Durability Design of Infrastructure Assets - Towards a Uniformed Approac...

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18 th International Corrosion Congress 2011 Paper 212 -

DURABILITY DESIGN OF INFRASTRUCTUREASSETS – WORKING TOWARDS A UNIFORM

APPROACH

F. Blin 1, S. Furman 1 and A. Mendes 1 1 AECOM

SUMMARY:

In the construction and building industries it has become increasingly frequent for infrastructure assetowners and operators to specify design life requirements for both capital and remediation works. Thistrend supports the development of more sustainable design and construction practices that take a life-cycle approach, such as including operational and maintenance considerations in the design solution

process, and not just focussing on minimisation of the initial capital cost. Whenever design lives arespecified there is a clear need to adopt and implement uniform durability design practices throughout the project so that consistency of approach is achieved through:

Developing a durability management plan outlining the approach needed to achieve the design liferequirements

Providing technical support to the design team so that durability is embedded in the design process

Supporting the construction team to manage issues that impact on the design life of the facility orstructure

Providing input to handover documentation such as the asset register, inspection and maintenance plans etc so that the integrity of the key design inputs and assumptions required to achieve thespecified service life are captured prior to commissioning and embedded in the operation phase.

This paper presents the findings of a literature survey of various documents that deal with durabilitydesign, especially ISO 13823 - “General principles on the design of structures for durability” which isdiscussed in detail. Of particular interest is the potential for ISO 13823 and the associated standard ISO15686 - “Buildings and constructed assets - Service life planning ” to be used more extensively inAustralia. Furthermore, this paper discusses and proposes the terminology and template that could beused when designing for durability in an effort to standardise this important design practice.

Keywords: Asset, Life-cycle, Durability, Corrosion.

1. INTRODUCTION

The term durability has been used for a very long time but has been increasing in importance over recent years in majorinfrastructure projects. This is due to asset owners and operators placing greater emphasis on whole-of-life performance bynominating the design lives that the assets being created during the project are required to achieve and requiring the projectdelivery team to justify and demonstrate how the design life will be achieved.

As asset owners become more sophisticated in applying asset management practices in their business it naturally generates arequirement to better assess and manage the social, environmental and economic risks associated with the construction andoperation of an asset. To this end durability can be an effective performance indicator and risk management tool, which takesinto account the consequences of the early failure of an asset (or its components) and tailors the materials selection to minimise

the likelihood of such event occurring within the required design life.




Similarly, the concepts of sustainability and whole-of-life-cycle costing are more commonly used when undertaking majorcapital works and tend to support the design and construction of more durable, less maintenance-intensive assets from the onset(i.e. allocating a higher budget in capital works to reduce operational costs during the service life). Interestingly, while thereseems to be a general agreement as to the definition of durability - typically the ability for an asset to achieve its requireddesign life in a given environment - there is at times some debate as to what level of maintenance would be consideredadequate. Generally though, so- called “minor” maintenance (such as c leaning/washing of stainless steel elements or touch-upsof a coating) seems to be widely seen as acceptable as opposed to “major”, more labour -intensive and costly activities such asconcrete repairs.

There have been numerous papers produced on durability and there are a number of useful resources to support durabilitydesign. There is also an overall agreement on what is meant by designing for durability and our industry is fortunate to have anumber of very experienced and knowledgeable practitioners in this field. However, in our experience there is not a common

process to undertake durability design. Having a common approach within the industry would make the process moreconsistent and transparent (currently too much reliance is placed on opinion) for designers, construction teams, operators, andasset owners in general. It would also allow for more effective verification of the deliverables produced by peers. The interestin developing a common approach to durability design led the authors to assess the standards and codes across the world thatsupport or guide durability design and propose a standard process template for the preparation of durability deliverables.

2. DURABILITY DESIGN AROUND THE WORLD

The concept of durability, and its importance when designing, constructing and operating assets, has gained importance over

the past decades. This is illustrated in the following examples for concrete elements. Back in 1963, the primary requirement forconcrete structures was a satisfactory compressive strength [1]. A few years later, according to Neville [1], the British Code ofPractice for Reinforce d Concrete in Buildings (CP 114) stated as a general comment: ‘The greater the severity of the exposurethe higher the quality of the concrete required’. Later, in 1973, Neville concluded: ‘Concrete of reasonable strength, proper ly

placed, is durable under ordinary conditions. But when high strength is not necessary and the conditions are such that highdurability is needed, the durability requirement will determine the water- cement ratio to be used’ [2]. In 2001 Neville [1]talked about the future in durability design and highlighted that for concrete an improvement in durability would be possiblethrough the use of correct placing methods, compaction techniques, finishing operations and adequate curing. In addition,

Neville stressed that in the future the importance of maintenance would be recognised as a way of achieving durability [1].

At present, most of the standards and codes available throughout the world have prescriptive and/or performance basedrecommendations, which rely on material requirements during design and their performance during construction. However,the majority of standards and codes appear to lack guidance on how to apply and manage the durability process from design,through construction and into the operational phase of an asset. Additionally, they do not seem to make recommendations onhow to link durability to the preparation of inspection and maintenance plans.

In Australia, a number of standards specify the minimum requirements necessary to achieve durability. For instance,galvanised and electro-galvanised zinc coatings in atmospheric exposures are covered by AS 2309 [3]. This standard specifiesthe corrosion rates and estimated service life for numerous systems in different atmospheric exposures and environmentalaggressiveness. AS 2309 Appendix E lists the items to be considered during the preparation of a specification for metallic

protective coatings, including both prescriptive and performance requirements (e.g. estimated corrosion patterns for a particular location and maintenance requirements). Furthermore, guidance for repair of metallic protective coating is providedin Appendix F.

Some of th e available concrete standards (AS 3600, AS 5100.5 and AS 3735) have a dedicated ‘Durability Design’ section [4-6]. For example, the ‘Durability Design’ section in AS 3600 [4] provides guidance for the durability design requirements ofreinforced and pre-stressed concrete structures and members with a design life of 50 years ± 20% by defining the differenttypes of exposures in Australia and the subsequent requirements (concrete strength grade and associated cover toreinforcement for the different environments) in order to achieve durability. In a similar manner, AS 5100.5 [5] providesguidance for the design for durability of concrete structures with a design life of 100 years, with the same focus as utilised inAS 3600. AS 3735 [6] provides guidance on the minimum cover to reinforcement required when the concrete is exposed todifferent liquids such as freshwater, seawater, corrosive liquids so a design life of 50 years ± 20% (as per AS 3600) can be met.In contrast to AS 2309, none of abovementioned concrete standards list requirements to be included in the preparation of aspecification, including maintenance requirements.

Another Australian standard to include a section in ‘Durability Design’ is AS 2159 [7] which addresses plain, reinforced and pre-stressed concrete and steel piles with a design life of 50 and 100 years, as well as timber piles. For concrete piles, AS 2159specifies the following main requirements: minimum concrete strength and cover to reinforcement, limitation in crack widthand selection of concrete aggregates. AS 2159 uses different terminology for the exposure environment in comparison to thatused by AS 3600 and AS 5100.5 (i.e. non-aggressive, mild, moderate and severe). For steel piles, the requirements relate tocorrosion allowance, application of coating systems or cathodic protection. The corrosion rate is specified as a range for each

environment, e.g. 0.04-0.1 mm/year for a severe environment (seawater submerged, tidal/splash zone and cold water south of




30’ S) , which makes it difficult to select a specific rate when designing. In addition, a linear corrosion rate is assumed, nottaking into account the higher rate usually observed for the first year (and mentioned for instance in ISO 9223 [8]). However,like the Australian standards related to concrete design [4-6], AS 2159 does not provide any guidance on how to structure adurability process from design into construction and maintenance during design life. For timber piles, AS 2159 discussestimber selection and treatment, while AS 5604 [9] provides life expectancy in-ground and above-ground for different classes oftimber.

Within Australia, state codes such as RTA B80 [10] and VicRoads Section 610 [11] also specify requirements to ensure

concrete durability. The RTA B80 concrete requirements include: cement type, minimum cement content, maximumwater/cement ratio, maximum water sorptivity, compressive strength, and curing regime. VicRoads Section 610 specifieslimits such as maximum volume of permeable voids (VPV), minimum cementitious content and maximum acceptable crackwidth.

A brief overview of codes and standards that were identified to provide durability guidance outside of Australia is presented below.

In Brazil, the durability of concrete assets is addressed by a section of ABNT NBR 6118 [12]. In this standard, only four mainexposure classifications are determined. However, in this case, the minimum cover to reinforcement is not the mainrequirement, but rather the focus is on the water/cement ratio and concrete strength grade. Interestingly, two additionalrequirements are made; one being that drainage must be considered during design to avoid accumulation of water on theconcrete surface, and the shape and format of the structure must allow for easy access for future maintenance.

In Colombia, the available standard that deals with concrete durability is slightly more detailed. NTC 5551 [13] specifies sevendifferent ex posure classifications including ‘no risk of corrosion’ and ‘high humid ity’. As expected, the durability requirementsdepend on the type of exposure linked to water/cement ratio, compressive strength and minimum cementitious content as wellas maximum allowable crack widths.

In the United States, the ACI 201.2R-01 Guide to Durable Concrete [14] describes the different deterioration mechanisms inconcrete including freezing and thawing, chemical sulphate attack, physical salt attack, carbonation and acid attack while somerecommendations are made in order to achieve durability. These recommendations refer to: water/cement ratio, quality ofmaterials, curing and attention to construction practices. A grading for severity of exposure is presented and subsequentrequirements are listed. In addition, a section on evaluation of damage and selection of repair methods is presented.

In South Africa, Alexander et al. [15] recently published a paper describing the South African approach to durability design forconcrete elements. According to the authors, in order for the South African concrete industry to address the need forappropriate performance indicators, it developed a Durability Index (DI). DI is based on the quality of the cover to

reinforcement/surface layer and a series of index tests (to cover a range of durability problems) [15]. It is stated, that the indextests are to be used for quality control purposes [15]. Also in accordance with Alexander et al. [15], correlation betweenindexes and actual structural performance allows for the prediction of the performance of concrete in the design environment.Furthermore, the authors also discussed the possibility of implementing such approach in India, as both countries have aextensive coastline and a similar internal geography [15]. Currently, durability of concrete in India is limited to therequirements of IS 456 [16], which prescribes a minimum grade of concrete strength, maximum water/cement ratio andminimum cement content for the different types of exposures.

In Japan, Tomosawa [17] assessed Japan’s approach to concrete durability. According to the author, while issues remainunsolved the approach for durability of concrete has significantly improved over the past 20 years. In 1997 JASS 5 [18] wasrevised to account for global environmental issues, such as global warming, waste disposal and natural resources. This code is

performance-based with compressive strength (assigned as durability design strength) being the main performancerequirement. Furthermore, Tomosawa commented on the durability recommendations provided by the Architectural Institute ofJapan in 2004 [19], whose recommendations established design and maintenance limit states as the criteria aiming to retain therequired performance of a concrete asset throughout a defined period. Carbonation, salt attack, frost attack, alkali-silicareaction, and chemical attack are considered as key factors for deterioration, and deterioration prediction models are presentedfor carbonation, salt attack and frost attack [17, 19].

In China, Li et al. [20] provided a detailed review of the Chinese national guide for the durability design of concrete assets – CCES01-2004. This guide also specifies durability requirements for different types of environment. Requirements include

binder type, binder content, water/binder ratio, curing condition, concrete strength, concrete cover, and crack control. Li et al.concluded that the requirements of CCES01 are generally at the same level with or stricter than codes such as EN 206-1:2000[21] .

In Europe, EN 206-1:2000 [21], like the standards described above, does not specify the need for maintenance requirementsnor does it provide guidance on how to structure a durability process. Rather it details requirements for concrete and

performance tests. This standard has a number of exposure classes for concrete elements varying from ‘X0’ – no risk ofcorrosion attack to XA1, XA2 and XA3 – chemical attack. Limiting values for the different exposure classes to avoid chemical




attack from aggressive substances in soil and groundwater are provided. ISO 9223 and 9224 provide guidance on how to assessatmospheric corrosivity categories as well as provide ranges of corrosion rate after one year (ISO 9223) as well as 10 years andsteady state (ISO 9224).

On a broader level, AS 5104 (ISO 2394) [22] does not relate to any specific material but for assets in general as it provides principles to design for their reliability. As well as composition, properties and performance of the materials, AS 5104 alsoconsiders that to achieve an adequate durable asset the following must be considered: its intend use, required performancecriteria, expected environmental condition, structural system, shape of members and structural detailing, quality of

workmanship and level of control, protective measures and maintenance during the design life.More recently, ISO 13823 [23] recommends the use of limit state methods for the design and verification of assets fordurability. It provides strategies for durability design, such as the development of maintenance/repair/replacement plan for theasset during the design phase, giving examples of how to structure procedures and communications to ensure durability isachieved. This document is discussed further in the next section.

3. TOWARDS A COMMON DURABILTY DESIGN APPROACH

3.1 Durability throughout an asset ’s life-cycleAs mentioned previously, durability often seems to be associated with the design phase of a project and at times even simplyconfined to the production of a plan or report. While the authors certainly agree that the preparation of a DurabilityManagement Plan (DMP) is an essential step early in the project, the following are also critical to achieving the asset owner ’s

requirements: Durability is embedded into the detailed design process, ensuring that all designs are compliant with the DMP and

satisfy the design lives specified in the project scope and requirements. The assets are constructed in compliance and to achieve the targets set by the DMP and design packages. The materials in situ achieve the levels of quality and consistency expected by the designers when formulating the

durability design. The assets are inspected and maintained in line with the requirements of the design so that they achieve their

designated service life.

Durability processes need to be designed into each phase of the asset life-cycle and the specific performance requirements foreach phase need to be clearly defined. Although the information that is needed in each phase of the life-cycle is different, theapproach must be consistent to optimise the durability and satisfy the design life requirements. To ensure consistency inapproach, an overall plan needs to be developed early in the project to map the overall durability process, the type of inputinformation necessary to develop the durability requirements for each phase and the output or deliverable (including its timing)that will be generated. With correct planning the deliverable from the previous phase will fo rm the input for the next phase.

From the start of the project it is necessary to have well-defined design life requirement and a proposed maintenance strategyfor the asset(s). In many circumstances the design life requirement will be nominated in bid documents, but the maintenancestrategy will not always be as well defined. As mentioned previously, it needs to be determined whether assets/structures will

be maintained and rehabilitated during their design life or will negligible deterioration and subsequent intervention beaccepted. Where the design life is not specified by the client, several international standards, namely ISO 15686.1 & 2 [24, 25]as well as AS 5104 (ISO 2394) can be used for guidance on the appropriate design life. Once the design life and themaintenance strategy are specified, the durability process can be formulated or defined. It is important that the assumptionsthat are used in developing the durability process are clearly stated to provide transparency and simplify verification of the

processes and outputs.

During the preliminary design phase the general approach to durability is proposed and preliminary materials guidelines aredeveloped. Parallel to this process, a basic asset hierarchy structure, which will later be used to create the asset register, needsto be developed into a working model that incorporates the asset and sub asset items in a logical manner. The asset hierarchystructure outlines the relationship between the broad asset category and the individual items that comprise the asset. Thehierarchy can also set out how unique identifying tag numbers can later be generated for each asset item or sub asset item.These steps become the building blocks for an asset register that contains detailed durability information.

Early in the detailed design phase a more comprehensive DMP needs to be produced, which defines clearly all the exposureconditions to which the materials will be subjected. Additionally, this document presents material selection guidelines withdetailed information enabling the identification of durability risks that are based on the likelihood of material degradation andits consequence. The minimum durability requirements for each material in each distinct environmental exposure are specified,to minimise the impact of materials degradation to the extent required by the selected maintenance strategy for the nominateddesign life. As the detailed design phase progresses, interdisciplinary durability reviews are undertaken which includesconsidering the suitability of alternative materials. The durability team needs to be well embedded into a project during the




detailed design phase to provide timely advice to the designers in an attempt to minimise any delays if construction has started.All these rapidly developing designs and material durability data should be captured in the evolving asset register that is

progressively developed (based on the asset hierarchy structure produced in the preliminary design phase). The asset registershould include the following as a minimum for each asset element: type, asset item/asset sub-item, design life, materials ofconstruction, environmental exposure conditions, durability risk, durability issues and minimum durability requirements,unique tag number and location of the asset. In addition to the development of the asset register, construction repair proceduresshould be developed during this phase so the documents are available before they are needed. Towards the end of the detaileddesign phase the preliminary requirements for post construction inspection and repair procedures should be nominated.

During the construction phase the durability team needs to respond quickly to requests for additional information, provideadvice on repairing construction defects to minimise changes to proposed frequency of maintenance and long term impact ondurability. Construction repair procedures may need to be modified to deal with specific site issues. The asset register needs to

be updated progressively with as-built information incorporating changes to the durability of the asset item resulting fromchange of materials or damage and repair of construction defects. The post construction inspection and repair proceduresshould be fully developed during this phase and the procedures should be linked into the asset register. The nominatedinspection and maintenance frequency should also be clearly nominated in the asset register.

In the operations and maintenance phase durability information is needed to assess the current condition of the asset, estimateits remaining life and predict its long term durability performance. The asset register is constantly updated as conditionassessments and maintenance are undertaken and more up to date information obtained.

Durability requirements and deliverables for each phase during the life-cycle are summarised in Table 1.

Table 1: Summary of durability in each phase of the life-cycle

Phase Required inputs for durabilitydeliverable

Durability deliverable Durability information requiredby other disciplines

Preliminarydesign – concept or BIDdesign

Preliminary environmental exposureinformation (e.g. preliminary soildata, process fluids information).

Preliminary design life.Proposed maintenance strategy.Preliminary asset information (typesand numbers of assets).

Preliminary DMP.Identify any key potential projectrisks in terms of materials/durabilitywith regards to design/construction;durability compliance and/ordelivery lead times.Asset hierarchy structure.

Preliminary guidelines for materialsselection and Materials/DurabilityRisk register/mitigation;

Detailed design Detailed soil testing data (soil type,composition/aggressive elements,

permeability, groundwatercomposition, etc.).Proposed construction method (e.g.cast in-situ vs. precast concrete).

Detailed design DMP (due early inthe phase).Design support (e.g. durabilitymemos, review of design packages).Preliminary asset register.Preliminary requirements for postconstruction inspection and repair

procedures.Construction repair procedures.

Detailed minimum durabilityrequirements and materials selectionguidelines.

Construction Detailed design DMP.Design support durability memos.Preliminary asset register.

Durability responses to constructionrequests for information (RFIs) andnon-conformance reports (NCRs).Procurement support andassessment of vendor durabilitydata.Repair procedures.

Inspection and maintenance procedures.

As-built asset register.

Equivalent durability of alternativematerials.Impact of construction damage ondurability.Repair strategies and suitable repairmaterials.

Operations andmaintenance

Maintenance inspection and repair procedures.As-built asset register.

Living asset register.Review of inspection andmaintenance procedures.

Predicted rates of deterioration.Likely deterioration mechanisms.Consequences of materialsdegradation.Condition assessment guidelines.




3.2 Understanding and using key durability standardsAmong the various standards and codes around the world, ISO 13823 (General principles on the design of structures fordurability) and ISO 15686 (Service life planning) provide clear directions to guide the durability design process throughout the

project life-cycle. ISO 13823 refers directly to ISO 15686-1 as the latter provides an overall framework and design procedurefor service-life planning. Note that while AS 5104 (General principles on reliability for structures) deals with durability, it is

primarily in relation to deterioration caused by actions such as gravity, wind, snow, and earthquake, rather than deterioration ofmaterial resulting from environmental exposure or action effects. This section combines an overview of ISO 13823 and,

wherever applicable, ISO 15686, as well as the authors’ comments on these standards and their application in durability design.The scope of ISO 13823 “specifies general principles and recommends procedures for the verification of the durability ofstructures subject to known or foreseeable environmental actions, including mechanical actions, causing material degradationleading to fa ilure of performance”. The definition of durability used in this standard aligns with that adopted across manystandards, codes and guidelines, namely the “capability of a structure or any component to satisfy, with planned maintenance,the design performance requirements over a specified period of time under the influence of the environmental actions, or as aresult of a self- ageing process”.

ISO 13823 can be used for the structural and non-structural elements of both new and existing assets as the durability processis similar in both cases, as previously shown [26]. The key difference is the possibility for an existing asset to collect historicaldata, which can be used to better estimate the remaining life and thus the need to undertake remediation. ISO 15686 Part 7 [27]

provides guidance on the use of information collected during performance assessments of an existing asset to estimate aservice life. Note that ISO 13822 [28] is also a very useful document that specifies how to assess existing assets taking into

account their reliability and the consequences of failure.At its core ISO 13823 proposes a limit-state approach to design for durability that can be summarised as follows:

Determination of the structure environment , which is defined as “external or internal influences (e.g. rain, UV,humidity, soil constituents) on a structure that can lead to an environmental action”. Examples of environments andagents are provided in Appendix B of this standard.

Identification of the transfer mechanisms , which is defined as a mechanism which promotes or prevent transfer ofenvironmental influences into agents resulting in environmental action. Transfer mechanisms are listed in Appendix Cand include direct exposure, condensation, diffusion etc.

Assessment of the environmental action , which is the “chemical, elect rochemical, biological, physical and/ormechanical action causing material degradation of a component”. Environmental actions for structural materials andtheir control are provided in Appendix D and include corrosion (of metals), sulphate attack (of concrete) and chemicalattack (of GRP and plastics) for instance.

Based on the action effects on a component of a structure, which “include damage, loss of resistance, internalforce/stress or change in appearance due to material deterioration, or displacement due to material deformation”, twolimit states can be considered:

o An ultimate limit state when the resistance of the structure or its components become equal or greater thanwhat it can withstand.

o A serviceability limit state when local damage or displacement affects the function or appearance of thestructure or its components.

Taking the above into account, durability requirements can be proposed to ensure that the structures and theircomponents achieve their required performance over their design lives with “sufficient reliability”. ISO 13823

provides guidance on service-life predictions based on data/experience/tests, probabilistic approach using a limit-states methods or mathematical modelling. More details and guidance on service-life modelling is provided in ISO15686-2.

Two durability examples that follow the process set out in ISO 13823 are presented in Table 2. ISO 15686-1 notes that the process of service life planning such as that listed above may need to be re-iterated a number oftimes in order to find the most appropriate and cost effective way to achieve the performance and maintenance requirements.

Both ISO 13823 and ISO 15686 make a clear dif ference between the design life of an asset (the “specified period of time forwhich a structure or a component is to be used for its intended purpose without major repair being necessary ”) and its servicelife (“actual period of time during which a struct ure or any of its components satisfy the design performance requirementswithout unforeseen major repair”). As mentioned previously the levels and/or possibility of maintenance activities to beundertaken need to be carefully considered to determine whether greater inherent durability or a more comprehensivemaintenance program is required. Interestingly, ISO 15686-8 [29] proposes a factor method as a way to empirically estimatethe service life of an element based on available information.




Table 2: Examples of the application of the approach described in ISO 13823

Examples of durability process

Process steps Concrete slab in a contact with ground Steel handrail on a balcony

Structure

environmentLocation Outside Outside – atmosphere

Influences Water, soil constituents, spills/leaks Rain, air constituents, contaminants, pollutants, temperature and humidity

Agents (Causeenvironmentalaction)

Sulphates, chlorides, acids, other chemicalsfrom spills/leaks

Moisture, oxygen, acid

Transfer mechanisms Direct exposure, capillarity/surface tension,diffusion

Condensation

Environmental action Sulphate attack/chloride attack Corrosion in atmospheric environment

Action effect Expansion followed bydisintegration/cracking and delamination

Failure, change in appearance, damage dueto corrosion product expansion

Durability requirements Design concrete characteristics (mix design)to resist attack and/or isolate from theenvironment

Drainage (avoid water traps), protectivecoatings

It has been th e authors’ experience that the terms “design life” and “service life” are sometimes used as if interchangeable, i.e.that an asset could be designed so that it becomes completely unserviceable when it reaches its design life. The aim of adoptinga durability approach is that the design life would be lower than the expected service life. This ensures that at the end of thedesign life an asset owner has adequate time to undertake inspections, estimate the remaining life and possibly plan for anextension of service life. It also provides a factor of safety for unforeseen increases in the aggressivity of an environment. Asstated in ISO 13823 “materials, components and design, including detailing and other reliable measures to lengthen the life,should be chosen so that the predicted service life, with a target probability of failure, exceeds the required design life”. Whilematerials selection typically focuses on minimising the likelihood of unacceptable material deterioration within the design life,it has to be also influenced by the consequence of any failure. ISO 13823 proposes four categories ranging from minor andrepairable damage without injuries to people (1), to loss of human life or serious injuries or considerable economic, social orenvironmental consequences (4). An asset failure and its consequence relates to the level of service or reliability that the assetowner deems acceptable to comply with its obligations.

The concept of risk management should be central to durability design and as such durability risks need to be identified as partof the process. This is done by assessing the likelihood of damage or failure of material/treatment options (from rare to almostcertain) and understanding from the designer/constructor/operator/owner its consequences. In addition to the four categories

proposed in ISO 13823 a fifth entitled “negligible” could be introduced to produce a symmetrical matrix that aligns with that presented in Table 6.6. of HB 436:2004 (Risk Management Guidelines - Companion to AS/NZS 4360) [30] as shown in Table3.

Table 3: Proposed durability risk matrix based on HB 436:2004 (AS/NZS 4360)

Likelihood ofdamage/failure

Consequence of failure/damage

Negligible Minor Moderate Major Severe

Almost certain Medium High High Very High Very High

Likely Medium Medium High High Very High

Possible Low Medium High High High

Unlikely Low Low Medium Medium High

Rare Low Low Medium Medium High




When the durability risk is considered too high it could be reduced by either or a combination of the following:

Decreasing the likelihood of failure by proposing a more durable option (e.g. more corrosion-resistant metal, greaterconcrete cover to reinforcement, use of cathodic protection, more robust coating system). However, this has to be

balanced by taking into account constructability and cost implications (i.e. can it be built, procured or even afforded). Putting in place mitigation strategies aimed at limiting the consequences of any damage/failure, e.g. having sufficient

redundancy in the process equipment, restricting access. Tailoring the inspection and maintenance plan (and especially the frequency of these activities) to detect early signs ofunacceptable deterioration in order to pro-actively plan for repair and/or replacement.

A similar approach can also be used when a number of materials options are available by rating (e.g. from 1 to 5) the risksassociated with durability, constructability and cost, adding the figures and selecting the one with the lowest risk value. It isworth mentioning that if no major maintenance is allowed to achieve the required design life, any option requiring more thanminor “refurbishment” should be eliminated.

In Appendix E, ISO 13823 provides an example of procedures that can be used for ensuring durability throughout the projectlife-cycle (with references to the relevant parts of ISO 15686) and who they should be communicated/worked with(owner/user, contractor, fabricator, supplier, designer, investigator). This example aligns with some of the comments made bythe authors in the previous section and is summarised below:

Design phase:o Undertake durability assessment (design life, environment, deterioration mechanisms) and materials

selectiono Design access to allow for inspection, maintenance and repair as well as based on constructability

considerationso Prepare life-cycle cost/assessment (as per ISO 15686-5 [31] and ISO 15686-6 [32]), if necessary, revise the

designo Prepare plans for inspection, maintenance, repair and replacement as well for quality control during

construction Construction phase:

o Review design and incorporate acceptable changes, which need to then be inspected and approvedo Review procurement information (this is not in ISO 13823 but in ISO 15686-9 [33])o

Mitigate the risk of damage to assets during construction Maintenance and operation:o Ensure that the environmental exposure does not adversely change during the design lifeo Implement inspection and maintenance plan (including cleaning, repair, replacement and monitoring)o If damages/defects are identified determine cause, record to provide feedback for future practice (as per ISO

15686-7).

4. PROPOSED TEMPLATES FOR KEY DURABILITY DEVILERABLES

As shown in the previous section, ISO 13823 and ISO 15686 can be used and referred to when undertaking durability design as part of service-life planning. Based on these standards, templates for the key durability documents listed in Table 1 (i.e. thedurability management plan and the asset register) are being proposed. As mentioned previously, design packages shouldclearly list out durability information (e.g. design life, materials, requirements) for each asset/sub-asset and their compliancewith the DMP so they can be extracted and added to the asset register. The latter can then progressively updated with as-builtinformation prior to handover to the asset owner and/or operator.

4.1 Durability Management Plan - Durability in Concept and Detailed DesignIt is suggested by the authors that a standardised durability management plan (DMP) contains the general categories ofinformation listed below. It is proposed the main report contains information on the approach taken to durability and overviewsof all the aspects. The detailed scientific and engineering information should be contained in discrete appendices where readersof the report can clearly find the required substantiations, justifications, assumptions and modelling. It is believed that a reporttemplate developed in this fashion will be more user friendly not only for the designer/engineer, who needs to includedurability information in each design reports, but also for the design verifier/proof engineer, contractor, operator and assetowner alike.




The proposed categories in the DMP are:

Project overview and overall durability objective: this is a general section that describes the project and the importancegiven to achieving durable outcomes in the contract document presenting the scope/technical requirements.

a) Approach and scope: in this section the extent and methodology of the durability design can be provided. It couldrefer to the process described in ISO 13823.

b) Referenced d ocuments: a table can present the list of standards, codes, guides as well as project-specific documentsupon which the durability design in the DMP is based.

c) Terminology: it is important to provide definitions for technical names, abbreviations and symbols (if used) used inthe DMP. Sections 3 of ISO 13823 and ISO 15686 provide good references for definitions.

d) General overview of assets: in the concept DMP this section may only provide a basic list of assets and sub-assetswhile the DMP prepared at the start of detailed design may present a more detailed asset hierarchy. However, thedevelopment of a full asset register should not be in the scope of the DMP.

e) Overview of environmental exposure categories for the assets: This section can present the structure environment(location, influences and agents), possibly in a table format and in accordance (in particular the terminology) with ISO13823 (and its Appendix B). For large projects this section could be split into geographical areas. Detailedinformation and pertinent test data should be provided in Appendix A of the DMP.

f) Overview of deterioration mechanisms for construction materials in each exposure: This section focuses on thetransfer mechanisms, environmental actions and action effects such as those described in ISO 13823, and in

particular, Appendices C and D of this standard. Detailed information regarding the mechanisms could be provided inAppendix B of the DMP.

g) Durability requirements: This section summarises the outcomes of the materials selection based on theassessment/prediction/modelling of future deterioration and the durability risk associated with material deterioration.The focus should be on assets critical to project operation or subject to high durability risk. More detailedinformation could be provided in Appendix C of the DMP. The approaches (e.g. based on standards, limit-statecalculations) followed should be clearly stated with appropriate references to documents as well as to the relevantappendices of the DMP, which provide greater details (see below).

Appendix A – Environmental exposure classification

Nominate reference standards (e.g. ISO 13823, AS 3600, AS 2159, AS 5100.5, AS/NZS 2312, ISO 9223

and ISO 9224) Define how the site specific test data for the location such as prevailing wind and temperature trends are

used to classify the various exposure conditions (including influences and agents as per ISO 13823):atmospheric, buried, immersed or tidal/splash. Include tables of relevant data

Appendix B – Materials degradation mechanisms

Provide technical information about the predicted deterioration mechanisms for the materials in eachexposure category

Appendix C – Durability requirements

Provide greater details of durability requirements for all asset types (including those with low andmedium durability risks)

List the methods employed to produce the requirements and make adequate references to the appendicesthat specifically address them (see suggestions below)

Appendix D – Durability risk

Assess the likelihood of deterioration of an asset item Conduct a consequence rating assessment of deterioration based on the effect it would have on the

operational capacity or performance of the asset (to be undertaken in co-ordination with the operationsteam and/or asset owner if possible)

List the risk mitigation strategies proposed and provide technical information about those selected for uson the project (e.g. corrosion control measures)




Appendix E – Predictive modelling / limit-state calculations

Explain the models used to predict the life of concrete assets in specific environmental exposures Outline the limitations of the models and define the assumptions used in process Provide limit-state calculations and make references to relevant standards

4.2 Asset register

As mentioned above the asset register can be created at the design phase and progressively updated with the durabilityinformation extracted from the design packages, which would typically be reviewed by the durability team. In these packagessuch information could be presented in simple tables that demonstrate compliance with the DMP either by direct reference to aspecific section (i.e. the type of asset in the particular environmental exposure is already presented in the DMP) or by the

provision of detailed requirements to ensure that the design life can be achieved.

Each asset register will be unique and the information contained in the register will depend on the number of fields that canexist in the asset management system for each asset, and specific contractual requirements from the asset owner. While it isnot considered feasible to provide a typical template for the asset register, the list of key durability-related parameters shouldinclude the following: unique tag number, duty description, design life, materials of construction, environmental exposure,durability risk, durability issues (deterioration mechanisms), minimum durability requirements, location of asset, manufacturer(if relevant), drawing references, links to pertinent inspection and repair procedures, inspection and maintenance cyclefrequencies and condition rating.

The inspection procedures nominated in the asset register need to be developed to ensure that the following information isavailable during the service life of the asset: any safety issue (especially if relating to materials deterioration such as concretespalling over pedestrian walkways), the current condition rating of the asset and any required repairs. In terms of maintenanceactivities, while the register would provide a schedule for minor maintenance (e.g. cleaning of stainless steel items), the timingfor any major repair would depend on the overall maintenance strategy (and especially the intervention levels set for the

project) and the evolution of the condition rating.

5. CONCLUSIONS

This paper was borne out of the interest and experience of the authors in durability design. In particular not having oneAustralian standard specifically providing guidance on the process of designing for durability led to a review of relevantdocuments around the world. While, like Australia, a number of countries have codes or standards that are performance-based,

prescriptive and/or have minimum requirements, ISO 13823 and ISO 15686 are focused on the actual processes of durabilitydesign and service-life planning. These documents also stress the importance of embedding durability throughout the entirelife-cycle of an asset.

In order to make the durability design process more consistent and transparent not only for practitioners but also for designers,construction teams, operators and asset owners alike, the authors have proposed a template for the preparation of a durabilitymanagement plan, which draws from the guidelines of ISO 13823. Equally important is the compilation of an accurate assetregister that adequately present the durability information including the type and frequency of inspection and maintenanceactivities that are necessary for the required design life to be achieved.

6. ACKNOWLEDGEMENTS

The authors would like to acknowledge the works of Dr Frank Collins and Dr Marita Berndt in the field of durability planning.We would like to also thank Miles Dacre for his valued feedback and comments.

7.

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6. AS 3735-2001: Concrete structures retaining liquids.

7. AS 2159-2009: Piling - Design and installation.




8. ISO 9223:1992 - Corrosion of metals and alloys -- Corrosivity of atmospheres -- Classification.

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14. ACI 201.2R-01:2000 - Guide to Durable Concrete.

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16. IS 456:2000- Indian Code - Civil Engineering for RCC.

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24. ISO 15686-1:2011 - Buildings and constructed assets - Service life planning - Part 1: General principles andframework.

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26. Blin F, Law D, Dacre MC, op'tHoog C, Gray B, Newcombe R (2008) Extension of Design Life of Existing MarineInfrastructure - A Durability Perspective. ACA Conference 1-13.

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8. AUTHOR DETAILS

Frédéric Blin is a Principal Engineer in the Advanced Materials Group at AECOM. He holds aPhD on corrosion inhibitors and has worked on numerous projects, including the conditionassessment of different types of structures exposed to various environments, non-destructivetesting, crack and corrosion monitoring, survey of compliance with Australian Standards,review, and advice on durability issues, technical specification for infrastructure repair woks,modelling and prediction of future deterioration. He has also managed several projects in the

field of civil and transport, especially maritime, infrastructure, and has authored and co-authored a number of publications, technical papers and technical reports.

Sarah Furman is a Principal Engineer in the Advanced Materials Group at AECOM. She has aMaster of Science in Corrosion Science and Engineering from UMIST in England. A materialsand corrosion specialist with a broad knowledge of both metallic and non-metallic materials,she specialises in durability planning for new infrastructure, performance assessments ofmaterials, materials selection, failure analysis, and cathodic protection design.

Alessandra Mendes is a Senior Engineer in the Advanced Materials Group at AECOM. Sheholds a PhD on fire resistance of concrete and has worked on numerous projects including

condition assessment of concrete assets, durability advice and review from design phasethrough maintenance phase (including inspection and maintenance planning), technicalspecification for infrastructure repair woks, modelling, and prediction of future deterioration.

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