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Performance Based Methods forService Life Prediction

Reports compiled byCIB W080 / RILEM 175-SLM Service Life MethodologiesPrediction of Service Life for Buildings and Components

CIB Report: Publication 294

State of the Art Reports

Part A

Author:Per Jostein Hovde

Part B

Author:Konrad Moser

ISBN: 90-6363-040-9

March 2004

Copyright © 2004 by CIB, NTNU and EMPA

CIB W080 / RILEM 175 SLM Service Life Methodologies Prediction of Service Life for Buildings and Components

Task Group Performance Based Methods for Service Life Prediction

State of the Art Reports

PART A FACTOR METHODS FOR SERVICE LIFE PREDICTION

PER JOSTEIN HOVDE

PART B ENGINEERING DESIGN METHODS FOR

SERVICE LIFE PREDICTION

KONRAD MOSER

March 2004

CIB W080 / RILEM 175 - SLM: Service Life Methodologies State of the Art Reports ii

Preface

The State of the Art Reports on methods of service life prediction have been arranged in two parts: A (Factor methods) and B (Engineering design). Each part, individually authored, is self-contained and includes a title page, table of contents, summary, and references and may contain additional information (e.g., preface, abbreviations). References are provided in author date style and are exclusive to the Part in which they are used – they have not been cross-referenced. In order that reference can be made to a specific Table or Figure in the text, these items have been enumerated using a prefix of either A or B pertaining the part in which they are presented. Finally, pagination is continuous over the two parts.

CIB W080 / RILEM 175 - SLM: Service Life Methodologies State of the Art Reports iii

Overview of Reports

Considerable work has been carried out in the area of service life prediction as requisite tools for helping assess long-term environmental effects, for maintenance management of infrastructure systems, such as roads, bridges, waterways, water distribution and waste-water removal systems, or indeed for maintenance of building envelope systems, envelope components and related materials. Increasingly, building material and component manufacturers are seeking systematic methods to assess the likely risk to premature deterioration of existing products given specific climatic effects, or the most vulnerable exposure conditions of new products in specified systems.

The current joint CIB / RILEM technical committee (CIB W080 / RILEM TC 175-SLM) on methods of service life prediction of building materials and components was created in September 1996. Prior to this, the joint CIB W080 / RILEM Committees (71-PSL, 100-TSL and 140-TSL) have been responsible for a preparing a series of useful working documents as well as co-ordinating the efforts required to bring about nine international symposia related to durability and service life issues and a tenth being planned for 2005 in Lyon, France. The number of significant contributions collectively presented in these conference proceedings provides a substantial depth of knowledge to the field. Full utilization of this body of knowledge for the benefit of manufacturers of building materials and components, designers, specifiers, constructors, as well as asset and property managers, requires the development of suitable guides and related information.

It is the aim of CIB W080 working jointly with the RILEM 175-SLM, to help develop the necessary guides, methods, and techniques that will enable practitioners to select the appropriate tools to predict service life. To achieve this aim, the focus of the technical committee is on integrating existing prediction and service life techniques, tools, and methods.

This publication comprises two parts of a State-of-the-Art report on performance-based methods of service life prediction. A task group to develop the reports was established over the course of the committee meeting held in Vancouver, Canada in June 1999. The objectives of the task group were defined during were to:

• Develop performance based methods for service life design based on models of degradation and environmental actions

• Develop a fundamental and scientific approach and provide framework for different levels of design

• Provide simplification of scientific models to engineering design

• Develop a simplified and practical design approach (“factor method”)

The work item was divided into three sub-tasks that focused on different approaches to service life prediction and included:

Sub-task 1 "Probabilistic" (also referred to as ‘theoretical’ and ‘stochastic’) Sub-task 2 "Engineering approach" Sub-task 3 "Factor method"

Of the three sub-tasks undertaken in 1999, reports for sub-tasks 2 and 3 (“Engineering approach" and “Factor method” respectively) are provided in this publication (Parts B and A

CIB W080 / RILEM 175 - SLM: Service Life Methodologies State of the Art Reports iv respectively). The third sub-task, “Probabilistic”, will perhaps be made available at a later date, but was incomplete at the time of publication.

The following provides a brief synopsis of the contents of each report in the order in which they are presented in the publication.

Part A: P. J. Hovde — Factor Methods for Service Life Prediction Sub-task item 3 — “Factor method”

The Factor method is one that has been promoted in the AIJ (Japanese) Guide for Service Life Planning of Buildings as well as in the subsequent ISO standard 15686-1 on Service Life Planning. Although this method has been suggested as an alternate means of estimating service life of components and materials, previous use of this method has not been documented. The State-of-the-Art report prepared for this publication contains the development, evaluation and use of factor methods for service life prediction as it is presented in ISO standard 15686-1. The introduction and background information provide a review of activities over the past decade that address the need for service life prediction tools given the increased focus on sustainable construction both internationally and on a national level. Mention is made of international standardization with ISO as well as the harmonization within the building and construction sector of the EC. Part B: K. Moser — Engineering Design Methods for Service Life Prediction Sub-task item 2 — “Engineering approach”

The scope of the sub-task item included the following steps.

• Gain an overview on the main methods applied to research and large engineering projects using a scientific approach. These methods often apply mathematical models and stochastic processing to the design data.

• Investigate possible modifications to the Factor method that enhances the scientific basis for the method.

• Define as to what an "engineering method" should be in terms of complexity of models applied and type and amount of data employed.

• Propose one, or several engineering design methods preferably developed on and applied to typical case studies.

The sub-task report provides a literature review and an appraisal of the state of the art. Three examples are shown to illustrate the proposed procedure for different basic equations and different quality of input data.

These reports represent the most recent advances in regard to the use of the “factor method” and related “engineering” methods to establish the service life of building materials and components. They provide useful background information on their development and offer practical approaches to the use of either method of service life prediction. These represent a valued contribution to the collection of practical technical information on service life prediction.

Chair/Coordinator CIB W080 /RILEM 175-SLM

Michael A. Lacasse

CIB W080 / RILEM 175 - SLM: Service Life Methodologies State of the Art Reports v

Acknowledgements

These reports have been compiled over the course of the 1999-2003 work programme of the CIB W080 /RILEM 175-SLM Commission and the authors are indebted to the many within the Task Group who made contributions, offered advice, to those who assisted in the review of the documents, and as well to those who helped prepare the final version of the reports.

The CIB W080 /RILEM 175-SLM Commission is likewise particularly grateful to the Norwegian University of Science and Technology and the Swiss Federal Laboratories for Materials Testing and Research for having unreservedly supported the Task group leaders in their endeavours over these years.

FACTOR METHODS FOR SERVICE LIFE PREDICTION State of the art

March 2004

CIB W080 / RILEM 175 SLM: Service Life MethodologiesPrediction of Service Life for Buildings and Components

Task Group

Performance Based Methods for Service Life Prediction

Per Jostein Hovde

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CIB W080 / RILEM 175 - SLM: Service Life Methodologies Factor Methods: State of the Art 2

NTNU – Norwegian University of Science and Technology Department of Civil and Transport Engineering March 2004


PREFACE

This report presents a state-of-the-art regarding the development, evaluation and use of factor methods for service life prediction, specifically the factor method presented in ISO 15686 Part1. Most of the report describes the background and development of the factor method, and some theoretical evaluation of the method. This may form a basis for further evaluation and implementation of factor methods as a simple and practical tool for estimation of service life of materials and components of buildings in the design and engineering phases. The technical approach to the development of the ISO factor method is not rigorous. It is in fact highly empirical and may be limited to specific types of building components (materials) used in specific contexts and under expected environmental conditions. This is to some extent emphasized throughout the report. However, the report mainly gives an overview of the development being presented, without giving a more comprehensive explanation and discussion of the factor method. This may be a task for future work. More recently, some projects have been carried out regarding practical application of the factor method. This seems to be an increasing activity around the world, and the challenge for the future will be to apply the factor method to estimate the service life of a wide variety of materials and components in buildings, in order to present the method and obtain real experience of how it works in practice and how it can be applied during the design and engineering phases of a building. This creates an immediate need for input data, both for reference service lives of materials and components, as well as practical values of the different factors included in the method. October 2003 Per Jostein Hovde



CONTENTS

SUMMARY 1. INTRODUCTION 2. BACKGROUND 3. THE NEED FOR SERVICE LIFE PREDICTION TOOLS 4. GENERAL REQUIREMENTS FOR

SERVICE LIFE PREDICTION METHODS

5. DESCRIPTION OF FACTOR METHODS

5.1 Development of factor methods for service life prediction in Japan 5.2 A factor method for evaluation of surface treatment of wooden windows and doors 5.3 Factor method for estimation of service life of components and assemblies as presented in ISO 15686 Part 1

6. EVALUATION OF FACTOR METHODS 7. APPLICATION OF FACTOR METHODS 8. FURTHER DEVELOPMENT OF FACTOR METHODS

REFERENCES

7

9

11

15

21

29

2931

32

35

41

47

49



SUMMARY This report contains a state-of-the-art regarding development, evaluation and use of factor methods for service life prediction, especially the factor method presented in ISO 15686 Part1. Chapter 1 and 2 give a short introduction and a background, respectively. In chapter 2, some important activities are described that have taken place during the last decade regarding the increased focus on sustainable development and the need for service life prediction tools, both internationally and on a national level. The international cooperation of standardization within ISO and the harmonization within the building and construction area in Europe is mentioned. Chapter 3 contains some examples in which the need for service life prediction is explained. Four different tables expressing design lives for different categories of buildings are provided. In chapter 4 some general requirements are presented that have been set up for service life prediction methods. There are needs on different levels of sophistication and for different purposes, and it is shown that service life prediction is encumbered with considerable uncertainties and that it is not an exact science. Chapter 5 contains a description and explanation of different factor methods, including the method described in ISO 15686 Part 1. Chapter 6 contains an evaluation of factor methods. So far, this is mainly based on theoretical considerations, since the ISO factor method has not been used very much in practice. In chapter 7, some examples of application of the ISO factor method are given. Some of the examples are theoretical in the way that they show how the use of factor methods can be incorporated in design for durability and development of sustainable buildings. There are also a few examples of specific application of the ISO factor method for service life prediction of buildings or building components (facades, windows). Chapter 8 contains some important aspects regarding further development of factor methods.



1. INTRODUCTION Buildings and the built environment represent a major amount of the national real capital in different countries. During the last century, there have been extensive construction activities, and the existing building stock represents a great value that requires continuous investments regarding management and maintenance as well as repair and replacement. Durability is one of the most important issues in the building and construction area, as it influences the following aspects of building materials, components, buildings and structures: sustainability service life cost of repair and refurbishment environmental impact

In 1999, the International Council for Research and Innovation in Building and Construction (CIB) published a report describing an Agenda 21 for sustainable construction [CIB 1999]. In the summary of the report it is stated that:

"The pursuit of sustainable development throws the built environment and the construction industry into sharp relief. This sector of society is of such vital innate importance that most other industrial areas of the world society simply fade in comparison. Proper housing and the necessary infrastructure for transport, communication, water supply and sanitation, energy, commercial and industrial activities to meet the needs of the growing world population pose the major challenge. The Habitat II Agenda lays stress on the fact that the construction industry is a major contributor to socio-economic development in every country. The construction industry and the built environment must be counted as two of the key areas if we are to attain a sustainable development in our societies."

In Norway, it has been shown that the building and construction sector has been responsible for approximately 40 % of the material consumption 40 % of the energy consumption 40 % of waste to landfill deposits

Similar figures have been shown for other countries. Compared to what is provided above regarding the total value of this sector, it is clear that even a limited reduction in the values for material and energy consumption, or waste, nonetheless represents significant values that have potential for greatly affecting the sustainability of building and construction activities.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Factor Methods: State of the Art 10 Likewise, it is expected that a major contribution to changes in this area will be caused by an ability to understand what influences durability and service life of materials, components and structures, to develop more durable materials and components and to establish reliable methods for testing of durability and for prediction of the service life.



2. BACKGROUND In recent years, there has been an ever-increasing focus on the needs to determine durability and service life of materials, components, installations, structures and buildings. This has been based on two important aspects: Environmental issues – scarcity of material and energy resources and the building and

construction sector as a big consumer of these resources, and the environmental impact caused by buildings

Economic issues – the total value of the built environment on a national level and the value of each specific unit (buildings, structures, roads, bridges, quays, etc.) for the owners (government, private sector or individuals). The conditions of the built environment, the annual costs of management and maintenance and the life cycle costs are of major importance be it for the economy of a country, or maintaining competitiveness within an industry or corporation.

The importance of these aspects is reflected in several initiatives and activities at both the international and national level. Some of these are briefly mentioned in the following paragraphs. Many of the activities were initiated after the UN Conference on Environment and

Development (UNCED) that was held in Rio de Janeiro, Brazil, in 1992. This conference resulted in two international agreements, two principal statements and an Agenda for a global sustainable development (Agenda 21) [CIB 1999].

International research and development activities within the International Council for

Research and Innovation in Building and Construction (CIB), especially the working commissions W60 “Performance concept in building”, W70 “Management, maintenance and modernisation of building facilities”, W080 “Prediction of service life of building materials and components”, and W094 “Design for durability”. The work of W080 is carried out in partnership with a Technical Committee (TC) of the International Association for Building Materials and Structures (RILEM). The number and title of the joint committee is CIB W080/RILEM 175-SLM "Service Life Methodologies".

The International Association for Building Materials and Structures (RILEM) has

published a Recommendation for prediction of service life of building materials and components [RILEM 1989]. This Recommendation was the basis for the development of standards for service life prediction within the International Organization for Standardization (ISO).


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Factor Methods: State of the Art 12 International standardization work within ISO. In 1984, ISO published a standard

describing the principles for preparation of performance standards in buildings and factors that must be considered (ISO 6241 [ISO 1989]). In the standard, agents relevant to building performance are presented (e.g. atmospheric exposure) that have a major influence on durability and service life. The most relevant technical committee regarding service life for the built environment is TC 59 “Building Construction”, where a sub committee (SC 14 "Design life") is working exclusively on service life. A series of new standards for planning of service life of buildings are being prepared. In Part 1 of the standard [ISO 2000] that contains general principles for the service life planning, the Factor Method is described in an Annex. Part 1 also contains a general suggestion of design life of various types of buildings. Part 2 of the standard [ISO 2001] describes general procedures for service life prediction that is based on the RILEM Recommendation [RILEM 1989]. ISO is also publishing a series of standards regarding environmental management. The work is done within ISO/TC 207 “Environmental Management”, and a set of standards describing life cycle assessment (LCA) is also being published. The first standard describing the principles and framework of LCA was published in 1997 (ISO 14040 [ISO 1998]).

In 1988, the Commission of the European Union (EU) adopted the Construction

Products Directive (CPD) (Directive 89/106/EEC [EU 1988]). Within the Directive, six essential requirements are defined that have to be fulfilled for a building during its economical working life, i.e. service life. The essential requirements are explained more in detail in the six Interpretative Documents [EU 1994]. The principal objective of the interpretative documents is to “establish the link between the Essential Requirements and the mandates which the Commission gives to European standardization bodies to establish harmonized standards and to the European Organization for Technical Approvals to establish Guidelines for European Technical Approvals”.

Development of European Standards within the European Committee for

Standardization (CEN). Although no technical committees are specifically working on durability and service life issues at this time, in accordance with the ISO/CEN Vienna Agreement, certain ISO standards published within this area may also become a European Standard (i.e. EN-ISO).

The European Organization for Technical Approvals (EOTA) was established under the

provisions of the EU Council Directive 89/106/EEC (Construction Products Directive) [EU 1988]. The scope is to produce European Technical Approval Guidelines (ETAG) for preparing and granting of European Technical Approvals (ETA). EOTA has published a document that describes how to assess the working life of products related to durability [EOTA 1999]. This document is based on documents published by RILEM and ISO as mentioned above [RILEM 1989, ISO 1984].

In 1999, the EU Commission published a Guidance Paper [EC 1999] regarding

durability and the Construction Products Directive [EU 1988]. The scope of this work addressed the issue of durability within the context of the implementation of Council Directive 89/106/EEC (Construction Products Directive). The paper is intended for specification writers, regulators and enforcement authorities within the European Economic Area.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Factor Methods: State of the Art 13 In Japan, work has been carried out for decades on how to deal with methods to predict

the durability and service life of materials and buildings both in the planning and the management phase of a building. The outcome of national activities has been published in a Principal Guide (1989) that was published in a short version in English in 1993 [AIJ 1993]. This Principal Guide is intended to show the fundamental concept of durability within each stage of the life cycle of buildings, such as planning, design, contract, construction, utilization, maintenance and modernisation and demolition. A new performance based building code is now being developed, which will also make use of service life prediction methods, and especially factor methods.

In the U.S., the result of a national study was published in 1993 regarding needs for

research and development to upgrade the civil infrastructure [NSF 1993]. A comprehensive plan has been proposed, consisting of a research programme and a programme for transfer of knowledge. The research activities are mainly focusing on technical matters, but they will relate to other subject areas such as the natural sciences, humanities and economics.

In many countries, there is a growing attention to establish regulations and standards to

address durability and service life issues during the planning, design, construction and use stages of a building. In New Zealand, quantitative requirements for the service life of building components were introduced into the Building Code in 1992 [BIA 1992]. The United Kingdom published a national standard in 1992 for prediction of durability and service life of buildings and building elements, products and components [BSI 1992]. In Canada, a similar standard was published in 1995 [CSA 1995]. In Norway, national standards have been published which describe specification texts for operation, maintenance and renewal of buildings and civil engineering works [NS 1994] and condition survey of construction works [NS 1995].



3. THE NEED FOR SERVICE LIFE PREDICTION TOOLS

Many of the activities and documents mentioned in chapter 2 state the needs for service life prediction of building products and components. Most of them describe general or overriding requirements at a national or regional level, but at the same time they reflect an important and increasing trend to focus on this issue. In this chapter, some of the specific requirements are presented to illustrate how these are expressed. In Europe, the Construction Products Directive (CPD) [EU 1988] has now been implemented in the European Economic Area (EEA) countries. In the CPD it is stated that any construction product which is covered by the CPD, shall have properties such that the building or structure is able to fulfil specific essential requirements regarding

1. mechanical resistance and stability 2. safety in case of fire 3. hygiene, health and environment 4. safety in use 5. protection against noise 6. energy economy and heat retention

The requirements shall be fulfilled during an economically reasonable working life of the products. The term working life is corresponding to service life. Each of the six essential requirements are explained more in detail in six corresponding Interpretative Documents [EU 1994], and these documents also contain a specification of what is meant by working life and how to take care of durability issues for the construction products. The following explanations are given for the working life in all the Interpretative Documents:

"1.3.5 Economically reasonable working life: (1) The working life is the period of time during which the performance of the

works will be maintained at a level compatible with the fulfilment of the essential requirements.

(2) An economically reasonable working life presumes that all relevant aspects are taken into account, such as: costs of design, construction and use; costs arising from hindrance of use; risks and consequences of failure of the works during its working life and costs of insurance covering these risks; planned partial renewal; costs of inspections, maintenance, care and repair; costs of operation and administration; disposal; environmental aspects."

The Interpretative Documents also contain the following comments regarding working life and durability:



"5.1 Treatment of working life of construction works in relation to the Essential Requirement

(1) It is up to the Member States, when and where they feel it necessary, to take

measures concerning the working life which can be considered reasonable for each type of works, or for some of them, or for parts of the works, in relation to the satisfaction of the essential requirements.

(2) Where provisions concerning the durability of works in relation to the Essential Requirement are connected with the characteristics of products, the mandates for the preparation of the European standards and guidelines for European technical approvals, related to these products, will also cover durability aspects.

5.2 Treatment of working life of construction products in relation to the

Essential Requirement (1) Category B specifications and guidelines for European technical approval

should include indications concerning the working life of the products in relation to the intended uses and the methods for its assessment.

(2) The indications given on the working life of a product cannot be integrated as a guarantee given by the producer, but are regarded only as a means for choosing the right products in relation to the expected economically reasonable working life of the works."

The Construction Products Directive is now a basis for introduction of performance based building regulations in European countries, and thereby requirements for durability and service life of construction products are implemented into national building regulations in Europe. In 1992, a new building code was published in New Zealand [BIA 1992] that contains specific requirements for the service life of various parts of buildings or for construction products. In the clause B2 Durability, the requirements are given in the following way:

"B.2.3 From the time a code compliance certificate is issued, building elements shall with only normal maintenance continue to satisfy the performance of this code for the lesser of; the specified intended life of the building, if any, or: (a) For the structure, including building elements such as floors and walls which

provide structural stability; the life of the building being not less than 50 years. (b) For services to which access is difficult, and for hidden fixings of the external

envelope and attached structures of a building: the life of the building being not less than 50 years.

(c) For other fixings of the building envelope and attached structures, the building

envelope, lining supports and other building elements having moderate ease of access but which are difficult to replace: 15 years.



(d) For linings, renewable protective clothing, fittings and other building elements to which there is ready access: 5 years.

Brand [1994] has described the importance of specifying the service life of various parts of a building in such a way that each part can be easily repaired or replaced, if the service life is shorter than the service life of the whole building. The relation between the various service lives has been illustrated by lines of different thickness, as shown in Figure A3.1. Figure A3.1: Optimal relation between the service lives of different building components

and functions. [Duffy and Henney 1989]). The Canadian Standard CSA 478-95 [CSA 1995] describes the relation between design life of a building or a building component, and the durability of the component. In chapter 6 of the standard it is stated that

"6.1 Buildings and Components Requirements for durability may vary from building to building and from one component to another. These requirements are related to intended use, to cost, and to frequency, difficulty and extent of maintenance, replacement and repair. Requirements for durability are expressed in terms of design service life. The design service life of the building provides one basis for the determination of the design service life of the building components."

Typical design service life categories for buildings that are given in the standard are shown in Table A3.1. In the Guidance Paper [EC 1999] published by EU in 1999, a table of assumed working lives of works and construction products is given and is provided in Table A3.2. The table has been developed by the European Organization for Technical Approvals (EOTA), and it is another


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Factor Methods: State of the Art 18 example of how quantitative values are given for service life which architects, consultants, authorities and manufacturers of building products have to take into consideration and be able to fulfil. Table A3.1: Categories of design service life for buildings. (From [CSA 1995]).

Category Design service life for building

Examples

Temporary Up to 10 years • non-permanent construction buildings, sales offices, bunkhouses • temporary exhibition buildings

Short life 10 to 24 years • temporary classrooms Medium life 25 to 49 years • most industrial buildings

• most parking structures Long life 50 to 99 years • most residential, commercial, and office buildings

• health and educational buildings • parking structures below buildings designed for long

life category Permanent Minimum period

100 years • monumental buildings (e.g. national museums, art

galleries, archives) • heritage buildings

Table A3.2: Assumed working lives of works and construction products (From [EC 1999]).

Assumed working life of works (years)

Assumed working life of construction products (years)

Category

Category Years Repairable or easily

replaceable

Less easily repairable or replaceable

Lifetime of works **

Short 10 10 * 10 10 Medium 25 10 * 25 25 Normal 50 10 * 25 50 Long 100 10 * 25 100

* In exceptional and justified cases, e.g. certain repair products, a working life of 3 or 6 years may be envisaged.

** Products not repairable or economically replaceable. The standard ISO 15686 Part 1 [ISO 2000] also contains a table of suggested minimum design lives for building components (DLC). This is given in Table A3.3. The European Organization for Technical Approvals (EOTA) published a guidance document [EOTA 1999b] that presents assumed service life (working life) for works and for construction products. The service lives are shown in Table A3.4. The document also states that

“By EOTA (as well as CEN) the assumed working life of a product should be understood as a basic assumption and reference to be considered when laying down the type and severity of verification methods (e.g. number of freeze-thaw cycles) and provisions relating to “durability”.”


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Factor Methods: State of the Art 19 Table A3.3: Suggested minimum design lives for components (From [ISO 2000]).

Components Design life of

building Inaccessible or structural

Replacement is expensive or

difficult* Major replaceable

Building services

Unlimited Unlimited 100 40 25 150 150 100 40 25 100 100 100 40 25 60 60 60 40 25 25 25 25 25 25 15 15 15 15 15 10 10 10 10 10

NOTE 1: Easy to replace components may have design lives of 3 or 6 years NOTE 2: An unlimited design life should very rarely be used, as it significantly reduces design options.* including below ground drainage Table A3.4: Assumed service life of works and construction products to be used by EOTA

(From [EOTA 1999a])

Assumed life of works (years) Working life of construction products to be assumed in ETAGs, ETAs and hENs (years)

Category Years Category

Repairable or easily

replaceable

Repairable or replaceable with

some more efforts

Lifelong 2

Short Medium Normal Long

10 25 50

100

10 1 10 1

10 1

10 1

10 25 25 25

10 25 50 100

1 In exceptional and justified cases, e.g. for certain repair products, a working life of 3 to years

may be envisaged (when agreed by EOTA TB or CEN respectively). 2 When not repairable or replaceable “easily” or “with some more efforts”.

Life cycle assessment (LCA) can be an important tool that is typically used for establishing more sustainable construction activities and achieving sustainable buildings. LCA techniques have been adopted for other product areas where the service life of the products often is much shorter (weeks or months instead of years and decades). Therefore, in the performance of LCA of a building product, component or element, the service life of the actual object or of each individual part of it has to be taken into consideration. The service life of a specific part will have a great influence on the outcome of an LCA of the complete object. Selection of alternative parts that have different service lives or where the service life varies depending on alternative maintenance procedures, may also have a great influence on the overall outcome of the LCA. The introduction of LCA into the building and construction sector will therefore increase the need for service life prediction of construction products.



4. GENERAL REQUIREMENTS FOR SERVICE LIFE PREDICTION METHODS

Service life prediction of buildings or building elements, components or products can be both a complex and time-consuming process. To date, the methods have not been developed into an exact science given the many different factors that must be considered that thereby make a thorough service life prediction an interdisciplinary activity. Service life prediction can be based on two different principal approaches: Deterministic approach Probabilistic approach

This gives the basis for development of service life prediction methods of various complexities and with different requirements of applicability and needs for input information. Three levels of service life prediction methods can be described as shown in Figure A4.1.

Research methods Probabilistic

Engineering

methods

Simple estimation methods Deterministic

Figure A4.1: Relation between different types of service life prediction methods. The factor methods that are discussed in this report are based on the deterministic approach. In 1987, Masters provided some general requirements to a service life prediction system [Masters 1987].

1. "You should define the problem explicitly before attempting to solve it. 2. You should define service life such that a) it can be measured (quantitatively) and

b) it can be related to in-service performance.



3. You should be open to new approaches and methods rather than blindly accepting those of tradition.

4. You should use simple and systematic procedures having a basis in logic, common sense, and material science.

5. You should be aware that unsystematic, qualitative accelerated ageing test data can be used to make anything look good, bad, or indifferent.

6. You should recognise that a) it is impossible to simulate all possible weathering stresses in the laboratory, and b) it is not necessary to do it anyway.

7. You should ensure that degradation processes induced by accelerated tests are the same as those encountered in-service.

8. You should measure the degradation factors. 9. You should be wary of the correlation trap. 10. You should recognise that, by using systematic, quantitative procedure, valid

acceleration tests can be developed." In chapter 2 a brief overview was offered of the development over recent decades of service life prediction methods. Based on these general requirements, a recommended procedure was developed in which a systematic approach to methodology for service life prediction of building materials and components is outlined [RILEM 1989]. The methodology is said to include the identification of needed information, the selection or development of tests, the interpretation of data and the reporting of results. It uses an iterative research approach, thereby permitting improved predictions to be made as the knowledge base grows. Mathematical analyses needed for prediction of service life are not described in detail, but either deterministic or probabilistic analyses may be used. The RILEM Recommendation is intended to be generic, and therefore applicable to all types of building materials and components. Specific test methods and equipment vary depending on the materials or components being evaluated as well as user requirements and for this reason, this information is not included in the Recommendation document. The Recommendation has been used as a basis for the development of the ISO Standard 15686 Part 2 [ISO 2001] The principle of this generic service life prediction method is given in Figure A4.2. In the Japanese Principal Guide [AIJ 1993] it is mentioned that various principles for the prediction of physical service life have been proposed. Reference is given to some national development and standards (US, France, Australia, Japan), along with the work of joint CIB/RILEM committees. It is stated that the system for service life prediction used in the RILEM Recommendation [RILEM 1989] is based upon the same principle as used in the Japanese Principal Guide. Some more information from the Guide is given in chapter 5. The problems of prediction of durability and service life are discussed in the British Standard BS 7543:1992 [BSI 1992]. It is stated that a designer needs to have information on durability to meet the building owner’s requirements and to develop a rational policy for the durability of the entire construction system. The necessary information can be obtained from

a) experience in the use of traditional materials b) certificates assessing the performance of products c) research publications d) predictions of the service life of products provided by their manufacturers



Exposure and evaluationShort-term Long-termexposure exposure

No Similardegradation

?

Yes

Analysis/interpretationProcess performance-over-timeor dose-response functions to

establish prediction models

Service life prediction

Field exposure

Inspection ofbuildings

Experimentalbuildings

In-use exposure

In-use-condition(non-acc.) exposure

Acceleratedexposure

Dose - response

Dose - environmental classes

Res

pons

e cl

asse

s (d

egra

datio

n in

dica

tor)

Critical review, Reporting

PretestingChecking mechanisms and loads, and verifying choice

of characteristics and techniques by short-termexposure

PreparationIdentification of degradation agents, mechanisms and

effects, choice of performance characteristics andevaluation techniques, feedback from other studies

DefinitionUser needs, building context, type and

range of agents, performance requirementsMaterials characterisation

Figure A4.2: Systematic methodology for service life prediction of building components.

(From ISO 15686 Part 2 [ISO 2001]). Further, in BS 7543 the following statements are made regarding prediction of durability and service life:

"4. Predictions of durability A designer needs to have information on durability to meet the building owner's requirements and to develop a rational policy for the durability of the whole construction. - - - - - It is important to note the following: (i) Prediction of durability is subject to many variables and cannot be an exact science.


A new building is a unique design to meet a specific set of conditions on a specific site. Unless these conditions are the same as those previously recorded for precisely the


same form of construction, the predicted life for the building and its parts can only be an estimate. (ii) Accelerated testing of components by itself can seldom be used to give an accurate basis for predicting service life. Accelerated testing is not usually feasible for large assemblies of components. (iii) Relevant test certificates are not always available from manufacturers and may have to be obtained by testing for a specific project." - - - - - "9. Predicted service life 9.1 Method of assessment The predicted service life of a building should be assessed in one or more of the following ways: (a) Assess by reference to previous experience with the same, or similar construction and in similar occupation or climatic circumstances. (b) Assess by measuring the natural rate of deterioration over a short period of use or exposure and estimating from the measurement when the durability limit will be reached. (c) Assess by interpolation from accelerated tests that have been devised to shorten the response time to the action of an agent. The science of accelerated testing is complex: care should be taken not to produce different effects by changing the natural intensity of agents." - - - - - "Prediction of service life will normally apply to components and small scale assemblies. Whole buildings and large assemblies are more often one-off designs that make previous experiences of durability less relevant and because of their size it is less easy to test their performance under controlled conditions. Whatever method is used to assess it, the predicted service life is unlikely to be a precise figure because the effect of an action in any building is not likely to be accurately predictable. More reliable predictions can be made when there is a correlation between the results of different assessments." - - - - - "In cases where the prediction of service life cannot be very accurate it may nevertheless serve as a useful purpose when items are being ranked in order of durability. The interpretation of data from tests requires skill and experience and knowledge of building maintenance. It is often necessary to rely on an informed opinion for service life prediction.”

Martin et al. [1994] have carried out a comprehensive study on methodologies for predicting the service lives of coating systems. They present a set of criteria for judging the adequacy of any proposed service life prediction methodology. These criteria include the ability to:

1. Handle large variability in the times-to-failure for nominally identical specimens



2. Analyse multivariate data 3. Discriminate among these variables. That is, the service life prediction

methodology should be able to separate the few significant variables from the many insignificant variables

4. Fit both empirical and mechanistic failure models to short-term laboratory-based exposure results

5. Establish a connection between short-term laboratory-based and long-term in-service results

6. Provide mathematical techniques to predict the service life of a coating system exposed in its intended in-service environment

In the Canadian Standard CS 478-95 [CSA 1995] there is also a general description of methods for service life prediction. The alternative methods are described in the following way:

"7. Predicted Service Life of Components and Assemblies 7.1 General It is understood that the predicted service life of any building component, including repaired as well as new components, is approximate based on the assumed environmental conditions and on installation, operating and maintenance procedures. 7.2 Methods to predict Service Life 7.2.1 The predicted service life of components or assemblies may be assessed by one or more of the following three methods: (a) demonstrated effectiveness, in accordance with Clause 7.3 (b) modelling of the deterioration process, in accordance with Clause 7.4; and (c) testing, in accordance with Clause 7.5

7.2.2 All methods used to determine predicted service life should be based on a sound understanding and application of the principles of building science, in accordance with Clause 7.6. 7.2.3 For the prediction of service life or an assembly, (a) demonstrated effectiveness may be applied where identical assemblies have been

used (i) successfully; and (ii) in the same environments

(b) modelling and demonstrated effectiveness should be applied where (i) a similar component or assembly has been used successfully in the same

environments; or (ii) proven components or assemblies have been used successfully, but in

moderately different environments; and (c) modelling and testing should be applied where

(i) innovative components and assemblies are to be used; or (ii) proven components or assemblies are to be used in significantly different

environments.



The degree to which an assembly or its components are innovative or the service environment is dissimilar to one previously experienced should be established by the application of building science principles."

A RILEM workshop [RILEM 1995] on environmental aspects of building materials and structures was held in Finland in September 1995. In a very brief summary of the workshop, it is concluded that:

“Environmental aspects form a very complex problem area with many factors to be included in the evaluation. This typically results in complicated evaluation and assessment methodologies which are difficult to apply." - - - - - "At present, there is no standard comprehensive methodology for assessing the environmental issues of the entire building process and the life cycle of building products. The methodologies in use today are incomplete and may give contradictory results. Many of the applied assessment methods concentrate only on some aspects of the total environmental performance."

Based on these conclusions, it is stated that there is a great need for design methodology and methods that are capable of analysing, evaluating and optimising the environmental impacts together with other multiple performance requirements.

Sarja and Vesikari [1996] have edited a RILEM Report on durability design of concrete structures. They also present a discussion of what they call durability models. These models may be:

Degradation models – mathematical presentations that show an increase in degradation

with time (or age) and with appropriate design parameters. Performance models – mathematical presentations that show decreased performance

as a function of time and appropriate design parameters. Service life models – mathematical presentations that show the service life of a

structure as a function of different design parameters. The authors state that there may be durability models for different levels such as materials, structural elements and buildings, and all of these can be used in durability design. Further, they state that:

"7.1.2 Deterministic and stochastic durability models Durability models can also be divided into deterministic or stochastic models. Deterministic durability models are used in deterministic durability design where the scatter of degradation (or performance or service life) is not taken into account. With known values of parameters the model yield only one value (of degradation or performance or service life) which is often the mean value. In some cases, deterministic models are formulated to give an upper or lower fractile value instead of the mean. In many cases the information yielded by deterministic models is insufficient to evaluate the risk of not reaching the target service life. Especially in the mechanical



design of structures, stochastic design methods are considered essential as the scatter due to degradation is normally wide and the degree of risk may be great."

The authors have identified the following needs for durability models:

1. technical material development 2. ecological evaluation of materials 3. network level management systems for the maintenance, repair and rehabilitation of

structures 4. planning of project level repairs 5. risk analysis of important structures 6. design of a material mix and quality assurance at the construction site 7. structural durability design.

In the report the following evaluation is given regarding durability models:

“7.2.2 Quantification of degradation, performance and service life The final step in the process of producing durability models is quantification and formulation. Statistical methods and theoretical reasoning are the tools used for these tasks. Simplifications, omitting irrelevant factors and limitation of relevant factors are often necessary actions. Durability models can be based on empirical or analytical grounds. Empirical models are based on experience and test results. They are developed from results of field surveys and laboratory tests by applying correlative and other statistical methods. Analytical models are based on laws of nature and fundamental reasoning. They are created as a thorough analysis of degradation mechanisms and kinetics. Before models can be applied, tests are usually required for determining values for some material properties. Very often, empirical models represent a viewpoint of engineers, and analytical models that of material scientists. A drawback of empirical models is that mechanisms of influence are poorly understood in models in general. Consequently, any deviation from the limits of the model may not be possible without entailing risk. Analytical methods are based on a deeper understanding of the characteristic features of damage, but their practical importance may be small if the parameters in the model are not measurable or the models cannot otherwise be brought to a level of practical utilization. Both the empirical and analytical viewpoints should be considered when developing durability models. Models can be considered good when based on an analysis of mechanisms and factors leading to degradation, yet subjected also to laboratory and field tests.”

In a discussion paper on service life prediction, Bourke and Davies [1997] present a list of essential and/or desirable characteristics of a service life prediction system. They state that



"the relative importance of each is arguable, but important features may be considered to include the following:

easy to learn easy to use quick to use accurate easy to update easy to communicate adaptable supported by data links with existing design methods and tools free of excessive bureaucracy recognises the importance of innovation relevant to diverse environments acceptable to practitioners and clients alike reflects current knowledge a flexible level of sophistication for either outline or detailed planning"



5. DESCRIPTION OF FACTOR METHODS 5.1 Development of Factor Methods for Service Life Prediction in Japan During the 1980’s, much research and development work was carried out in Japan to develop methods and tools for prediction of service life of buildings, parts of buildings, and of elements, components and equipment. As mentioned in chapter 2, a Principal Guide for service life planning of buildings was published in 1989, and in 1993 a shorter version was published in English [AIJ 1993]. In this Principal Guide the following principles are presented for the service life prediction: Evaluation of the physical deterioration Evaluation of obsolescence

A general method for prediction of the service life based on the physical deterioration is presented in the Principal Guide. The description contains a listing of the conditions which influence the service life, and which are reflected in the factors used within a factor method.

"2.4.2 Set of given conditions

The given conditions shall, as a rule, be arranged according to the following classification concerning the deterioration factors in addition to the type and usage of building, space, part of building, building elements, components and equipments. a) Items relating to the inherent characteristics of performance over time:

1) Performance of materials 2) Quality of designing 3) Quality of construction work 4) Quality of maintenance and management

b) Items relating to the environmental deterioration factor: 1) Site and environmental conditions 2) Condition of building

The method for prediction of service life determined by deterioration is presented more in detail by use of some examples in Appendix 2 of the Principal Guide. The following terms are used: Standard service life: the time until a deteriorated stage is reached when the whole

building or its parts, elements, components or equipment have degraded under any one of specified conditions, under the circumstances of “normal” design, construction, use,



maintenance and climate exposure. The standard service life has to be predicted on the basis of experience.

Estimated service life: the standard service life multiplied by a variety of factors based

on a more careful consideration of the actual design, construction, use, maintenance and climate exposure of a specific building, part of the building, element, component or equipment.

In the Appendix 2 of the Principal Guide, eight examples are given of prediction of service life of building elements and components. The examples are Wooden buildings (in the case of biological deterioration) Structural elements of reinforced concrete buildings Steel frame buildings - paint coated steel elements Waterproofing layer - exposed asphalt waterproofing system External finishings (coatings) of reinforced concrete building External wall tiling of reinforced concrete buildings Aluminium fittings Piping

For each example, the following factors are listed: Factors relating to inherent durability characteristics

Performance of materials Design level Work execution level Maintenance level

Factors relating to the deterioration

Site and environmental conditions Building conditions

The factors are then quantified and combined in different equations dependent on an evaluation of how and to what extent they influence the service life of the actual building element and component. The estimated service life is calculated as the standard service life multiplied by the various factors combined in different ways depending on the actual product to be evaluated. In order to illustrate the method, two examples from the Principal Guide are presented.

"2. Example of the method for estimating the service life of wooden buildings (In the case of biological deterioration)

4) Estimation of durability value of the structural member in a unit (y). The "y" is calculated by the following expression:

y = ys x B x C x D + M where



ys: The standard durability value of structural members, which is calculated according to Item 5)

B: The coefficient of the design level, which is calculated according to Item 6) C: The coefficient of the work execution level, which is decided according to Item 7) D: The coefficient of the site, environment and building conditions, which is

calculated according to Item 8) M: The coefficient of the maintenance level, which is calculated according to Item 9)"

The references to specific items in the quotation above refer to the Principal Guide [AIJ 1993]. The different items mentioned are explained in the Principal Guide, and it means that each of the given factors can be described in more detail and quantified by other equations taking care of the various aspects which influence each of the factor.

"3. Example of the method for estimating the service life of structural steel frame buildings - paint coated steel elements

4.3 Procedure for estimating service life

The service life (Y) is calculated on the basis of the following equation: Y = (Yss x Bs x Cs x Ms) + (Ysp x Dp x Bp x Cp x Mp) where Yss: Standard service life (years) of steel, according to Item 1) Bs: Part of building where the steel element is installed, according to Item 2) Cs: Execution level of steel element, according to Item 3) Ms: Level of maintenance, according to Item 4) Ysp: Standard service life (years) of coating film on steel component, according to

Item 5) Dp: Area and environment for deterioration of coating film, according to Item 6) Bp: Part of building where the coated element is installed according to Item 7) Cp: Execution level of coating film, according to Item 8) Mp: Level of maintenance, according to Item 9)

The references to different items are similar to what is explained for example 2 on wooden buildings above. From the two examples we see that the number and combination of factors are varied, based on what is regarded to be of importance for the different constructions. 5.2 A Factor Method for Evaluation of Surface Treatment of Wooden

Windows and Doors In Germany, the Association of Window and Facade Manufacturers (Verband der Fenster und Fassadenhersteller e.V.) have published a guideline for prediction of the durability of wooden windows, based on the use of surface treatments [Merkblatt 1997]. In the guideline, some factors are used to evaluate the durability of surface treatments, and the factors are:

fB: exposure conditions



fS: material qualities fR: conditions to reduce the durability

The following conditions to reduce the durability are listed: critical construction spray application of the priming treatment, instead of immersion too light or colourless treatments

The basis for estimation of the durability of the windows is a durability factor h, which is given by a figure in the interval 1.0 to 4.0. The value of h is fixed based on evaluation of a number of conditions, such as material quality, pre-treatment of the wood, colour of the surface treatment and number of layers, and intervals of inspection. The resulting durability factor hges is then calculated based on the following equation:

hges = h x fB x fS x (fR)i For each of the modifying factors the following intervals are given:

fB: 1.0 – 4.0 fS: 1.0 – 1.2 fR: 0.7

For the modifying factor fB, the value is also influenced by use of colourless of coloured surface treatment. The guideline has been established based on systematic studies and experience, and the application of factors has no reference to the factor method of ISO 15686. However, it is an interesting example of a similar treatment of durability and service life of some specific building components by quantifying and combining important factors that are of main influence. It is also interesting to notice that the factors are combined by multiplication. The values of the factors are based on experience and are of course related to actual conditions in Germany. 5.3 Factor Method for Estimation of Service Life of Components and

Assemblies as Presented in ISO 15686 Part 1 The concept of the factor-based evaluation of the service life as described in the Japanese Principal Guide [AIJ 1993] has been introduced in the International Standard for service life planning of buildings, ISO 15686 Part 1 [ISO 2000]. The method is presented in the following way:

"9. Factor method for estimating service life 9.1 Outline of the factor method

The method allows an estimate of the service life to be made for a particular component or assembly in specific conditions. It is based on a reference service life (normally the expected service life in a well-defined set of in-use conditions that apply to that type of component or assembly) and a series of modifying factors that relate to the specific conditions of the case. - - - - -



The method uses modifying factors for each of the following: - factor A: quality of components - factor B: design level - factor C: work execution level - factor D: indoor environment - factor E: outdoor environment - factor F: in-use conditions - factor G: maintenance level

Any one (or any combination) of these variables can affect the service life. The factor method can therefore be expressed as a formula: ESLC = RSLC x factor A x factor B x factor C x factor D x factor E x factor F x factor G."

The reference service life is similar to the standard service life as defined in the Japanese Principal Guide [AIJ 1993], see chapter 5.1. In ISO 15686 Part 1 [ISO 2000], the reference service life is defined as:

"service life that a building or parts of a building would expect (or is predicted to have) in a certain set (reference set) of in-use conditions.”

In the Standard there is also a brief discussion of the use of the factor method, and a discussion of the reference service life as well as each of the modifying factors. This is referred in chapter 6.



6. EVALUATION OF FACTOR METHODS The evaluation of factor methods that is referred in this chapter, regards the factor method as described in the International Standard ISO 15686 Part 1 [ISO 2000]. Since the introduction of the method in the first draft of the International Standard, it has been evaluated in several papers, both on a theoretical basis as well as based on some straightforward applications, some of which are presented in chapter 7. The ISO Standard itself contains a chapter that describes the factor method. In that chapter, there is a general discussion as well as a discussion of the reference service life (RSL) and each of the factors. In the general comments of the factor method, it is said that

“9.2 Use of the factor method The factor method is a way of bringing together consideration of each of the variables that is likely to affect service life. It can be used to make a systematic assessment even when reference conditions do not fully match the anticipated conditions of use. Its use can bring together the experience of designers, observations, intentions of managers, and manufacturers’ assurances as well as data from test houses. - - - - - The factor method does not provide an assurance of a service life: it merely gives an empirical estimate based on what information is available. It is different from a fully developed prediction of service life (as described in clause 8), which will ideally provide the reference service life for a factored estimate. The distinction between estimated and predicted service life should be made when a forecast of service life is given. The information taken into account should also be recorded, so that it is clear whether the estimate is particularly robust or not. - - - - - The factor method can be applied to both components and assemblies. When applied to assemblies it is necessary to consider the interfaces (e.g. joints) between components as well as the components themselves. For example, different external environment and maintenance factors may apply to a whole assembly that relies on sealants to weatherproof the joints between factory made cladding units than that which would apply to each of the individual cladding units.”

In the discussion of the reference service life, it is pointed out that the most reliable way of establishing this is by use of the service life prediction procedure that is briefly described in chapter 8 of ISO 15686 Part 1 [ISO 2000] or fully described in ISO 15686 Part 2 [ISO 2001]. This procedure is based on the RILEM Recommendation [RILEM 1989] that is mentioned in chapters 2 and 4.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Factor Methods: State of the Art 36 An interesting background for the evaluation of the factor method is a study that was published by Assaf et al. [1995]. The authors do not refer to the ISO factor method, but they describe the results of a survey of 90 contractors, 30 architectural/engineering firms and 20 owners from the eastern province of Saudi Arabia. In this survey, 35 defect factors were found during the construction stage of large buildings. All the defect factors are gathered in the six following groups: Construction inspection Civil construction Contractor administration Construction materials Construction equipment Construction drawings

These groups are mainly related to a few of the factors included in the ISO factor method. This indicates that in practice a large number of factors and parameters that influence the service life of a constructed facility, building or building component will likely need to be considered. It will therefore be quite difficult to evaluate the influence of all these factors and parameters, and to suggest reliable values of each of the factors of the ISO factor method. A thorough discussion of the factor method has been presented by Bourke and Davies [1997]. The report is intended to give a contribution to the further development of the method described in ISO 15686 Part 1 [ISO 2000]. The main content of the various chapters of the report are: Conclusions including listing of further research needs Introduction with a brief survey of alternative approaches to predicting service lives for

components Features of factorial approach to service life prediction, with a main focus on the ISO

factorial system Analysis of critical factors affecting the service life Experience of factor systems and recommendations on number of factors, including

presentation of some previous British Standards Three worked examples that were presented in an informative annex in the ISO/DIS

15686 Part 1 In the general summary and conclusions of the report, the authors state that:

“The system would serve initially as a means of permitting objective comparison and analysis rather than as a firm prediction of anticipated years on service. This however should not disguise that the effect of adoption of such a system should be to optimise the selection of components, making large-scale, expensive and disruptive remediation unnecessary. Equally, excessively durable specifications for short-life buildings could be reduced. It would also highlight the ease with which durability could be improved “on the drawing board”, thereby achieving enhanced performance for minimal cost. The many benefits claimed for costs-in-use analysis could finally be achieved, as the critical issue of how long the components should last could be answered. As such, it could contribute to reducing the overall costs of construction and improving the competitiveness of the industry.”


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Factor Methods: State of the Art 37 Lounis et al. [1998] describe the state-of-the-art and the possibilities of standardization of service life prediction of roofing membranes. The main part of the paper focuses on the development towards standardization of service life prediction methods based on a quantitative methodology using stochastic modelling of the performance of the roofing membranes through the use of a Markov chain. However, in the paper are also presented alternative efforts to establish service life prediction methods that may be applied for roofing membranes. The authors refer to the development and evaluation of the factor method that has been done within CIB W080 “Prediction of Service Life of Building Materials and Components”. They state that:

“Despite its practicability, this approach has many shortcomings, which include: (i) It is not performance based and as such no identification of adopted minimum performance requirement; (ii) Arbitrary choices of standard lives and adjusting factors; and (iii) Deterministic approach, despite the large uncertainty and variability of the service life. It appears then, that this method is neither readily available, nor is it likely to be attainable in the near future.”

In the conclusions, the authors state that the use of in-service performance data to develop this stochastic model overcomes the difficulties associated with accelerated life testing and empirical factorial approaches. Hovde [1998] has presented an evaluation of the factor method as presented in ISO 15686 Part 1 [ISO 2000]. It is not based on a practical application of the method, but just based on considerations and discussions, e.g. within CIB W080/RILEM 175-SLM. Hovde stated that there is a strong need for further evaluation of the method. In the short range, he asks for input data both for the quantification of the reference service life (RSL) as well as for the different factors in the equation. In the long range, there will be a need for a more comprehensive evaluation of the factor method, including possibilities of quantitative description of the RSL and the factors. Hovde also pointed out that the method should be evaluated according to the general requirements for service life prediction methods, such as the requirements given in chapter 4 of this report. He gives a brief discussion of the following items that ought to be further evaluated: Estimation of the reference service life (RSL) Important factors Necessary number and type of factors Use of the factors in an equation Reasonable span of the values of the different factors Relative importance of the factors Uncertainty of the factors Factor dependency on material or component to be evaluated Important considerations for practical use

The last item is illustrated in the Figure A6.1. Teplý [1999] describes the possibilities and limitations for development of the factor method into a simple and yet sufficiently general method for service life prediction of structural members. After a presentation of the factor method, the author illustrates the use of the method by estimation of the service life of a reinforced concrete structure of a one-story


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Factor Methods: State of the Art 38 factory hall. In the conclusions, Teplý states that the following critical circumstances affect the service life and therefore require attention: A structure may be exposed to a combined effect of aggressive processes Combination of materials and the design of joints Movable parts and the tolerances of these movements Protective layers Accessibility for inspection, repair or replacement of some parts.

The author also concludes that in order to achieve a further development and use of the factor method, building contractors and prospective owners have to start using the method based on the existing data and experience. COMPLEXITY SIMPLICITY APPLICABILITY CREDIBILITY Figure A6.1: Relations to take into consideration in evaluation of the factor method

(From [Hovde 1998]). An improvement of the factor method by introduction of a statistical evaluation of the individual factors has been studied by Aarseth and Hovde [1999]. A “step-by-step” principle has been applied, and this was developed within the project planning area. Initially, it was developed in Denmark in the 1970’s, but has since been further developed in Norway. It is a tool for improving the quality of the basis for decisions in project planning under uncertainty. As with estimating service life, a basic problem in project planning at an early stage is the lack of relevant information. A value of each of the individual factors of the factor method is given by use of a triple estimate, a minimum value, a maximum value and the most expected value. In order to give a reasonably good statistical representation of the triple estimates, an Erlang density function is used. The authors have applied this modified factor method for estimation of the service life of a wooden window, which is also used as an example in ISO 15686 Part 1 [ISO 2000]. By using the simple factor method as shown in the ISO Standard, the estimated service life of the window is 62.2 years, i.e. 60 years. By using the step-by-step principle and a statistically modified factor method, the estimated service life is calculated to 50 ± 6 years. In the conclusions, the authors state that:

“The “step-by-step” principle enables a stochastic handling of the modifying factors in the ISO factor method by performing a triple estimate for each factor. After the statistical calculation the estimated service life is expressed as three figures: the expected value plus/minus one standard deviation.



The “step-by-step” principle also demonstrates a systematic approach to the estimating process, including the process of defining, subdividing and estimating the modifying factors in the ISO factor method.”

Moser [1999] has also carried out an evaluation and improvement of the factor method by use of statistical methods. Instead of a joint statistical treatment of all the factors as shown by Aarseth and Hovde [1999] in the “step-by-step” principle, he applies an individual statistical treatment of each factor. This is done by using different statistical distributions for each factor (i.e. deterministic, normal, lognormal or Gumbel), and by giving individual figures for the minimum, most probable and maximum value of each factor. Moser has given an example of estimating the service life of windows on all four sides of a building. In his conclusions, he states that:

“The use of probabilistic tools for planning maintenance or replacement costs is no longer only restricted to projects having large funding requirements or numerous assets. Making full use of the information, e.g. given in ISO/CD 15686 and modified by professional opinion, permits the use of variables instead of deterministic factors in the equation for the estimated service life. The results give a much more detailed insight into the service life of building components involved and allow a far better planning of the investments required.”

Rudbeck [1999] has made an extensive discussion of service life prediction methods in which he describes the assessment of the reference service life (RSL). Different methods for determination of RSL are presented, and a guideline for choosing between these methods is suggested. The author also presents a discussion of the factor method for service life prediction and concludes that:

“So before the most correct method can be determined, assisted by the field data, one can only look at the possible advantages and disadvantages that the methods present. From this viewpoint, the methods based on the ISO proposal with a probabilistic approach, described by Aarseth and Hovde (1999) and Moser (1999), seem to be the most usable. The requirement for input to develop the needed functions in the two methods is the same, but they report the input (i.e. the functions) in different ways. The method suggested by Aarseth and Hovde (1999) reports the data in a very aggregated form (a low, a medium and a high estimate for each parameter), whereas the method described by Moser (1999) enables the use of all available data. From a statistical point of view, the latter method therefore seems to be the most reliable.” - - - - - “The conclusion of the discussion regarding the different methods for estimating service life of components is that unless very large sample sizes are considered a throughout probabilistic approach may not be the best solution. Some building components are produced in large numbers, but as they are applied in numerous ways, measurements of performance over time may not be comparable. Instead, the focus may be on the hybrid methods, the coupling of the factor approach and the probabilistic approach, due to the advantages this way to proceed can offer.” - - - - -



“If the probability transition matrices for the Markovian model can be developed and validated, that model would be the one recommended when predicting service life of building components.”



7. APPLICATION OF FACTOR METHODS To date, the use of the ISO factor method for prediction of service life of building materials and components has been very limited. Most of the published cases are described in research papers or reports where examples of the use of the applications are provided. Widespread practical application of the method has been limited due to the lack of knowledge of the method among practitioners (i.e. architects, consultants or building owners and managers) or due to the need for useful values of the various factors used in the method. Some examples of application of factor methods are presented here. There is also an extensive research and development going on regarding design for durability of buildings, sustainable construction and development of sustainable buildings. Several papers have been published during the last years describing different challenges in this area, and they show the necessity of implementing service life of materials and components into the overall methods. Several examples are Wyatt [1998], Lucchini and Wyatt [1999] and Wyatt and Lucchini [1999]. In the later paper, the authors state in the conclusions that

“Whilst there is a complex but important relationship between the building performance the designer may have pre-defined and the level of reliability achieved one cannot guarantee a building’s life future. One can however, be prudent and accept the place of a service life practice and life care to at least seek a building product life. So adopting the service life approach that reflects materials, components and systems service loss and durability would help to improve cost certainty of building ownership and begin to address the challenges posed in striving for sustainable cities and buildings. For it has become clear that designing for durability has an immensely important contribution to make to both the work of CIB W094 and CIB W080 as well as ISO task group responsible for developing the Design Life of Building’s Standard. It is now believed that service life and its practice will come to form and be seen as the corner stone of building asset management’s life care.”

Strand and Hovde [1999] have carried out at study of how service life data of exterior surface materials (wood and brick) influence the LCA of the materials. The authors wanted to emphasize the need for service life data in LCA, how the data occur and how they might influence results. Building materials and components are used for a longer period of time than most other products. LCA of a building product therefore necessitates gathering of data that will be valid for a longer period of time. The building, components and materials also have different life spans, as illustrated in chapter 3 in this report. The authors apply the factor method as described in ISO 15686 Part 1 [ISO 2000], but mainly highlight the use of factors E (outdoor environment) and G (maintenance level). LCA is carried out for two climates (industrial and rural inland) and for facades facing north or south.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Factor Methods: State of the Art 42 Different intervals for painting, cleaning and replacement are also used. In the conclusions it is stated that:

“Service life prediction methods (SLP) and life cycle analysis (LCA) deal with some common problems. Most of the data in SLP are also needed in LCA. Information regarding the variation associated with the ESLC is crucial information in LCA. It is extremely important that the basis of both the stated SL and the performed LCA is clear, like e.g. maintenance type and interval. In this study it looks as if the maintenance is in fact the most important information. - - - - - It is a problem that LCA techniques themselves give rise to large variation e.g. due to for example evaluation methods. LCA and SLP can be used together to optimise the service life and the resulting environmental load from different materials. Further development is needed for both systems.”

Hovde [1999] has presented the need for service life prediction of passive fire protection systems. He refers to the factor method as described in ISO 15686 Part 1 [ISO 2000]. Passive fire protection has got an increasing interest and importance in relation to the introduction of performance based building and fire codes. This makes it important to predict the durability and service life of the fire protection, and this will be a specific area for application of service life prediction methods. A joint committee between RILEM and CIB (RILEM TC 172-EDM/CIB TG 22 “Environmental design methods in materials and structural engineering”) has been working on development of methods for environmental design of materials and structures. A progress report of the work has been presented by Sarja et al. [1999]. In the presentation of the progress report it is explained that the incorporation of an environmental viewpoint into the design of materials and structures, it is necessary to reconsider the entire context of the design process in order to integrate environmental aspects into a set of other design aspects. Further, this kind of process is called integrated life cycle design, and it is said that the aim of the process consists of assimilating, in a practical manner, the multiple requirements of functionality, economy, performance, resistance, aesthetics and ecology all into the technical specifications and detailed designs of materials and structures. The joint RILEM/CIB committee is producing a manual that will provide methods and methodologies for structural design in order to meet the requirements of sustainable development over the entire service life of the structures. The scope of the manual includes both bearing and non-bearing structures of buildings, bridges, towers, dams and other structural facilities. The main phases of the life cycle design procedure are said to be: Analysis of the actual requirements Interpretation of the requirements into technical performance specifications for

structures Creation of alternative structural solutions Life cycle assessment and preliminary optimisation of the alternatives Selection of the optimal solution between the alternatives Detailed design of the selected structural system


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Factor Methods: State of the Art 43 In the progress report a summary of the integrated life cycle design phases and the specific design methods described in the manual are presented in a table as shown in Table A7.1. Table A7.1: Integrated life cycle design process and central methods for application.

(From [Sarja et al. 1999]).

Design phase Life cycle design methods 1. Investment planning Multiple criteria analysis, optimisation and

decision making. Life cycle (financial and natural) economy.

2. Analysis of client’s and user’s needs Modular design methodology. Quality Function Deployment Method (QFD).

3. Functional specifications of the buildings Modular design methodology. Quality Function Deployment Method (QFD).

4. Technical performance specifications Modular design methodology. Quality Function Deployment Method (QFD).

5. Creation and sketching of alternative structural solutions

Modular design methodology.

6. Modular life cycle planning and service life optimisation of each alternative

Modular design methodology. Modular service life planning. Life cycle (financial and natural) economy calculations.

7. Multiple criteria ranking and selection between alternative solutions and products

Modular design methodology. Quality Function Deployment Method (QFD). Multiple criteria analysis, optimisation and decision making

8. Detailed design of the selected solution Design for future changes. Design for durability. Design for health. Design for safety. Design for hygrothermal performance. User’s manual. Design for re-use and recycling

In the description of the design process in the progress report, there is also a presentation of alternative methods that can be applied for durability design. These are: Durability design with structural detailing rules Design of the environmental conditions of the structures for durability Protection of the materials and structures against deterioration Lifetime safety factor method Reference factor method

The reference factor method is the same as the ISO factor method. In the conclusions of the progress report, it is stated that

“Concerning materials and structures, new basic knowledge will be needed especially regarding environmental impacts, hygrothermal behaviour, durability and service life of materials and structures in varying environments. Structural design methods that are capable of life cycle design, multiple analysis decision-making and optimisation will have to be further developed. Recycling design and technology demand further research in design systems, recycling materials and structural engineering. The knowledge obtained will have to be put into practice through standards and practical guides.”


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Factor Methods: State of the Art 44 As presented in chapter 6, Rudbeck [1999] has described alternative service life prediction methods in his Ph.D. thesis. The thesis also comprises a description of integrating durability in future building design, and this gives a demand for application of the service life prediction methods. The author discusses the following two methods for integrated life cycle design which were under development in 1999: Integral Building Envelope Performance Assessment (IBEPA), developed by the

International Energy Agency (IEA) Annex 32. Building Envelope Life Cycle Asset Management (BELCAM), developed by the

National Research Council, Canada. Rudbeck also shows how the durability aspect can be integrated into the design process of two flat roof constructions. Hed has carried out a study of service life planning for a multi family building, which was built in Gävle, Sweden, in 1999. The results have been presented in different publications [Hed 1998, 1999, 2000]. ISO 15686 Part 1 [ISO 2000] was used as a basis for the study, and the final results are given in [Hed 2000. The service life planning was integrated into the design of the building and followed the building process from the design phase to the beginning of the construction of the building. The report comprises three separate papers, and in one of the papers is given a presentation and discussion of the application of the factor method as presented in ISO 15686 Part 1[ISO 2000]. The author states that:

“A problem is that there are still few tests performed of material and component service life, comprising all the effects required of the building component when it is in operation in the building, i.e. following the service life prediction methodology (ISO 1999). - - - - The accuracy of the estimated service life is of course suffering from this fact, so one has to discuss if it is worth the effort of doing the estimations or not. If the goal is to find a precise value it is clear that the goal is not reached. But if the goal is to improve the general situation in service life planning the answer is yes. The factor method in ISO/DIS 15686-1 is meant to be a tool to improve the estimation of the service life. It was found in the project that this method did not improve service life estimations. This opinion is summarised in the following. Uncertainty of RSLC and values of Factors. The factorial formula (1) comprises in the right side of a reference value (RSLC) and the adjusting Factors, A to G. If the reference value cannot be determined accurately it is not appropriate to adjust these values with a set of uncertain Factors. Uncertainty of the effect by combination of Factors. The method does not support the thoughts that one needs knowledge of cause and effect to estimate the service life. The estimation will be based on uncontrollable occurrences, which can act independently of each other.”

In Finland, a project has been carried out to develop an information management system concerning service life of building products. The purpose of the system is to serve for designers, contractors and organizations responsible for the care and maintenance of buildings. The subject is dealt with from the point of view of product manufacturers paying


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Factor Methods: State of the Art 45 attention on the content, formulation and delivery of the service life information offered by the manufacturers. The project results are presented in a report [Häkkinen et al. 2001] in which a comprehensive information system of service life behaviour of building products is introduced. The idea is that this information is given by the manufacturers and it is needed in building design, building and assembling and within the use, care and maintenance of buildings. However, the report does not deal with the service life prediction principles and methods. An important part of the project was aimed at developing service life prediction methods for facades and roof coverings. It covered facades made of concrete, masonry, timber and steel plate and roof coverings made of bitumen membrane, steel plate and concrete tiles. The factor method described in ISO 15686 Part 1 [ISO 2000] was used. Application of the factor method to estimate the service life of concrete in facades was presented by Vesikari [2000]. As a new method of producing the factor values a computer simulation was applied. By this simulation the environmental stresses, temperature and moisture content in a structural cross-section and degradation of materials were described. This made it relatively easy to determine out the essential properties and structural factors and their effects on service life. The study showed that the use of only a single of a factor was not sufficient to cover all the material effects or the environmental effects. Two partial factors were therefore established for material parameters, i.e. A1 (dependent on air porosity) and A2 (dependent on water-cement ratio). Similarly, two partial factors were established for environmental parameters, i.e. E1 (dependent on the direction) and E2 (dependent on the geographical location). These practical factors are multiplied to obtain the total effect due to material and environmental parameters. The application of the factor method was illustrated for a multi-storey dwelling facade in southern Finland having a design life of 50 years. The factor method was found to be a suitable prediction method for practical service life design. In a Nordic Research Training Course funded by the Nordic Academy for Advanced Study (NorFA) that was carried out during 2001 regarding service life of buildings and building products, some of the participants gained interest in the factor method as a simple tool for service life prediction. The title of the course was "Service life of buildings - from theory to practice", and the research students carried out individual project tasks related to the main topic of the course. The project reports are at a preliminary stage, and the intention is that they will form the basis for papers to be published in journals, conferences, etc. In the reports are discussed the possibilities to apply the factor method for service life prediction of different building products, components, structures and installations, such as surface products, exterior wood products, external renderings, sulphur concrete, a solar collector and a fibre reinforced polyester pedestrian bridge deck. This clearly underlines the fact that there is a need for practical and simple service life prediction tools, and that the factor method may be evaluated for practical use on a wide variety of products in the future. 1. One of the reports at the Research Training Course was presented by Marteinsson [2001]

in which results of an extensive condition survey of wooden windows in Iceland were presented. The factor method was applied, as described in ISO 15686 Part 1, to estimate the service life of the window components. Results from the condition assessment and the house owners' answers to a questionnaire are combined, and a Weibull probability distribution is then used to evaluate the estimated service life of the windows. In the conclusions of the study, Marteinsson states that



"The results show that for some materials at least, the synergy between agents that affect the durability of materials is so great that it is difficult to give each and one of the factors in the standard a value even based on results gained by systematic study of the object in use. The results show furthermore that for materials where the durability gains very much from good care and maintenance, then it is a good way to decide on the probability distribution of the service life from information from the user. - - - - - The realistic span in multiplication factor is thus considerable and the user of the methodology will not be able to choose appropriate values for the factors without extensive knowledge about materials and local building practice. In any case he needs information about the main factors for the component and material considered, (and) what span is normal for the factors. The methodology is far from easy to use correctly and at the risk of results being evaluated as being more precise than is reasonable at the present time. - - - - - Handbook of worked examples would be of great value for the designer, without this freedom for each user to define the factors is too great."



8. FURTHER DEVELOPMENT OF FACTOR METHODS

The state of the art of factor methods, with main focus on the method described in ISO 15686 Part 1 [ISO 2000] that is presented in the previous chapters, should be a good basis for further development and application of such methods. However, there are still many topics that have to be evaluated further before the methods will come into a practical application. The following topics will be of importance:

Determination and collection of data for the reference service life (RSL) and the

individual factors Development of sound engineering methods that combine the benefits of more

sophisticated probabilistic methods and simple deterministic methods. A practical approach seems to be to describe the different factors by use of statistical distributions.

Practical use of the methods in case studies of specific building materials and components or of specific buildings

Application of the methods in life cycle assessment of building materials and components and environmental evaluation methods for buildings

Application of the methods in integrated life cycle design and design for durability of buildings



REFERENCES

Aarseth, L.-I. and Hovde, P. J. (1999): A stochastic approach to the factor method for estimating service life. 8th International Conference on Durability of Building Materials and Components, Vancouver, Canada, 30 May - 3 June.

Architectural Institute of Japan (1993): The English Edition of Principal Guide for Service Life Planning of Buildings, Architectural Institute of Japan, Japan.

Assaf, S., Abdulmohsen, A-H. and Al-Shihah, M. (1995): The effect of faulty construction on building maintenance, Building Research and Information, V. 23 N. 3, pp. 175-181.

Bourke, K. and Davies, H. (1997): Factors affecting service life predictions of buildings: a discussion paper. Laboratory Report. Building Research Establishment, Garston, Watford, UK.

Brand, S. (1994): How buildings learn. What happens after they are built? Viking, UK.

British Standards Institution (1992): BS 7543:1992 Guide to Durability of Buildings and Building Elements, Products and Components. British Standards Institution, London, UK.

Building Industry Authority (1992): New Zealand Building Code. Clause B2 Durability. Building Industry Authority, Wellington, New Zealand.

Canadian Standards Association (1995): CSA S478-1995 Guideline on durability in buildings, Ottawa, Canada.

CIB - Council for Research and Innovation in Building and Construction (1999): Agenda 21 for Sustainable Construction, CIB Report Publication 23, Rotterdam, The Netherlands.

Duffy, F. and Henney, A. (1989): The Changing City, London Bullstrode, London, UK.

European Commission (1999): Durability and the Construction Products Directive, Guidance Paper F, European Commission, DG III, Brussels, Belgium.

European Organization for Technical Approval (1999a): Assumption of working life of constructional products in guidelines for European Technical Approval, European Technical Approvals and harmonized standards. EOTA Guidance Document 002, European Organization for Technical Approval, Brussels, Belgium, December.

European Organization for Technical Approvals (1999b): Assessment of working life of products. EOTA Guidance Document 003. European Organization for Technical Approvals (EOTA), Brussels, Belgium, December.

European Union (1988): Council Directive of 21 December 1988 on the approximation of laws, regulations and administrative provisions of the Member States relating to construction products, European Union, Brussels, Belgium, December.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Factor Methods: State of the Art 50 European Union (1994): Interpretative Documents of Council Directive 89/106/EEC. Official

Journal of the European Communities, C62. Vol. 37, 28. February 1994.

Häkkinen, T., Vares, S., Vesikari, E. and Karhu, V. (2001): Service life planning. Product specific service information, Report VTT-JULK-848, VTT, Espoo, Finland, February.

Hed, G. (1998): Service Life Planning in Building Design. CIB World Building Congress 1998, Gävle, Sweden, 7-12 June.

Hed, G. (1999): Service life planning of building components. 8th International Conference on Durability of Building Materials and Components, Vancouver, Canada, 30 May - 3 June.

Hed, G. (2000): Service Life Planning in Building Design. RD-report No 4. Centre for Built Environment, University of Gävle, Gävle, Sweden, April.

Hovde, P. J. (1998): Evaluation of the Factor Method to estimate the service life of building components, Materials and Technologies for Sustainable Construction, CIB World Building Congress 1998, Gävle, Sweden, 7-12 June.

Hovde, P. J. (1999): Needs for service life prediction of passive fire protection systems. 8th International Fire Science and Engineering Conference (Interflam '99), Edinburgh, Scotland, 29 June – 1 July.

International Organization for Standardization (1984): ISO 6241 Performance standards in building - Principles for their preparation and factors to be considered, Geneve, Switzerland.

International Organization for Standardization (2000): ISO 15686-1 Buildings and constructed assets - Service life planning - Part 1: General principles. International Organization for Standardization, Geneve, Switzerland.

International Organization for Standardization (2001): ISO 15686-2 Building and Construction Assets - Service Life Planning - Part 2: Service Life Prediction Procedures. International Organization for Standardization, Geneve, Switzerland.

International Organization for Standardization (1998): ISO 14040 Environmental management - Life cycle assessment - Principles and framework. International Organization for Standardization, Geneve, Switzerland.

Lounis, Z., Lacasse, M. A., Vanier, D. J. and Kyle, B. R. (1998): Towards standardization of service life prediction of roofing membranes. Roofing Research and Standards Development: 4th Volume, ASTM STP 1349, American Society for Testing and Materials, Philadelphia, PA, USA.

Lucchini, A. and Wyatt, D. P. (1999): A systematic vivendi for design for durability. 8th International Conference on Durability of Building Materials and Components, Vancouver, Canada, 30 May - 3 June.

Martin, J. W. et al. (1994): Methodologies for predicting the service lives of coating systems, NIST Building Science Series 172, National Institute of Standards and Technology, Gaithersburg, MD, USA.

Marteinsson, B. (2001): Durability of wood windows and the factorial method of ISO15686-1. Project report, Nordic Research Training Course "Service life of buildings - from theory to practice". The Icelandic Building Research Institute, Keldnaholt, Iceland.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Factor Methods: State of the Art 51 Merkblatt (1997): Klassifizierung von Beschichtungen für Holzfenster und Haustüren. VFF

Merkblatt HO.01 - Entwurf. Verband der Fenster- und Fassadenhersteller e.V., Frankfurt, Germany, October.

Masters, L. W. (1987): Service life prediction – A state of the art. 4th International Conference on Durability of Building Materials and Components, Singapore, 4-6 November.

Moser, K. (1999): Towards the practical evaluation of service life - illustrative application of the probabilistic approach. 8th International Conference on Durability of Building Materials and Components, Vancouver, Canada, 30 May - 3 June.

National Science Foundation (1993): Civil Infrastructure Systems Research: Strategic Issues, National Science Foundation, Washington D.C., USA, January.

Norges Standardiseringsforbund (1994): NS 3422 Specification texts for operation, maintenance and renewal of buildings and civil engineering works. Norges Standardiseringsforbund, Oslo, Norway.

Norges Standardiseringsforbund (1995): NS 3424 Condition survey of construction works. Contents and execution, Norges Standardiseringsforbund, Oslo, Norway,.

RILEM (1995): Workshop on environmental aspects of building materials and structures, VTT, Espoo, Finland,.

RILEM (1989): Systematic methodology for service life prediction of building materials and components. RILEM Recommendation, Materials and Structures, Vol. 22, pp. 385-392.

Rudbeck, C. (1999): Methods for designing building envelope components prepared for repair and maintenance. Ph.D. thesis. Report R-035, Department of Buildings and Energy, Technical University of Denmark, Lyngby, Denmark..

Sarja, A. and Vesikari, E. (1996): Durability Design of Concrete Structures. RILEM Report 14, E & FN Spon, London, United Kingdom.

Sarja, A., Fukusima, T., Kümmel, J. and Müller, C (1999).: Environmental design methods in materials and structural engineering - Progress report of RILEM TC 172-EDM/CIB TG 22. Materials and Structures, Vol. 32, pp. 699-707, December.

Strand, S. M. and Hovde, P. J. (1999): Use of service life data in LCA of building materials. 8th International Conference on Durability of Building Materials and Components, Vancouver, Canada, 30 May - 3 June.

Teplý, B. (1999): Service life prediction of structures - Factor Method. Stavební Obzor (Structural Horizon), Vol. 8, pp. 137-139 (In Czech).

Vesikari, E. (2000): Estimation of service life of concrete facades by the factor approach. International RILEM Workshop on life prediction and ageing management of concrete structures. Cannes, France, 16-17. October.

Wyatt, D. P. (1998): Building for life - a sustainable objective. The Design Agenda. CIB W96 Architectural Management, Brighton, United Kingdom.

Wyatt, D. P. and Lucchini, A. (1999): A service life design for lifecare management. 8th International Conference on Durability of Building Materials and Components, Vancouver, Canada, 30 May - 3 June.


Swiss Federal Laboratories for Materials Testing and Research Überlandstrasse 129 CH-8600 Dübendorf Switzerland

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ENGINEERING DESIGN METHODS FOR SERVICE LIFE PREDICTION

CIB W080 / RILEM 175 SLM: Service Life Methodologies Prediction of Service Life for Buildings and Components

Task Group

Performance Based Methods for Service Life Prediction

Konrad Moser

EMPA – Swiss Federal Laboratories for Materials Testing and Research Laboratory for Concrete and Construction Chemistry March 2004

CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 53



SUMMARY After a literature review and an appraisal of the state of the art, the subtask-group proposes a basic approach that can be applied to the factorial method for standard cases as well as to other service life prediction methods that employ mathematical relations for service life.

As opposed to using simple numerical factors, as is done in the original factor method, this approach incorporates the use of probability density functions for factors as well as for the service life of individual components to arrive at an overall estimate of a building system’s service life. The density distributions are established using reliable and understandable engineering techniques applied in a systematic and straightforward manner.

Three examples are shown to illustrate the proposed procedure for different basic equations and different quality of input data.



CONTENTS

SUMMARY..................................................................................................................................... 54

CONTENTS ................................................................................................................................... 56

1. SCOPE ........................................................................................................... 58

2. LITERATURE REVIEW AND STATE OF THE ART ..................................... 59 2.1 General ............................................................................................................................. 59 2.2 Need for Service Life Design........................................................................................... 59 2.3 End of Service Life .......................................................................................................... 60

2.3.1 General definitions................................................................................................. 60 2.3.2 Definitions in structural engineering..................................................................... 60 2.3.3 Design code and legal definitions.......................................................................... 61 2.3.4 Project definitions.................................................................................................. 61

2.4 Factor Method .................................................................................................................. 61 2.5 General Aspects of the Probabilistic Methods ................................................................. 62 2.6 Application of Probabilistic Prediction Methods ............................................................. 62

2.6.1 Markov model for the deterioration....................................................................... 62 2.6.2 Variables defined as distributions.......................................................................... 65 2.6.3 Practical examples of probabilistic methods ......................................................... 71

2.7 Developments of the Factorial Method Towards Probabilistic Methods......................... 74 2.8 Other Concepts................................................................................................................. 78

3. PROPOSED ENGINEERING DESIGN METHODS........................................ 80 3.1 Basic Requirements for Engineering Design Methods .................................................... 80 3.2 Proposed Principle ........................................................................................................... 80 3.3 Examples of Engineering Design Methods ...................................................................... 80

3.3.1 Data acquisition by the recursive Delphi method.................................................. 81 3.3.2 Application of engineering design methods ........................................................... 81 3.3.3 Example 1: Engineering design method based on equation of factorial

method .................................................................................................................. 82 3.3.4 Example 2: Engineering design method based on modified equation

method and scarce data ....................................................................................... 86 3.3.5 Example 3: Engineering design method based on simplification of the

probabilistic method............................................................................................. 89 4. CONCLUSION AND OUTLOOK ................................................................. 91

4.1 Proposed Engineering Design Methods ........................................................................... 91 4.2 Further Developments ...................................................................................................... 91 4.3 Research Needs ................................................................................................................ 91

REFERENCES ................................................................................................... 92 Abbreviations and database.................................................................................................... 95



1. SCOPE The scope of work of the sub-task group “Engineering Method” was defined by the joint commission CIB W 80 / RILEM 175 SLM: Service Life Methodologies in 1999 and entices the following four steps:

1. Gain an overview on the main methods applied to research and/or large engineering projects using the scientific approach. (These methods often apply mathematical models and stochastic processing to the design data.)

2. Look for modifications of the factorial method towards the methods of the scientific approach.

3. Define the level of complexity of models and type and amount of data to be used in an engineering design method.

4. Propose an engineering design method or several engineering design methods, preferably developed on and applied to typical case studies.

This report summarised the work done under the above scope.



2. LITERATURE REVIEW AND STATE OF THE ART

2.1 General A literature review was performed, mainly concentrating on conference proceedings (see references for details). Reviewing the available literature, it was noted that, although titles of papers in earlier conferences indicated quite specific topics on service life prediction, their contents appear to be fairly general. Earlier papers mainly give outlines and point at areas of work to be done [Masters and Brandt 1989].

As a consequence, this state of the art review concentrates on publications as far back as about 1996. In the following text, the relevant and in general the most recent references to the topics dealt with are reviewed.

This report is limited to techniques for the prediction of service life. The term service life often appears within life cycle analyses (LCA). Service life can be part of an LCA, but an LCA is rather more comprehensive and comprises at least a calculation of all costs from cradle to grave inclusive of all investments over the entire lifetime and an assessment of the environmental impacts. LCAs as such are not dealt with in this report.

2.2 Need for Service Life Design In 1996 the need for service life design was identified and standardisation was suggested [Frohnsdorff 1996, Frohnsdorff and Martin 1996]. Nireki [1996] showed several approaches to solve the durability and service life issues respectively and identified needs for further research. In 1997 the need for a state of the art report to service life design was stated [Jernberg et al. 1997].

In studies to the cost benefit analysis with regard to road bridges, several authors give estimations of the functional service life of these bridges [De Brito and Branco 1998, Thoft-Christensen 1997].

Aikivuori in “Critical loss of performance – what fails before durability” [1999] points out, that service life limited by durability is seldom reached, as components are refurbished earlier due to other reasons:

Empirical research has been carried out to find out the actual reasons for initiation of repair projects on buildings. This research has shown that the owners of the buildings actually experienced the user requirements predominantly outside of the range of durability failures. Only 17 % of the repair projects were initiated primarily because of deterioration. The critical loss of performance seems to primarily be in the range of a subjective perception of the building. Very little technical or economical rationality can be seen in the actual decisions made on building refurbishment. In most cases the limiting factor for service life is not durability.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 60 Life cycle economics of the buildings have (according to the empirical findings) been evaluated on a technically/economically irrational basis. Decision-makers pay little attention to the condition and remaining potential for service life of the building components. They pay very limited attention to economical expectations. Optimisation of economical factors of the buildings is the primary goal in less than 10 % of repair projects.

If service life is seen as the actual time in service of the building components, the basis of service life prediction models should not be based on durability or economics of the building components only. Durability is of course the limiting factor for service life in the sense that service life can not exceed the limitations set by durability, but in fact the actual service life seldom reaches the full potential life time of the components limited by durability. The forecasting of the refurbishment requirements should therefore not rely on the durability-based concepts only. Asset and maintenance management should pay more attention to the more critical perception of the perceived quality of the buildings.

The problem of service life design has attracted more and more attention, mainly due to pressure from owners requiring such a design, supported by the CPD [1988] and EOTA requiring the topic to be addressed properly (see [Sjöström et al. [2002]).

2.3 End of Service Life

2.3.1 General definitions All design methods require clear definitions of the end of the service life. This is however not a universal and easily defined value. In general terms it is the point in time, when the foreseen function is no longer fulfilled. The properties of a building part can be split up into several sub-properties, e.g.

Safety: The integrity of the building part is maintained at the standard level of safety, • •

•

Function: The required function is fulfilled, (i.e. deflections are still within limits, a window can easily be opened and closed, etc.), Appearance: The expected appearance is given (i.e. the surface of the building part is of acceptable appearance, the glazing of windows has not deteriorated or turned opaque, etc.).

2.3.2 Definitions in structural engineering In structural engineering, depending on the function of the part of a building or structure, engineering criteria for the end of the service life are often used, in order to permit a meaningful calculation of the service life as such.

Typical engineering criteria are:

• A minimum concrete cover for a given environment. This definition represents standard practice based on experience, but is not based on a specific clearly defined service life.

• The arrival of the carbonisation front to the outside face of the reinforcing steel is considered to be the end of service life,

• The arrival of the front of the chloride ingress to the outside face of the reinforcing steel is considered to be the end of service life. These two definitions are rather conservative.



• The onset of spaulling is considered to be the end of service life. This definition assumes, that either spaulling is impairing the appearance beyond acceptance, or that shortly after the onset of spaulling, the required level of safety is not maintained any more due to corrosion of the reinforcement now lacking the protective concrete cover.

2.3.3 Design code and legal definitions The Swiss code of practice code SIA 160 [1989] requires that both the safety limit state and serviceability limit state be checked. As soon as properties of a structure are time dependent, this requirement would actually enforce a service life calculation.

The European Construction Products Directive [CPD 1993] and the Swiss Bauproduktegesetz (National law covering building products, 2001) both address service life as the period of time during which the essential requirements have to be fulfilled.

2.3.4 Project definitions The designer has to set up the relevant criteria for the building or structure considered in consultation with the owner, depending on the requirements of safety, function and appearance. For this purpose Quilling et al. [2002] propose a framework for service life design which concentrates on a holistic approach and defines the necessary steps throughout the design process in order to ensure the data exchange between the parties involved: BRE (Building Research Institute) are currently developing a generic service life design system which will provide specific guidance for concrete structures. This paper describes the work that has been carried out to date in the development of the overall framework of the design system. This framework will be expanded as the project progresses to provide targeted guidance and tools to assist the practising engineer in achieving a structure that is durable for its required service life and in optimising whole life costs.

2.4 Factor Method The factor method and it’s developments are covered comprehensively in the respective part of this state of the art report: Factor methods for service life prediction: A state of the art. A summary can be found in Hovde [2002], a recent application in Abu-Tair et al. [2002].

The factorial method according to ISO/CD 15686 identifies the main factors of influence with regard to service life and there from, a plain figure for the service life of the building or building component can be calculated. Knowing the main factors of influence and the overall behaviour of a component facilitates the understanding of the relevant issues, but does not reflect reality very closely.

Examples on the factor method can be found in many publications, e.g. Strand [1999]. The shortcomings of the method are discussed in some of them. These main shortcomings can be summarised as follows:

• • • •

The plain multiplication of factors, which in reality might have a different weight, The result being a single figure instead of a result to reflect variance of reality, The data still to be accumulated, The lack of a direct relation to data gathered e.g. on environment, climate, installation quality, in use conditions, etc. The factors are usually set basing directly on the behaviour of the component in a given set of conditions, rather than basing on the



influence of individual parameters such as regimes of rainfall, temperature, wetting time, type of use, etc.

Considering the efforts in gathering input data, the simple figure result of the factor method, when executed as set out in ISO 15686, seems not to be adequate.

2.5 General Aspects of the Probabilistic Methods Examples of service life predictions using probabilistic tools can be found in numerous publications. Most deal with a single material or exposition. The main fields of application are the service life of reinforced concrete, the service life of pavements (streets or airports), see McNerney et al. [1997], Flintsch et al. [1997] and the service life of (wooden) building envelopes such as windows, wall claddings and roofs.

Apparently concrete is a dominant material as far as durability under severe conditions is concerned. Examples of durability in view of chloride ingress into concrete are quite common. Most studies deal with steady states as far as exposition is concerned. Some of the authors however, have considered transient (humidity, wetting, drying, etc., see e.g. Vu and Stewart [2002]).

In many cases, data has been collected and variables fitted to them. Open databases to draw upon seem not to be available yet. Those methods being applied in projects that have paying clients may result in the preparation of reports that are not freely disseminated.

2.6 Application of Probabilistic Prediction Methods Degradation is generally regarded as a stochastic process and the main parameters are in most cases known. Variations or secondary parameters on the other hand are often not explicitly and numerically taken into account, but their influence results in a considerable scatter of the behaviour of the structure.

2.6.1 Markov model for the deterioration The Markov model assumes deterioration to be a stochastic process governed by random variables. The structure may be split into a number of components, which deteriorate randomly. The main parameters of the deterioration are established for each component, together with the deterioration variables versus time. In Figure B2.1 deterioration using seven stages for condition rating is shown.

Figure B2.1: Markov deterioration function Research projects and large engineering projects often rely on models like the Markov model:


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 63 • Abraham and Wirahadikusumh [1999] in “Development of prediction models for sewer

deterioration” treat service life of sewer lines: Due to their low visibility, rehabilitation of sanitary sewers is often neglected until catastrophic failures occur. Neglecting regular maintenance of these underground utilities adds to life-cycle costs and liabilities, and in extreme cases, stoppage or reduction of vital services. Incorporating condition data and deterioration patterns of the city's sewer system is pivotal for obtaining a realistic assessment of the city's infrastructure. The paper explores the probability-based Markovian approach for modelling deterioration. This approach is based on the assumption that since the behaviour of sewer lines (i.e. the rate of deterioration) is uncertain, the selection of an appropriate repair strategy is also an uncertain procedure. Probability-based prediction models enable the comparison of the expected proportions in given condition states with the actual proportions observed in the field, and in this way possible defects in construction, materials, quality control, etc., can be identified. Expert opinions from engineers, who have developed the sewer assessment surveys for the City of Indianapolis, Department of Capital Asset Management (DCAM), are used for validating the deterioration models developed in the research. More realistic deterioration models will assist asset managers in improved performance modelling of the sewer infrastructure and also in determining this infrastructure's rehabilitation costs based on improved estimates of deterioration.

• Leira et al. [1999] in “Degradation analysis by statistical methods” treat various methods: Several utilities experience a great future challenge due to deterioration of properties, this being both buildings and infrastructure: As service life ends, there will be an increasing need for rehabilitation (i.e. renewal and maintenance). Most maintenance decisions up to now have been based on the so-called fire brigade strategy, i.e. to make spot repairs after the failure has occurred, or based on rules of thumb. In order to enhance maintenance and rehabilitation decision-making, it is essential to improve our understandings about the deterioration processes. A set of tools should be developed for decision support. These should be based on, or take into account, existing knowledge of failures. Statistical methods can be regarded as a way to organise this knowledge. This paper describes how statistical methods can be applied for forecasting rehabilitation needs. Examples of trend plots, survival methods, condition class transition and stochastic model parameter analyses from concrete structures, roads and water networks are shown. It should be emphasised that there are similarities in the way various construction outputs can be analysed.

• Ansell et al. [2002] report a Markov approach in estimating the service life of bridge elements in Sweden: The service life of Swedish road bridges has previously been studied by collecting inspection reports and other significant information from 353 bridges. A total of 3747 bridge inspection remarks were gathered and the type and cause of damage were stated and each element was given a condition class. This information was then inputted into a relational database. ... Deterioration of bridge elements can be analysed numerically using the Markov chain theory. The deterioration of a particular structural member must be defined by a number of states, in this case given by the assessed condition classes. A state vector that gives rise to a new state after multiplication by a transition probability matrix defines the states of a population of elements. It is demonstrated how a transition probability matrix can be numerically determined to describe the deterioration process of a bridge element from data in the relational database. The numerical method used, is based on an iterative stepwise combination of the matrix elements until the error between a known deterioration average



curve and a curve given by the Markov chain is minimised. The method is computationally demanding for small steps but will, with larger steps, quickly converge towards an approximation close to the transition probability matrix. It is also demonstrated how the remaining service life of a bridge element can be estimated by studying the variation of the state vector with bridge age.

• Kaempfer et al. [2002] have applied a somewhat simplified deterioration model to sewer lines: The condition of sewer pipes and pipe joints are evaluated according to the scale and the effects of damage. The determined damages are assigned to one of five different damage classes. The damage classes range from very serious to negligible. In a second stage, the status of sewer sections is evaluated according to the greatest damage. These evaluation data are installed in a sewer database according to belonging functionality and stability variables such as significance of sewer section, hydraulic capacity, overflow frequency, material, construction year, geometry, size of covering and traffic load situation. In a third stage the correlation is graphically described between the network sections and the year of construction and different functionality and stability variables. The aging curves were derived from the available inspection data and the construction year for each status class (see Figure B2.2). The average residual service life of the sewer section is represented by a vertical line between the real age of the sewer section and the point of intersection with the aging curve of intervention status class. The different intersections on the horizontal line with the aging curves of different status classes indicate the ages at which the section is likely to drop to the next class or, going back in time, came from the previous class. The example of a small town shows how the acquisition of data and the evaluation of damaged sewers in a municipality is carried out and illustrates which priorities have to be established during the maintenance of sewer networks. The model city of 8,000 inhabitants is situated in the middle of Germany. The sewer network comprises around 25 kilometres with 700 individual sewer reaches. In 1998 the total sewer system was optically inspected. The results of the inspection serve as the basis for a database.



Figure B2.2: Status transition functions for concrete and stoneware sewers in Stadtilm, Germany

2.6.2 Variables defined as distributions The probabilistic methods often quantify uncertainties in the form of density distributions. Some examples are looked at in this section, as this concept could prove to be worth wile for application in the engineering design methods. For the assessment of service life using formulas with several variables, distributions can be used instead of plain values.

• Enright and Frangopol [1998] studied the deterioration of highway bridges using time-variant series reliability approach where both load and resistance are time dependent. The minimum level of safety governs the end of service life. The purpose of the analysis lies in the development of a reliability-based maintenance strategy: Experience has demonstrated that highway bridges are vulnerable to damage from



environmental attack, such as alkali-silica reaction, corrosion, and freeze-thaw. To make rational decisions in a life cycle cost perspective, reliable prediction of the service-life of deteriorating highway bridges is necessary. To obtain an accurate insight into this problem, time-variant reliability methods have to be used. The application of these methods in the performance and safety assessment of deteriorating structures is relatively new. In this study, the reliability of reinforced concrete highway girder bridges under aggressive conditions is investigated using a time-variant series system reliability approach in which both load and resistance are time-dependent. Monte Carlo simulation is used to find the cumulative-time system failure probability. An existing reinforced concrete T-beam bridge located near Pueblo, Colorado is investigated. The effects of various parameters such as variability in dead and live loads, live load occurrence rate, strength loss rate, degradation initiation time, resistance correlation, and number of girders under attack on the time-variant bridge reliability are studied. The results can be used to better predict the service life of deteriorating reinforced concrete bridges, and to develop optimal lifetime reliability-based maintenance strategies for these bridges.

• Lounis et al. [1998] in “Further steps towards a quantitative approach to durability design” presents further steps in the development of reliability-based approaches for the durability design and service life prediction of building components which integrate the requirements of safety, serviceability and durability: In General, the load and resistance should be modelled as stochastic processes and the resulting durability problem is formulated in a time-dependent probabilistic format. Using the classical reliability approach, the resulting time-dependent reliability problem is transformed into a time-independent reliability problem through the adoption of an extreme-value probability distribution for the maximum lifetime load. The resistance degradation and its variability are included in the model, and the probabilistic design problem is transformed into a deterministic (or semi-probabilistic) problem using the first-order second moment theory. This semi-probabilistic integrated approach to durability design and prediction overcomes the shortcomings of the empirical factorial approach and the complexities of a fully time-dependent probabilistic method. An alternative approach using stochastic process theory is proposed to formulate the durability design problem as a crossing problem for which the probability of failure within the component lifetime is obtained from the first-passage probability for the stochastic process. In addition, a service life-based formulation of the durability design and prediction problems is presented in order to illustrate its equivalence with the perfor-mance-based formulation. It is shown that in principle the same probabilistic approaches used for the development of structural design approaches for safety and serviceability are also applicable for durability design. The durability design objective is to keep the probability of failure within a specified time interval (or service life) below a certain threshold value that depends on the consequences of failure of the component or system. It is expected that in the near future, further simplifications of the proposed approaches will be made leading to practical and reliability-based methods to durability design or service life prediction. These simplified methods will be implemented in the design of durable new structures and optimal life-cycle maintenance management of existing structures. The Markov model considers steadily degrading systems, where for each property, during each time period, a probability of deterioration is defined. This method thus requires fairly sophisticated inputs in the form of probabilities, which are not easily estimated, as they cannot be read directly off the real behaviour of the structure in the field. The Markov model requires an in depth knowledge of the system dealt with or on the other hand has to rely on significant simplifications.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 67 • Fagerlund [1999] treats frost attack using this variant of probabilistic approach:

The main parameters: saturation, critical and effective, frost, are introduced as distributions. Significant frost damage will not occur until a certain critical moisture level is transgressed over a sufficiently big portion of the structure. The critical moisture level is a “fracture value” which can be compared with the load carrying capacity in structural design. It is a materials property that seems to be rather uninfluenced by normal variations in environmental properties, such as number of freeze-thaw cycles and minimum freezing temperature. The moisture content inside the structure depends on the outer moisture conditions; the more moist the environment, the larger the inner moisture content, and the larger the risk of frost damage. The actual moisture content in the structure can be compared with the actual load in structural design. The risk of frost damage can be calculated when the frequency functions of the two parameters, critical moisture content and actual moisture content are known (see Figure B2.3). Some hypothetical cases are treated in the paper showing that the probability of frost damage might actually decrease with increasing exposure time in moderately moist environments, but that it normally increases with increasing exposure time in continuously moist environments where the structure has no possibility to dry.

Figure B2.3: Distributions of critical and effective saturation in concrete versus time

• Flourentzou [1999] uses four typical degradation schemes to quantify the behaviour of an element. The choice of the respective degradation curve or combination thereof is somewhat theoretical: The service life of buildings is an important factor e.g. in life cycle assessment and the assessment of global costs. Based on experience much information is available regarding the service life of building elements. However, for existing buildings such information is of little use as the key question is the probable date of repair/replacement. MEDIC (“Prediction Method of probable Deterioration Scenarios and Refurbishment Investment Budgets”) is developed on the theories of conditional probabilities to help assess the residual service life and thereby the necessary investments in refurbishment.



When passing from working on general products, like the life span of wooden windows, to specific objects, for example 29-year old wooden windows, the current condition of the object must be taken into account. An evaluation of residual service life must for that reason be closely connected to a good diagnosis method. In the European project EPIQR (“Energy Performance Indoor Environment Quality Retrofit”, see also Brandt et al. [1999]) the deterioration of building materials and components is described by the use of a classification system with four classes for the qualitative condition (e.g. of a facade or window, see Figure B2.4). MEDIC calculates the probability to change from one class to another with time. The prediction is based on the combination of the a priori probability based on experience from a large number of previous investigations/refurbishments and the current state of the object under study.

Figure B2.4: Four representative resultant degradation curves

• Dotreppe [1999] uses a two stage degradation scheme for modelling the behaviour of reinforced concrete bridge decks as shown in Figure B2.5:

1. Initiation and then 2. Propagation (depassivation) Composite steel-concrete constructions are presently widely used, and certainly in the field of composite bridges where they appear quite competitive. Most of the structural problems related to this type of construction are presently solved, concerning particularly the design of the steel girders. Nowadays particularly in the northern temperate zone where de-icing salts and freeze thaw are a problem studies are focused on the durability of the concrete slab, which controls the performance of these bridges. The model commonly accepted for the description of the corrosion process in a reinforced concrete element is presented. The evolution regarding the problem of the influence of the crack width on the durability of concrete is discussed. The factors leading to cracking of the concrete slab are examined, with special attention to thermal and autogenous shrinkage involving early cracking, and the results of a practical example are presented. The most essential requirements regarding durability are mentioned. Concerning reinforcement of the slab the classical solution consists in using standard reinforcing steel. However, as the slab is cracked, durability will be controlled by the corrosion development, which leads to uncertainty regarding service life. Prestressing can ensure a satisfactory performance during a sufficiently long period. Several parameters have to be assessed carefully, such as the type of prestressing and the amount of prestress to be introduced in the slab.



Figure B2.5: Two stages of the degradation scheme used by Dotreppe

• Siemes [1999] presents a probabilistic method of predicting the behaviour of concrete structures. He defines four stages relevant to service life:

1. Depassivation, 2. Cracking, 3. Spaulling and 4. Collapse.

For chloride-induced corrosion, an equation giving the required concrete cover as a function of the chloride concentration has been derived for a chosen time of service. The variables are introduced as distributions: Due to the high construction costs and the social importance the durability demands for large infrastructures is becoming more and more important. Service life requirements of 100 years or even more are quite common. For the bored reinforced concrete tunnel under the Western Scheldt in the Netherlands the requirement was a service life of at least 100 years. No method had been specified to prove this service life. Since the concrete codes are only based on deem-to-satisfy rules for the durability, without any specification for the service life, it was not possible to base the design on existing codes. The service life design has been made on the basis of the methodology that has been developed in a research project of the European Community. This project with the name ‘DuraCrete’ has further improved the existing reliability and performance based structural design method by introducing the modelling of degradations and environmental actions. It is believed that the service life design of the Western Scheldt Tunnel is the first project were the DuraCrete approach has been applied in practice.

• In Siemes [2002] a further refined five stage model is shown for concrete from its initial state to collapse, see Figure B2.6:



Figure B2.6: Adverse events during corrosion of the reinforcement to collapse

• Lair et al. [1999] and [2001] predict the service life using two approaches. On the one hand, they perform a Failure Mode and Effects Analyses (FMEA). This method allows the identification of the failure modes, i.e. the failure to fulfil one of the functions for which the building part was designed. On the other hand, they collect service life information form all available sources (expert opinion, statistical studies, modelling, artificial and natural ageing, etc.), assess their quality, and, by means of a data fusion procedure, give a probability of failure, together with optimistic and pessimistic values of this probability (upper and lower bounds). These two approaches give a band of service life as shown in Figure B2.7.

In the past decades much effort has been put into the improvement of the durability of concrete structures. This has resulted in a reasonable understanding of the main degradation processes or experience with measures to prevent degradation. The results of this effort can be found in the present concrete codes and in manuals on durability design. The design rules are in general presented as deem-to-satisfy rules. If the rules are followed it may be assumed that the structure is durable. The present approach does not give direct insight into the service life, the necessary maintenance or the probability of premature failure.

Further it is clear that a lack of durability can have an influence on the structural behaviour. The direct relationship between durability and safety and serviceability of concrete structures has however not been made in the concrete codes. In the Brite-EuRam project ‘DuraCrete’ the durability design has been developed into a service life design based on performance and on reliability for reinforced concrete structures. This offers the possibility to present the design on the same level as the structural design, also based on performances and reliability. The structural and service life design can even be integrated. The ‘DuraCrete’ approach can be modified for the service life design of other structural materials and building materials.



Figure B2.7: Range of service life defined by the two approaches used by Lair et al.

Lair et al. [1999]: Assessing the service life of building products is relevant for all building actors (insurers, manufacturers, building owners and architects). Indeed, the knowledge of building product service lives leads to a reduction of maintenance costs and environmental impact, and an improvement of safety. This paper deals with a methodological approach for durability assessment. The major steps are:

• Research of available durability data and their organisation in a graph structure followed by the assessment of belief and plausibility distribution of service life.

• A Failure Mode and Effects Analysis, including a structural and a functional analysis in order to search all potential failures (weathering factors, product design and setting up).

The proposed method is a multi-model and multi-scale approach; multi-model in order to adjust the model with our knowledge and our aim (modelling real life of building, but not a too complex and unusable model), multi-scale to take into account the links between the three geometric scales materials/products/building. Finally, it gives

(1) Distribution of nominal service life, for normal weathering processes, with corresponding belief and plausibility degrees,

(2) Details on the design and setting up problems, on exceptional weathering phenomena, which could lead to a shorter service life.

• Faber and Gehlen [2002] describe the probabilistic concept for the assessment of the durability of existing reinforced concrete structures with special emphasis on the spatial variability of the parameter dominating deterioration. They use the fault tree and decision tree concept and four levels of damage (see also [Siemes 1999]). The method is illustrated on the problem of chloride diffusion where even the chloride concentration on the surface is treated probabilistic.

2.6.3 Practical examples of probabilistic methods From literature, a few examples are extracted and commented upon in the following paragraphs, showing the main lines of application.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 72 Selected examples In practical applications of the probabilistic methods, the main parameters have to be identified and often, the theoretical models are modified and simplified, most often reducing the parameters included in the model to a minimum:

• Breitenbüchner et al. [1999] present the design for service life of the Western Scheldt tunnel, the governing factor being the concrete cover. Assuming the chloride attack being the main parameter for degradation the required cover was derived at. Inputs for this calculation such as chloride concentrations, diffusion factors, etc. were defined as stochastic variables (density distributions, see Table B2.1). As a limiting property the reliability index was chosen, for onset of corrosion 1.5-1.8 up, for onset of spalling 2.0-3.0 up to collapse 3.6-3.8. : Due to the high construction costs and the social importance the durability demands for large infrastructure are becoming more and more important. Service life requirements of 100 year or even more are usual. For the bored reinforced concrete tunnel under the Western Scheldt in the Netherlands the requirement was a service life of at least 100 years. No method has been specified to prove this service life. Since the concrete codes are only based on deem-to-satisfy rules for the durability, without any specification to the service life, it was not possible to base the design on existing codes. The service life design has been made on the basis of the methodology that has been developed in a research project for the European Community. This project with the name ‘DuraCrete’ has further improved the existing reliability and performance based structural design method by introducing the modelling of degradations and environmental actions. It is believed that the service life design of the Western Scheldt Tunnel is the first project were the DuraCrete approach has been applied in practice.

Table B2.1: Input distributions used in the design of Scheldt Tunnel by [Breitenbüchner et al. 1999]


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 73 • Helland [1999] shows how the remaining service life of existing concrete structures was

assessed based on the chloride ingress and taking into account a decreasing chloride diffusion over time (see Figure B2.8): After reviewing the present technology on performance-related durability criteria, CEN TC-104 concluded for the coming EN 206 and ENV (execution standard) that none of these were sufficiently mature to be incorporated in a technical standard. For this reason, all the clauses in these two standards will be of the traditional ”deemed to satisfy” type. A technical standard shall not deal with responsibilities. However, they will normally be the main references for contractual agreements. Hence the fulfilment of the technical requirements will heavily influence the producers’ liability. The present ”deemed to satisfy” requirements define the quality of the product purchased by the owner. Future performance-related requirements have to be suited both for the producer and the client as a reference to split the risk of liability for possible future shortcomings of the structure. This split must be acceptable for both parties, and the final evaluation of conformity should be concluded as early as possible after the construction. The paper describes a scenario where the evaluation of conformity might be done within a fairly short period concerning a structure’s ability to withstand the ingress of chlorides in its specific environment.

Figure B2.8: Service life solutions using different materials

• Wisemann [1999] also gives an example of structural service life prediction using distributions for the parameters to assess durability of designs using different materials: Parking garage configurations in Canada present one of the most adverse climates for reinforced concrete. The historic excessive use of de-icing salts on North American roadways has, by exposing the structural elements to a saline solution at a heightened ambient temperature, enhanced the rate of deterioration in parking structures. Various rehabilitation options may be considered for different circumstances, from simple stripping and repair of affected areas, to the chloride extraction or re-alkalinisation of structural elements, and ultimately to demolition and reconstruction. The emergence of "innovative" materials and methods promising extended or altered in-service performance has left designers without a clear view of the relative benefits of the more traditional approaches. This paper examines the degradation models available for service life prediction of



parking structures, in particular methods to evaluate systems using innovative materials (for results of service life prediction see Figure B2.8). This paper also presents the projected service life extensions provided by design, rehabilitation and material options and evaluates the life-cycle economics of their application to a specific parking garage scenario. The assessment considers the total life-cycle costs, including capital, operations and maintenance costs, re-capitalisation as well as the projected rehabilitation costs for each scenario.

• Teply [1999] deals with practically all variables influencing the service life of a reinforced concrete beam (variations in beam properties and degradation (corrosion) due to carbonation and chloride attack): For a specific RC beam, the depassivation of reinforcement is assessed and a consequent corrosion process in time is evaluated using numerical models. The cross-section of the beam is analysed using the “layer approach”. The efficient statistical modelling of the carbonation process and the consequent corrosion of reinforcement is updated utilising the in-situ measurements. The influence of deterioration on failure probability is assessed and sensitivity analysis is performed.

• Hong [2000] considers the degradation of reinforced concrete structures due to both aggressive environment and in-service loading for service life prediction: Aggressive environment and in-service loading can cause the resistance degradation of existing reinforced concrete structures. They must be considered for the safety evaluation and service-life prediction of deteriorating structures. This paper presents an integrated approach for time-dependent system reliability analysis considering the stochastic nature of load processes, uncertainties in strength, degradation initiation time, and strength degradation mode. The approach takes into account the partial correlation among the failures of structural elements, which arises from the fact that elements of a structural system are subjected to the same load processes and/or depend on some common basic random variables. The approach is efficient, since it does not require simulation. Analysis results obtained by using the proposed approach indicate that the reliability of series systems is relatively insensitive to the correlation between the failure of structural elements. However, the reliability of parallel system is highly sensitive to the correlation. The use of the time-dependent reliability obtained by neglecting possible correlation as a safety measure, therefore, should be avoided.

Further applications The topic of service life of concrete structures namely depending on the ingress of chlorides is widely covered in the papers of 8DBMC and 9 DBMC (see references).

2.7 Developments of the Factorial Method Towards Probabilistic Methods The momentum for of developing more realistic models based on the factorial method is gained on the one hand from the ease of understanding and on the other hand from the need for a more satisfying result of the service life prediction.


• From this background Moser [1999] uses the definitions of the factorial method, but employs variables with density-functions instead of plain figures. The variables are based on data given by the manufacturer, by tests, by experience, by expert opinion, and others. Reliable data from expert opinion can be derived by application of the so-called recursive Delphi method. Experts are required to estimate the minimum (say 5%), the average (50%) and the maximum (say 95%) fractals of the variable considered. These estimates are fitted into density distributions of any kind such as: standard, symmetric, asymmetric, custom-defined, or others (or even deterministic, here for design level) see Figure B2.9.


Figure B2.9: Input variables defined by various types of distributions

The result of the computation using existing simple software such as VaP [1996] is a density function of the expected service life of a similar lot of elements (see Figure B2.10). These resulting distributions indicate for e.g. the average or mean service life, but fractiles such as the first 5% or 70% of the elements at the end of their service life can also be read off the distribution curve at once.

These functions can be processed through to for e.g. a replacement cost versus time function of a building or other works. The experts’ estimates are reconsidered after calculation of the resulting service lives or replacement demand. The necessary fine-tuning based on the experts’ experience leads to a realistic model and powerful engineering tool.

This method is proposed for application in the current draft of ISO 15686-4 – Service Life prediction data requirements.



Fig. 5: Density distributions for the windows in the four faces

Figure B2.10: Resultant service life distributions for four sides of a building.

• Aarseth and Hovde [1999] discuss a similar principle in broader terms based on inputs in meetings in Gävle 1998. The distribution function is restricted to the Erlang-distribution and based on estimates of the 1% and 99% fractals, see Figure B2.11. (These fractals have according to experience of the author proved to be rather difficult to assess for experts.) Furthermore, the equation in ISO 15686 is modified from a multiplication of factors to a summation of delta years starting at the reference service life. It is doubtful, whether this principle adequately covers the real variation of the individual factors:

Figure B2.11: Erlang function showing estimated values

The ISO/CD 15686-1 “Service life planning” describes a deterministic method that allows an estimate of the service life to be made for a particular component or assembly in specific conditions. In “real life” the service life has a big scatter and should be treated as a stochastic quantity. In this paper we introduce the “step-by-step” principle as a stochastic approach to the ISO factor method. The “step-by-step” principle provides a more systematic approach to the estimating process and makes possible a stochastic handling of the factors. For each factor three estimates shall be made, the minimum, maximum and the most expected value of the factor. In this way the uncertainty is identified and estimated for each factor. The most uncertain factors should, if possible, be



divided into sub-elements and more information should be gathered in order to reduce the uncertainty. In this stochastic approach the “factors” are treated as elements that finally are summed up.

Also unlike the proposed ISO factor method, the estimates are expressed in years, instead of in numbers close to 1. These changes facilitate seeing the consequences of the estimates during the estimating process. After a statistical calculation the estimated service life is expressed as three figures, the expected value plus/minus one standard deviation. Two examples are shown where the service life is estimated for a window: first in a deterministic way according to the proposed ISO factor method, then in a stochastic way according to the proposed “step-by-step” principle, see Figure B2.12.

Figure B2.12: Example service life calculation

This method, although somewhat similar to the one proposed by Moser [1999], deviates substantially from ISO 15686. The restriction to a set form of density distribution and the different basic equation are further drawbacks of this method.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 78 2.8 Other Concepts Some authors have used other than the above concepts for more sophisticated investigations.

• Estes and Frangopol [1999] have set up a model for bridges as a series-parallel combination of failure modes. Using time-dependent deterioration models and live load models, lifetime repair can be optimised. The authors state however, that considerable research effort is required to develop accurate input data: As reliability based methods gain increased acceptance, there is greater opportunity to use scarce resources more efficiently while maintaining a prescribed level of reliability of a structure throughout its service life. The goal is to provide management decisions that will balance lifetime system reliability and expected life-cycle cost in an optimal manner. This study proposes a system reliability approach for optimising the lifetime repair strategy for highway bridges. The approach is demonstrated using an existing Colorado State highway bridge. The bridge is modelled as a series-parallel combination of failure modes, and the reliability of the overall bridge system is computed using time-dependent deterioration models and live load models. Based on an established repair criterion, available repair options, repair costs, and updating, the optimum lifetime repair strategy is developed. The sensitivity of the optimum strategy to changes in various problem parameters including the prescribed service life, system failure criterion, and net discount rate is studied. Finally, the conclusions reveal that the proposed approach demonstrates real potential for practical applications, needs frequent updates through inspection, and requires considerable research effort to develop accurate input data.

• Raj [2000] presents the framework of a method, which partitions the systems into sub-systems linked by multidimensional variables. System analysis is done in the linking variable space (LVS) yielding detail information on how the sub-fields influence the overall variability of the service life. The example works on a light bulb and serves to illustrate how design regime is created in the LVS by overlaying the results from engineering design and materials science sub-systems.

• Liang et al. [2001] use a multiple layer fuzzy method model for the assessment of the service life of bridges. Thereby the deterioration is modelled using the fuzzy theory and the end of the service life is defined by the minimum safety index: The principal objective of this paper is to set up an evaluation multiple layer fuzzy method model for evaluating the damage state of existing reinforced concrete bridges. After establishing the detailed evaluation supportability and anti-seismic ability of existing bridges items, the fuzzy mathematical theory is adopted to evaluate the damage state of any member of an existing bridge. The damage state of any member of an existing bridge is systematically and completely composed as an evaluation model of multiple layer fuzzy mathematics. The evaluated results may be used for the safety index and reference index for repair or reinforcement in existing bridges. In addition, the evaluated results may also be used as a design reference for service life in future bridges. The evaluated model may be divided into the degrees of grades I, II, III, IV, and V, which are described as non-damage, light damage, moderate damage, sever damage, and unfit for service, respectively. Using the proposed model, the Huey–tong bridge, Jzyh–Chyang Bridge, Ay–gwo west road viaduct, and old Hwan–Nan viaduct in Taipei was chosen for evaluation. The results of the present investigation indicate that the order of repair and reinforcement in the Ay–gwo west road viaduct, Huey–Tong bridge, old Hwan–Nan viaduct, and Jzyh–Chyang bridge. Thus, the multiple layer fuzzy method appears to be of advantage for evaluating the damage of existing reinforced concrete bridges.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 79 • Vu and Stewart [2002] use a reliability model which includes the spatial and random

variability of both chloride diffusion, concrete cover and concrete strength: In this paper, the service life of structures exposed to aggressive environments is measured by the probability of cracking and spalling of concrete cover. The time to corrosion cracking/spalling is experimentally investigated from accelerated corrosion testing of RC slabs with the emphasis on trying to quantify the relationship between concrete quality (w/c ratio; or strength), concrete cover, crack propagation and time. The probability of cracking and spalling of concrete cover is calculated by using a structural deterioration life-cycle reliability model. The reliability model includes the random spatial variability of concrete compressive strength, concrete cover and the surface chloride concentration. The reliability model also includes a stochastic deterioration model that considers the random variability of chloride diffusion, threshold chloride concentration and corrosion rates. Therefore, the reliability model can be used to predict the proportion of a concrete surface likely to spaull for any reference period (see Figure B2.13). This is a useful criterion for predicting the service life of RC structures.

Figure B2.13: Input data and resulting effects of concrete cover and w/c ratio on concrete cracking and spalling

Some of these concepts are even more sophisticated and more elaborate than probabilistic methods. The concepts are, at the present time, only suitable for research or specific use in singular and unique projects, the latter case that would warrant the cost of undertaking elaborate investigations.



3. PROPOSED ENGINEERING DESIGN METHODS

3.1 Basic Requirements for Engineering Design Methods As far as more common engineering designs are concerned, only a few papers are available, but a general engineering design method as such was found. The methods under section 2.5 above, however, are deemed to satisfy the main requirements for engineering design methods, as they are:

1. The method is (for engineers) easy to understand, 2. The method is easy and swift to apply, 3. The results are (for the given simplifications) realistic. Attempts to improve the factorial method seem to be the most promising path leading to engineering design methods. Introducing stochastic data into the factor method according to ISO 15686-1, or other relations defining service life, by defining the factors as density distributions, could well prove to produce powerful engineering design methods.

3.2 Proposed Principle The general principle of the proposed engineering design methods can be defined as follows:

1. Establish an equation, describing the service life of the building or component, taking into account all identified relevant parameters. For standard cases, the equation of the factorial method as set up in ISO 15686-1 can be used. In other cases, modified or tailor-made equations have to be set up.

2. Gain data on the parameters of the above equation from experience, from expert opinion, etc. Set up any kind of probability density distribution for the individual parameters identified.

3. Perform the service life calculation.

4. Review the plausibility of the results using experts’ opinion, and when deemed necessary, modify the input data accordingly, i.e. go into greater detail in setting up the parameters for the variables dominating the service life.

3.3 Examples of Engineering Design Methods Moser and Edvardsen have published a paper titled “Engineering Design Methods for Service Life Prediction” at the 9DBMC [2002] demonstrating the application of the engineering design method to three different examples. The following text is an extract of this paper.

This method, applied plainly as set up in the code, yields one single value for the average service life. A customer however, is not only interested in the average value; he has to know


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 81 when substantial renovations or replacements can be expected. The latter is usually the case long before the average service life is reached.

3.3.1 Data acquisition by the recursive Delphi method In many cases, data are not available for ready use in the form of values for the main factors of influence defined in the equation for service life, let alone in the form of distributions. A very valid method is the so-called (recursive) Delphi method (see Figure B3.1). (The method has been used for data acquisition as far back as the 1980’s in the field of (industrial) risk engineering.)

1. In a first step, a panel of experts is called together and asked for their professional opinion on the distributions of the different factors, their type of distribution (normal, log-normal, Gumbel, etc.), their mean values, standard deviations. Usually, it is easier to define fractiles based on experience and professional judgement, say 5 or 10%, and mean values and 90 or 95%. Experts can quite well define these values, when asked precisely.

2. The second step involves the service life calculation using the expert panel’s input. The distributions are used instead of plain factors in the mathematical formulations for service life.

3. The third step is a thorough discussion of the results and of the dominating parameters. Sensitivity analysis has proved to be an important tool at this stage. Very often, as a consequence of this appraisal, the data or the models have to be adjusted to yield results judged reasonable in those areas of the problem where the experts have sufficient practical experience. After this fine-tuning of the model and the density distributions of the factors, the general problem can be tackled successfully.

Model Parameters Results and and Model Input Sensitivity Analysis

Panel of Experts

Figure B3.1: Recursive Delphi Method

3.3.2 Application of engineering design methods The general engineering design method (EDM), at this stage, is defined as:

“Any simple mathematical relation (as simple as possible, but not too simple) worked on, using distributions of any kind for the individual factors in the relation”.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 82 This procedure yields as result distributions for the expected service lives, information, which can easily be understood and interpreted by the decision-makers.

The following examples show three variations of the engineering design method EDM:

1. The first example uses all seven factors of ISO 15686-1, 2000 under the assumption, that the information for defining the respective distributions is readily available.

2. The second example is working on limited information only. The equation has to be modified, and the respective distributions are set up indirectly, partially from information on the resultant differences of service life.

3. The third example bases on a completely different equation, which is established using common engineering sense. It is normalised using an average result calculated on the basis of an equation based on an error-function.

3.3.3 Example 1: Engineering design method based on equation of factorial method This example demonstrates the basic procedure with the following full equation (1). All factors are applied as indicated, using four different distributions in order to demonstrate the ease of application of this expanded factorial method. The example is based on the worked example for softwood windows given in ISO/CD 15686-1 [1997] but the factors are indexed as in the edition 2000 of the code:

(1) GFEDCBA fffffffRSLCPSLDC ⋅⋅⋅⋅⋅⋅⋅=

PSLDC: is the predicted service life distribution of the component based on the reference service life RSLC.

The factorial indices are for: A: quality of the component, B: design level, C: work execution level, D: indoor environment, E: outdoor environment, F: in-use condition, G: maintenance level.

Estimated service lives for the windows in all four faces The basis of the numerical example is a squared building of a length of 50 m, a width of 25 m and a height of 30 m. The long sides are facing due south and north respectively. The windows of the four façades of the building are in the example treated separately. The assumed relevant conditions for all factors and faces are shown in Table B3.1. From this Table, the factors for the three fractiles 5%, 50% and 95% are defined in the sense of the Delphi method, in this case based on the factors and their description given in ISO/CD 15686-1:2000.



Table B3.1: Estimated fractile values of factors

Factor

Face

Relevant conditions

Factors for the fractiles

5% / 50% / 95% fA Quality of component all General variations of components 1.2 / 1.5 / 1.8 fB Design level all Good, identical value 1.2 fC Work execution level all General variation,

but insufficient quality repaired 1.0 / 1.2 / 1.5

fD Indoor environment S W N E

Occasional risk of condensation Medium risk of condensation

High risk of condensation Medium risk of condensation

0.9 / 1.0 / 1.2 0.8 / 0.9 / 1.1

0.7 / 0.8 / 0.95 0.8 / 0.9 / 1.1

fE Outdoor environment S W N E

Occasional cycling dry / damp Regular cycling dry / damp

Sheltered from rain Occasional cycling dry / damp

0.8 / 1.0 / 1.3 0.6 / 0.8 / 1.0 1.0 / 1.2 / 1.5 0.8 / 1.0 / 1.3

fF In use conditions S W N E

Occasional access by children 1) Regular access by children 1)

Occ. / reg. access by children 1) Occasional access by children 1)

0.8 / 1.0 / 1.2 0.6 / 0.8 / 1.0 0.7 / 0.9 / 1.1 0.8 / 1.0 / 1.2

fG Maintenance level all Painted on judgement of caretaker 0.9 / 1.0 / 1.1

Note: 1) according to example in ISO/CD 15868-1 [1997], other descriptions for wear and tear may appear more realistic.

The values for the fractiles given in the Table B3.1 are approximated by the density functions given in Table B3.2 for the ease of processing. The functions chosen represent those generally used: deterministic, normal, lognormal and Gumbel (extreme-value) distributions. The program used for the data processing, VaP 1.6 [1996], supports 11 types of distributions and user defined functions. It only requires 2.2 MB of disc-space in the expanded state.

The factors for C reflect the fact that a quality of workmanship usually is such, that all parts not being sufficient are upgraded to meet the requirements, whereas those exceeding the requirements are naturally left at their higher level. This procedure leads typically to asymmetric distributions (in this example approximated using an extreme value “Gumbel” distribution) with very few components below the satisfactory level of 1.0 and a fairly wide spread upper area of the distribution.

The variables m and s are the first and second moments of the respective distributions (see VaP 1.6 1996].



Table B3.2: Predicted service life distribution of the components (PSLDC)

Face Factor

Type of

Distribution South m / s

West m / s

North m / s

East m / s

RSLC fA fB fC fD fE fF fG

Deterministic Normal

Deterministic Gumbel

Lognormal lognormal

normal normal

25 years 1.5 / 0.185

1.20 1.25 / 0.10 1.05 / 0.10 1.05 / 0.20 1.0 / 0.12 1.0 / 0.06

25 years 1.5 / 0.185

1.20 1.25 / 0.10 0.95 / 0.10 0.80 / 0.20 0.80 / 0.12 1.0 / 0.06

25 years 1.5 / 0.185

1.20 1.25 / 0.10 0.80 / 0.10 1.25 / 0.20 0.90 / 0.12 1.0 / 0.06

25 years 1.5 / 0.185

1.20 1.25 / 0.10 0.95 / 0.10 1.05 / 0.20 1.0 / 0.12 1.0 / 0.06

PSLDC (years) Lognormal 1) 62.0 / 20.4 34.2 / 11.8 50.6 / 14.8 56.1 / 18.6

Note: 1) close fit

The results were calculated by direct methods using VaP 1.6 [1996] and are shown on the last line of Table B3.2. The average results of two runs of a Monte Carlo simulation match the mathematically calculated results by a maximum difference on the average value of 0.1 years. These simulations yielded detail results as graphically shown for each facade as in Figure B3.2. Dividing the relative densities shown on the vertical axis through the number of runs, in this case by 100’000, derives at the absolute densities.

Comparison of the four façades The results for the estimated service lives of the four façades in Figure B3.2 are different with respect to several aspects. Firstly one notices the different widths of the distributions, in accordance to the values (second moments) in Figure B3.2). The spread is largest for the south face and narrowest for the west face. Some of this effect is relative: Due to the higher average value, the same relative spread is larger in years.

The west face shows the shortest service life, as expected, for a northern temperate location, mainly due to the unfavourable outdoor climate and the in-use conditions. The effect of the higher risk of condensation, assumed for the north face indoor climate, is offset by the more favourable outdoor climate. The main difference originates from the in-use conditions. Both effects combined yield some 15% less estimated service life.



Note: Densities are the result of 105 runs of a Monte Carlo simulation

Fig. 5: Density distributions for the windows in the four faces

Figure B3.2: Distributions of predicted service lives PSLDC for all four facades

Financial demand For the planning of the maintenance funds, the functions for the service lives of the similar building parts can be superimposed. In general, this has to be done for all parts of a building considered. For the superposition, costs have to be allocated to the different groups of building parts.

In this example, the superposition of all window areas to be replaced is executed only, in order to be able to show typical results. (It is assumed for this purpose, that the windows cover 40% of the area of the respective façades.)



0

10

20

30

40

50

60

0 20 40 60 80 100

service life [years]

win

dow

are

a [m

2 ]

Total area of windows(=integral of function)

A = 1800 m2

120

Figure B3.3: Financial demand

The superposition yields an asymmetric function having a steep increase up to a peak demand of replacements of 48 m2/year after 37 years (see Figure B3.3). Then the demand decreases at a gentle slope down to 10 m2/year after 70 years.

In a next step, the same service life functions can again be applied to the replaced windows, and the results of the multiple replacements are summed up, leading to a fairly constant replacement function. These steps are omitted here for clarity.

In general, the financial demand for similar parts tends to merge into a one-peak function. The superposition of the functions of all different parts of a building is more likely to result in several peaks or even a relatively steady demand over the lifetime of the building considered, starting at a certain age of the building.

3.3.4 Example 2: Engineering design method based on modified equation method and scarce data

This example deals with service life of fibre cement slates used as wall cladding. The input data is fairly scarce, far from being complete and not directly suited for application in service life calculation. The basis of this calculation is the factorial method as set up in ISO 15686-1:2000, modified to suit this specific case. The example shows that interpretation of available limited data can nevertheless lead to a coherent and satisfactory service life prediction.

Available data A manufacturer supplied data from his experience as follows:

• The quality of production of the slates can be derived from the bending strength, assumed to be characteristic value for the mechanical strength. The mean value m lies 20% above the strength required and the standard deviation s is very small, normalised: v = m/s = 0.015.

• The design level is such, that out of all designs some 10% to 15% are considered to be inadequate.


• The quality of work execution is at a fairly high level and some 5% are judged to be inadequate.

CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 87 • Outdoor environment under normal conditions results in the modifications of service life

shown in Table B3.3.

Table B3.3: Example 2: Service lives and equivalent factors for different expositions

Exposition East North West South Difference in service life ± 0 years - 2 to -3 years -7 to -10 years - 5 to -7 years

Equivalent factor 1.0 0.95 0.85 0.90

The equivalent factors above have been estimated knowing that the expected service life lies somewhere between 50 and 60 years.

• In-use conditions do not have to be considered, as only direct mechanical destruction can be a result of use. These cases are however not of statistical significance.

• Maintenance level does not have to be considered, as basically no maintenance is required. Flat sheets are installed and do not receive any, or minimal maintenance only, throughout their entire service life.

Input data For the calculation of service life, the following equation is set up according to the factorial method:

(2) ECBA ffffRSLCPSLDC ⋅⋅⋅⋅=

From the above inputs, the following mean factors and standard deviations, or second moments respectively, are derived at:

• The density distribution of the factor for the quality of the component is, on the basis of the mechanical strength, set to a mean value of fA = 1.2. The standard deviation is, on the basis of the normalised standard deviation from production, set to sA = 0.02.

• The density distribution of the factor for the design level is set to a mean of fB = 1.1 with a standard deviation of sB = 0.12, resulting in some 13% of the cases being below 1.0, i.e. exhibiting insufficient quality.

• The density distribution of the factor for the work execution level is from experience asymmetric and a lognormal distribution is defined by a mean value (first moment) of fC = 1.1 and a second moment of sC = 0.06, resulting in some 5% of the cases being below 1.0, i.e. insufficient.

• The density distribution of the factors for the outdoor environment are set to the mean values fE in Tab. 3 above. The standard deviation is, for the sake of simplification, set to an estimated sE = 0.1 for all four expositions.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 88 Calculation of service life The calculation using VaP [1996] yields the results for the predicted service life PSLC as shown in Table B3.4, basing on a RSLC of 50 years (deterministic value, for the purpose of this example):

Table B3.4: Service lives and equivalent factors for different expositions

Exposition East North West South

PSLC: mean value [years]

m: standard deviation [years]

72. 6

11.5

69.0

11.2

61.7

10.5

65.3

10.9

SLC ≈ PSLC - m (16% of the slates damaged) 61 58 51 54

The distributions for the service lives of two facades are shown in Figure B3.4.

East facade [years] West facade [years]

Note: Densities are the result of 105 runs of a Monte Carlo simulation

Figure B3.4: Distributions of predicted service lives (PSLDC) for East and West facades

Under the assumption, that damage to about one out of every six of the slates requires replacement of the entire respective cladding (i.e. SLC for a fractile of about 16% of damaged slates), the service life of the four facades is shown in the above table, varying from 61 to 51 years.

Discussion of results The differences in service life given from experience can be found in the results of the prediction. For the purpose of investment planning, the 16% fractile (or any other fractile deemed to be reasonable) seems to be a good indication of the point in time of replacement.

This example shows that even on the basis of relatively scarce input, quite sensible service life prediction are possible.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 89 3.3.5 Example 3: Engineering design method based on simplification of the probabilistic

method This example demonstrates, that service life prediction using a relatively plain formula, different to the one in ISO 15686, 2000, can be done in the same way, again by introducing densities for the factors involved.

This example deals with chloride ingress into concrete. It is based on a paper using the probabilistic approach on the basis of error-functions [Edvardsen and Mohr 2000]. In the paper, the authors compare the results of deterministic and of probabilistic calculations of service life of reinforced concrete structures in two climates (+10°C and +30°C). The necessary concrete cover for a service life of 50 years is determined. A chloride content of 0.1 % chloride by mass of concrete at the reinforcement is assumed to define the end of service life.

Procedure for simplified equation In terms of an engineering design method, the following procedure is used:

1. The mean value of the chloride ingress depth is calculated as x = 34 mm, using the equation for diffusion for a time of 50 years:

−⋅−−=

tDxccctxc ss 2

erf1)(),( 0 (3)

where: c: concentration on chlorides, cs: concentration at the outer face and c0: initial concentration in the concrete. The front of the ingress is defined by the critical value c = ccrit = 0.1 % of mass of concrete.

2. A simplified diffusion equation is set up for the depth x of chloride ingress. By this, all constants in the equation of diffusion are rounded up into one single constant K:

( ) DcccKx crits 0−−≈ , (4)

3. The constant K is calculated by solving the equation for the mean value x. Using the mean value of 34 mm this results in K = 38⋅103 [s0.5 / wt.-%].

Table B3.5: Example 3 - Values used for the diffusion calculations

Variable Distribution Mean value Standard Deviation

Surface chloride concentration

Critical chloride content

Initial chloride content

Eff. chloride diffusion coefficient (10°C)

Eff. chloride diffusion coefficient (30°C)

cs

ccrit

c0

D1

D2

Lognormal

Normal

Normal

Normal

Normal

1.0 [wt-%]

0.1 [wt-%]

0.01 [wt-%]

1.0·10-12 [m2/s]

4.0·10-12 [m2/s]

0.3 [wt-%]

0.025 [wt-%]

0.002 [wt-%]

0.1·10-12 [m2/s]

0.4·10-12 [m2/s]

Solving equation using distributions The equation reads now as

( ) [ mass /%s1038 03 Dcccx crits −−⋅= ] (4)

Solving this equation using the same density distributions as used in the detailed probabilistic solution (Table B3.5), yields results as shown in Figure B3.5. The slight skewness of the resultant density distribution is neglected for indicating standard deviations.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 90 Fractile value required The probability of exceeding the critical content ccrit is set to 10% in Edvardsen and Mohr [2000] Assuming a normal distribution, the value of the fractile of 90% is derived at by adding λ = 1.28 standard deviations to the mean: sxx λ+=90 . These results are also shown in Figure B3.5 and are compared to the exact values of the original paper. The mean values are identical as well as the standard deviation for Diffusion constant D1. For Diffusion constant D2, the fractile value of this prediction exceeds the exact value by some 5%.

This accuracy is deemed to satisfy the needs of the customer, bearing in mind, that all input values, although being set up as distributions, are still never perfectly exact.

Fractile of 10% exceedance Diffusion constant

Mean value

Standard deviation Eng. Design

Method Edvardsen and

Mohr 2000

D1 D2

34 mm 68 mm

12 mm 23 mm

49 mm 97 mm

49 mm 91 mm

x [mm] for D1 x [mm] for D2

Note: Densities shown are the results of 105 runs of a Monte Carlo simulation

Figure B3.5: Engineering design method: density distributions for the depth of chloride ingress



4. CONCLUSION AND OUTLOOK

4.1 Proposed Engineering Design Methods After a review of the recent literature, an engineering design method (EDM) is proposed for the calculation of the service life of buildings or components.

In order to calculate the service life, an equation containing the relevant factors at their relevant levels has to be set up first. This equation may be derived from ISO 15686-1 or other sources. It can be made up to specifically suit the problem to be solved.

In the equation for service life, density distributions instead of simple factors are used, thereby greatly improving the information content and the relevance of the results. The intellectual, mathematical and time wise input compared to the quite often elaborate original equations or to a thorough stochastic design, is greatly reduced.

As opposed to using simple numerical factors in the original factor method, this approach incorporates the use of probability density functions for factors as well as for the service life of individual components to arrive at an overall estimate of a building system’s service life. These are established using reliable and understandable engineering techniques applied in a systematic and straightforward manner.

By making the trail from input to results clearly understandable, fewer errors will occur and fewer traps stepped into. Thus the engineering design method can be applied by the plain engineer and yields nearly as good results as finely tuned sophisticated probabilistic models.

4.2 Further Developments The method as proposed has been applied so far to a few examples only. It is hoped that for distinct fields of application, standard equations and factors can be defined, together with, for each climate, the respective input data.

These tools will help to make service life design a standard engineering technique thus fulfilling the requirements set by owners or the European Commission [CPD 1988].

4.3 Research Needs Further research is needed, to identify the relevant parameters governing the service life of structures of all kinds of materials, as well as to set up workable mathematical relations for the application of the engineering design method.



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Frohnsdorff, G.J.C. (1996): Predicting the service lives of materials in construction, 4th Materials Engineering Conference: Materials for the new millennium, Washington DC, ISBN 0-7844-210-8, 1776 p., pp. 38-53, ASCE, Nov 10-14 1996.

Helland, S. (1999): Assessment and prediction of service life of marine structures, a tool for performance based requirements? Workshop by Duranet on "Design and durability of concrete", Berlin, June 1999.

Hong, H.P. (2000): Assessment of reliability of aging reinforced concrete structures, Journal of Structural Engineering, December 2000, pp. 1458-1465.

Hovde, P.J. (2002): The factor method for service life prediction from theoretical evaluation to practical implementation, 9DBMC, paper 232.

ISO 15686-1:2000, Buildings and constructed assets - Service life planning- Part 1: General principles, International Standard Organisation, Geneva.

ISO 15686-2:2001, Buildings and constructed assets: Service life planning, Part 2 – Service life prediction procedures, International Standard Organisation, Geneva.

Jernberg, P., Sjöström, C., Lacasse, M.A (1997): State-of-the-Art-Report, Materials and Structures, Suppl. 196 March 1997, p 22-25.

Jernberg, P. (1999): Overview and notional concepts on performance and service life, 8DBMC, pp. 1417-1425.

Kaempfer, H.W., Berndt, M., Voigtlaender, G. (2002): Estimation of residual service life for existing sewerage systems, 9DBMC, paper 164.

Lair, J., Le Teno, J.F., Boissier, D. (1999): Durability assessment of building systems, 8DBMC, pp. 1299-1308.

Lair J., Chevalier J.L., Rilling J. (2001): Operational methods for implementing durability in service life planning framework, CIB World Building Congress. Wellington, paper INF 11, 10 p., April 2001.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 94 Lair, J., Le Teno, J.F., Chevalier, J.L., Boissier, D. (2000): Service life prediction under

uncertainty as a criteria for decision-making, Eds. Mangin, J.C., Miramond, M., 2nd Conference on Decision Making in Urban and Civil Engineering, 2 volumes, 1270 p., Lyon, 20-22 November 2000.

Leira, B., Lingard, J., Nesje, A., Sind, E., Saegrov, S. (1999): Degradation analysis by statistical methods, 8DBMC, pp. 1436-1446.

Liang, M.T., Wu, J.H., Liang, C.H. (2001): Multiple layer fuzzy evaluation for existing reinforced concrete bridges, Journal of Infrastructure Systems, 4 December 2001, p 144-159.

Lounis, Z., Lacasse, M.A., Siemes, A.J.M., Moser, K. (1998): Further steps towards a quantitative approach to durability design, proc. Materials and Technologies for Sustainable Construction, Construction and Environment, CIB World Building Congress, Gävle, Sweden, pp. 315-324.

Masters, L.W., Brandt, E. (1989): Systematic methodology for service life prediction of building materials and components, Materials and Structures, Sept 1989, pp. 385-392.

McNerney, M.T., McCullough, B.F., Stokoe, K.H., Lee, N., Bay, J., Wilde, J. (1997): Prediction of remaining life on airport pavements, Airfield Pavement Conference, Seattle, Washington, ASCE, ISBN 0-7844-0286-8, 392 p., pp. 77-93.

Moser, K. (1999): Towards the practical evaluation of service life – Illustrative application of the probabilistic approach, 8DBMC, pp. 1319-1329.

Moser, K., Edvardsen, C. (2002): Engineering design methods for service life prediction, 9DBMC, 12p.

Naus, D. (2000): Life prediction and aging management of concrete structures, D. Naus, Ed., International RILEM Workshop, Cannes, France.

Nireki, T. (1996): Service Life Design, Construction and Building Materials, nr. 5, July 1996, p. 403-406.

Quillin, K.C., Somerville, G., Hooper, R., Nixon, P. (2002): A framework for service life design of concrete structures, 9DBMC, paper 128.

Radojicic, A, Bailey, S.F., Brühwiler, E. (2001): Probabilistic models of cost for the management of existing structures, US-Japan Workshop on Life-Cycle-Cost Analysis and Design of Civil Infrastructure Systems, Hawaii, ISBN 0-7844-0571-9, ASCE, pp. 251-270.

Raj, R. (2000): An interdisciplinary framework for the design and life prediction of engineering systems, Journal of Engineering Materials and Technology-Transactions of ASME, July 2000, pp. 348-354.

Rudbeck, C. (2002): Service life of building envelope components: Making it operational in economical assessment, Construction and Building Materials, pp. 83-89, March 2002.

SIA 160: Actions on Structures (1989), Swiss Association of Engineers and Architects, Zürich.

Siemes, T., Edvardsen, C. (1999): DuraCrete: Service life design for concrete structures, 8DBMC, pp. 1343-1356.

Siemes, T, de Vries, H. (2002): Overview of the development of service life design for concrete structures, 9DBMC, paper 261.


CIB W080 / RILEM 175 - SLM: Service Life Methodologies Engineering Design Methods: State of the Art 95 Stewart, M.G. (2000): Optimisation of durability design specifications for RC structures,

Structures Congress: Advanced Technology in Structural Engineering, Philadelphia, Pennsylvania, ISBN 0-7844-0492-5, ASCE, section 21, chapter 2, May 8-10, 2000.

Strand, S.M., Hovde, P.J. (1999): Use of service life data in LCA of building materials, 8DBMC, pp. 1948-1958.

Teply, B., Novak, D., Kersner, Z., Lawanwisut, W. (1999): Deterioration of reinforced concrete: probabilistic and sensitivity analyses, Acta Polytecnica and 8DBMC, pp. 1357-1366.

Thoft-Schristensen, P. (1997): Estimation of the service life time of concrete bridges, Structures Congress XV: Building to Last, ISBN 0-7844-0229-9, Portland, Oregon, 1736 p., pp. 248-252, April 13-16 1997, ASCE.

VaP 1.6 for Windows™ (1996): Short course in Variables Processing, ETH (Swiss Federal School of Engineering) Zürich, Manual 13 p. in English; Papers 33 p. and Examples 22 p. in German.

Vu, V.A.T., Stewart, M.G. (2002): Service life prediction of reinforced concrete structures exposed to aggressive environments, 9DBMC, paper 119.

Wiseman, A., Kyle, B.R. (1999): Service life prediction and economic assessment of parking garage options, 8DBMC, pp. 1493-1505.

Abbreviations and database ASCE: American Society of Civil Engineers, Reston, Virginia, www.pubs.asce.org

Brite-EuRam project DuraCrete: Selected publications.

CIB World Building Congress 1998: Materials and technologies for sustainable construction, Gävle, Sweden, M. Lacasse, ed., NRC, Ottawa, Canada.

CIB World Building Congress 2001: Performance in product and practice, CD by Building research association of New Zealand (BRANZ), Eds., New Zealand.

7DBMC: Proceedings of the 7th international conference on the Durability of Building Materials and Components, Sjöström Ch., Ed., Stockholm, Sweden, E and FN Spon, London, 19-23 May 1996.

8DBMC: Proceedings of the 8th international conference on the Durability of Building Materials and Components, Lacasse and Vanier, Eds. NRC research press Ottawa, Canada, 30 May – 3 June 1999.

9DBMC: Proceedings of the 9th international conference on the Durability of Building Materials and Components, Burn S. CSIRO ed., on CD ROM, ISBN 0-943-06828-7, Brisbane, Australia, 17-19 March 2002.

SL/AM IT CD ROM, containing the full papers of the 8DBMC and the abstracts of all earlier DBMC conferences back to 1DBMC in 1978, Lacasse and Vanier, Eds. NRC research press Ottawa, Canada, 1999.


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CIB Task Groups (TG) and Working Commissions (W)(as at 1st January 2004)

Task GroupsTG23 Culture in ConstructionTG28 Dissemination of Indoor Air Sciences (joint CIB-ISIAQ Task Group)TG31 Macro-Economic Data for the Construction IndustryTG33 Collaborative EngineeringTG34 Regeneration of the Built EnvironmentTG37 Performance Based Building Regulatory SystemsTG38 Urban SustainabilityTG39 DeconstructionTG40 Informal SettlementsTG42 Performance Criteria of Buildings for Health and Comfort (Joint CIB-ISIAQ Task Group)TG43 MegacitiesTG44 Performance Evaluation of Buildings with Response Control DevicesTG46 Certification in ConstructionTG47 Innovation Brokerage in ConstructionTG49 Architectural EngineeringTG50 Tall BuildingsTG51 Usability of WorkplacesTG52 Transport and the Built EnvironmentTG53 Postgraduate Studies in Building and ConstructionTG54 Standardisation of Fibre Reinforced Polymers (FRP) Use in Building and ConstructionTG55 Smart and Sustainable Built Environments

Working CommissionsW014 FireW018 Timber StructuresW023 Wall StructuresW040 Heat and Moisture Transfer in BuildingsW051 AcousticsW055 Building EconomicsW056 Sandwich Panels (joint CIB - ECCS Commission)W060 Performance Concept in BuildingW062 Water Supply and DrainageW063 Affordable HousingW065 Organisation and Management of ConstructionW067 Energy Conservation in the Built EnvironmentW069 Housing SociologyW070 Facilities Management and MaintenanceW077 Indoor ClimateW078 Information Technology for ConstructionW080 Prediction of Service Life of Building Materials and Components (Joint CIB-RILEM

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W084 Building Non-Handicapping EnvironmentsW086 Building PathologyW087 Post-Construction Liability and InsuranceW089 Building Research and EducationW092 Procurement SystemsW094 Design for DurabilityW096 Architectural ManagementW098 Intelligent and Responsive BuildingsW099 Safety and Health on Construction SitesW100 Environmental Assessment of BuildingsW101 Spatial Planning and Infrastructure DevelopmentW102 Information and Knowledge Management in Building (Joint CIB-UICB Commission)W103 Construction Conflict: Avoidance and ResolutionW104 Open Building ImplementationW105 Life Time Engineering in ConstructionW106 Geographical Information SystemsW107 Construction in Developing CountriesW108 Climate Change and the Built Environment

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