D5x14 Service Life Design

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PAPERS IN STRUCTURAL ENGINEERING AND MATERIALS- A Centenary CelebrationDepartment of Structural Engineering and MaterialsTechnical University of Denmark, 2000.

SERVICE LIFE DESIGN OF CONCRETE STRUCTURES

- AN EXPERIENCE-BASED DISCIPLINE BECOMING

SCIENTIFIC

Steen Rostam

ABSTRACT

Concrete has been the most important building material for more than 100years, and is likely to remain so for the next century. The engineering challenges

of this building material and its development into world wide domination has in-

trigued researchers, designers and architects throughout the years. Therefore, no

other building material represents the centennial celebration of a technical uni-

versity institute as legitimately as concrete.

Durability issues were threatening the general recognition of the merits of

concrete. Recent years valuable research will allow today's grossly over-simplistic

deem-to-satisfy design methods to be replaced by scientifically sound probabilis-

tic service life design methods. The same reliability-based design procedures are

being used as used for structural design.

The problems to be met are multidisciplinary in nature. This requires co-

ordinated efforts of all parties involved, which will reflect on future engineering

curricular now on the verge of being updating in this direction.

Introduction

Concrete is the most versatile and robust construction material available and has obtained a

dominating position in construction.

The formability of structural concrete represents its unique usefulness with remarkable ar-

chitectural challenges as exemplified in Figure 1. Thus it becomes an economic disaster when

urban dwellings, large bridges, or major marine structures deteriorate just after a few years inservice.

The reasons for premature deterioration are very complex, Figure 2, but the main causes

have now been identified. It is essential to have these causes highlighted aiming at rectifying

design methods, construction procedures, material compositions as well as maintenance and

repair procedures, to ensure more reliable structures in the future.


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Steen Rostam

132

With respect to deterioration, concrete structures have some important characteristic proper-

ties, which differ fundamentally from structures made from other structural materials. These

properties are the following (fib1999):

The quality and the performance of concrete adopted at the design and contracting

stages are assumed values.

The true quality and performance characteristics of structural concrete are created

through the actual execution process during construction on site. Hence, this very short

period of time (hours and days) constitutes the most important phase during which the

true initial qualities are established.

If durability performance turns out to be sub-standard, this is most often not apparentnor detectable until some time has passed due to the nature of the deterioration of con-

crete structures. The time passed before premature deterioration becomes apparent may

often be longer than the contracted liability period, but very much shorter than the ser-

vice life expected by the owner.

To manage these special properties of concrete structures a durability based design concept, a

conscious execution, and a planned inspection and maintenance is needed.

Structural design versus durability design

The generally accepted aim of a design is "to achieve an acceptable probability that the struc-ture being designed will perform satisfactorily during its intended life".

When designing a structure today the designer first defines the loads to be resisted. As

these loads usually vary, he applies some safety factors to be on the safe side. These factored

loads must then be resisted by the structure through selecting a combination of structural sys-

tems, element geometry, material types and material's strengths. As it is difficult to predict the

precise properties of the materials, the geometry and the qualities, which are achieved in the

structure, safety factors are applied to account for this by limiting the maximum allowable

Figure 1: Sydney Opera House.

Architect: Utzon

Figure 2: Collapsed car park deck due to

chloride corrosion of reinforcement caused by

de-icing salts. The deck below was also fully

occupied!


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Service Life Design of Concrete Structures

133

stresses. Mathematical equations are then used to verify that the probabilities of the loads,

which exceed the resistances, are maintained at an acceptable low level.

When it comes to durability design to verify that the intended life can be achieved with an

acceptable level of reliability, the situation is entirely different. It seems to be acceptable with-

out question to use a grossly over-simplistic approach. The codes provide only qualitativedefinitions of exposure and they fail to define the design life in relation to durability, (Bamforth

1998). In particular, they fail to quantify the durability limit states that must be exceeded for

the design life to be ended.

The traditional design follows a deem-to-satisfy approach. This means to specify require-

ments to substitute parameters such as the cement type and quantity, concrete mix, maximum

water/cement ratio, minimum air content, concrete cover, type of curing, control of early age

cracking, limitation of crack widths. The values chosen depend on the assumed aggressivity of

the environment but have been chosen based on engineering judgement without any factual

calculation to verify the consequences.

Previous approaches fail to recognise that, in relation to durability, it is not the properties

of the materials or components alone that define performance, but the condition of the struc-

ture in its environment as a whole, and its need for maintenance. This performance can be de-

fined by functional requirements such as fitness-for-purpose, which includes issues such as

deflections, cracks and spalling, vibrations, aesthetics and structural integrity.

When it comes to residual service life assessment of existing structures the same situation

of using over-simplistic procedures prevail as for service life design of new structures. This is

in spite of the fact that once the structure is a reality a large part of the inherent uncertainties

associated with new designs, can be eliminated or limited to required levels through appropri-

ately planned inspections, measurements and testing. The longer the structure has served, and

the longer it has interacted with the environment the more reliable could the residual service

life forecast be. This is the condition, which shall be kept in mind when performing assessmentand residual service life evaluation.

Durability and service life design

The operational way of designing for durability is to define durability as a service life require-

ment. In this way the non-factual and rather subjective concept of "durability" is transformed

into a factual requirement of the "number of years" during which the structure shall perform

satisfactorily without unforeseen high costs for maintenance.

Designing for a specified service life requires knowledge of the parameters determining the

ageing and deterioration of concrete structures. Hence, the pre-condition is to have scientifi-cally sound mathematical modelling available of the:

Environmental loadings

Materials and structural resistances.

This is the only rational way of performing a quantified service life design for new con-

crete structures - and a residual service life design for existing structures.

The materials and structural resistances include transport mechanisms for substance into

and within concrete, and deterioration mechanisms of concrete and reinforcement.


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Steen Rostam

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The main issue when deciding upon a specific service life is to clarify the event, which will

identify the end of the service life. The requirement for a specific service life performance of a

structure is therefore closely associated with the short and long-term costs of fulfilling this re-

quirement. The owner must therefore acknowledge that he has to take decisions on both the

service life and on the performance requirements, and he must accept both the short and thelong-term costs associated with his decisions.

For the everyday building the national codes and regulations have defined society's service

life requirements - often not explicitly but implicitly through the standards and codified design

requirements. 50 years seems to be a general objective of most codes.

The Technical service life is the time in service until a defined unacceptable state of dete-

rioration has been reached by the structure, see Figure 3. In the following the main focus will

be on how to design for a specific technical service life.

Environmental loading

With respect to service life design one of the most important decisions to be taken by the de-

signer is the determination of the exposure conditions for which each member of a structure

shall be designed, as the structure itself has decisive influence on the future micro-climate to be

expected.

The exposure shall be related to the type and severity of deterioration that may result from

the exposure. In this respect a differentiation is needed between mechanisms deteriorating con-

crete and mechanisms leading to reinforcement corrosion.

Different parts of a structure may therefore be in different exposure conditions. Obvious

examples are the submerged, the tidal, the splash and the atmospheric zones of a marine struc-

ture, but also different geographic orientations (north/south/east/west, or seaward/landwardorientation) may be in different exposure classes. Even very local differences can be taken into

account such as vertical faces, horizontal surfaces facing upward (risk of ponding) or facing

downward.

Materials and structural resistance

Having identified the environmental aggressivity the next step of the durability design is to

identify the relevant degradation mechanisms. Mathematical models describing the time de-

Figure 3: Service life of

concrete structures. The well-

known two-phase modelling

of deterioration.


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1

1

2

3

4

2 3 4

Time

Initiation

Events

Depassivation

Cracking

Spalling

Collapse

Damage

Propagation

pendant degradation processes and the material resistances represent the big step forward to-

wards performance related durability design.

Among the main deterioration mechanisms relevant for concrete structures chloride in-

duced reinforcement corrosion is by far the most serious. In the following service life design

will therefore focus primarily on chloride ingress and chloride induced reinforcement corrosion.

Deterioration mechanisms

The two-phase diagram illustrated in Figure 3 may model the development in time of nearly all

types of deterioration mechanisms of concrete structures. The two phases of deterioration are

the following:

The initiation phase. During this phase no noticeable weakening of the material or the

function of the structure occurs, but the aggressive media overcomes some inherent protec-

tive barrier.

The propagation phase. During this phase an active deterioration develops and loss of

function is observed. A number of deterioration mechanisms develop at an increasing rate

with time. Reinforcement corrosion is one such important example of propagating deterio-ration. The propagation phase may be divided into several events.

Figure 4 shows in principle the performance of a concrete structure with respect to reinforce-

ment corrosion and related events. Points 1 and 2 represent events related to the serviceability

of the structure, point 3 is related to both serviceability and ultimate limit states and point 4

represents collapse.

Concrete resistance tochloride ingress

To focus on the main principles of modern service life design the calculations in the following

have been limited to defining the service life of new structures to be equal to the initiation pe-

riod. This means that the time for the chlorides to reach the reinforcement and induce depas-

sivation and initiate corrosion is equal to the design service life.

Figure 4:Corrosion depend-

ent events related to service

life.


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Steen Rostam

136

The initiation phase ends when the chloride concentration at the reinforcement reaches a

critical threshold value. Carbonation of concrete can be treated in a similar manner. Depassiva-

tion does not necessarily represent an undesirable state, as illustrated in Figure 4. However,

this event must have occurred before corrosion will begin.

Fick's 2nd law of diffusion is commonly used to model the penetration of chlorides intoconcrete. The model provides also the basis for modelling the transition between passivation

and onset of corrosion by giving as a result the time until the critical chloride concentration

reaches the level of the reinforcement.

Solving the diffusion law leads to the following expression:

C(x, t) = Cs[1 - erf (ttD

x

)(2)]

C(x,t) = Chloride concentration at depth x at time t

Cs = Chloride concentration on the exposed surface

t = Exposure timex = Depth

erf = Error function

D(t) = Diffusion coefficient

The corrosion process starts (onset of corrosion), when the chloride concentration at the level

of the reinforcement is higher than the critical value, Ccr.

In this equation the calculated surface chloride concentration Cs represents the environ-

mental load and is taken as time independent. The chloride diffusion coefficient, D(t)

characterises the concretes ability to withstand the ingress of chlorides.

The chloride diffusion coefficient was earlier considered as time independent. However, it

has been confirmed both in laboratory and in-situ testing that this resistance is improved withtime. The time dependent diffusion coefficient may be expressed as:

D0 is a measured reference chloride diffusion coefficient at the age t0. In a design phase the

concrete resistance D0may be measured in the laboratory by a bulk diffusion test or by a mi-

gration test.

The exponent governs how fast the diffusion coefficient is improved with time. The de-

crease of the diffusion coefficient with age is due to a combined effect of continued hydration

of the concrete and ion exchange with the environment, such as physical and chemical bindingwithin the concrete.

Based on these formulae, the time to onset of corrosion may be calculated. This is a de-

terministic way of calculating service life using the mean values or fixed characteristic values of

the relevant parameters (concrete cover, surface chloride concentration, diffusion coefficient,

and critical chloride concentration). This type of calculation does not include the obvious un-

certainty of the various parameters in a consistent manner.

=

t

tDtD

00)(


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Sensitivity of the parameter

The effect of the ageing factor on the service life calculation is considerable.

The examples presented in the following sections illustrated by Figure 5 and Figure 11 are

with kind permission reported from a special study of the critical parameters of the service life

models presented in a recent Report (Larssen 2000).

The calculations are based on the following input parameters representing typical values for

good quality concrete:

service life of 50 years (time to corrosion initiation) diffusion coefficient D0of 7.0 10

-12m

2/s

surface chloride concentration of 1.0 % of concrete weight

critical chloride concentration of 0.10 % of concrete weight

A constant diffusion coefficient ( = 0) results in a cover thickness of 244 mm. For an -

value of 0.6 the necessary cover thickness becomes 37 mm for a service life of 50 years.

Corrosion of reinforcement

Normal reinforcement is very efficiently protected against corrosion when cast into a good

quality alkaline and chloride free concrete. This is the well-known unique benefit of using rein-forced concrete structures in building and construction. Only when carbonation reaches the

level of the reinforcement, or more seriously, when chlorides in sufficient quantity reach the

surface of the reinforcement will the passivating effect be eliminated and corrosion may start.

Available corrosion models

Corrosion leads to the formation of expansive rust products leading eventually to cracking and

later spalling of the concrete cover. Models are available to calculate the rate of corrosion as

well as to determine the time to cracking and subsequent spalling of the concrete, see

(DuraCrete 1998, DuraCrete 1999).

Figure 5: Calculated chlo-

ride profiles after 50 years

of exposure. Assuming achloride threshold value of

0.1% of chlorides by weight

of concrete the figure gives

the required concrete cover

depending on the value of

. (Larssen 2000)


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The corrosion rate is very sensitive to the electrolytic resistivity of the concrete, which in

turn is governed by the concrete mix and in particular by the moisture content in the concrete

and the oxygen availability. Hence, the corrosion rate is very low or negligible in dry concrete

due to low conductivity, and the corrosion rate is also very low in water saturated concrete

due to the low availability of oxygen.Field tests and laboratory investigations have shown that a crack width at the surface of

about 1 mm could be a crude assumption of spalling being imminent.

Although such assumptions are very crude at this stage, it allows a rational calculation to

be made, and on this basis improved models can be developed. Basically it shall be emphasised

that a crude model based on some engineering evaluation is better than no model, and certainly

better than pure guesswork.

Design strategy

In principle two basically different design strategies for durability can be followed, (Rostamand Schiessl 1994):

A. Avoid the degradation threatening the structure due to the type and aggressivity of the

environment.

B. Select an optimal material composition and structural detailing to resist, for a specified

period of use, the degradation threatening the structure.

Strategy Acan be subdivided into three different types of measures:

A.1. Change the micro-environment, e.g. by tanking, membranes, coatings etc.

A.2. Select non-reactive, or inert, materials, e.g. stainless steel reinforcement, non-

reactive aggregates, sulphate resistant cements, low alkali cements.

A.3. Inhibit the reactions, e.g. cathodic protection. The avoidance of frost attack by air

entrainment is also classified in this category.

Most of the measures indicated above do not provide a total protection. The effect of the

measures depends on a number of factors. For example, the efficiency of a coating depends on

the thickness of the coating, and on its permeability relative to the permeability of the concrete.

Strategy Brepresents different types of design provisions. For example corrosion protec-

tion could be achieved by selecting an appropriate cover and a suitable dense concrete mix. In

addition, the structure can be made more resistant against aggressive environments of different

sorts by appropriate detailing such as minimising the exposed concrete surface, by rounded

corners, and by adequate drainage.

Modelling of deterioration processes is only relevant for Strategy B. An outline of a pro-

cedure for Design Strategy B could be the following: Start with the definition of the performance and service life criteria related to the envi-

ronmental conditions to be expected.

The next important element is the realistic modelling of the actions (environment) and

the material resistance against these actions.

Based upon the performance criteria, performance tests are indispensable for quality

control purposes. The performance tests must be suitable both to check the potential

quality of the material under laboratory conditions and, even more important, the in

situ quality.


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From this approach the design procedure can be established.

Strategy A and Strategy B can of course be combined within the same structure but for

different part with different degrees of exposure (foundations, outdoor exposed parts, indoor

protected parts, etc).

Multi-stage protection strategy

The approach of the service life design following strategy B is to select intelligently an appro-

priate number and types of co-operating measures to ensure the required service life. This is

considered a multi-stage protection design strategy, or a multibarrier approach, (Rostam

1991, Rostam 1993):

1. Identify the type and aggressivity of the environment of the structure

2. Forecast the possible movement and accumulation of the aggressive substance

3. Determine which transport mechanism govern (permeation, diffusion, capillary action) and

which parameters control the mechanisms

4. Select barriers that can co-operate in slowing down or prevent the transport and accumula-

tion process.This was a so-called 1

st- Generation service life design approach introduced first time for the

Great Belt Link in Denmark, Figure 6, (Rostam 1993). Later this design has been evaluated

using the reliability-based service life design (2nd

- Generation service life design approach) and

currently it seems as if 150 years service life could be expected.

Durability enhancing measures

For the majority of ordinary structures in aggressive environments the design approach for

durability described in this paper will ensure a satisfactory service life.

However, it must also be recognised up front that in some cases the usual choice of design

parameters for durability will not provide adequate service life - as painful experience has

shown. Often the damage is only developing on a very small or local part of the structure,

which is particularly exposed.

A large number of durability enhancing measures may be used. The following are of par-

ticular interest due to their novelty or special consequences for design and execution:

High performance concrete (HPC)

Figure 6: The Great Belt East

Bridge, Denmark, with the second

longest bridge span,1624m. This 6.8

km bridge is part of the Great Belt

link comprising also a 6.6 km lowlevel combined road and rail bridge

and a dual bored railway tunnel of

6.3 km. A 100 year service life de-

sign was specified for this link, fol-

lowing the multi-stage-protection

strategy for service life design.


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Permeability controlled formwork liners (PFL)

Self-compacting concrete (SCC)

Stainless steel reinforcement (SSR).

High Performance Concrete (HPC)

The demand for increased strength and improved durability of concrete structures has led to

the development of HPC. This development has had three main objectives:

1. Protect the reinforcement against corrosion; in particular provide protection against

ingress of chlorides by creating dense impermeable concrete in the cover zone with

very low penetrability of aggressive ions such as chlorides, sulphates and CO2.

2. Resist deterioration of the concrete itself when exposed to the aggressiveness of the

environment such as sulphates, seawater and other chemical attacks, as well as resist

freeze/thaw attack.

3. Provide adequately high strength to fit the structural requirements.

This development has been very successful in many respects. The advanced HPC products

available have met very complex and demanding structural challenges, where the practicalstrength requirements usually remain within the range of say 50 - 80 MPa.

One drawback has been that these more refined concrete mixes become more sensitive

towards the actual handling during execution. They set high demands on the competence, ex-

perience and workmanship of the workforce. To varying degrees these concretes differ from

the behaviour of the long term known types of structural concrete.

The nature of deterioration of concrete structures highlights that high performance con-

crete does not necessarily provide high performance concrete structures, (Rostam 2000).

Permeability controlled formwork liners (PFL)

The most important part of the structure protecting it against ingress of aggressive substance is

the concrete cover, also considered the "skin" of the structure. PFL has proven effective inenhancing the denseness of the outer mm and cm of the cover by reducing the water-cement

ratio and improving the curing of this outer concrete layer.

The main drawback of PFL is the difficulty in fixing the liner to the formwork so attractive

appearance is maintained. PFL cannot compensate for local bad compaction and honeycombs,

and require therefore a skilled workforce to be successful. Combining PFL with self compact-

ing concrete could be one valuable solution to ensure high quality concrete being less depend-

ent of the skills and experience of the workforce.

Self-compacting concrete (SCC)

The development of a concrete mix where the placing and compaction has minimal dependence

on the available workmanship on site would improve the quality of the concrete in the finalstructure. This has been a main driving force in recent years development of SCC. With the aid

of a range of chemical admixtures and optimal grading of the aggregates, concrete with very

low water/cement ratio can be made to flow without segregation through complicated form

geometry and around complex reinforcement layout. The form can be filled and a uniform

compaction without honeycombs can be achieved, also in the cover zone of the concrete, with

no or only minimal additional contribution to the compaction and levelling of the concrete from

the workforce on site. The flowing concrete will usually increased the pressure on the form,

which shall be taken carefully into account when designing the formwork.


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The use of SCC is also an environmentally friendly technology as the noise level from vi-

brators is nearly eliminated and the concrete workers need only minimal work with the vibra-

tors, with all the adverse effects vibrating concrete has on the body.

The main current drawback with this technology is the sensitivity of such concrete to the

precise dosing, mixing and transporting of the concrete and the dependence on the weatherconditions while casting the concrete. Under adverse conditions the ability to flow may sud-

denly be lost. In addition, the increased cost of such concrete is noticeable, and the demand for

expertise and experience is moved nearly fully to the mixing plant.

Stainless steel reinforcement (SSR)

In recent years SSR has become an interesting option in then design against reinforcement

corrosion. The strengths and the dimensions available are now fully compatible and interchangeable

with ordinary black steel reinforcement.

Of particular importance is the often-overlooked fact, that SSR can be coupled with nor-

mal mild steel reinforcement (carbon steel) without causing galvanic corrosion, (FORCE 1999,

Pedeferri 1998). This leads to the possibility to use SSR only in those parts of the structurewhere this is considered necessary, and then reinforce the remaining parts with ordinary mild

steel reinforcement. Such highly exposed zones could be splash zones of marine structures,

foundations in contaminated soils, lower parts of columns above ground, balconies, etc.

In general terms all steel alloys with a content of chromium more than 12 %, are called

stainless. Stainless steel is normally divided into 4 major families or groups, namely Martensitic,

Ferritic, Austenitic and Duplex. The latter has a combined Austenitic and Ferritic Molecular lattice.

Only Austenitic steels with 18-26% Chromium (Type AISI 304 and 316) and Duplex steels

with 21-28% Chromium (Type AISI 318) have major importance as chloride corrosion resistant

reinforcement. These steels contain also varying amounts of nickel and molybdenum.

The use of SSR in zones being exposed to high chloride concentrations is a highly reliable

solution following design Strategy A. This can ensure a long problem-free service life in that

part of the structure, provided the concrete itself is made sufficiently resistant to avoid other

types of deterioration (Markeset et. al. 2000, Concrete Society 1998, Abbott 1999).

Steel type and quality Relative cost per unit weight

Carbon steel 1.0

Austenitic (1.4301) / 304 4.5

Austenitic (1.4401) / 316 5.5

Duplex (1.4462) / 318 5.5 6

Table 1: Price comparison. Relative costs per unit weight.

SSR is often considered a very expensive solution, and thus not given sufficient attention

when evaluating optimal solutions in such aggressive environments. The relative costs per unit

weight of SSR compared to normal carbon steel are presented in Table 1. When cut, bent and

placed in the form the difference in cost diminishes.


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142

Figure 7: Comparative costs for the design of three new concrete jetties in Norway

1995-1996, two large and one small jetty. The total costs for reinforce-

ment placed in the form relative to the total construction costs. The in-

creased construction costs if 50% or 10% of the reinforcement is replaced

with SSR at a unit weight price being 5 times the cost of ordinary mild

steel reinforcement, (Markeset 2000).

In Figure 7 the total initial cost of replacing some of the mild steel reinforcement with SSR is

presented for three real constructions made during the years 1995-1996. Although the cost of

SSR in this case is 5 times the cost of ordinary reinforcement the effect of introducing this

highly reliable and durable element in the most critical zones of an exposed structures turns

out to be marginal. Such experience will have a very positive economic effect if a whole lifecycle cost optimisation is performed.

A very valuable and convincing documentation of the performance of SSR in highly chlo-

ride contaminated concrete is presented by the 60 year old 2 km long concrete pier out into the

Mexican Gulf at Progreso in Mexico reinforced with stainless steel, see Figure 8. No corrosion

has taken place within the structure, despite the harsh environment and poor quality materials

used in the construction. The chloride levels, at the surface of the reinforcement were between

10 and 20 times the traditionally assumed corrosion threshold level, (Knudsen et al 1999).

The Danish contractor Christiani & Nielsen made this very foresighted design for durabil-

ity in a very aggressive environment for concrete structures.

From a practical point of view the SSR technology is particularly interesting because it"only" solves the corrosion problem. All other techniques and technologies within design, pro-

duction and execution of reinforced concrete structures remain unchanged, a fact that is very

attractive to the traditionally very conservative construction industry.

As it is recognised that the most serious durability problem for concrete structures in the

Gulf Countries is reinforcement corrosion. It becomes evident that (fib 1999): The reliable

and readily availability of stainless steel reinforcement at reasonable and foreseeable prices

may change - or revolutionise - major parts of the building sector in aggressive environments,

simply by solving the corrosion problem.

R ein fo rc em en t C on cr ete w or ks

Remaining works

R ein fo rceme nt C on cre te w orks

Remaining works

R ei nf orc emen t C on cr ete wor ks

Remaining works

Reinforcement = 6.1 % Reinforcement = 8.3 % Reinforcement = 13.1 %

Additional initial costs replacing black steel (BS) with stainless steel (SS)

50% BS to SS: 5.0% 7.0% 11.0%

10% BS to SS: 1.1% 1.4% 2.3%

LargeLarge LargeLarge SmallSmall


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The main reason is also the added value which follows from the possibility of accepting the use

of locally available materials, even with chloride contamination, and also accepting the qualifi-cations of the local workforce as it is, and still produce highly durable long lasting reinforced

concrete structures intelligently designed.

Reliability-based service life design

The theories of probability and reliability in structural design have been developed and matured

remarkably during the past five years. These theories have been transformed from the level of

research and development to be directly applicable and operational in practical engineering

design. The methodology has been internationally recognised and used for many decades as

basis for the structural safety design through the well-known load-and-resistance-factor design

(LRFD).

However, the factors and mechanisms governing the durability and performance of struc-

tures throughout their service life have only recently been developed in similar ways. This has

among others been achieved through a European research project 1996-1999 "DuraCrete",

(DuraCrete 1999).

This has allowed the treatment of transport and deterioration mechanisms to be modelled

on a probabilistic level and introduced in the general service life design. Thus, design for safety

and for durability can be performed using similar procedures.

This new durability design methodology is based on the reliability theory as traditionally

used in structural design. The purpose of a reliability analysis is to determine the probability of

a given event, e.g. the end of the service life.The DuraCrete Design Guide, (DuraCrete 1999), aims at obtaining a sufficient level of

safety of the design service life with respect to the considered events. The guide is developed

taking into account:

The geometry of the structure

The materials used for construction

The environment in which the structure is located

The quality of the execution of concrete works

The main deterioration mechanisms, but especially highlighting reinforcement corrosion

Figure 8: 60 year old

stainless steel reinforced

pier at Progreso in Mexico

still fully intact without anymaintenance whereas the

remains in the foreground

is what is left of an only

about 30 year old pier

reinforced with ordinary

black steel (Knudsen et al

1999)


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Steen Rostam

144

The planned inspection of the structure

Deterministic versus probabilistic service life design

The merits of the probabilistic approach to durability design is illustrated by the following ex-

ample of a marine structure (Rostam 1999). Two different environments are considered, repre-

senting yearly average temperatures of 10 oC (exemplified by Northern Europe) and 30 oC (ex-

emplified by e.g. the Gulf Countries) respectively. The design requirement is 50 years service

life. For simplicity the service life is also in this example defined as the length of the initiation

period, i.e. the time until corrosion initiation due to chloride ingress.

Figure 9 depicts the required concrete covers in each of the two environments, based on a

traditional deterministic approach.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 20 40 60 80 100

Distance from surface [mm]

Chlorideconcentra

tion[%]

Northern Europe

Middle East

Critical concentration

34.1 68.2

Figure 9:Deterministic approach.

Required concrete cover to ensure 50

years service life and assuming a chloridethreshold value of 0.1% by weight of

concrete.

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100

Cover thickness [mm]

Probabilityofcorrosio

n[%]

Northern Europe

Middle East

Acceptance criteria

48.6 91.034.1 68.2

Figure10:Probabilistic approach.

The deterministic approach provides only

50% probability of avoiding corrosion atthe age of 50 years. Accepting 10%

probability of having corrosion initiated

after 50 years results in considerably lar-

ger covers.

Figure 10 highlights the fact that the deterministic approach only provides a 50% probability of

achieving the required 50 years corrosion free service life. This fact is often overlooked in

usual design for durability. If, say only a 10% risk of having corrosion initiated before 50 years

is considered acceptable, then much larger covers are required, as seen from Figure 10.

The deterministic approach used here is based on mean values of the governing parame-

ters. In the probabilistic approach the mean values and their known or assumed uncertainties

are used together with the relevant distribution functions. This latter approach makes it not

only possible to relate cover thickness to probability of corrosion but also to quantify the con-

sequences of differently chosen risks of corrosion. These consequences relate not only to con-

crete quality and cover thickness, but more importantly also to the economic consequences.

The Load-and-Resistance-Factor-Design (LRFD) for Service Life

Having decided the level of reliability required for the service life, then the design basis is

equivalent to the well-known situation for structural design. This means that the input parame-


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ters (the environmental loads) shall be multiplied by a load factor and the resistance against

ingress and transport of aggressive substance to initiate and nourish an ongoing deterioration

shall be divided by a resistance factor. The values of these factors are determined directly from

the available uncertainties and the distribution functions describing the individual parameters.

The example above is in fact the calculation of the cover needed to achieve 50 years ser-vice life until corrosion initiation in a marine structure exposed in the two environments men-

tioned. The calculation is based on a modified Ficks 2nd

Law of diffusion taking the ageing

effect into account.

It can be shown that the same calculation can made using the well-known LRFD ap-

proach. The calibrated partial safety factors in the two environments thus become:

10oC: fx= 1.27; fCcr= 1.21; fCs= 1.0; f= 1.12; fDo= 1.0

30oC: fx= 1.11; fCcr= 1.26; fCs= 1.0; f= 1.14; fDo= 1.0

With respect to the concrete cover in the two situations, the cover is considered a resis-

tance. Hence, the design value, xd , being the characteristic value divided by the partial safety

factor, becomes 48.6/1.27 = 38.3 mm for 10

o

C, and similarly 91.0/1.11 = 82.0 mm for 30

o

C.Effect of corrosion threshold value

A rather large reinforced concrete jetty is designed for a service life of 50 years, (Larssen

2000). The target service life is defined as the time to onset of corrosion. Acceptance limit for

onset of corrosion is given as 10 %. The structural system of the jetty was a beam-slab type

solution with concrete piles resting on rock surface. Concrete piles with permanent steel casing

are found to be a very durable solution.

The statistical parameters approved by the owner and used in the calculations of required

cover thickness for the remaining part of the concrete structure were:

Cs = N(0.4, 60 %) [% of concrete weight]

Do = N(7.0 10

-12

, 15 %) [m

2

/s] = N(0.4, 15 %)

X = LN (x, 10) [mm]

Two deterministic values were applied for the critical chloride concentration:

Ccr = 0.05 and 0.10 [% of concrete weight]

Based on these data a probabilistic calculation of required concrete cover was performed. The

results of these calculations are shown in Figure 11. With a probability of 50% for initiation of

corrosion the needed concrete cover varies between 50 and 66 mm for a service life of 50

years. This corresponds of course to the deterministic results based on average of the input

parameters.


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Steen Rostam

146

Figure11: Required concrete cover for service life of 50 and 100 years for the con-

crete jetty in the example, (Larssen 2000).

For the given acceptance limit (probability of corrosion initiation of 10 %) the cover thickness

needed for 50 years service life becomes 75 and 93 mm for a critical chloride concentration of

0.10 and 0.05 %, respectively. If the critical chloride concentration is 0.05 % the required

cover thickness becomes 93 mm for achieving the target service life of 50 years. However, if

the critical chloride concentration is 0.10 % a cover thickness of 93 mm gives 100 years ser-

vice life. This illustrates the importance of the concrete cover, and the importance of the as-

sumed threshold value for corrosion initiation.

Durability monitoring as part of planned maintenance

Figure 12: Installation of corrosion sensors in Norwegian quays, (Markeset 2000).

For existing structures it is recognised that in order to minimise maintenance and repair costs a

regular and systematic inspection and recording of the performance is needed.

It has been recognised that the rate of chloride penetration (diffusion coefficient with age-

ing factor alpha) and the local threshold value for chloride initiation (Ccr) constitute the two

most important parameters for corrosion related service life of concrete structures. Sensors are


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147

available which can record the depth of penetration of the critical chloride content (threshold

value) in the actual structure.

Chloride monitoring has, among others, being investigated in (Markeset 2000), by remote

sensoring of exposed marine structures. Three existing quays aged 38, 19 and 13 years respec-

tively, have been provided with special corrosion sensors in order to determine the values ofthese two critical parameters, see Figure 12. It is the intention to utilise such information as

feed-back data for further improving the service life designs of new structures as well as to

optimise the operation, maintenance and repair costs during the lifetime of the structure,

through a rational life cycle cost optimisation.

The corrosion sensors constitute six separate thin steel rings spaced at 10 mm and

mounted in a stainless steel cylinder, see Figure 13. The sensor is placed in a hole drilled into

the concrete at the place selected for monitoring.

When chlorides in sufficient quantity penetrate to the level of a ring depassivation will oc-

cur and corrosion may take place on this ring and the depth of ingress of the chloride threshold

concentration can be determined within the 10 mm ring spacing.

Figure 13:Mounting of proprietary corrosion sensors in existing quays. The pictureto the right shows the sensor, a temporary micro-amp meter and the

small titanium pin as cathode placed just above the instrument, (Schiessl

and Raupach 1992).

By taking sensor readings as part of a regular inspection routine, say each half-year or so, the

rate of chloride penetration can be determined with time, and updating of the assumed chloride

diffusion coefficient - or of the value of the ageing factor alpha - can be made. This will then

provide data for an updating of the residual service life of the structure.

Reliability updating of service life

A very valuable consequence of adopting the probabilistic method in practice is that the uncer-

tainties to be expected are taken into account at the initial design stage, where the concrete and

the construction and maintenance conditions are not known. When the initially only assumed

values have been determined through testing, the tools are available to update the service life

expectations, and take necessary precautions.

Therefore, one important means to increase the knowledge of the expected performance

and service life of the structure is to establish a base-line-study of the finally achieved values of


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Steen Rostam

148

the critical parameters in the finished structure. This could conveniently be done and reported

as part of the handing-over of the structure from the contractor to the owner. This would pro-

vide a Birth Certificate, (Rostam 1999), which includes a first forecast of the service life,

based on factual data.

The Birth Certificate would then provide the basis for - and be part of - the future Main-tenance Planin support of the operation and maintenance of the structure in use.

At later inspections the service life data can be updated through renewed testing, and used to

update the expected residual service life as well as provide the necessary data for updating also

the Maintenance Plan, allowing preventive maintenance introduced at optimal timing. This

procedure will result in an ever-increasing accuracy of the residual service life forecast and help

minimise the maintenance costs.

Hence, the residual service life and the economic consequences of any decisions taken (savings

or additional costs) can be readily updated when inspections are made and additional informa-

tion becomes available.

Such measures for maintaining structures in satisfactory form and preserving pleasant vis-

ual appearance is a precondition for continued acceptance an reputation of concrete as the

primary building material for the future, see Figure 14 and Figure 1.

Conclusions

Reliability based service life design is a multi-disciplinary engineering challenge

Transition from deem-to-satisfy durability design to reliability based service life design is a

multi-disciplinary challenge for the materials and structural engineering profession. Society

demands sustainable and environmentally friendly engineering solutions and owners demandreliable service lives of structures. This will set new standards for future Research, Develop-

ment and Education (R,D&E). Universities engineering curricular must adapt to the multidisci-

plinary demands, which will face future engineers. A Centennial celebration of an internation-

ally renown R,D&E Institute is an appropriate occasion to synthesis its valuable past and pro-

ject this into a new multidisciplinary format spearheading the new developments now so much

in demand.

Modelling of deterioration mechanisms is a key issue

Figure 14: The "Cube" at La De-

fence, Paris, France. Architect:

S reckelsen


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149

Deterioration mechanisms, in particular corrosion of reinforcement, sulphate attack, alkali-

aggregate reactions and frost action have caused serious premature damage to concrete struc-

tures. Recent years development of these deterioration mechanisms and the identification of

their governing parameters have transformed service life modelling into a realistic tool for pro-

viding quantified service life designs.

Probabilistic treatment of uncertainties is needed

The introduction of probabilistic methods to treat the inherent uncertainties in concrete

quality and environmental aggressivity does now allow the level of reliability of such service

life designs to be quantified. During subsequent durability testing as part of the maintenance

procedures an updating of the service life forecast can then be made with ever increasing reli-

ability.

The success of service life design depends on recognition by all involved parties

The new probabilistic service life design procedures may lead to much more well-

performing structures as well as to very considerable savings in future maintenance and repair

costs of concrete structures. However, this requires the owners accept of the true potentials

and that the construction industry adopts the necessary measures where relevant within each

their fields of work. These will require recognition of concrete as a living material in the sense

that it changes its properties with time, which highlights the need for monitoring performance.

The fact that the true and decisive qualities of concrete and of the structures are only deter-

mined during the execution phase, is a strong motivating factor to shift from traditional means

of managing durability to modern service life management of structures.

Revised means of co-operation and contracting are needed

Fundamental changes in the very traditional ways of approaching service life requirements

from the owners' side are needed, and revised types of contracting and liabilities are needed, if

true improvement shall be obtained. Close co-operation and co-ordination among all involved

parties is needed if improvements shall be obtained, maybe through new types of partnering.

The Owner is the key responsible

To ensure the required service life performance the Owner is obliged to define the quality

and service life he require, he must check that the quality of the materials and execution is sat-

isfactory, and he must pay for that quality.

References

Abbott, C. J.: (1999). Steel Corrosion Solution. Concrete Engineering International, Vol. 3,

No. 4.

Bamforth, P. (1998). Double Standards in Design. CONCRETE.

Concrete Society (1998). Guidance on the use of stainless steel reinforcement. Technical Re-

port no. 51, Report of a Concrete Society Steering Committee, October.

DuraCrete (1998). Modelling of Degradation. The European Union - Brite EuRam III, Project

No. BE95-1347, Probabilistic Performance based Durability Design of Concrete Struc-

tures, Report No. 4-5.


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DuraCrete (1999). General Guidelines for Durability Design and Redesign. The European Un-

ion - Brite-EuRam III, Project No.BE95-1347, "Probabilistic Performance based Durabil-

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Markeset, G., Rostam, S. and Skovsgaard, T. (2000). Stainless Steel Reinforcement - An

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Pedeferri, P. et al. (1998). Behaviour of stainless steels in concrete. Repair and rehabilitation of

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Feedback into New Designs. Regional Conference on Damage Assessment, Repair Tech-

niques and Strategies for Reinforced Concrete held in Bahrain 7 th-9thDecember.Rostam, S. (1993). Service Life Design - The European Approach. ACI ConcreteInterna-

tional, Vol. 15, (7), pp 24-32.

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D5x14 Service Life Design