Design for Service Life

Technical Paper Steinar Helland

DOI: 10.1002/suco.201200021

Design for service life: implementation of fib Model Code 2010 rules in the operational ope rational code ISO 16204 CEB/FIP Model Code 1990 (MC-1990) [1] did represent the tech- nology and focus some 20 years ago. However, it soon became evident that the document had some notable lacunas. In 1995 the general assemblies of the two organizations endorsed CEB/FIP bulletin No. 228 [2], extensions to MC 1990 for high-strength concrete, and in 2000 a similar extension to MC 1990 for lightweight aggregate concrete as bulletin No. 4 [3]. The fib approved bulletin No. 34 Model Code for Service Life De- sign (MC SLD) [4] in 2006. All these three additions have since matured and are now incorporated in the new fib Model Code for Concrete Structures 2010 (MC-2010) [5, 6, 7]. The main purpose of an fib Model Code is to act as a model for operational standards. The obvious counterpart for a body such as fib operating worldwide is ISO. The initiative taken by MC SLD has therefore further matured in ISO TC-71/SC-3/WG-4 and it was accepted as ISO 16204 “Durability – Service Life Design of Con- crete Structures” [8] during the summer of 2012. According to the obligations given in the WTO Agreement on Technical Barriers to Trade [9], it is hoped that these principles will be further implemented in national and regional standards. This article describes the need for a transparent methodology when dealing with service life design, and the process – originat- ing from a group of enthusiasts one decade ago – through fib and finally reaching international consensus in ISO. fib Model Model Code 2010, ISO 16204, service life design Keywords: fib

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Background

Durability of concrete structures, and in particular the lack of such, has been in the focus of society in general over the last few decades. Excessive repair needs have challenged our industry. The traditional approach in most national and regional concrete standards is to specify the provisions to ensure a certain design service life by limiting values for material composition and geometry based on the expert opinion of the code committee. There are several weaknesses in this approach: – It is often unclear as to which condition represents the end of the service life.

– The required level of reliability for the design is often unclear as well. – The criteria should be based on long-term field experience. Such experience is, however, not normally available for modern materials and design concepts, and concepts with service records > 50 years are seldom in use any more.

In 1998 a group of 19 European enthusiasts, all of us with a long record within CEB and FIP, signed a contract with the European Commission to develop a platform for dura bility design of concrete structures that contained the same elements and philosophy as that of modern structural design. This European network was named “Duranet”, and the contract lasted until 2001. At DuraNet’s final workshop in Tromsø, Norway, in 2001, attendees from Europe and North America worked out a plan for how to progress to get this methodology standardized and implemented in the industry worldwide (Fig. 1). The obvious environment for this was ISO. ISO. Some of us therefore met at the ISO TC-71 meeting in Norway that autumn and presented our visions. TC-71, responsible for concrete-related standardization within ISO, endorsed the initiative, but quite correctly made us aware of the fact that ISO normally starts its work on the basis of existing documents. We therefore agreed to ask the International Federation for Structural Concrete, fib (formed by the merger of CEB and FIP) to work out such a model for a standard.

Corresponding author: [email protected] Submitted for review: 03 August 2012 Revised: 23 August 2012 Accepted for publication: 23 August 2012

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Fig. 1. The “Duranet” workshop in Tromsø, 2001, which came up with a roadmap for how to implement limit state and reliability-based service life design in standards.

© 2013 Ernst & Sohn Verlag für Architektur und technische Wissenschaften Wissenschaften GmbH & Co. KG, Berlin · Structural Concrete 14 (2013), No. 1

S. Helland · Design for service life: implementation of fib Model Code 2010 rules in the operational code ISO 16204

Table 1. Comparison of some European durability requirements to ensure 50 years design service life (from [12])

Range of XC3 provisions for CEM I in Europe

UK → w/c < 0.55 and 25 mm minimum cover

Germany → w/c < 0.65 and 20 mm minimum cover

Range of XC4 provisions for CEM I in Europe

Netherlands → w/c < 0.50 and 25 mm minimum cover

Germany → w/c < 0.60 and 25 mm minimum cover

Range of XS2 provisions for CEM I in Europe

UK → w/c < 0.50 and 35 mm minimum cover

Norway → w/c < 0.40 and 40 mm minimum cover

Thereupon, fib set up Task Group 5.6 with experts from Europe, North and South America and Japan. In 2006 fib “Model Code for Service Life Design” (bulletin No. 34) was endorsed by the fib’s General Assembly in Naples, Italy. fib TG 5.6 was headed by Prof. Peter Schiessl from Germany. The other members were Gehlen (DE), Baroghel-Bouny (FR), Bamforth (UK), Corley (US) (present chair of ISO TC-71), Faber (DK), Helene (BR), Ishida (JP), Markeset (NO), Nilsson (SE), Rostam (DK) and Helland (NO). The group decided early on to produce a document fully parallel with ISO 2394 “General principles on relia bility for structures” [10]. This standard today forms the reference for fib MC-2010 and most modern standards for structural design. ISO 2394 is also the “parent document” for the European Eurocode 0 “Basis of structural design” (EN 1990) [11]. fib based its approach on a limit state (LS) and relia bility-based concept. This approach recognizes that the nature of the deterioration of concrete structures over time must be treated in a statistical way. This is due to the natural spread in material characteristics and also to the spread in the mesoclimatic and microclimatic conditions a concrete structure is exposed to. Since 2006 this initiative has progressed in close cooperation between fib’s group working on fib MC-2010, Special Activity Group No. 5 (SAG-5) and ISO TC-71/SC3/WG-4. fib MC-2010, including its elements for service life design, is currently being finalized. ISO 16204 “Dura bility – Service life design of concrete structures” achieved a positive international vote in summer 2012. These two documents are today – except for the cover and references – close to being identical when it comes to service life design. 2

How service life design is handled in most standards today

Provisions to ensure sufficient durability are today normally embedded in the concrete standards. In Europe durability is still regarded as coming under national authority and its provisions are expected to be given in a national annex to the European standard. In CEN TR 15868 [12], Tom Harrison has compared how the 31 European countries cooperating in CEN have solved the request in EN 1992/EN 13670/EN 206-1 [13, 14, 15] to give national provisions for a service life of 50 years based on requirements mainly linked to maximum w/c ratio, minimum cover to the reinforcement and cement type. The spread of requirements for structures expected to be subject to similar conditions is striking. Some exam-

ples for exposure classes XC3 (exposed to carbonation – sheltered from rain), XC4 (exposed to carbonation – exposed to rain) and XS2 (submerged in sea water) for 50 years design service life are given in Table 1. The differences in actual performance for these extremes are very large. Comparisons of durability-related provisions from other parts of the world demonstrate a similar spread. Bearing in mind that the technical expertise on these matters is more or less at the same level in these countries, the explanation must be that the different national standardization bodies have different understandings of what actually represents the “end of service life” as well as the intended level of reliability. 3

Limit state concept for service life design

The limit state concept recognizes the need to be specific about what condition represents the “end of service life”. The application of reliability-based and LS-based service life design is specifically excluded from both ISO 2394 and EN 1990. The task for fib TG 5.6 was therefore to come up with the amendments needed in these reference documents. At first sight these ideas might be considered as revolutionary, but actually that is not true. All code writers in the past must have had some idea of what they considered to be the “end of service life” when they came up with their provisions. They must have known whether they were considering just rust stains or full structural collapse. They then applied a “limit state” concept. They must also have had in mind whether they expected the statistical average of the building population to stand this design service life length, or whether they expected the great majority of the population to meet this requirement. They then applied a probabilistic approach. However, it is fair to say that these processes are very seldom applied in a transparent way. ISO 2394 defines the serviceability limit state as “a state which corresponds to conditions beyond which specified service requirements for a structure or structural element are no longer met”. fib MC SLD, fib MC-2010 and ISO 16204 apply the same definition, but fib MC-2010 has a further group “Limit states associated with durability” as a separate category. In principle, this may be any condition that makes the building owner feel uncomfortable. For concrete structures, corrosion of the reinforcement is often the critical deterioration process. The LS could then be depassivation, cracking, spalling or collapse (ultimate LS). Due to the problem of developing reliable time-dependent models for the rate of corrosion (after depassiva-

Structural Concrete 14 (2013), No. 1

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tion), LS depassivation is the choice of convenience for most engineers. 4

Level of reliability – consequences of failure

fib MC SLD, fib MC-2010, EN 1990 and ISO 2394 all suggest a three-level differentiation of the consequences upon passing an LS: a) risk to life low, economic, social and environmental consequences small or negligible b) risk to life medium, economic, social and environmental consequences considerable c) risk to life high, economic, social and environmental consequences very great Based on the relevant consequence class, combined with a consideration of the cost of safety measures, a relevant level of reliability for not passing the LS during the design service life should be selected. Within the limitations normally found in national building legislation, the reliability level used in the design should be agreed with the owner of the structure. fib and ISO suggest a probability of failure pf = 10–1 for depassivation of reinforcement (by carbonation or ingress of chlorides) in cases where the presence of oxygen and moisture makes corrosion possible. If collapse is the LS considered, pf = 10–4 to 10–6 may, as for traditional structural design, be the relevant level if the possible consequences are in classes b) and c).

Collapse of structure pf ≈ 10 -4- 10 -6

20

) n o i s o r 15 r o c ( n o i t a r10 o i r e t e D

Spalling

Formation of cracks Depassivation pf ≈ 10 -1

5

0 0

5

10

15

20

25

30

35

Time

Fig. 2. Various limit states and related reliability levels shown for corrosion of reinforcement

100

C 50% ) 75 % ( e r u l i a f e v i t a l u m u c

B 30%

A 2%

25

10%

0

5

End of service life

0

50

100

150

years

Based on the above, a main element in the fib and ISO documents is therefore an amended quantitative definition to the qualitative one we find in traditional standards such as the ones in ISO 2394 or EN 1990: Traditional qualitative definition: The design service life is the assumed period for which a structure or part of it is to be used for its intended purpose with anticipated maintenance but without major repair being necessary. Quantitative amendment by fib and ISO: The design service life is defined by: – A definition of the relevant LS – A number of years – A level of reliability for not passing the LS during this period

Fig. 2 indicates how various limit states may be associated with corresponding levels of reliabilities for not passing the LS within the design service life in the case where corrosion of reinforcement is the critical case. In principle, the verification of the design has to demonstrate that the structure will satisfy all combinations of LS and pf . For practical design, however, we do not have time-dependent models with international consensus to predict the corrosion phase after depassivation. The calculation therefore often has to be based on the time up to depassivation. The corresponding pf must then be sufficiently low to ensure that this LS results in equal or stricter requirements for material and depth of cover than the other combinations.

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Fig. 3. Time until depassivation of surface reinforcement (example derived from [16]). The Norwegian Standardization body applied a 10 % acceptance for depassivation as a criterion when determining its durability provisions, whereas countries A, B and C applied 2, 30 and 50 % respectively.

When considering the effect of corrosion of the reinforcement after its depassivation, splitting stresses in the cover zone from the reinforcement due to the effects of other mechanical actions/loads should also be considered. Wherever there are bond stresses in the reinforcement there are also “bursting stresses” in the concrete of the same nature as those from the expanding corrosion product, ultimately leading to the same type of cracking and spalling of the cover. This is another argument for avoiding the minefields of using cracking and spalling as the LS for service life deign. If we are pursuing the example of depassivation due to carbonation, all the characteristics that determine when the individual reinforcing bars will depassivate in a structure will have a statistical spread. This includes the actual depth of cover, the microclimatic conditions, the humidity of the concrete, its curing, etc. As a result, the initiation period will also exhibit a statistical spread. Fig. 3, derived from Bamforth [16], indicates the accumulative time for depassivation of the surface rebars in a structure subjected to carbonation. To assess the actual service life of this structure, the depassivation LS has to be

S. Helland · Design for service life: implementation of fib Model Code 2010 rules in the operational code ISO 16204 100

6

) 75 % ( e r u l i a f

50

v i t a l u m u c 25

0 0

50

100

150

years Fig. 4. The same example as in Fig. 3, but with10 years of active corrosion added to reach cracking and spalling of the rebar cover. The limit state at 50 % probability for depassivation then implicitly results in an approx. 35% probability of failure for the limit state of cracking and spalling.

matched with a level of reliability. In fib Commission 5, TG.5.11, we are presently developing supporting documents to fib MC-2010/ISO 16204. The work has revealed that Norway applied a pf of 10–1 when working out its present deemed-to-satisfy requirement. In this case a service life of 70 years is reached. However, representatives from three other European countries stated that experts in their standardization bodies had in mind a p f of 2*10–2, 3*10–1 and 5*10–1 (2, 30 and 50 % respectively). This then gives a range of nominal service lives from 50 to 109 years for the same structure exposed to the same environment. This lack of consistent use of the reliability-based limit state concept is probably a main reason for the aforementioned major differences in durability provisions among the European standards. The present lack of transparency must also be very confusing for the stakeholders when service life design is discussed. In Fig. 4 I have included an assumption often used of 10 years active corrosion until cracking and spalling occurs. In this case the nation accepting a 50 % probability for depassivation implicitly also accepts an approx. 35 % probability of cracking and spalling. Although it might be easy for a client to accept a high probability for passing an undramatic event such as depassivation during the design service life, it will be much harder to accept excessive cracking and spalling. The implicit consequences of linking an excessively high proba bility of failure for depassivation should therefore be clearly communicated.

What is the appropriate length of a design service life?

ISO 2394 gives guidance on the appropriate choice of the length of the design service life (Table 2). The same guidance is referred to in the European standard EN 1990 and in practice dominates the application in many parts of the world. However, the table provides general guidance for all structural materials and should be used with utmost care for concrete structures. This is particularly the case for class 3 comprising “buildings”. This is a very diverse group. Some buildings, e.g. factories, will often have an economic service life corresponding to the installed machinery. On the other hand, structural parts of residential buildings will, in general society, normally have an expected service life of much more than the 50 years indicated in the table. ISO 16204 therefore strongly advises users to be more ambitions for at least those structural parts of a concrete building where repairing or replacing elements will be complicated and expensive. 7

Design of service life and its verification

“The design of a structure includes all activities needed to develop a suitable solution, taking due account of functional, environmental and economical requirements.” (definition in fib MC-2010) This implies that the flow of activities for service life design will follow the flowchart given in Fig. 5. A similar graph is given in fib MC SLD and in text form in fib MC-2010. The serviceability (performance) criteria have to be agreed with the owner within boundaries given in the legislation. The documents are not specific regarding how the designer comes up with a general layout, dimensions and materials. However, the verification of the proposed design is strictly regulated. The fib and ISO documents offer four formats for verifying the ser vice life design: The full probabilistic method: The time to reach the LS with the required level of reliability is calculated based on statistical data for the environmental load and structural resistance. The partial factor method: As for the full probabilistic method, but the statistical data for load and resistance are substituted by characteristic values and partial coefficients.

Table 2. ISO 2394, Table 1 [10], gives examples of design service lives. The same table is given as guidance in EN 1990 [11]. ISO 16204 [8] states that class 3 should be used with care for structural parts of buildings where repair is complicated or expensive.

Class

Notional design working life

Examples

1 2 3 4

1 to 5 25 50 100 or more

Temporary structures Replacement structural parts, e.g. gantry girders, bearings Buildings and other structures, other than those listed below Monumental buildings and other special or important structures, large bridges


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Establishing the serviceability criteria

Establishing the general layout, the dimensions and selection of materials

Verification by the “Full probabilistic” method. Involving: • Probabilistic models – resistance – loads/exposure – geometry • Limit states

Verification by the “Partial factor” method. Involving: • Design values – characteristic values – partial factors • Design equations • Limit states

Verification by the “Deemed-to-satisfy” method. Involving: Exposure classes, limit states and other design provisions

Verification by the “Avoidance of deterioration” method. Involving: Exposure classes, limit states and other design provisions

Execution specification Maintenance plan Condition assessment plan

Execution of the structure

Inspection of execution

Maintenance

n g i s e d e r , l a a i t r i r e t a i r p c r o e l c l n u a f m o r t o t f r c e e j p b e u s h t r o o t e t y e t i l o m s r b o f o n s o e c - m n o o c n e f b o e r e u s t a c c u r e t h s t e n t h I

Condition assessments during operational service life

Fig. 5. Flowchart for service life design (from [8])

The deemed-to-satisfy method: A set of requirements (normally w/c, cover to the reinforcement, crack width, air entrainment, etc.) that are prequalified by the code committee to satisfy the design criteria. The avoidance-of-deterioration method: This method implies that the deterioration process will not take place due to, for instance: separation of load and structure by, for example, cladding or membrane, using non-reactive materials, suppressing the reaction with electrochemical methods, etc.

The fifth format offered by fib MC-2010 for verifying the structural capacity, the “global resistance”, is not mobilized for service life design. The partial factor and deemed-to-satisfy methods both need to be calibrated, either by the full-probabilistic method or on the basis of long-term experience of building traditions. Of these four options, the full probabilistic method is obviously the most complicated and sophisticated. For this reason, many academics have regarded it as the most prestigious and precise one. This is fundamentally wrong. Due to the normal lack of good and representative data, and uncertainty in modelling, the full probabilistic method will seldom be feasible for the design of new structures; however, the method is well suited to assess the remaining service life of existing structures where data might be derived from the actual structure. By assessing the remaining service life of existing structures by means of the full-probabilistic method, we al-

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so have a powerful tool for verifying deemed-to-satisfy provisions for the design of new structures with similar exposure and design conditions. The partial factor method is a semi-probabilistic approach where the calculation is deterministic and the statistical spread of the input parameters is taken care of by partial factors. The calibration of these partial factors for service life design for general use is very challenging, and its practical application is therefore not envisioned in the near future. Both MC-2010 and ISO 16204 assume that the deemed-to-satisfy and avoidance-of-deterioration methods will continue to dominate the practical service life design of new structures in the future, but the provisions for the former will be linked to a specific LS and reliability. These two methods should be further verified by the code committee and communicated to the stakeholders. 8 8.1

Modelling General

We need models describing the deterioration process over time in order to be able to apply the full probabilistic and partial factor methods. There are not too many of models in our field which enjoy general international consensus. In fib MC SLD, fib MC-2010 and ISO 16204 we have dared to suggest Fick’s 2nd law, modified by a time-dependent diffusion coefficient, for the ingress of chlorides, and the traditional square-root-of-time model for carbonation. These two models, as described and explained in the three documents, are described in sections 8.2 and 8.3.


However, the documents are also open for the use of other models provided they are sufficiently validated through full-scale experience. 8.2

Carbonation

The ingress of the carbonation front might be assumed to obey the following equation:  xc(t) = W · k √ t

(1)

where k is a factor reflecting the basic resistance of the chosen concrete mix (such as w/c ratio, cement type, additives) under reference conditions and the influence of the basic environmental conditions (such as mean relative humidity and CO2 concentration) against ingress of carbonation. It also reflects the influence of the execution. W takes into account the varying mesoclimatic conditions for the specific concrete member during the design service life, such as humidity and temperature. For the design of a new structure, the factors W and k might be derived from published data or existing structures where the concrete composition, execution and exposure conditions are similar to those expected for the new structure. When assessing the remaining service life of an existing structure, the product of W and k might be derived directly from measurements on the structure. 8.3

α

Chloride ingress

The ingress of chlorides in a marine environment may be assumed to obey the following equation:

  x  C(x, t) = C s − (C s − C i ) ⋅ erf ( )  2 ⋅ Dapp(t) ⋅ t   

(2)

In this modified Fick’s 2nd law of diffusion, the factors are: C ( x,t) chlorides content in the concrete at depth x (structure surface: x = 0 mm) and time t [% by wt./binder content] chlorides content at the concrete surface [% by C s wt./binder content] initial chlorides content of concrete [% by C i wt./binder content] depth with a corresponding chlorides content x C(x,t) [mm] apparent coefficient of chloride diffusion Dapp(t) through concrete [mm2/year] at time t, see Eq. (3) t time of exposure [years] erf error function α

 t  Dapp(t ) = Dapp(t0 )  0   t  where: Dapp (t0)

(3)

apparent diffusion coefficient measured at a reference time t0

ageing factor giving the decrease over time of the apparent diffusion coefficient – depending on type of binder and micro-environmental conditions, the aging factor is likely to lie between 0.2 and 0.8

The “apparent” diffusion coefficient after a period t of exposure to chlorides Dapp(t) represents a constant equivalent diffusion coefficient giving a similar chloride profile as the measured one for a structure exposed to the chloride environment over a period t. The decrease in the apparent diffusion coefficient is due to several reasons: – Ongoing reactions of the binder – Influence of reduced capillary suction of water in the surface zone over time – Degree of saturation of concrete – Effect of penetrated chlorides from seawater or de-icing salts (leading to ion exchange with subsequent blocking of pores in the surface layer) For the design of a new structure, the parameters C s, C i, α and Dapp (t0) may be derived from existing structures where the concrete composition, execution and exposure conditions are similar to those relevant for the new structure. When assessing the remaining service life of an existing structure, the factors, with the possible exception of α, may be derived directly from measurements on the structure. For both the design of new structures and the assessment of the remaining service life of existing structures, the ageing factor α should be obtained from in situ observations of structures where the concrete composition, execution and exposure conditions are similar to those of the actual structure. Observations during at least two periods of exposure (with a sufficient interval between the observations) are needed for the calculation of the ageing factor. 8.4

Other deterioration mechanisms

For acid and sulphate attack, as well as for alkali-aggregate reactions, fib MC-2010 and ISO 16204 conclude that no time-dependent models with general international consensus are available and that full probabilistic and partial factor approaches for service life design are in these cases not feasible at present. For these mechanisms, deemed-to-satisfy and avoidance-of-deterioration approaches have to be applied. We have formulated a general time-dependent model for freeze-thaw cases, but this will hardly be usable due to the complexity of the input parameters. Therefore, deemed-to-satisfy and avoidance-of-deterioration will again be the practical approaches here. As mentioned above, the fib and ISO committees did have problems with recommending time-dependent models for the rate of corrosion after the steel is depassivated. Even if such models for predicting the total volume of corrosion products exist, they have problems in distinguishing between concentrated corrosion (pitting) and corrosion spread over a greater area with less severe consequences.


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8.5

Influence of cracks

Intuitively, we assume that cracked structures will deteriorate faster than uncracked structures. However, neither the fib nor the ISO committee was able to come up with any general model to take this effect into account. The committees therefore decided to stick to the simplified approach used by most operational concrete standards today. This implies that corrosion of the reinforcement is not influenced by crack widths below a certain size. Depending on the severity of the environment and sensitivity of the structure, this limiting crack width is normally given as a characteristic value (5 % upper fractile) in the range of 0.2 to 0.4 mm. In harsh exposure conditions (e.g. exposure classes XD3/XS3 as defined in ISO 22965-1 [17] and EN 206-1), if functionality or structural integrity is affected, and if inspection and possible intervention is impossible, an avoidance-of-deterioration approach is recommended. 8.6

Uncertainties in model and data

As engineers, we should be humble and accept that the models we are applying are only approximations of how the real thing operates. As with traditional structural design, model uncertainties must be taken into account in our calculations, and their consequences should be reduced if possible. We also have an inherent problem when trying to characterize a structure’s long-term resistance by way of accelerated testing on young concrete specimens in the laboratory. fib MC-2010 and ISO 16204 therefore warn the user not to rely, uncritically, on predictions based on laboratory tests involving specimens just a few months old extrapolated to the end of the design service life without taking due account of the uncertainties in both model and data. One obvious way of reducing these influences is to use the models to extrapolate observations from structures exposed in the field for a certain period. The Norwegian code committee used this approach when verifying the present deemed-to-satisfy requirements given in the Norwegian standards. Maage and Smeplass [18] analysed and extrapolated in situ observations of carbonation in structures with an age of about one decade. Helland, Aarstein and Maage [19] analysed the remaining service life of 10 North Sea concrete structures based on 180 chloride profiles taken after 2 to 26 years of exposure (Figs. 6 and 7) . Both studies were carried out according to the models and principles based on LS (depassivation) and level of reliability as described for the full probabilistic method in MC-2010 and ISO 16204. 9

Design assumptions concerning execution, maintenance and repair

Some important assumptions have to be made when designing a new structure (or redesigning an existing one). The execution of the structure must ensure that the finished work achieves the properties on which the design is based. The quality level for the workmanship, and the quality management regime at the construction site, must

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Fig. 6. Oseberg A platform in stormy weather [19]


Fig. 7. An inspector assessing the condition of a concrete shaft on a North Sea petroleum installation [19]

therefore be at a certain level. fib MC-2010 and ISO 16204 have therefore assumed that the minimum requirements given in ISO 22966 “Execution of concrete structures” [20] are complied with. This standard is more or less identical to its European counterpart EN 13670. It should be stressed that any special requirements regarding materials or execution that affect durability and are not already covered by the execution standard should be communicated from the designer to the constructor as part of the “execution specification”. It is further anticipated that the completed structure will be subject to an inspection. It is advised that the design and construction be provided with “as-built” documentation. The part of this documentation containing the direct input parameters for the service life design, and therefore acting as a basis for condition assessments during the service life, is often called the structure’s “birth certificate”. If inspections reveal deviations from the specifications outside the given tolerances, a non-conformity process should be initiated.


The assumptions concerning following up the struc- sustainability. The main element for service life design is, ture during its use are given in fib MC-2010 in chapter 9 however, found in section 7.8 “Verification of limit states “Conservation”, and in ISO 16204 as the series of stan- associated with durability”. fib MC-2010 does not include dards to be denoted ISO 16311 [21]. the informative annexes of fib MC SLD, but refers to this The ISO 16311 series for maintenance and repair of document for interested readers. concrete structures is under development in ISO TCThe text in fib MC-2010 is basically the same as in 71/SC-7, chaired by Prof. Tamon Ueda, one of the main the normative part of fib MC SLD, but has somewhat maauthors of chapter 9 of fib MC-2010. This is another exam- tured based on experience with fib MC SLD and the fact ple of fib MC-2010 provisions being implemented in oper- that the core of the old fib TG.5.6 was reinforced with ational ISO standards, and vice versa. some additional 25 experts in the ISO committee working Another requirement is that the designer should in parallel with fib SAG-5 preparing fib MC-2010. communicate a “maintenance plan” to the organization ISO 16204 is close to an equivalent to the elements that manages the structure. This plan should give the in- on service life design in fib MC-2010, but contains less structions on those activities assumed during the design, commentary. Owing to the fact that ISO 16204 is an operwhich may include activities such as general cleaning, en- ational standard, its scope also differs from that of fib MCsuring that the drainage system works, applying sealants at 2010: regular intervals, etc. “This International Standard specifies principles and The design work should also result in an inspection recommends procedures for the verification of the duraplan to be applied by the operator. This plan should state: bility of concrete structures subject to: – what types of inspection are required, – known or foreseeable environmental actions causing – what components of the structure are to be inspected, material deterioration ultimately leading to failure of – the frequency of the inspections, performance; – the performance criteria to be met, – material deterioration without aggressiveness from the – how to record the results, and external environment of the structure, termed self-ageing. – the actions to be taken in the event of non-conformity with the performance criteria. NOTE The inclusion of, for example, chlorides [in] the concrete mix might cause deterioration over time Since the reliability level on which the verification of the without the ingress of additional chlorides from design is based is chosen on the basis of the possible conthe environment. sequences if the structure does not satisfy the relevant LS, the extent of inspections during the service life is very “This International Standard is intended for the use by naimportant. If the structure will be subjected to frequent tional standardization bodies when establishing or validatdetailed inspections by qualified personnel, deficiencies ing their requirements for durability of concrete strucwill be noticed at an early stage, enabling strengthen- tures. The standard may also be applied: ing/repair of the structure. Severe consequences will then – for the assessment of remaining service life of existing be avoided. On the other hand, if the structure will not be structures; and subjected to any inspections (the case with many founda- – for the design of service life of new structures provided tions), the possible consequences of underperformance quantified parameters on levels of reliability and design will be much more severe. This must be reflected in the parameters are given in a national annex to this Internadesign. tional Standard; 10

Differences between fib MC SLD, fib MC-2010 and ISO 16204

In this suite of documents, the MC SLD was the first stage. Due to its mission to introduce a new concept, it contains an extensive commentary as well as a number of informative annexes giving examples of applications. These examples have been very helpful for readers, but some have misinterpreted the examples and regarded them as generally valid. Such misuse has caused some disappointments as it often produced results that were regarded as unrealistic. Some in our community have also incorrectly associated MC SLD only with the use of modelling using the full probabilistic method. It is for these reasons that there is some scepticism of the concept in the industry and the standardization bodies. In contrast to the fib MC SLD, fib MC-2010 is a general document covering all aspects of design, execution, conservation and dismantling. The various elements of relevance for service life design here are spread and dealt with fully in parallel with structural design and design for

“In annex E to ISO 16204 we have given guidance for the content of such a national annex.” 11

Further fib activities regarding service life design

Commission 5 “Structural service life aspects” is the prime fib committee dealing with this theme. The Task Groups currently working on documents providing direct support for fib MC-2010 and ISO 16204 are: – TG.5.08 “Condition control and assessment of reinforced concrete structures exposed to corrosive environment” – TG.5.09 “Model technical specifications for repair and interventions” – TG.5.10 “Birth and rebirth certificates and through-life management aspects” – TG.5.11 “Calibration of code deemed-to-satisfy provisions for durability” – TG-5.13 “Operational documents to support service life design”


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12

Conclusion

fib MC-2010 considers the design of a concrete structure for loadbearing capacity, service life and sustainability in parallel. The main author of the sustainability related elements in MC-2010 is Prof. Koji Sakai. He is also the chairman of the parallel ISO TC-71 subcommittee implementing these provisions in the ISO 13315 [22] suite of standards ensuring compatibility between these two sets of documents. The design service life of a structure is the prime denominator in all calculations regarding cost and sustainability as applied by the owner and society. As chairman of ISO TC-71/SC-3/WG-4, it is my hope that the LS and reliability-based concept developed by fib and implemented by ISO will improve the present situation and enable the industry to make more rational decisions. In Europe we have started the process of revising our main concrete-related standards. The result is expected to appear at the end of this decade. The joint working group from CEN TC-104 (materials and execution) and TC-250/SC-2 (design) dealing with overlapping issues have already taken this methodology on board in their discussions. A similar intention to include the fib/ISO methodology on service life design was expressed by TC-250/SC-2 when starting the process of revising EN 1992 [23]. It is the author’s hope that this methodology will also be included in the “light” revision of the European standard for concrete production, EN 206, scheduled for 2013, thus enabling the 31 national standardization bodies in the CEN community to make their national annexes more harmonized and transparent than is the case today. References 1. CEB/FIP Model Code 90 . fib – fédération internationale du béton, International Federation for Structural Concrete. Lausanne, 1993. 2. FIP/CEB Bulletin No 228. High Performance Concrete. Extensions to the Model Code 90. fib – fédération internationale du béton, International Federation for Structural Concrete. Lausanne, 1995. 3. fib Bulletin No. 4. Light Weight Aggregate Concrete – part 1: Recommended extensions to Model Code 90. fib – fédération internationale du béton, International Federation for Structural Concrete. Lausanne, 2000. 4. fib Bulletin No 34. Model Code for Service Life Design. fib – fédération internationale du béton, International Federation for Structural Concrete. Lausanne, 2006. 5. fib Bulletin No. 65. Model Code 2010, Final draft, vol. 1. fib – fédération internationale du béton, International Federation for Structural Concrete. Lausanne, 2012. 6. fib Bulletin No. 66. Model Code 2010, Final draft, Volume 2. fib – fédération internationale du béton, International Federation for Structural Concrete. Lausanne, 2012. 7. Walraven, J., Bigaj-van Vliet, A.: The 2010 fib Model Code for concrete structures: a new approach to structural engineering. Structural Concrete, Journal of the fib, vol. 12, No. 3, Sept 2011. 8. ISO 16204 Durability – Service Life Design of Concrete Structures. International Organization for Standardization, Geneva, 2012.

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9. WTO Agreement on Technical Barriers to Trade (TBT), Uruguay Round Agreement, World Trade Organization, https://www.wto.org/english/docs_e/legal_e/17-tbt_e.htm. 10. ISO 2394 General Principles on reliability for structures. International Organization for Standardization. Geneva, 1998. 11. EN 1990, Eurocode – Basis of structural design. CEN – European Committee for standardization, Brussels, 2002. 12. Harrison, T .: CEN/TR 15868 Survey of national requirements used in conjunction with EN 206-1:2000. CEN – European Committee for standardization, Brussels, 2009. 13. EN 1992-1-1, Eurocode 2: Design of concrete structures – Part 1-1: General – Common rules and rules for buildings. CEN – European Committee for standardization, Brussels, 2004. 14. EN 13670 Execution of concrete structures. CEN – European Committee for standardization, Brussels, 2009. 15. EN 206-1 Concrete – Part 1: Specification, performance, production and conformity. CEN – European Committee for standardization, Brussels, 2000. 16. Bamforth, P.: Enhancing reinforced concrete durability. Concrete Society Technical Report No. 61. The Concrete Society, 2004. 17. ISO 22965-1 Concrete – Part 1: Methods of specifying and guidance for the specifier. International Organization for Standardization, Geneva, 2007. 18. Maage, M., Smeplass, S. : Carbonation – A probabilistic approach to derive provisions for EN 206-1. DuraNet, 3rd workshop, Tromsø, Norway, June 2001. Reported in “Betongkonstruksjoners Livsløp”, report No. 19, Norwegian Road Administration, Oslo, 2001. 19. Helland, S., Aarstein, R., Maage, M.: In-field performance of North Sea offshore platforms with regard to chloride resistance. Structural Concrete, Journal of the fib, vol. 11, No. 2, June 2010. 20. ISO 22966 Execution of concrete structures. International Organization for Standardization, Geneva, 2009. 21. ISO/DIS 16311 Maintenance and repair of concrete structures. International Organization for Standardization, Geneva, 2011. 22. ISO 13315 Environmental management for concrete and concrete structures. International Organization for Standardization, Geneva, 2012. 23. CEN TC250/SC2 document N 833 Future development needs in EN 1992’s. Secretariat, DIN, Berlin.

Steinar Helland Skanska Norge as Post box 1175 Sentrum 0107 Oslo Norway

Design for Service Life

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