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Condition control and assessment of reinforced

concrete structures exposed to corrosive environments

(carbonation/chlorides)

State-of-Art Report prepared by

Task Group 5.8

May 2011


Subject to priorities defined by the Technical Council and the Presidium, the results of fib‟s work in Commissions and Task Groups are published in a continuously numbered series of technical publications called

'Bulletins'. The following categories are used:

category minimum approval procedure required prior to publication

Technical Report approved by a Task Group and the Chairpersons of the Commission

State-of-Art Report approved by a Commission

Manual, Guide (to good practice) or Recommendation

approved by the Technical Council of fib

Model Code approved by the General Assembly of fib

Any publication not having met the above requirements will be clearly identified as preliminary draft.

This Bulletin N° 59 was approved as an fib State-of-Art report by Commission 5 in November 2010.

This report was drafted by Task Group 5.8, Condition control and assessment of reinforced concrete structures exposed

to corrosive environments, in Commission 5, Structural service life aspects:

Christoph Gehlen (Convener, TU Munich, Germany)

Carmen Andrade (Instituto Eduardo Torroja, Spain), Mike Bartholomew (CH2M HILL, USA), John Cairns

(Heriot-Watt University, United Kingdom), Joost Gulikers (Rijkswaterstaat Centre for Infrastructure, Netherlands), F.

Javier Leon (Univ. Politecnica de Madrid, Spain), Stuart Matthews (BRE, United Kingdom), Philip McKenna

(Halcrow Group Ltd., United Kingdom), Kai Osterminski (TU München, Germany), Ainars Paeglitis (Techn.

University Riga, Latvia), Daniel Straub (TU München, Germany)

Full address details of Task Group members may be found in the fib Directory or through the online services on fib's website, www.fib-international.org.

Cover images: Left: Research reactor at TU München, Garching, Germany. Right: “Schlüpfbachtalbrücke” highway

bridge, BAB A81, Tauberbischofsheim, Germany, 2008. Photographs by Schießl, Gehlen, Sodeikat

(www.ib-schiessl.de)

© fédération internationale du béton (fib), 2011

Although the International Federation for Structural Concrete fib - fédération internationale du béton - does its best to ensure that any information given is accurate, no liability or responsibility of any kind (including liability for negligence) is accepted in this respect by the organisation, its members, servants or agents. All rights reserved. No part of this publication may be reproduced, modified, translated, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission. First published in 2011 by the International Federation for Structural Concrete (fib) Postal address: Case Postale 88, CH-1015 Lausanne, Switzerland Street address: Federal Institute of Technology Lausanne - EPFL, Section Génie Civil Tel +41 21 693 2747 • Fax +41 21 693 6245 [email protected] • www.fib-international.org

ISSN 1562-3610

ISBN 978-2-88394-099-4

Printed by DCC Document Competence Center Siegmar Kästl e.K., Germany


fib Bulletin 59: Condition control and assessment of reinforced concrete structures iii

Preface

fib and its preceding organizations, CEB and FIP, have a long tradition of addressing durability aspects and durability design. In 1978 CEB created a first working group, the “Task

Group Durability”. Milestones in CEB and FIP work on durability are CEB Bulletin 148

“Durability of concrete structures”, 182 “Durable concrete structures” and 238 “New

approach to durability design”. In the latter document, the framework for a probabilistic

design approach was established. In 2002 fib established Task Group 5.6 “Model code for service life design of concrete structures”, with the objective of developing a model code

document on service life design. This approach can be used for the design of new structures

as well as for the assessment of existing structures. The latter topic is addressed in this

document in detail.

The following members of Task Group 5.8 actively contributed to the work (in

alphabetical order):

- Carmen ANDRADE

- Mike BARTHOLOMEW

- John CAIRNS

- Christoph GEHLEN* (Convener)

- Joost GULIKERS

- F. Javier LEON

- Stuart MATTHEWS*

- Philip McKENNA

- Kai OSTERMINSKI* (Secretary)

- Ainars PAEGLITIS

- Daniel STRAUB*

Further acknowledgements are made to (in alphabetical order):

- Christian CREMONA

- Carola EDVARDSEN

- Sylvia KESSLER*

- Till Felix MAYER*

- Rachel MUIGAI

- Frank PAPWORTH

- Stefanie VON GREVE-DIERFELD*

- Marc ZINTEL*

* Members of the Drafting Board

Christoph GEHLEN

Convener of fib Task Group 5.8


iv fib Bulletin 59: Condition control and assessment of reinforced concrete structures

Contents

Terminology vi

Part I: Framework 1

1 Scope 1

2 Introduction 1

3 Proactive/reactive strategies 3

4 Economic considerations 6

Part II: Procedures for condition assessment 8

5 Deterioration modelling 8

5.1 Deterioration mechanisms 9

5.2 Deterioration models 11

5.2.1 Initiation period models 12

5.2.2 Propagation period model 13

5.3 Limit states of reinforcement corrosion (definitions) 15

5.3.1 Limit state equations 17

6 Modelling of spatial variability 18

7 Condition control 23

7.1 Framework of condition control 23

7.2 Collection of existing data 26

7.2.1 Data from the design/construction phase 26

7.2.2 Data from former repair interventions 26

7.2.3 Data from former inspections 27

7.3 Analysis of existing data 27

7.3.1 Assessment of data quality 27

7.3.2 Probability of Detection (PoD) 28

7.3.3 Uncertainty of measurement 34

7.3.4 Bayesian update 37

7.4 Condition assessment 38

7.5 Planning of inspections and monitoring 40

7.6 Documentation 42

Part III Final remarks 43

Part IV Examples 44

8 Remaining service life evaluation 44

8.1 Example I: Carbonation of concrete (SLS) 44

8.1.1 Collection of existing data 45

8.1.2 Analysis of existing data 45


fib Bulletin 59: Condition control and assessment of reinforced concrete structures v

8.1.3 Condition assessment 46

8.1.4 Inspection planning/documentation 47

8.2 Example II: Chloride-induced corrosion (SLS) 47




8.2.4 Intervention 53

8.3 Example III: Industrial hall with prestressed concrete frames (ULS) 54




8.3.4 Inspection planning/monitoring 61

References 61


vi fib Bulletin 59: Condition control and assessment of reinforced concrete structures

Terminology

The terms and definitions according to fib Bulletin 17:2000, fib Bulletin 22:2003; fib Bulletin 34:2006.

Birth certificate A document, report or technical file (depending on the size and

complexity of the structure concerned) containing engineering

information formally defining the form and the condition of the

structure after construction. This document/report should provide specific details on parameters important to the durability and service

life of the structure concerned (e.g. cover of reinforcement, concrete

permeability, environmental exposure conditions, quality of

workmanship achieved etc) and the basis upon which future

knowledge of through-life performance should be recorded. This

framework should provide a means of comparing actual

behaviour/performance with that anticipated at the time of design of

the structure. The document/report should facilitate ongoing (through-life) evaluation of the service life which is likely to be achieved by the

structure.

Condition

assessment

A process of reviewing information gathered about the current

condition of a structure or its components, its service environment and

general circumstances, whereby its adequacy for future service may be

established with respect to specified performance requirements for a

defined set of loadings and/or environmental circumstances.

Condition control The overall through-life process for conserving the condition of a

structure involving condition survey, condition assessment, condition

evaluation, decision-making and the execution of any necessary

interventions; performed as a part of the conservation process.

Condition

evaluation

Similar to condition assessment, but may be applied more specifically

for comparing the present condition rating with a particular criterion,

such as a specified loading. Condition evaluation generally considers

the requirement for any later intervention which may be needed to

meet the performance requirements specified.

Condition survey A process whereby information is acquired relating to the current

condition of the structure with regard to its appearance, functionality

and/or ability to meet specified performance requirements with the aim of recognizing important limitations, defects and deterioration. A

wide range of parameters may be included within condition survey

with data being obtained by activities such as visual inspection and

various forms of testing. Condition survey would also seek to gain an

understanding of the (previous) circumstances which led to the

development of that state, together with the associated mechanisms

causing damage or deterioration.


fib Bulletin 59: Condition control and assessment of reinforced concrete structures vii

Conservation Activities and measures taken which seek to ensure that the condition

of a structure remains within satisfactory bounds to meet the

performance requirements for a defined time; that is in respect of

structural safety, serviceability and sustainability requirements, which

may include considerations such as aesthetics. Conservation activities

may involve restoring the current condition of a structure to a

satisfactory state, or include proactive measures which aim to ensure

that the future condition of a structure remains within satisfactory

bounds, or improvements to meet revised performance requirements.

For this consideration may need to be given to the effects of potential

future deterioration.

Conservation plan The overall plan for controlling and conserving the condition of a

structure; i.e. condition survey, condition assessment, condition

evaluation, decision-making and the execution of any necessary

interventions.

Inspection A primarily visual examination, often at close range, of a structure or

its components with the objective of gathering information about their

form, current condition, service environment and general

circumstances.

Intervention A general term relating to an action or series of activities taken to

modify or preserve the future performance of a structure or its

components. Interventions may be undertaken as a proactive

intervention (applying some form of treatment / taking action to ensure

that the condition of a structure remains within satisfactory bounds /

that an unsatisfactory performance condition is not reached) or as a

reactive intervention (taking action after damage has become visible

e.g. cracking or spalling of concrete). Interventions may be planned or

unplanned. Planned interventions tend to be classified as a

maintenance intervention. Unplanned interventions tend to be

classified as repair interventions/actions. Thus intervention activities

might be instigated for the purposes of e.g. repair, rehabilitation,

remediation of the structure concerned.

Maintenance A set of planned (usually periodic) activities performed during the

service life of the structure intended to either prevent or correct the

effects of minor deterioration, degradation or mechanical wear of the

structure or its components in order to keep their future performance at

the level anticipated by the designer. This term is commonly applied

in the context of building fabric components with a limited life,

components associated with water management and rainwater run-off,

items where regular intervention is required to maintain their effective

operation etc. This term is commonly applied to ancillary items such

gutters, drains, seals, movement joints, bearings, etc.

Monitoring Keeping watch over and recording progress and changes in materials

and/or structural properties with time; possibly also controlling the functioning or working of an associated entity or process (e.g. warning

alarms based upon parameters such as applied load, element deflection

or some aspect of structural response).


viii fib Bulletin 59: Condition control and assessment of reinforced concrete structures

Proactive

(preventative)

intervention

A proactive conservation activity concerned with applying some form

of treatment or taking action prior to a change in a material property

(such as that caused by carbonation or chlorides) adversely affecting

the ability of the structure, or parts thereof, to meet the required

performance levels because of deterioration. The situation may include

circumstances where the performance requirements have changed over

time or where the planned service life has been extended. It is implied

that the treatment or action will be taken prior to deterioration and

damage becoming apparent/visible on the structure; e.g. cracking or spalling of concrete.

Reactive

intervention

A reactive conservation activity undertaken after deterioration and/or

damage has become apparent/visible (e.g. cracking or spalling of

concrete) such that, because of the deterioration, it has adversely

affected the ability of the structure, or parts thereof, to meet the

required performance levels (which may include consideration of

issues like aesthetics).

Rehabilitation Intervention to restore the performance of a structure or its component

parts that are in a changed, defective, degraded or deteriorated state to

the original level of performance, generally without restriction upon

the materials or methods employed. The aim of rehabilitation is in

principle similar to the aim of reconstruction, but possibly with greater

emphasis upon the serviceability requirements associated with the

existing or proposed revised usage of the structure. In some instances,

the rehabilitation may not be intended to bring the structure or

component parts back to the original level of serviceability or

durability. The work may sometimes be intended simply to reduce the

rate of deterioration or degradation, without significantly enhancing

the current level of serviceability.

Reliability The ability of a structure or a structural member to perform its

intended function satisfactorily (from the viewpoint of the customer)

for its intended life under specified environmental and operating

conditions. Reliability is usually expressed in probabilistic terms. In

the context of performance-based design of structures, reliability refers

to the ability of a structure or a structural member to fulfil the

performance requirements during the service life for which it has been

designed at a required failure probability level corresponding to a

specified reference period.

Remedial

intervention

A conservation activity undertaken after a change in a material

property (e.g. such as that caused by carbonation or chlorides) has

adversely affected the ability of the structure, or parts thereof, to meet

the required performance levels because of deterioration. The

situation may include circumstances where the performance

requirements have changed over time or where the planned service life

has been extended.


fib Bulletin 59: Condition control and assessment of reinforced concrete structures ix

Repair Intervention to reinstate to an acceptable level the current and future

performance of a structure or its components which are either

defective, deteriorated, degraded or damaged in some way so their

performance level is below that anticipated by the designer; generally

without restriction upon the materials or methods employed.

Restoration Intervention to bring the structure or its component parts back to their

original condition not only with regard to function and performance

level anticipated by the designer, but also with regard to aesthetic

appearance and possibly other (historical) considerations.

Serviceability limit

state (SLS)

A state that corresponds to conditions beyond which specified service

requirements for a structure or structural member are no longer met.

Strengthening An intervention made to increase the strength (load resistance / load

capacity) and/or possibly the stiffness of a structure or its components,

and/or to improve overall structural stability and/or the overall robustness of an existing structure to achieve a performance level

above that anticipated by the designer.

Survey The process, often involving visual examination or utilising various

forms of sampling and testing, aiming at collecting information about

the shape and current condition of a structure or its components.

Survey is taken to mean the range of activities used to evaluate

conformity with the design data for actions and/or material and/or product properties used in the service life design (SLD) on a periodic

basis during the service life of the structure. Survey activities would be

expected to include visual inspection undertaken in conjunction with

various forms of localised condition testing and measurements (e.g.

measurement of depth of concrete cover). The term survey may be

applied to the inspection of a number of similar structures/components to obtain an overview. The term is also used to describe the formal

record of inspections, measurements and other related information

which describes the form and current condition of a structure and its

components.

Ultimate limit state

(ULS)

State associated with collapse or with other similar forms of structural

failure.

Note: In general the ultimate limit state corresponds to the maximum

load-bearing resistance of a structure or structural member.

Up-grading

(retrofitting)

Intervention to enhance the functionality of a structure or its

components so as to improve some aspect of future performance above

that defined/achieved during design and construction; typically undertaken to achieve an improved (higher) load resistance against

specified loads/actions.


fib Bulletin 59: Condition control and assessment of reinforced concrete structures 1

Part I: Framework

1 Scope

The present bulletin summarizes relevant information in fib Bulletins 17, 22, 34 and 44 and develops a practical concept of how, when and where to control the condition of an

existing concrete structure in order to facilitate structural management. In fib Bulletin 34 the general principles of full-probabilistic service-life design for new and existing structures are

described. fib Bulletins 17, 22 and 44 focus particularly on the efficient management of existing structures, including the numerous prerequisites and tools needed.

As part of ongoing condition assessment it is necessary to identify the extent, nature,

cause and prognosis of any deterioration using a range of tools and methods, including

prediction models. Combined with the original design and construction details this gives a

vast amount of information over a long time period which is why a framework concept is

needed to process suitably the complete information in order to make sound investment

decisions on future maintenance management.

This bulletin provides a basis for processing the information in order to make decisions on

taking an appropriate course of action for condition control.

2 Introduction

Figure 2-1 shows the life of a structural component starting with its design, continuing

through construction and followed by the operational service life until the structure is

demolished. Each stage may be assessed by appropriate evaluation procedures, the results of

which are used to establish appropriate condition control.

Stage 1 - Design phase

In the design phase the specific client requirements, relevant standards stipulations,

exposure conditions and implication of construction materials and methods are considered to

identify applicable methods of design, including durability assessment based on deterioration

models. The client and standard requirements define the structures targeted

as-constructed-quality and long term performance which are expressed in terms of the

minimum required condition and reliability.

Stage 2 - Construction phase

The construction phase is the most important stage for ensuring that the target service life

of a structure is achieved. The conformity of the structure to the design specifications depends

highly on the experience and training of the construction team. The actual material properties

and geometries on completion of construction are likely to deviate, for better or worse, from

those assumed in the durability design. Hence the “birth certificate” of the structure is

intended to reflect the actual properties of the concrete in the new structure including an

update of the service life design to indicate if the maintenance plan has to be modified and to

identify critical components/parts of the structure.


2 2 Introduction

Co

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n (

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Co

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Stage 1:

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Stage 2:

Construction Stage 3: Operational service life

or

Maintenance Maintenance Maintenance

Lowest condition allowed

Costs

Con-

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after

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Cond

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Cond

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Inte

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Time

Technical service life

Prolonged service life

Realised service life

Ow

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req

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Desig

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Figure 2-1: Complete service life from birth to death, adapted from [26]

Stage 3 - Operational service life of a structure

During this period the structure is in service, should be regularly maintained, assessed and

have appropriate intervention measures applied.

Maintenance is commonly applied to limited life items such as gutters and seals, while the

fabric of the building is designed not to require maintenance over its design service life or

replacement interval. However, the building fabric does require condition assessment at

intervals so that interventions can be applied as necessary to ensure no unexpected, and

potentially costly, deterioration occurs. These condition assessments are also used on

extending the original design service life of the original components and interventions.

The measures applied during maintenance are translated into structural reliability by the

assessment of the structure using established mathematical deterioration models. Data of a

statistical nature obtained by performance testing (condition control) are used to perform the

calculation. The calculated reliability level is compared to the minimum level required.

Additional measures may be necessary if the reliability is less than required.

The accuracy of the deterioration model, as well as the available amount of measured data

and their quality have a major effect on the validity of the calculated structural reliability level

and therefore on the predicted structural condition and further maintenance.



Intervention measures may be necessary to prolong structural life at an acceptable

condition level (reliability). Depending on the strategy applied, either proactive or reactive

intervention will be necessary. This strategy has to be agreement between the persons

involved, especially owner/asset managers. The actual condition of the repaired/strengthened

structure should be used for the evaluation of prolonged service life.

As shown schematically in Figure 2-1, the condition of the structure must be controlled

throughout its service life to guarantee an acceptable condition level which lies above a

permissible level. The data resulting from performance testing on existing structures is used,

for example, to estimate the remaining service life of existing structures, to indicate the

interval before further condition assessment and/or to evaluate conformity with performance

design requirements, i.e. for actions and/or material and/or product properties. In the case of

non-conformity, this data is often used as a basis for decisions on the extent/volume of a

repair/strengthening measure.

However, for condition control of existing structures most of the data obtainable in stages

1 and 2 is missing and alternative data must be supplied at greater cost during the operational

service life. Firstly, the condition of the existing structure has to be assessed by detailed

survey. The inspection following construction yields data on the properties at a specific point

in time. Secondly, this data is used to determine various aspects of the structure‟s reliability.

Thirdly, this calculated reliability is compared to the required one which is usually specified

in standards or required by the owner. If the reliability is less than required, it is recalculated

based on more precise information/knowledge. The structure‟s reliability is improved by an

intervention measure or a lower reliability is chosen which accepts a higher risk level.

Following the service life stages in Figure 2-1, condition assessment is the main basis for

decision making, irrespective of whether intervention has to be made or not. Therefore this

report provides methods of assessing the extent and type of information to be gathered during

surveys and the interval between those surveys. It does this by discuss in independent section

strategies for condition control (Chapter 3), economic considerations (Chapter 4), modelling

of deterioration (Chapter 5) and spatial modelling (Chapter 6). Chapter 7 shows how to

combine these aspects to develop a plan for a structures condition control. Examples of how

the aforementioned procedures could be implemented can be found in Chapter 8.

3 Proactive/reactive strategies

The objective of condition assessment is to determine the reliability of the structure or of

structural elements over the remaining service life.

A number of different strategies can be pursued when investigating the

condition/performance of a structure. Each strategy aims to identify the best means/approach of maintaining or repairing the structure, depending on the severity and extent of damage

found. In this respect, "best" means optimal from a whole-life economical point of view,

which means all costs have to be considered (actual cost of work, repair materials, durability

of the repairs themselves and repetition of repair).

During recent years, the level of understanding of the deterioration of concrete structures

during exposure in service has increased substantially, and modelling of transport and

deterioration mechanisms has improved to a level where mathematical calculation of rates of


4 3 Proactive/reactive strategies

deterioration can be made with increasing reliability; compare fib Bulletin 34. The ability to model deterioration under different environmental exposure conditions allows a more rational

approach to condition assessment than in the past. This improvement/development should be

reflected in upgrading of the traditional approach to inspecting, testing and assessing concrete

structures and in the strategies for maintenance, repair and ongoing condition assessment.

The proactive and reactive approaches to condition control of concrete structures are

summarized in Figure 3-1. The graphs of reinforcement corrosion with time follow the

conceptual approach of Tuutti [1] to reinforcement corrosion but which apply to most

deterioration mechanisms. During the initiation period no damage is visible. Non-destructive

testing (NDT) or monitoring can provide information about the actual state of the structure.

Once deterioration can be foreseen a proactive intervention prolongs the service life of the

structure by keeping the corrosion negligible. Following this a reactive strategy intervention is

applied on occurrence of visible damages when significant corrosion exists. A more detailed

technical explanation of the Tuutti approach can be found in Chapter 5.3.

Figure 3-1: Contribution of inspection and monitoring methods to proactive/reactive strategies

for condition control, fib Bulletin 44

In more detail the reactive strategy typical of structures where only visual observations

are made, generally has the following hallmarks:

A reactive strategy implies that inspections are based on visual observations. More in-

depth investigations may then be made when deterioration has been observed and the

results determine the types of repair needed. The type and optimal point in time of

repair are mainly based on the results of visual inspection.

The optimization of the repair work is usually based on the first cost of the repair

alone. In some cases the durability of the repairs and the repaired structure are

included in the optimization. This is done by basing the optimization on the sum of



actual repair costs and mostly excludes future repair costs so present day costs can be

used for the calculation. However, as a consequence of the choice of a high discount

rate, a short-term perspective is taken.

The disadvantages of a reactive strategy based primarily on visual observations are,

among others:

When damage of concrete structures has become apparent on the surface, the

degree of deterioration is generally well-advanced. Accordingly the extensive

damage that is likely to occur within a short time will require expensive repair

work.

The possibility to intervene with proactive maintenance has usually been passed,

although proactive maintenance will be economically more beneficial in the long

term.

Although the delay of intervention until the time at which more expensive repairs

is necessary may be economically advantageous in the short term as it postpones

repairs, which increases the extent and unit cost or repair considerably. Such a

strategy, combined with the increasing stock of structures accelerates the need for

maintenance funds. Hence, this reactive approach adds to a potentially serious

social and financial threat for the prosperity of future generations which might be

avoided by a more rational long-term maintenance and repair strategy, combined

with a philosophy for more appropriate design based on an economic rational that

incorporates public costs.

Delaying intervention until cracking, discoloration, spalling or other types of

damage are apparent on the concrete surface also makes the degradation clearly

visible for the users of the structure and the public in general. This reduces the

general attractiveness of concrete as a construction material and the confidence of

the public in concrete structures. Since concrete is an important construction

material, this is considered a very unsatisfactory situation.

Better knowledge of the deterioration mechanisms, as provided by suitable mathematical

models with data provided by NDT and monitoring, can provide the basis for more proactive

strategy when maintaining concrete structures. The proactive strategy has the following

hallmarks:

A proactive strategy may be followed which will allow early quantification of the

deterioration rate long before visual damage occurs. This will enable the owner to plan

and optimise proactive interventions aimed at delaying or retarding further

deterioration.

The optimization of such proactive intervention may be based on several approaches,

such as the following:

Technical-economic optimization, that can make better use of discounting methods to provide a comparison of present day values of anticipated future costs for

various condition control options.

User-availability of the facility. The issue of „public cost‟ is gaining more and more weight for design, construction and maintenance of the public infrastructure.

For example, the user costs due to traffic delays are rated realistically (e.g. high

costs for redirections, traffic jams) when selecting the type and timing of

maintenance and repairs.


6 4 Economic considerations

Environmental issues as well as requirements favouring sustainable solutions can be a legitimate additional basis for optimization.

The benefits of following a proactive strategy based on modelling, condition control

and taking action prior to visual deterioration include the following:

Proactive intervention may eliminate the need for future remedial/corrective intervention. A proactive intervention will usually be cheaper than the extensive

repairs potentially needed following the traditional (reactive) approach of an

assessment and maintenance strategy based on assessment of deterioration by

visual inspections.

The performance and the visual appearance of the structures are impaired to a lesser extent, thus enhancing the value, trustworthiness and reliability of the

structure and asset manager while maintaining public confidence. This will benefit

the owner and public and improve the reputation of the construction engineering

profession.

The residual value of the asset is improved.

4 Economic considerations

Since a decision on the necessity and required extent of a repair and strengthening action

is of high economic importance (Figure 4-1), the information used to make the decision must

be as accurate as possible and of known reliability. The uncertainty of the information

provided also requires appropriate treatment.

Figure 4-1: Relative costs of reactive and proactive structure management, fib Bulletin 44

Due to the deterioration the relative structural performance decreases and can only be

regained by intervention measures. As explained in Chapter 3 a proactive or reactive strategy

can be followed whose economic impact is demonstrated in Figure 4-1. The proactive strategy

usually requires frequent expenditure resulting in a structure that stays in good condition

throughout its operational service life whereas the reactive strategy is accompanied by intense

investment in rehabilitation as soon as the structure reaches poor condition.



Consequently, the condition assessment of ageing structures is gaining in importance for

civil infrastructure management systems. Extending the use of existing structures is becoming

more and more important due to environmental, economical and socio-political

considerations. The condition of many structures has declined over the years so the inherent

level of safety could be insufficient with respect to current design specifications or owner

requirements. Structural integrity has to be guaranteed by considering the most extreme

service conditions in order to ensure the safety of the structure and its users. In order to

develop appropriate life extension and replacement strategies rules for through-life

performance assessment, target safety levels and optimum maintenance strategies must be

formulated and resolved from the perspective of lifetime reliability and lifecycle cost.

In order to maintain the integrity of the structure up to a given future date, a cost-effective

maintenance strategy must be used. The optimization of management of existing structures

aims at minimizing the total expected cost under the reliability constraints as follows:

T des con osl demC C C C C (4-1)

CT: total cost of a structure from design until demolition [Currency]

Cdes: design costs (Stage 1) [Currency]

Ccon: construction costs (Stage 2) [Currency]

Cosl: costs throughout operational service life (Stage 3), Equation 4-2 [Currency]

Cdem: demolition costs (including e.g. nuclear decommissioning) [Currency]

osl M I R FC C C C C (4-2)

CM: maintenance costs (also e.g. traffic delays, loss of tenants, etc) [Currency]

CI: inspection and monitoring costs including access costs [Currency]

CR: repair costs [Currency]

CF: failure costs including all consequential costs [Currency]

Structure-related life-cycle costs can be reduced at all stages:

Stage 1: Design (by eliminating components requiring labour-intensive or complex

maintenance)

Stage 2: Construction (by optimizing quality)

Stage 3: During operational service life in the categories:

Inspection (by reducing uncertainty by more conclusive structural evaluation),

Maintenance (by optimizing condition control) and

Management (by optimal assessment, maintenance strategy selection and implementation)

Cost varies according to the maintenance method selected and the residual structural value

will depend on the improvement in relative structural performance (Figure 4-1). Maintenance

expenses invested in the past affect both the current system reliability and the maintenance

cost budget requested for the future. Thus the interaction between maintenance cost and

system reliability over the lifetime of the structure has to be considered.


8 5 Deterioration modelling

Part II: Procedures for condition assessment

5 Deterioration modelling

Reinforced concrete structures have to bear loads and endure exposures. The former is

guaranteed by load design (stage 1, Chapter 2). Occasionally the load bearing situation might

change due to adaptions in the use/function of the structure. This situation is usually

accompanied by a preliminary re-design for proving the load bearing capacity of the structure.

The endurance versus exposures is a strongly time-dependent process for which generally

empirical knowledge is used during design. As deterioration proceeds the structures become

less reliable/durable. As emphasised in „PART I: FRAMEWORK‟ a need for tools/models

exists that allows prediction of the various deterioration processes affecting the durability of

reinforced concrete structures. In general, the durability of reinforced concrete structures

depends on the durability of the reinforcement material and the concrete (cover concrete).

Consequently Figure 5-1 divides deterioration mechanisms into those affecting the durability

of reinforcement and those affecting the durability of concrete.

Figure 5-1: Deterioration mechanisms and its consequences for reinforced concrete structures

In 1989, in a study carried out by Wallbank [2], the causes of deterioration of 200

randomly chosen bridges in the U.K. were evaluated. It was reported that the deterioration of

the examined bridges was frequently caused by chloride induced corrosion. A second study

reported on the main deterioration problem of German bridges throughout their operational

service life, [3]. In Figure 5-2 the corrosion of reinforcement can clearly be identified as the

main cause of deterioration. In addition to these numbers, the damage caused by other

mechanisms in Figure 5-1 and others such as stress corrosion cracking, fatigue, etc., must not

be forgotten. Since damage caused by reinforcement corrosion represents the most frequent

damage to reinforced concrete structures, the following chapters concentrate on the

mechanisms, models and limit states for durability design concerning carbonation and

chloride induced corrosion.



Figure 5-2: Causes of damage to bridge structures of the German motorway network [3]

5.1 Deterioration mechanisms

Embedded reinforcement is protected against corrosion by the high alkalinity of the

concrete pore solution (pH > 12.6 [4]). In this environment a reaction between hydroxides

(OH-) and iron ions forms a thin iron oxide layer on the surface of the steel. This so-called

passive layer can be destroyed either by carbonation of concrete (Figure 5-3) or by ingress of

chlorides. The carbonation of concrete takes place, when structures are exposed to an

atmosphere containing CO2 and a promoting relative humidity (highest rate of carbonation

between 60% and 80% [5]) is present. Carbon dioxide diffuses through the pore system of the

concrete and finally forms calcium carbonate. In this reaction, hydroxides in the pore solution

are consumed, resulting in a pH value drop to below 9. The passive layer is destroyed and

corrosion can occur.

Figure 5-3: Carbonation of concrete



Structures that are exposed to de-icing salts or seawater may suffer from corrosion due to

chloride attack. Different transport processes can be involved. If the pore system is

permanently water-saturated, chlorides penetrate mainly by diffusion. In non-saturated

exposure chlorides at the concrete surface still penetrate by diffusion, but to a lesser extent

through water in partially filled pores. In the case of intermittent wetting, chlorides penetrate

the surface layers by convection and diffusion. In almost all cases of chloride ingress a

combination of these transport processes can be found. If a critical chloride concentration [6]

is exceeded at the steel surface, the steel depassivates resulting in a locally destroyed passive

layer.

After depassivation active corrosion begins by releasing iron ions (Fe2+

) into the pore

solution and freeing electrons in the steel grid. The beginning of this process is accompanied

by a dramatic change in the electrochemical potential at the local anode which may shift to

negative (active) values resulting in a potential difference between anodic and the adjacent

passive cathodic surfaces. Anodes and cathodes are usually electrically connected enabling an

unhindered transport of the free electrons to the cathode areas. Here, hydroxide ions are

formed by reacting with oxygen and water in the pore solution. The positively charged iron

ions and the negatively charged hydroxide ions seek equilibrium. Due to the high conductivity

of the pore solution [7] the negative charges are transported from the cathode back to the

anode. These processes are shown in Figure 5-4 and the corresponding oxidation and reduction

given as Equations 5-1 and 5-2. In Figure 5-4 (left) a coplanar (in-line) setup of anodic and

cathodic areas is schematically depicted. In addition, a face-to-face setup of anodes and

cathodes is also possible, especially when considering an initiating front (chlorides or CO2)

depassivating an outer reinforcement layer, which is electrically connected to another deeper

passive layer, Figure 5-4 (right).

Figure 5-4: Schematic figure of reinforcement corrosion in concrete as a coplanar setup (left)

and an example for a planparallel (face-to-face) setup in a bridge deck (right)

Fe Fe e 2 2 (5-1)

e H O O OH2 24 2 4 (5-2)

During corrosion the steel diameter decreases in the anodic area. Beside this, depending

on the nature of the iron oxides formed (mineral), the material properties of the corrosion

products differ from those of the original steel. As the volume of the corrosion products

formed is several times higher [1] it causes expansion induced strains in the concrete matrix

leading to cracking and spalling of the cover. In addition, the corrosion might lead to a further

degradation of the bond strength between concrete and reinforcing bars due to the loss of

concrete cover or degraded ribs on the reinforcing bars.



5.2 Deterioration models

In order to model the complex processes involved in reinforcement corrosion, the causes

and consequences of corrosion have to be taken into account. Figure 5-5 shows a fault tree

analysis for reinforcement corrosion. Herein, corrosion modelling is related to two subsequent

periods of time, the initiation period with models for the determination of the point in time

when depassivation occurs and the propagation period with a deterioration model providing

the input for different structural limit states.

Figure 5-5: Fault tree analysis of a reinforced concrete structure due to corrosion

As mentioned in „PART I: FRAMEWORK‟ a strong need for suitable full-probabilistic

design models for all deterioration processes, in this specific case the reinforcement corrosion,

exists. The main technical requirements for these models are specified in [8]:

validation: the result of the calculation must be reliable/represent values observed in

practice;

quantification: input parameters applied and their uncertainty must be quantifiable by

means of tests, observations and/or experience;

reproducibility: test methods must be available for quantification of input parameters,

e.g. material resistances;

uncertainties: knowledge about the uncertainties of the model/test methods must be

provided.

Besides providing results from reliability prediction of structures, the models should

provide comparative calculation options so that technical-economical decision making can be

supported.



5.2.1 Initiation period models

Full-probabilistic design models that meet the requirements in Chapter 5.2 for carbonation

and chloride induced depassivation of steel for uncracked concrete are provided by

fib Bulletin 34.

Therein the carbonation depth is calculated according to Equation 5-3.

c e c t ACC t sx t k k k R C t W t1 ,0( ) 2 ( ) (5-3)

xc(t): carbonation depth [m]

ke: environmental function [-]

kc: execution transfer parameter [-]

kt: regression parameter (test method) [-]

RACC,0-1

: inverse effective carbonation resistance [(mm²/year)/(kg/m³)]

εt: error term [(mm²/year)/(kg/m³)]

Cs: CO2-concentration of the ambient environment [kg/m³]

W(t): weather function [-]

t: time [year]

In this formula diffusion of CO2 is considered the dominating transport mechanism. The

inverse carbonation resistance of the concrete RACC,0-1

has been introduced as a decisive

material parameter. This material property can be obtained from existing performance data of

various concretes. Usually this data is taken from other projects, where comparable concretes

to those of interest have already been tested, Hereby a first rough reliability calculation with

the knowledge of concrete composition and exposure can be processed with the existing data

basis. In case further knowledge is needed for a more reliable calculation, the inverse

carbonation resistance of concrete made with local ingredients can be provided by laboratory

testing. All input parameters of the model are of a stochastic nature. Table 8-1 (Chapter 8.1.2)

shows an example of a quantification of all parameters used for design purposes with respect

to SLS “carbonation induced depassivation of steel”.

The time and depth dependent chloride content is modelled according to fib Bulletin 34:

ge

S xa

e RCM t

x xC x t C C C erf

tk D k t

t

0 , 0

0,0

( , ) 1

2

(5-4)

C(x,t): chloride content at depth x and time t [wt.-%/cem.]

C0: initial chloride content of concrete [wt.-%/cem.]

CS,Δx: substitute surface chloride concentration at depth Δx [wt.-%/ cem.]

x: depth with a corresponding content of chlorides C(x,t) [mm]

Δx: thickness of the convection zone layer [mm]

ke: factor for considering temperature impact on DRCM,0 [-]

DRCM,0: rapid chloride migration coefficient [mm²/year]

kt: transfer parameter (test method) [-]

t0: reference testing time (t0 = 28d) [year]

age: ageing exponent [-]



This chloride ingress model is based on a solution of Fick‟s second law of diffusion

assuming that diffusion is the dominant transport mechanism. While diffusion is not

appropriate for covering the transport mechanisms for the near-surface layer exposed to

intermittent wetting and drying, the solution of Fick‟s second law has been modified by

neglecting the data until reaching the thickness of the convection layer Δx whereupon

modelling starts with a substitute surface concentration of CS,Δx. This simplification provides

results in good accordance with in situ test results. A complete matrix with quantified input

parameters can be found in Table 8-2 (Chapter 8.2.2).

5.2.2 Propagation period model

According to DURACRETE [9] the propagation of steel corrosion in concrete can be

modelled by using Equation (5-5).

ini

t

corr corr

t

x t v t dt( ) ( ) (5-5)

xcorr(t): loss of steel radius [cm]

vcorr(t): degradation rate in radial direction [cm/year]

In order to quantify the loss of steel radius xcorr or the degradation rate vcorr the following

uncertainties must be overcome:

The beginning of the propagation period tini

The beginning of the propagation period is equal to the end of the initiation period

using the models explained in chapter 5.2.1. Thus all uncertainties that are

incorporated in the input parameters of these models must be taken into account.

The evolution of the degradation rate vcorr

This input parameter varies depending on ageing/quality of concrete and on climatic

influences, e. g. temperature and relative humidity. The degradation rate vcorr(t)

describes the loss in steel radius per year. It can be obtained by using the following

methods:

o Inspection: the loss of cross sectional area can be measured/estimated from corroded reinforcing bars in representative hot-spots of a structure. Therefore, the

cover concrete has to be removed and the freed reinforcement bars have to be

processed systematically. As reinforcement corrosion hugely scatters and varies

with time a sufficient amount of rebars at different points in time should be

processed.

o Models: the degradation rate vcorr(t) can be expressed as a function of corrosion current density icorr. Considering Faradays 1

st law of electrolysis with the molar

mass of iron and its number of liberated electrons, the calculation of the

degradation rate due to reinforcement corrosion is enabled, Equation (5-6).

corr

3

corr i1016.1v (5-6)

icorr: corrosion current density [µA/cm²]



Nowadays, some full-probabilistic models for the calculation of the

corrosion current density exist, e.g. [9, 10]. Currently, no agreement on one of

these models can be achieved, which is why no further detailed explanation is

included in this document. In general, the corrosion current density itself

depends on many parameters of which one is the electrical resistance of

concrete [11]. Figure 5-6 shows results for carbonated mortar specimens in

different exposures. Therein an increase in the resistance leads to a decrease in

corrosion current density.

Figure 5-6: Influence of resistance of carbonated mortar on the corrosion current

density of embedded steel electrodes [11]

The correlation of the corrosion current density and the loss in

reinforcement diameter (dS = 12 mm) is shown in Figure 5-7. After finishing

construction (t = 0 years) the mean corrosion current density nearly equals zero

and increases when the reinforcement depassivates (tini = 29 years). Active

corrosion causes loss of steel cross section.



Figure 5-7: Correlation between corrosion rate and degradation of reinforcement

cross section following Faradays 1st law of electrolysis for a rebar with

a diameter of 12 mm (schematic)

5.3 Limit states of reinforcement corrosion (definitions)

Limit states can be set to suit whoever sets them. Three common limit states for durability

are characterised by:

„a specific condition‟ (e.g. a defined level of cracking), Definition 1;

„the optimum point for application of an intervention‟ (e.g. coat at a time such that

corrosion activation will not be reached in the design life), Definition 2;

„a defined performance level‟ (e.g. a defined chloride concentration at a certain depth),

Definition 3.

Definition 1: Condition related limit states

There are four consecutive points of marked condition change during the reinforcement

corrosion process (Figure 5-8):

limit state of depassivation: rebar changes from non-corroding (passive) to corroding

behaviour;

limit state of cracking: initial corrosion induced cracks reach the concrete surface and

can be observed;

limit state of spalling: concrete cover spalls for the very first time;

limit state of collapse: the final point in time reached e.g. by loss of bond or rebar

cross section.



Figure 5-8: Damage of a reinforced concrete structure due to corrosion, modified from [1]

According to [12] these states can technically be differentiated between serviceability

limit states (SLS) and ultimate limit states (ULS). The latter defines the collapse of the

structure, whereas the former restricts the usability or the appearance of the structure without

leading to severe damage or casualties. For reinforcement corrosion the limit states indicated

in Figure 5-8 can be presented in a time-dependent damage diagram according to Tuutti [1]. In

Chapter 5.3.1, probabilistic formulations for modelling depassivation and the limit state of

loss of rebar cross section are summarized. The corresponding models for determining the

input parameters are presented in Chapter 5.2.

Definition 2: Optimal point for intervention limit states

The optimal point in time for intervention can also be derived from Figure 5-8, but it

depends on the strategic approach. For example measures for protecting the structure from

ingress of aggressive substances must be performed before reaching the limit state of

depassivation while the (most economically) optimal point in time for restoring steel passivity

after depassivation is at later stages, where some damage has already accumulated. The

decision making process for this limit state is shown in Chapter 7.5.

Definition 3: Defined performance limit states

The service life of a structure can also be defined relative to performance and functionality

of the structure (Figure 5-9). For example a certain maximum chloride content allowed at

depth x could be set and as soon as this target performance level is reached, the defined

functionality (performance) may be considered as lost (end of service life).



Figure 5-9: Performance of a reinforced concrete structure due to corrosion

5.3.1 Limit state equations

Initiation period

The serviceability limit state of depassivation can be reached either by carbonation of

concrete or by ingress of chlorides. As soon as the carbonation front reaches the

reinforcement (xc(t) ≥ xcover) it will result in depassivation of the corresponding reinforcement

surface, Figure 5-10 (left) and Equation 5-7. For chloride ingress a critical corrosion inducing

chloride content Ccrit at the depth of the reinforcement must be reached to initiate corrosion,

Figure 5-10 (right) and Equation 5-8.

Figure 5-10: Limit states of depassivation due to carbonation (left) and chloride ingress (right)

cover c targetp x x (t) p 0 (5-7)

p{ }: probability [-]

xcover: concrete cover [m]

xc(t): carbonation depth in dependence of time t, Equation 5-3 [m]

ptarget: target probability selected, Chapter 5.3 [-]


18 6 Modelling of spatial variability

crit cover targetp C C(x , t) 0 p (5-8)

Ccrit: critical corrosion inducing chloride content at depth of

reinforcement [wt.-%/cem.]

C(xcover,t): chloride content at cover depth at time t, Equation 5-4 [wt.-%/cem.]

Propagation period

Progressive corrosion leads to a successive series of limit states being reached that have

increasingly severe consequences. The loss of reinforcement cross section ultimately affects

the structural reliability in areas with high tensile strains, leading to the last in the series of

limit states, the limit state of collapse of the structure. Here, the limit state function in

Equation 5-9 can be used. This limit state is reached, when the remaining reinforcement cross

section falls below a critical cross section needed for load bearing.

Since no reliable model for cracking, spalling or loss of bond exists so far, a limit state

formulation cannot be given here.

S,0 S,st corr targetp D D x t 0 p (5-9)

DS,0: original rebar diameter [cm]

DS,st: rebar diameter needed for load bearing [cm]

6 Modelling of spatial variability

In contrast to deterministic models, input parameters for probabilistic models are generally

quantified by statistical distributions rather than just a set of mean values or characteristic

values. These statistical distributions account for local and time-dependent scatter of both

loads and resistance as well as model uncertainties. These statistical distributions characterize

the deterioration behaviour at a particular point in space and are not intended to reflect

systematic and random differences of loads, resistances or workmanship over the structure.

For a more realistic and accurate description of the deterioration progress, spatial variability

of loads and resistances has to be taken into account. For complex structures, hierarchical

systems are recommendable that permit subdivision from the structural level over a module

level and an element level down to a sub-element level which is then used for deterioration

modelling [13]. Figure 6-1 exemplifies possible subdivisions of bridges or of multi-storey car

parks.



Figure 6-1: Subdivision of structures with respect to durability [13]

In order to gain reliable results from deterioration modelling, the structure at the sub-

element level has to be subdivided further into zones of comparable load and resistance. The

condition assessment has to be carried out for each of these zones separately. It is assumed

that there is no interdependence of different zones. For example, the lower parts of the

columns at a road that are exposed to splash are considered to be different zones which are

different from the zones in the upper parts of these columns that are exposed to spray.

Thereby the lower parts of all columns are considered as one zone, i.e. the interdependence

among these parts for different columns is accounted for; the same applies to the upper part of

the columns.

Before the condition assessment can be made, the relevant deterioration mechanism and

the corresponding limit states have to be identified for each zone individually. Inspection

results from previous inspections, information on earlier repair interventions etc. have to be

related to the corresponding zones before this information can be used in the condition

assessment.

Depending on the available data, zones can be identified as hot-spots. Hot-spots are

surface areas that are exposed to harsh environmental conditions and/or imply very low resistances (e.g. low cover depth) to which special attention has to be paid to. Therefore, hot-

spots have to be treated individually in the analysis.

Spatial variability of physical properties includes systematic spatial variation (variation of

the mean value and standard deviation) and random spatial variation. Consider the case of a

concrete bridge deck: the chloride content in the two side-areas of the deck is normally higher

than in the middle part of the deck due to the spray of chloride by passing vehicles. This

implies that the mean of the chloride content is likely to be higher at two side areas than in the

middle. This effect is termed “systematic spatial variation”. At the same time, the chloride

content varies from point to point around its corresponding mean value, independent of the

area under consideration. This property is referred as “random spatial variation” [14]. The

random spatial variability of the characteristic physical property can be modelled by spatial

probabilistic models (random fields). The systematic spatial variation can be eliminated by

the subdivision of the structure into zones.



A simple random field model is obtained by further subdividing the zones into

zone-elements (Figure 6-2). It is assumed that there is no spatial variation within a single zone-

element, i.e. the condition within a zone-element is assumed to be constant in space. The

dependence among zone-elements is then described either through a covariance matrix or

through common uncertainties, the so-called hyperparameters.

Figure 6-2: Illustration of the system model. P(Si) indicates the probability of different condition

states [15]

In order to assess the size of these zone-elements, as well as their dependence structure, a

study of the so-called correlation length or radius of the parameter governing the degradation

process must be carried out. Correlation length is a measure of the range over which

fluctuations in one zone-element are correlated with those in another zone-element. Two

points, which are separated by a distance larger than the correlation length, will each have

fluctuations that are relatively independent. A problem in estimating the correlation length is

generally a lack of data. In most studies where the spatial variability of concrete material

properties is accounted for, values of the correlation radius and hence of the size of the zone-

elements are based on practical considerations and experience, Table 6-1 [16].

One way of modelling the random spatial variability is to introduce a hyper-parameter ω.

This parameter is a random variable that is universally valid for all zone-elements. The

probabilistic models of deterioration in the zone-elements are then defined conditionally on

the realizations of these hyper-parameters; i.e., the statistical characteristics of the degradation

of zone-elements are conditionally independent for a given ω. For example the hyper-

parameter ω may include the mean value of the different diffusion coefficient µD and the

mean value of the chloride surface concentration µCs. Note that the implementation of hyper-

parameters, in principle, is also the same for identical zones in different structures; identical

zones being zones with the same initial condition indicators. Information obtained on one

structure thus can also affect the reliability of another structure [15].



Table 6-1: Indicative values of the zone-element size [16]

Reference Element size

[m]

Based on Property

Li [17] 2 Measurements Chloride content

Vu and Stewart [18] 0.5 Assumed

Compressive strength,

cover depth, surface

chloride concentration

Sterritt et al. [19 ] 0.5 - Cover depth, chloride

concentration

Engelund and Sørensen

[20] 0.35 Measurements

Surface chloride

concentration

Lentz [21] 1 Inspection grid size

Half cell potential

measurements Rusch [22]

Malioka and Faber [23] 0.48 Measurements Air permeability

Malioka et al. [24]

Straub et al. [25] 0.8 Measurements Chloride conductivity

The probability that a certain surface percentage of a zone, Δ(t), exhibits a critical degree

of deterioration (depending on the individually selected limit-state: visible cracking, spalling

etc.) may then be expressed as [15]:

N n i

K m

j

i 1 j 1

np t p g t 0

N

X,

( ) ( , ) (6-1)

Δ(t): part of a surface exhibiting a critical degree of deterioration [-]

n: number of zone-elements that exhibits a critical level of

degradation [-]

N: total number of zone-elements [-]

KN.n: number of combinations of zone- elements of the critical

percentage of the zone [-]

mi: number of zone-elements in the ith

combination [-]

gj(X, t): limit state function describing critical degradation for

zone-element j [-]

X: random field [-]

Given the hyperparameter ω, the formula can be simplified into:

np t 1 E B n 1 N t

N( ) ( , , ( ))

6-2

Eω: expected value operator in regard to the hyperparameters ω [-]

B(): cumulative binominal distribution [-]

θ(t|ω): state of an individual element at the time t belonging to a specific

condition state [-]



In the following, three theoretical cases of spatial variability of concrete structures are

introduced:

Case one: All zone-elements of one zone have identical physical properties. These

zone-elements can be described by the same distribution function and the performance

of the individual zone-elements is calculated by the same limit state function. The

consequence is that all zone-elements can be merged to a single zone-element. Here,

no further information is given about the spatial variability within the zone since the

degradation process is considered identical at all points/zone-elements within. In other

words, the full zone will be either in the safe state or in the failure state. This is, of

course, an extreme simplification of the real world.

Case two: The physical properties of each zone-element are statistically independent

of each other. The zone-elements are then totally separable, i.e. they can be analysed

individually. The inspection result is assumed to be dependent only on the condition

state of the zone-element at the time of inspection and independent of the results

obtained from other zone-elements. The degradation process has to be calculated for

each zone-element. If the condition of one zone-element is known, it is not possible to

make a prediction about the condition state of adjacent zone-elements.

Case three: Some physical properties are correlated over the whole zone and some are

not. For example, it is assumed that the chloride diffusion coefficient or the

carbonation resistance are correlated, and that the chloride impact varies randomly. So

the condition state of all zone-elements will be identical before an inspection has been

performed. But after the first inspection and the subsequent updating, the condition

states of the zone-elements are at different stages of the degradation process. When the

number of zone-elements is sufficiently large, the probability density function of the

number of zone-elements in a particular condition state can be approximated by a

normal distribution. This is the most common case in realistic circumstances.

The evaluation of spatial variability can provide necessary information for determining the

spatial extent of inspection or the optimal number of samples that should be taken. It allows

prediction of the proportion or percentage of the surface area that shows concrete

deterioration such as the area with initiation of reinforcement corrosion, cracking, spalling

etc. over the whole operational service life. This information can facilitate the optimization of

repair or maintenance strategies for concrete structures, based on the percentage of the

structure surface that shows visible signs of deterioration.

To consider spatial distribution of loads and resistances, the subdivision of structures is a

reasonable approximation and provides sufficient information for inspection and maintenance

planning. The calculation with spatial variability reflects the actual situation more realistically

and has the flexibility to implement spatial differences in the structural properties. The

approach with spatial variability provides more differentiated information about the condition

of the structure and so the service life time can be predicted more accurately.



7 Condition control

7.1 Framework of condition control

The concept of condition control presented here aims to assess the condition of a

structure in service by means of full probabilistic deterioration models as described in

Chapter 5.2. To minimize the costs for condition assessment, the input parameters for these

models are quantified on the basis of available data from the construction phase, former

inspections and former repair interventions (Chapter 7.2) if possible. If an analysis of the

available data establishes that it is insufficient or incomplete or does not fulfil the quality

requirements postulated for a full-probabilistic condition assessment in Chapter 7.3,

additional inspections should be carried out to complete the data set. The extent of the

additional inspection is determined on the basis of available data in order to economically

generate the additional information needed (Chapter 7.4). Once sufficient information is

available to quantify all the input parameters for deterioration modelling, a full-probabilistic

condition assessment can be carried out (Chapter 7.5).

One possible result of the condition assessment can be that the state of the structure

strongly deviates from the condition level assumed (e.g. during design) and thus the

inspection methods chosen previously might not have been adequate to fully assess the state

of the structure. In this case further inspection has to be carried out in order to gain additional

information to allow for an update of the condition assessment. Once the condition

assessment gives a comprehensive and complete description of the state of the structure, the

last step of the condition control procedure is the documentation of the results as the basis for

future inspections or repair actions should these become necessary (Chapter 7.5).

A general flow chart of this condition control procedure is presented in Figure 7-1.

The steps in the condition assessment of a structure are illustrated in Figure 7-1 for a

simple example of a column situated in a parking garage as shown in Figure 7-2. The column

has a rectangular cross section, is constructed of reinforced concrete and placed in a sleeve

foundation. The column has been patch repaired at height of the top ground surface.


24 7 Condition control

Figure 7-1: Flow chart illustrating the condition control procedure



Figure 7-2: Example for a condition assessment – reinforced concrete column

Based on the information given in Chapter 6, this column can be subdivided into three

main zones. The footing inside the sleeve foundation is exposed to chlorides over a long

period and usually displays a high degree of water saturation. It is expected that chloride-

induced corrosion will be the governing deterioration mechanism if freeze-thaw-attack can be

neglected (Figure 7-2, zone 1).

The column up to a height of approximately 50 cm above the parking deck surface is

subjected to chlorides from spray water and maybe capillary suction from the foundation.

Again, chloride induced corrosion will be the dominant deterioration mechanism (Figure 7-2,

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