concrete in the immidiate surrounding should be unaffected by the installation of the sensors due to the fact that if the environment was changed the results from the corrosion monitoring system were not representative for the concrete structure. Furthermore the installation of the sensors should not introduce cracks into the structure which would accelerate the ingress of chloride and carbon dioxide into the concrete.

At present at least two post mounted corrosion monitoring systems are available "CorroRisk probes" Force Technology and "Expansion-Ring-System" from S+R Sensortech GmbH, Ref. [11].

The experience from using these corrosion monitoring systems is still limited, but they seem very promising.

A corrosion monitoring system will provide the bridge Owner with detailed information about the current deterioration state of his structure. The corrosion monitoring technique will ensure detection of any critical initial stages of deterioration and unacceptable rates of deterioration can be detected at an early stage

The results from a corrosion monitoring system might be used for updating of service life models, giving the bridge Owner a valuable tool for optimization of the maintenance effort allowing the Owner to make cost-optimal maintenance decisions.

A corrosion monitoring system is especially recommended if there is limited access to the structure or where the impact of any degradation is substantial.


Acknowledgments This guideline has been drafted on the basis of Contract No. TIP3-CT-2003-001653 between the European Community represented by the Commission of the European Communities and the Skanska Teknic AB contractor acting as Coordinator of the Consortium. The authors ac-knowledge the Commission of the European Communities and COWI A/S for its financial support.


Table of Contents 1 Introduction..........................................................................................................................7

1.1 Background ................................................................................................................7 1.2 Purpose ......................................................................................................................7

2 Corrosion Monitoring System ..............................................................................................8 2.1 Principles of a corrosion monitoring system...............................................................8 2.2 Corrosion monitoring system guideline ......................................................................9

3 Corrosion monitoring system.............................................................................................10 3.1 Sensors ....................................................................................................................10

3.1.1 Post mounted sensors "CorroRisk probe" ....................................................10 3.1.2 Cut off reinforcement....................................................................................11

3.2 Design ......................................................................................................................12 3.3 Measurements..........................................................................................................13 3.4 Equipment ................................................................................................................13 3.5 Half-cell potentials ....................................................................................................14

3.5.1 Evaluation of the corrosion sensor half-cell potentials and reinforcement half-cell potentials............................................................................................................14 3.5.2 Evaluation of the noble counter electrode half-cell potential ........................15

3.6 Corrosion rates.........................................................................................................15 3.7 Macro cell currents ...................................................................................................15 3.8 AC resistance ...........................................................................................................16

4 Use of the corrosion monitoring system ............................................................................17 5 Examples...........................................................................................................................19

5.1 Post mounted sensors..............................................................................................19 5.2 Cut reinforcement sensors .......................................................................................21

6 References ........................................................................................................................23


1 Introduction

1.1 Background Traditionally, an evaluation of the corrosion risk for reinforced concrete structure has been based on visual inspection supported by measurements e.g. half-cell potential measure-ments.

Deterioration of concrete structures takes place below the concrete surface, and due to this the Owner may well be unaware of the potential problems building up inside his structure.

Deterioration mechanisms in concrete structures including corrosion mechanisms are well described in the literature e.g. Ref. [12], [13] and [14].

Visual inspection will only show when corrosion is developing, not when there is a risk that it will initiate. Similarly half-cell potential measurement only predicts the corrosion risk in the propagation phase when the degradation has been introduced to the reinforcing bar. A bridge owner may well be unaware of the need for investments in maintenance building up inside his structure. The longer deterioration is allowed to develop without being discovered the higher becomes the operation and maintenance costs of structural rehabilitation.

As an alternative to conventional visual inspection, a corrosion monitoring system will provide the Owner with detailed information about the current deteriorated state in his structure. The corrosion monitoring technique will ensure detection of the critical initial stages of deteriora-tion and unacceptable rates of deterioration can be detected at an early stage, allowing the owner to make cost-optimal maintenance decisions.

1.2 Purpose This guideline addresses itself to technicians who work with design, monitoring and supervi-sion of the structural safety, maintenance and serviceability of railway bridges.

The purpose of this report is to give guidelines for implementing a corrosion monitoring sys-tem on a railway bridge.


2 Corrosion Monitoring System

2.1 Principles of a corrosion monitoring system The corrosion monitoring systems described in this guideline is based on the following theo-retical considerations:

- Reinforcement embedded in healthy concrete is passivated. - When passivation of the reinforcement is destroyed e.g. due to carbonation or ingress of

aggressive substances into the concrete, corrosion is initiated involving a decrease in the half-cell potential of the rebar.

- While connecting two pieces of metals with different half-cell potential an electric current will be generated.

- A passivated rebar will easily be polarised by an impressed current, while a corroding rebar will not be considerably affected.

- Electric current and the ability to polarise steel can be measured. These measurements can be used as indicators for initiation of reinforcement corrosion.

The principle is that three electrodes: A piece of black steel (steel sensor), a piece of a noble metal (counter electrode) and a reference electrode are placed in the cover to the reinforce-ment within a concrete structure in direct contact with the concrete matrix.

With the three electrode set up a number of measurements are possible among other macro-cell current, corrosion rate, half-cell potentials and electric resistance.

Where the measurement of the macro-cell current or the corrosion rate is the primary meas-urements, the half-cell potential measurements and the electric resistance measurements are supportive measurements.

The macro-cell current between the steel electrode and the counter electrode can be meas-ured by connecting these electrodes over an ammeter.

If the black steel electrode is passivated the macro-cell current will be close to zero due to a negligible difference in the electrode potentials, while a macro-cell current different from zero will be generated, when the black steel electrode is active due to the difference in electrode potentials.

If the steel electrode surface area is small, the size of the macro-cell current will be to small (the noise to signal ratio is high), in these cases corrosion rate measurements can replace the macro-cell current measurements. The corrosion rate is measured by the GalvaPulse technique where the steel is impressed with an electric current, and the steel sensors ability to be polarised is determined.

In practice a corrosion monitoring sensor includes a number of black steel sensors located in the concrete cover at varying depths, together with the counter electrode and the reference electrode. When the macro-cell current or the corrosion rate exceeds a trigger value at the outermost steel sensor, the passivation has been destroyed for this sensor, and corrosion has been initiated. When there is a signal from the next outermost steel sensor this sensor has been activated and corrosion has been initiated in this depth, etc. In this manner, the ingress of the "corrosion front" into the concrete cover can be followed. This is illustrated in figure 1.


Figure 1 Principle for detecting the ingress of the "corrosion front" into the cover to the rein-forcement.

In this manner the corrosion monitoring system ensure detection of critical initial stages of deterioration and unacceptable rates of deterioration is detected at an early stage.

The corrosion monitoring system is a tool for assessing the current condition of the embed-ded reinforcement, and quantifying the risk of reinforcement corrosion due to carbonation or ingress of aggressive substances.

2.2 Corrosion monitoring system guideline In the following section the elements of the corrosion monitoring system is described:

- Corrosion monitoring system: Sensors, design, measurements, equipment - Evaluation of measurements: Macro-cell currents, corrosion rate, half-cell potentials and

Ac-resistance - The use of the corrosion monitoring system

Finally the guideline gives two examples illustrating the use of a corrosion monitoring system.

Reinforcement corrosion

S1

S2

S3

S4

Time

Depassivation depth

Signal 1 Signal 4

Reinforcement

Concrete surface


3 Corrosion monitoring system

3.1 Sensors This guideline includes two types of corrosion sensors, but others are commercial available among other an expansion ring system, Ref. [2].

Two different sensor principles are used:

- Post mounted steel corrosion sensors, Ref [3] - Cut off reinforcement, Ref.[4]

3.1.1 Post mounted sensors "CorroRisk probe" Each sensor includes:

- 4 to 8 carbon steel electrodes each with an active surface area of approximately 8 cm2.

- 1 counter electrode for each set of steel electrodes. The counter electrode is a ribbon of titanium coated with mixed metal oxide. The counter electrode is mounted around the reference electrode.

- 1 permanent embeddable reference electrode (Manganese/ Manganese dioxide). - 1 connection to the reinforcement (optional).

The corrosion sensor shall be installed within the concrete cover zone, from the concrete surface to the outer reinforcement layer in two or more depths, depending on the actual cover size. The innermost steel electrode usually has the same cover as the outer reinforce-ment layer. After sensor mounting the sensor top and cable connection shall be well sealed (7 years of experience has been reported, but the expected life time is 20-25 years).

Data logger

Cl- / CO2

control box

reinforcement

counter electrode

steel electrode

reinforcementconnection

reference electrode

Data logger

Cl- / CO2

control box

reinforcement

counter electrode

steel electrode


reference electrode


Figure 2 Principle of post mounted corrosion monitoring system.

The individual items of the corrosion monitoring system shall be installed according to the supplier's recommendations. There shall be no short circuits, i.e. electrical connections be-tween steel electrodes, counter electrode, reference electrode and the reinforcement. The cables from the individual items shall be lead to a control box with socket compatible with the measuring equipment. The cables shall have an adequate length (no cable extensions are permitted due to the risk of introducing a failure). The control box shall be placed in an ac-cessible location. To avoid AC- noise the cables shall be kept as short as possible, depend-ant on noise level and frequency.

3.1.2 Cut off reinforcement Each sensor includes:

- E.g. 4 separate reinforcement rods with an active surface area of approximately 60 cm2 each (depending of the actual reinforcement diameter and length).

- 1 counter electrode for each set of reinforcement rods. The counter electrode is a mesh of titanium coated with mixed metal oxide.

- 1 permanent embeddable reference electrode (Manganese/ Manganese dioxide).

In each location the reinforcement is cut at 4 reinforcement crosses leaving 4 separate rein-forcement rods insulated from the rest of reinforcement. In the centre of the 4 reinforcement rods (se figure 3) a counter electrode and a reference electrode is to be installed. Electrical connections shall be made to the individual items, and all cables shall be lead to a control box with socket compatible with the measuring equipment. The cables shall have an ade-quate length (no cable extensions are permitted due to the risk of introducing a failure). The control box shall be placed at an accessible location.


Figure 3 Typical setup where the existing reinforcement is used as a corrosion sensor.

The reinforcement can e.g. be cut by diamond core drilling and after establishment of electri-cal connections the holes shall be sealed with a membrane (to avoid any change in chemis-try in the environment around the reinforcement pieces) and succedingly filled with repair mortar. Cutting of reinforcement should always be in agreement with a structural engineer.

3.2 Design The design of the corrosion monitoring system includes:

- Sensor type - Number of sensors - Location of sensors - Location of control box - Cable routing - Execution of measurements

- Measurement program - Execution of measurements

The choice of sensor type depends on the purpose of the corrosion monitoring. E.g. if the purpose is to get an early warning as part of a preventive maintenance strategy the post mounted sensors must be used, as initiation of reinforcement corrosion in general are unac-ceptable depending of the strategy. If the purpose of the corrosion monitoring system is to get knowledge of the corrosion rate of the reinforcement as basis for estimating the optimum time for repair the cut reinforcement sensor principle is suitable. The total number of corro-sion monitoring locations depends of the individual bridge structure, and whether both the superstructure and substructure are reinforced concrete. Each corrosion monitoring system has to be tailor made. The following issues shall be taken into consideration:

Environmental parameters:

data logger

Reinforcementrod

counter electrode

reinforcementrod

reinforcementrod

reference electrode

control box

cut –diamant drill

data logger

Reinforcementrod

counter electrode

reinforcementrod

reinforcementrod

reference electrode

control box

cut –diamant drill


- Humidity - Availability of chloride (marine environment and de-icing salts) - Carbon dioxide - Access to Oxygen (submerged, tidal/splash, atmosphere) - Temperature (climate region)

Construction parameters:

- Structural element types - Joints - Concrete quality - Reinforcement geometry including cover - Access ability

Condition:

- Degradation mechanism (chloride initiated reinforcement corrosion, carbonation, other mechanism such as ASR or frost)

- Available data from condition surveys (half-cell potentials, cover, corrosion rate, resistance, carbonation depth, chloride profile, delamination, petrographic analysis, resistance, etc.)

- Earlier repairs - Assessment of the load carrying capacity

In general representative areas as well as special critical zones of the structure shall be monitored. To obtain adequate accuracy in determining the corrosion risk it is recommended to triple the number of sensors in each location. Due to the complexity of design, it is advis-able to use an experienced reinforcement corrosion engineer/specialist.

3.3 Measurements In general there is several types of measurement possible with a 3 electrode setup inde-pendent of the sensor type, among other:

- corrosion rate of the corrosion sensors by the galvanostatic polarisation method (GPM) - half cell potentials of corrosion sensors, counter electrode and reinforcement vs. the

reference electrode - AC-resistance between all electrodes

For corrosion sensors with a large surface area the measurements further include:

- macro-cell current between corrosion sensors and counter electrode or between corrosion sensors and reinforcement

The non-stationary macro-cell current shall be recorded in optional intervals from 0-20 sec. The stationary corrosion current shall be recorded every 5 sec. up to 1 minute, and subse-quently every minute until the current has stabilised (increase below 5 %). The measure-ments shall be carried out using a zero-resistance ammeter (ZRA).

All these measurements are normally supplied with a temperature measurement.

3.4 Equipment The equipment is either manually handled or the measurements are conducted automatically by a data logger. The latter is outside the scope of this guideline and dealt with in D5.1


"Evaluation report on monitoring techniques". The equipments used to conduct manual measurements are:

- a GalvaPulse equipment (used for half-cell potentials and corrosion rate measurements) Ref.[5]

- an AC resistance meter using a frequency of 128 Hz (used for resistance measurements).

- a zero ammeter with data-logger (used for macro-cell currents). - Ohm-meter used for temperature sensor type PT100

Equipment shall produce reproducible and accurate measurements. Special attention must be given to the principle of logging the low corrosion currents. Requirements to the accuracy of the measurements are given in Table 1.

Table 1: Range and requirements to the accuracy of measurements.

Measurement Measure range Accuracy of measure-ment

Resolution

Potential -1500 mV to + 500 mV

±1 mV 0.1 mV

Current (ZRA/ 1 Ω) - 2 mA to + 2 mA ±0,2 μA 1 μA

LPR ±1 mA ±2 μA 30 nA

Resistance (AC) 0 kΩ to 100 kΩ ±1 kΩ 0.1 kΩ

Temperature 0-100 oC ±0,2oC 0.02 C

3.5 Half-cell potentials

3.5.1 Evaluation of the corrosion sensor half-cell potentials and reinforcement half-cell potentials.

The steel potential is dependent of the corrosion state of the steel the oxygen level around the steel and the pH. ( Ref.[6] )

According to ASTM C 876-91 the following guidelines can be used for evaluation of half-cell potential measurements on concrete structures:

Half-cell potential Evaluation

Less than -350 mV High corrosion risk

Between –350 & –200 mV Medium corrosion risk

Above –200 mV Low corrosion risk

These values are valid when measured against a cupper/cupper sulphate electrode (CSE) These values are only true for dry/semi-dry constructions where oxygen diffusion is not lim-ited by water filled pore structures or very dense concrete and where the concrete is not car-


bonated. At wet or very dense concrete structures the access of oxygen is limited and this will affect the half-cell potential to more negative values, but the lack of oxygen will on the other hand stop the risk of corrosion although the potential is low.

In carbonated concrete the pH value can be so low that corrosion risk will be high at poten-tials above -200 mv vs. CSE. In addition to this it is important to take into consideration that the half-cell potential measured by the reference electrode on the concrete surface or at a distance from the reinforcement/corrosion sensor may be very different from that measured adjacent to the reinforcement.

3.5.2 Evaluation of the noble counter electrode half-cell potential The noble counter electrode is normally a platinum plated titanium rod or a mixed metal ox-ide plated titanium mesh. Both are corrosion resistant, but their half-cell potentials are de-pendant of the oxygen level. With limited access to oxygen their potential will drop often to lower potentials than the steel corrosion sensors and their use as part of a macro-cell is not possible. The information from the noble electrodes half-cell potential values does however give important information on the oxygen level and therefore possibility to evaluate steel half-cell potentials.

3.6 Corrosion rates Due to the half-cell potential evaluation problems, and the need of quantifying the corrosion process galvanostatic pulse measurements have been applied to the corrosion sensors us-ing the noble electrode as a counter electrode supplied with an ERE20 reference electrode. Available corrosion equipment is normally using a guard ring system to focus the applied current field to the reinforcement. In the setups given in this guideline the sensors all have well known surface areas and the use of a guard ring technique is not needed. At the Gal-vaPulse equipment there is an option of turning off the guard ring and use the instrument for post mounted sensors. Still the corrosion rate measured is an average corrosion rate of the over-all surface area exposed to the concrete.

Experience from on-site investigations has led to the following evaluation of corrosion rates by the GalvaPulse equipment:

- corrosion rates less than 0.5 µA/cm2 - negligible - corrosion rates between 0.5 and 5 µA/cm2 - slow - corrosion rates between 5 and 15 µA/cm2 - moderate - corrosion rates above 15 µA/cm2 - high

3.7 Macro cell currents Macro-cell currents measurements are only possible with a zero ammeter at currents larger than 1µA. This fact rules out the possibility of making macro cell currents at the corrosion risk sensors produced by FORCE Technology due to the very limited steel surface area of the steel corrosion sensors.

Experience from The Copenhagen Metro, the Great Belt Link and other large structures show however that macro-cell current suffer from the same oxygen problem as the half-cell potentials. The driving force in the macro-cell is the potential difference between the steel corrosion sensors and the noble metal electrode (counter electrode). In dry and semi dry concrete, where oxygen is available the counter electrode will act as cathode and the steel corrosion sensors as anodes. When a steel corrosion sensor starts corroding its half-cell po-tential drops but the noble electrode remains at the same potential. The potential difference decreases as well as the corresponding macro cell current. The macro-cell current technique is a very sensitive technique to detect the corrosion initialisation.


In wet and dense concrete lacking of oxygen access the noble "cathode" drops to a lower potential than the steel corrosion sensors and the macro-cell current will flow in the opposite direction giving no information on the sensors corrosion state.

3.8 AC resistance In systems where current is due to both electron and ion flow only AC-resistance techniques can be used. The resistance values between reference electrode and corrosion sensors give information on the reliability of the half cell potential readings. Resistances above 50-100 K Ohm indicates a very dry environment for corrosion monitoring setup and unreliable poten-tials, but it also indicates that the concrete is very dry and that the corrosion risk is therefore very low. If the resistances between the counter electrode and the steel corrosion sensors are low and the resistance between the reference electrode and the steel corrosion sensors is high indicates reference electrode problems e.g. bad contact to the concrete. The different resistance measurements give useful information of electric connections and humidity changes in the structure.


4 Use of the corrosion monitoring system

The results from the corrosion monitoring system can be used directly or they can be used as input to service life models.

Service life models for chloride ingress and carbonation are available. It is outside the scope of this guideline to describe these models, but informations can be seen in among other DuraCrete Ref. [7], Lightcon Ref [8], Mejlbro-poulsen Ref [9] or Clincon [10].

The information's from the corrosion monitoring system are used to update a probability-based service life assessment.

An equality limit state modelling that the chloride concentration in the depth where corrosion has been detected, at a certain time, are equal to the chloride threshold value can be used for a Bayesian updating. It is also possible to include the complementary information that corrosion has not been detected at a certain depth at a certain time. However this informa-tion has not the same value or weight as this would be modelled as an inequality.

In figure 4 the result of such an updating of the service life is shown. As can be seen from the sketch in figure 4 the effect of updating with the result from a corrosion monitoring system is that the expected service life (average service life) can be changed and furthermore that the uncertainty on the service life is reduced. The reduction in uncertainty can be seen as a mere narrow density function. In a probability-based assessment of the service life the time to chlo-ride initiated corrosion will be quantified as a mean value and a standard deviation. The mean value is used in connection with evaluation of the expected maintenance cost, while the standard deviation indicates the uncertainty of the maintenance cost in proportion to the corrosion risk. The acceptable safety level can be determined/choosen depending on the corrosion risk accepted by the Owner.

Mean value of service life

Mean value of updated service life

Time

Figure 4 Density function for corrosion initiation time with or without information's from a corrosion monitoring system.


It is recommended that the service life models are based on a probabilistic model as just described, where the results from the corrosion monitoring system can be included on a ra-tional basis via bayesian updating taking into account the inherited uncertainty.


5 Examples

5.1 Post mounted sensors To determine the chloride threshold value in a coastal bridge pillar built in 1981, the bridge was monitored with post mount corrosion sensors in 2003. Sensors were located in different depth from the concrete surface (0 mm, 35 mm and 50 mm depth) at 5 levels above sea-water level, see figure 5 and 6. The concrete cover is 50 mm.

Figure 5 A corrosion monitoring system installed on a bridge pier.

East side


0.2 0.2 0.3 0.3 0.3 m level

1.1 m

0.9 m

0.7 m0.5 m

1.5 m

3 m

steel corrosionsensors

humidity probes in same level in depth30, 60 and 90 mm

ERE20 with counterelectrode

East side


0.2 0.2 0.3 0.3 0.3 m level

1.1 m

0.9 m

0.7 m0.5 m

1.5 m

3 m

steel corrosionsensors

humidity probes in same level in depth30, 60 and 90 mm

ERE20 with counterelectrode


Figure 6 Sensors for determination of the corrosion risk at different levels and depths.

The corrosion sensors are placed in predrilled holes in close contact to the concrete and sealed to avoid any chloride diffusion along the electrical wires. The symmetric setup allows reference and counter electrode to be placed with the same distance to the corrosion sensor making resistance measurement comparable. All electrical connections were lead into the hollow pillar shaft easy accessible from the inside of the bridge.

After 1 year of exposure the sensors from this bridge pillar already give interesting informa-tion of the corrosion rate in different levels and depths. It is obvious from the data in figure 6 that access to oxygen is a very important factor in the splash zone area where the highest corrosion rates are found at level 90cm.

Figure 7 Corrosion rates at different levels above seawater and depths in concrete cover.

0,00

1,00

2,00

3,00

4,00

5,00

6,00

0 50 100 150

Level, cm

20mm

35mm

50mm

Icorr., µA/cm2

0,00

1,00

2,00

3,00

4,00

5,00

6,00

0 50 100 150

Level, cm

20mm

35mm

50mm

Icorr., µA/cm2


It is planned to make automated data sampling to give information on the corrosion rate be-haviour over the year and to determine the chloride levels at sensor depths to estimate the chloride threshold value and hereby improve the life time models.

5.2 Cut reinforcement sensors The next example is a carbonated reinforced concrete face wall built in 1934

COWI was in 2003 allowed to test 3 different types of rehabilitation methods on a carbonated reinforce concrete wall built in 1934:

1. Inhibitor in the repair mortar

2. Surface treatment with inhibitor

3. Realcalisation

The test was carried out on 3 comparable areas and a comparable reference area.

In each area the reinforcement was cut in 4 reinforcement crosses leaving 4 separate rein-forcement rods insulated from the rest of the reinforcement. In the centre of the 4 reinforce-ment pieces (se figure 1) a piece of mixed metal oxide mesh and an ERE20 reference elec-trode was mounted. Electrical connections where made to all reinforcement pieces, refer-ence electrodes and counter electrodes and lead to a control box. The reinforcement was cut by diamond core drilling and after establishment of electrical connections the holes where sealed (to avoid any change in chemistry in the environment around the reinforcement pieces) and filled with repair mortar.

Figure 8 Concrete surface after cut of reinforcement and installation of reference electrode and counter electrode.

cut off reinforcement

reference and counter electrode


Corrosion rate vs. time

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

0 50 100 150 200 250 300

Time, days

µA/c

m2

0

2

4

6

8

10

12

14

16

18

20

Reference AreaInhibitor in repair mortarInhibitor treated surfaceRealkalisation AreaTemperature

realkalisationday

inhibitor treatment day

Figure 9 Average corrosion rates and temperature over time for the 4 test areas

The tests at carbonated reinforced concrete face wall are planned to continue for a 5 year period and the results have not yet been evaluated but the goal is to document the effect of the different repair methods.


6 References

1 Smart Structures: Embeddable sensors for use in the integrated monitoring systems of con-crete structures", O. Klinghoffer, P. Goltermann and R. Bässler,

IABMAS 2002, Barce-lona, Spain

2 Smart Structures: Development of sensors to monitor corrosion risk for the reinforcement of concrete bridges", M. Raupach.

IABMAS 2002, Barce-lona, Spain

3 Frølund, T., Klinghoffer, O. and Poulsen, E., “Rebar Corrosion Rate Measurements for Service Life Estimates

ACI Fall convention 2000. Toronto Canada

4 Klinghoffer, O.; Rislund, E.; Frølund, T.;Elsener, B.; Schiegg, Y.; Böhni, H.: Assessment of Reinforcement Corrosion by Galvanostatic Pulse Technique

Proc. Int. Conf. on Repair of Concrete Strictures, Svolvaer, Norway, 1997 pp 391 - 400

5 Danish Patent 171925B1 1997

6 Pourbaix, M. Atlas of electrochemical equilibria in aqueous solutions

Brussels: Pergamon Press, 1900.

7 DuraCrete, General Guidelines for Durability Design and Redesign,

Doc. no. BE95-1347/R15, Feb. 2000.

8 M. Maage, S. Halland, J. E. Carlsen, Chloride penetration into concrete with light weight ag-gregates

Report FoU Lightcon 3.6, STF22 A98755 SINTEF, Norge 1999

9 L. Mejlbro, The complete solution of Fick's sec-ond law of diffusion with time-dependent diffu-sion coefficient and surface concentration

Durability of Concrete in Saline Environment, Ce-menta, Sverige 1996

10 L. Tang, Chloride transport in concrete - Meas-urement and prediction

Chalmers University of Technology. Publication P-96:6

11 M. Raupach, Corrosion Behaviour of the Rin-forcement under On-site-Conditions.

15th International Corro-sion Congress, Frontiers in Corrosion Science and Technology. Granada, 2002.

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