Concrete Repair, Rehabilitation and Retrofitting II – Alexander et al (eds)© 2009 Taylor & Francis Group, London, ISBN 978-0-415-46850-3

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Design for service life: How should it be implemented in future codes

J.C. WalravenDelft University of Technology, Delft, The Netherlands

ABSTRACT: Design for durability has gained the same level of importance as design for safety and design for serviceability. Many individual contributions to design for service life have been noted in recent years. Such initiatives should, after some time, result in a consistent approach. When trials are made to develop such an approach, inevitably “blind spots” are discovered. This paper gives an overview of the most important develop-ments and moreover traces areas were knowledge is still inadequate and further research is needed.

2 THE DEVELOPMENT OF A STRATEGY FOR SERVICE LIFE DESIGN

2.1 Deterioration mechanisms and their influence on service life

A treatment will be given of the various methods to extend the service life by means of a rational design strategy. Design for service life requires, that all rel-evant deterioration mechanisms are identified and that those, which are relevant for the structure to be designed are selected. Structures can be prone todeterioration due to:

− Chloride penetration− Carbonation− Frost-thaw effects, whether or not in combination

with de-icing salts− Chemical influences (attack by acids or sulphates)− Ettringite formation− Alkali silica reaction, Fig. 1

In this list damage due to the penetration of chlo-rides and carbonation are by far the most impor-tant causes for damage to concrete structures. It is therefore understandable, that a substantial number of research projects have been focused on a better description of those mechanisms. With regard to the prediction of durability with mathematical models the Duracrete models are well known. These mod-els describe the penetration of chloride and carbon dioxide on the basis of diffusion models. In this

1 INTRODUCTION

In the light of the developments in the field of con-crete structures the increasing attention for “design for service life” is remarkable. The background of this development is clear. Damage due to insufficient attention for durability, or due to the use of inade-quate criteria, have led to expensive repair and even demolition and replacement. De Sitter (1984) made a statement which perfectly touches the core of the matter. This statement is known as the Rule of Fives: “If no maintenance is carried out the later repair costs will be five times the saved maintenance costs. If no repair is carried out, the cost of renovation will be five times the money saved by not repairing”. Mean-while we have experienced the truth behind this say-ing: expensive maintenance and repair measures are necessary there, where in the past the need for the aspect durability was ignored.

The following question is how maintenance can be minimized by appropriate design. It is clear that this may result in appreciable cost savings in time. Now it also becomes clear that in the design stage not only the costs of the new building, but most of all the integral costs, including maintenance, adaptation and demolition have to be considered.

In order to develop a suitable approach to design for service life it is necessary to know which aspects play a dominant role and where still open questions stand out. This paper gives a survey of developments and needs for further research.

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way the deterioration of concrete structures in time can be calculated. Fig. 2 shows the reduction of R (Resistance) in time due to deterioration. In the same diagram the increasing loads (S) on the structure are represented. If as well the scatter of R(t) and S(t) are regarded, it can be calculated until when the struc-ture can carry the load with specified reliability. It is then as well clear when repair and/or strengthening of the structure should be carried out.

2.2 Practical methods for the extension of service life

Contractual specifications for the service life to be regarded in design can be met in two ways (Fig. 3).

On the one hand it is possible to reach the specified service life by avoiding the deterioration mechanisms. Possibilities are:

− the application of membranes or coatings (exten-sion of reaction)

− the application of materials with low sensitivity to deterioration (like stainless steel)

− preventing the reaction (by for instance cathodic protection)

On the other hand materials and design meth-ods can be chosen with the aim to delay or extend the chemical reaction that leads to corrosion. This method of design means that the sensitivity of the structure is reduced by appropriate choices for mate-rial and structural detailing. By the selection of a material with suitable properties in combination with the minimum structural dimensions the deterioration process is delayed. In this respect two possibilities are distinguished:

The deem to satisfy approach (recipe based). The deem-to-satisfy method implies, that design criteria are met, which guarantee a certain specified service life. These criteria like minimum cover, or concrete composition to limit permeability for the governing environmental class, are mostly based on experi-ence. Up to now optimization is not possible, because the rules do not offer the possibility for exchange (for instance smaller cover for improved concrete impermeability).

The method of probabilistic modelling. According to this method the environmental load is compared with the resistance of the structure, taking account of the influence of time. On this basis the probability is cal-culated that damage will occur to the structure. With this method optimization is possible: the option for exchange is built in and new materials can be used as soon as their properties are known.

Figure 1. Damage due to ASR.

Figure 2. Probabilistic determination of service life, Rostam (2001).

Figure 3. Methods for extending the service life of a con-crete structure.

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It should be noted that the option “avoiding dam-aging reactions” should not be regarded with blind confidence. Fig. 4 shows the condition of a protective membrane that was applied on a structure in order to stop the penetration of chlorides: on the one hand the membrane has become brittle due to ultra violet radiation, on the other hand behind the membrane a moisture pressure has been developed, which leads to peeling-off of the membrane.

The application of stainless steel is an interesting possibility too for durable design, especially since less expensive types of stainless steel are available on the market. Savings on the quality of the concrete can be accepted with confidence and economic solutions on another basis are possible. Also the possibility of combining stainless steel with normal “black” steel is a possibility.

Most actual design recommendations are based on a deem-to-satisfy approach. EN 206 distinguishes 5 main environmental classes, including 18 subclasses. Design parameters to satisfy the requirements are the thickness of the cover and the concrete strength. The combination of the two has to satisfy certain criteria.

Meanwhile considerable progress was made with regard to modelling the deterioration processes. The-oretically it is possible to describe the deterioration process on the basis of physical models. For the prac-tical application of those models in design a number of basic elements are necessary:

− limit state criteria− a defined service life− deterioration models− compliance tests− a strategy for maintenance and repair− quality control systems

The method of probabilistic modelling implies that a concrete composition is chosen with regard to its capacity to control the speed of penetration of for

instance chloride or carbon dioxide. By optimizing the concrete composition and applying an appropri-ate minimum concrete cover the structure can then be designed for a specified service life.

The probabilistic approach is now only applied for structures with large relevance, such as tunnels and important bridges. For “all-days” structures the deem-to-satisfy-method is sufficient. However, it is impor-tant to make sure that the results of both methods are compatible. In EN 206 a minimum strength class is given without any relation to the type of cement, and/or the use of fillers. Moreover the quality of curing and the length of the curing period have a signifi-cant influence. Here parameter studies and compari-son with experiments are necessary to increase the understanding.

2.3 Dealing with the quality of execution

Every design method should take into account the large number of influencing factors which influence service life. The most important factors, in sequence with their occurrence in the building and construc-tion process are the environmental conditions, the conceptual design, the material choice, the dimen-sions, detailing, execution, maintenance and quality of archiving the data.

The best available models for design for service life consider predominantly environment, material and geometrical dimensions: hardly any attention is given to detailing, execution and maintenance. Espe-cially the quality of execution is a “blind spot”. Here a number of aspects play a role, such as placing the rein-forcement, the casting procedure, compaction, curing, storage of materials, formwork, demoulding, labour conditions, quality control, organisation at the site, education and training of the executing labourers.

Formulating requirements with regard to the quality of execution and the introduction of quality control procedures are no absolute guarantee that the criteria for service life are met. So, at handing over the structure to the client it should be controlled that the specified quality has indeed been achieved. At the start of its life the structure should be inves-tigated with regard to the most important influenc-ing properties governing durability. These findings should be laid down in reports. This initial control fits very well in the total plan of intermediate con-trols. On the basis of this inspection it is possible to determine whether the maintenance plan and the quality of the structure are in agreement with each other. Not reaching the specified quality should have consequences for the contractor, like for instance the obligation of repair or the payment of capitalized additional maintenance cost.

In Japan the flow chart shown in fig. 5 is used. At handing-over the initial inspection is carried out. On

Figure 4. An unsuccessful attempt to stop the deterioration process with a coating on a quay wall.

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the basis of the results of this inspection a mainte-nance category is defined and the deterioration of the structure is predicted. Intervals of control testing are programmed in advance. On the basis of these control tests the need for repair is determined. For small-scale repairs the prediction of deterioration is updated. For large scale repairs the initial control is carried out again. The recording of the data is very essential. In stead of inspections, or additional to those, the condi-tion of the structure can be followed by monitoring.

3 EXPERIENCE WITH DESIGN FOR DURABILITY UP TO NOW

3.1 Experience with infrastructural projects

In The Netherlands various large infrastructural proj -ects have been realised in the last decades. Durability has been a point of growing attention.

The sluices of Haringvliet, Fig. 6, were finished in 1960. With regard to the durability the following requirements have been formulated in the design stage:

− use of blast furnace cement− water-cement ratio ≤ 0,45− concrete cover ≥ 70 mm− prestressing where possible

Until now no problems with regard to durability have been noted: after nearly 50 years, only lim-ited chloride penetration was measured. The prob-ability that the chloride content at the reinforcement reaches a critical value during the first 100 years was calculated to be smaller than 3,5%.

The Eastern Scheldt Barrier was finished in 1980. The specified design service life was 200 years. Cal-culations for determining the durability have been carried out until an age of 80 years on the basis of mean values. After 25 years no visual damage was observed. There was some chloride penetration. According to probabilistic calculations, based on physical models and inspection the probability of reaching a critical chloride content at the reinforcement was calculated to be less than 6,6% after 50 years and less than 14% after 200 years.

The Maesland Storm Surge Barrier, was deliv-ered in 1990. The design service life is 100 years. The design for durability was made on the basis of an extrapolation of the governing design code (which was based on a maintenance free service life of 50 years) with the √t formula. The calculation showed that for a maintenance free service life of 100 years a minimum cover of 35 √(100/50) = 50 mm is necessary.

The Western Scheldt Tunnel and the High Speed Railway Amsterdam-Brussels were finished in 2000 and 2001 respectively. For the design probabilistic models according to the DuraCrete method were used. Further criteria were:

− A maintenance free service life of 100 years− The end of service life was defined as the start of

corrosion (end of initiation period) due to pen-etration of sea water (outside) or frost thaw salt (inside)

− An accepted failure probability of 3,6%− Effect of exposure and curing to be regarded in

calculations.

3.2 Verification of the DuraCrete models

The Duracrete models contain a number of param-eters. For those parameters a number of indicative

Figure 5. Flow chart for inspection and maintenance of concrete structures.

Figure 6. Haringvliet Sluices (1960).

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values have been given. Whether these values are sufficiently representative has not yet been verified. A research project was carried out by CUR Com-mittee B82. The project was entitled “Durability of Marine Structures”, abbreviated as DuMaCon.

The most important topic of research was the chlo-ride penetration as a function of time. The equation to be investigated on suitability was:

C x t C x KD t t tsnCl( , ) [ ( / { ( / ) })]= −1 2 0 0erf (1)

where Cs is the chloride content at the surface, x is the

distance from the surface, K is a coefficient taking account of various influences (environment, curing), D

0 is the diffusion coefficient for chloride at the refer-

ence time t0, n

Cl is the material dependant “reduction

exponent” and t is the time. Also the chloride content which is initially in the concrete should be regarded. With the aid of this equation it can be predicted when the chloride content at the reinforcement reaches the critical content C

cr and corrosion is initiated (limit

state G(t) = Ccr – C(x,t)). This happens at time t

i.

Field research was carried out at six structures, in which the thickness of the concrete cover, the steel potential and the electric resistance were measured. Drilled cores were used in order to establish the chlo-ride penetration (profiles), the microstructure (by microscopy), the resistance and the strength. Meas-urements were carried out at and specimens taken from 17 places with an area of 1 × 1 m2. Subjects of investigation were i.e. the Pier of Scheveningen, the Haringvliet Sluices, the Eastern Scheldt Storm Surge Barrier and three quay walls in Rotterdam. These structures are now between 25 and 45 years old.

It was remarkable that in most cases a consider-able scatter was found in chloride penetration, even

between 6 cores taken from one m2, see fig. 7. Also along the length of the Eastern Scheldt Storm Surge Barrier a considerable scatter was found. With regard to the backgrounds of this variation no clear expla-nation was found yet. In this respect more data are necessary in order to take full profit of new calcula-tion models, like described in the PhD thesis of Li (2004).

The chloride profiles as determined from the tests were compared with those from the theoretical model. In most cases the agreement was good. On the basis of those observations, in combination with laboratory tests, it was proposed to modify a number of parameters for blast furnace cement in a marine environment. This referred to the value of the expo-nent nCl and another calculation of the environment factor K: in this calculation the temperature is a deci-sive factor and not anymore the curing. Moreover a value for the surface chloride content was found which describes well the chloride load from about 10 years exposure for concrete from the water level to 7 meters above sea level: an average chloride content of 2,9% of the cement weight with a standard deviation of 0,8% was measured. Above 7 m for C

s strongly

diverging results were found: between 1% and 5%. Apparently at a larger height the surface chloride content is influenced by the contrary effects of wet-ting by sea water and raining. Rain-protected areas (which are only in contact with fine seawater vapour) are therefore exposed to significantly higher chloride concentrations.

4 FURTHER NECESSARY STEPS FOR ENHANCING THE RELIABILITY OF SERVICE LIFE DESIGN

4.1 Improvement of physical models like the DuraCrete models

When verifying transport models like the DuraCrete models on existing concrete structures, like described in 3.1, it turned out that in certain respects knowledge was lacking and understanding was not sufficiently developed:

• There is no reliable value for the critical chloride content in practice (this holds true as well for Portland cement with fly ash). The B82 research project did not give sufficient evidence. This is felt as a major lack in knowledge. The significance of this parameter for taking the right decisions with regard to maintenance is large. Further research is therefore necessary.

• The existing model for chloride transport con-tains simplifications and uncertainties. Improved models are needed, which are able to simulate the

Figure 7. Scatter in chloride profiles within an area of 1 m2, see CUR B82 (2004).

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effect of moisture variations better. Also a new generation of models should be verified. Moreo-ver it is useful to follow structures in time. The penetration of chlorides and carbon dioxides can then be estimated with better accuracy. In The Netherlands it was therefore advised to investigate the same structures another time after 10 years. In this respect use can be made from improved for-mulations for the penetration of chlorides, like for instance developed by Meijers, (2003). In his model account is taken of variable boundary con-ditions with regard to moisture, temperature and chloride, Fig. 8.

• Exposure to chlorides at an early age of the con-crete (<28 days) leads to increased penetration. A high speed of construction can therefore negatively influence the service life. This effect should be studied more in depth.

• The background of spatial variation in chloride penetration is not yet well known. This should be further investigated, both with regard to the vari-ation in exposure and the variation in resistance (macro scale and meso scale). This information is necessary to be able to evaluate results of local inspection measurements.

• Maintaining concrete structures can be improved by better recording relevant data from the construc-tion period. Here it should be formulated which data are necessary, to which degree they are meas-urable and to which extent they should be addition-ally determined. Relevant information concerns design criteria, information on the materials used, the way of producing and curing and further con-struction aspects including deviations and eventual control tests. The total collection of data should be the basis of the “Birth certificate”.

• The “maintenance limit state” needs to be further specified. Mostly here the age at initiation of corro-sion is taken, although for the realisation of corro-sion more time is required (propagation period). An

open question is which reliability index belongs to that limit, what the consequences for maintenance decisions are and how anticipating maintenance versus reacting maintenance works out on the cost along the service life.

4.2 Simulation of the behaviour after depassivation of reinforcing steel

Until now it has always been assumed that the instant of depassivation of the steel defines the end of the damage-free service life. For this limit state a reliabil-ity index in the range 1,5–1,8 is defined. It is, how-ever, important to find out what happens in reality after depassivation. Now qualitatively the following stages in the further deterioration process are distin-guished: the occurrence of splitting cracks, spalling of the cover, and loosening of parts of the concrete. With regard to the length of those stages only limited information exists. This is important because it is not sure that depassivation is synonymous to the start of an accelerated corrosion process which finally leads to unacceptable damage. There is uncertainty with regard to the question after how much time dam-age will develop in relation to internal and external factors.

4.3 Better accounting for the quality of execution

The quality of execution has a large influence on the durability of concrete structures. A better understand-ing is necessary into the role of the quality of com-paction and curing. In the DuraCrete models those influences are only treated with a simple factor. A better solution is hardly possible because the quality of execution can only be determined after finishing the structure. Deviations from the specified concrete cover and the water-cement ratio have an important effect. For projects of high relevance the quality of the structure should be determined at the “initial inspection”, upon delivery of the structure. Only then a definite maintenance plan can be made. The initial inspection, see also Fig. 5, is an important instrument for the determination of the initial state of the struc-ture. On the basis of these data the development of the deterioration can be predicted and a maintenance plan can be made in combination with an inspection plan. The initial inspection is a significant element in the quality control. It can give an indication whether the execution has been carried out with sufficient quality. It should be further specified what is controlled at this initial inspection, what should be measured and how measurements are carried out. Further to that the con-sequences should be clear for the case that the speci-fied quality has not been reached.

Figure 8. Comparison of experimentally and calculated chloride profiles after 2, 8 and 16 years according to Meijers (2003).

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4.4 The significance of crack width criteria for the durability of structures and their economic impact

In the past a governing role for durability was attrib-uted to the widths of the cracks. Later on it was dem-onstrated that the thickness and the quality of the cover play a much more important role. Neverthe-less in many codes still maximum crack width lim-its are given in relation to the environmental class. These demands are not very well supported by evi-dence. Especially when the crack widths should be small the influence of this requirement on economy (required reinforcement) can be large. Whether the specified maximum crack width is 0,2 mm or 0,1 mm can mean an increase of the steel volume with 100%. Another important aspect is that the crack width con-trol mainly pertains to the effect of loads and imposed deformations. These are the most harmless cracks, because they mainly cross the reinforcement per-pendicularly. Cracks, however, can have their origin as well in many different causes which are more rel-evant. Especially cracks occurring before hardening should be considered.

4.5 Taking better account of probabilistic aspects

With the PhD thesis of Li (2004) first impetus was given to the introduction of probabilistics into inspec-tion and maintenance. In this thesis the significance of a good probabilistic basis for the choice of the optimum repair strategy was well demonstrated. Deviations from the specified values should be taken into account and their effect on durability should be regarded. Measurements at the building site showed, that the deviations are often significant. At a Dutch building site it was found, that for a specified value of the water-cement ratio of 0,50 mm an average value of 0,53 was measured, with an upper limit of 0,60 and a lower limit of 0,48 mm. Also for the concrete cover deviations were noted. At a site where the specified minimum cover thickness was 35 mm, an average thickness of 40,4 mm was measured, with a stand-ard deviation of 9,5 mm. The measured minimum thickness was 17 mm and the maximum was 75 mm. Such imperfections can have a large influence on the durability.

4.6 Improvements in design methodology

For a number of important criteria no generally accepted values or definitions are available, such as:

− Definition of the end of service life.− Determination of acceptable probability of exceed-

ing a limit state.

− Critical chloride content for various cement types used in practice, as well as blends.

− Better definition of the exposure conditions (chlo-ride, moisture variation).

4.7 Robust criteria in stead of “advanced” calculations?

Many mechanisms that cause deterioration are based on penetration of chemicals in concrete. The micro-structure of the material plays therefore an important role. However, the variation in the concentration of chemicals (like Cl− ions and CO

2) and the variation

in environmental conditions (temperature, relative humidity, wind, and solar radiation) are uncertainties. Moreover the properties of the material are determined by the way of casting in relation to the workability of material and quality of curing. Summarizing, there is a multitude of processes and boundary conditions, which cannot always be defined very accurately.

In cope with this situation different approaches are possible. On the one hand one can try to describe all processes as accurately as possible and carry out parameter studies in order to get an idea about types of deterioration, their speed and their probability of occurrence.

On the other hand one can argue that, because of the availability of so many unknown aspects, it may be better to distinguish a limited number of durabil-ity classes for the concrete. In a cooperation program between a number of South African universities, see Beushausen (2003), a proposal was formulated to classify the concrete in durability classes, on the basis of penetration tests for three different media, being oxygen, water and chlorides. On the basis of the results of those tests the concrete ends in one of the durability classes “excellent”, “good”, “moderate” or “bad”. This method is as well suitable for the classi-fication of existing structures. It seems worthwhile to further investigate and evaluate such methods.

5 DETERMINING THE REAL BEARING CAPACITY OF EXISTING STRUCTURES

In Fig. 2 it was shown that the bearing resistance of structures is reduced in time due to deterioration processes. On the other hand the traffic increases. This means that the reliability of the structure decreases. The decision to upgrade the structure, however, has considerable financial consequences and should there-fore be taken on the basis of sound considerations. A very important aspect that has not been treated yet is that many structures have a residual bearing capac-ity. If it can be proven convincingly that the curve

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representing the real bearing capacity is higher than the curve representing the design-capacity, this means that strengthening can be extended. Fig. 9 shows this schematically.

Residual bearing capacity can occur as a result of several reasons. A number of those reasons are given in the following survey:

a. Compressive membrane action. If a bridge deck is loaded by a heavy wheel load the punching shear capacity may be expected to be governing. In many bridge decks the real bearing capacity is larger than the theoretical punching load. The reason is the development of compressive mem-brane action. This occurs since deflection under the wheel load goes along with lateral extension of the concrete around the loaded area.

b. Design calculations are mostly carried out accord-ing to lower bound models. An example is the strip method, assuming simplified load bearing paths. This method is transparent for the designer and leads generally to simple reinforcement geometries, appropriate for construction. When, however, a structure designed in this way is recal-culated by e.g. the theory of elasticity, it is often found that the real bearing capacity is larger than determined on the basis of the design rules.

c. After having calculated the necessary reinforce-ment on the basis of ULS calculations, often practical reinforcement is added. Examples are horizontal web reinforcement or reinforcement along free edges. This reinforcement adds to the real bearing capacity.

d. At the time that the bearing capacity of a struc-ture becomes subject of discussion the age is mostly in the range of 30–60 years. The concrete strength is then, as a result of continued hydration, much higher that the 28-days strength on which the design has been based. Many bridges in The Netherlands showed concrete compressive

strengths of 60 MPA, whereas the design strengths was not higher than 25 MPa in the past. This may count especially in the case that the shear capacity is concerned.

e. Solid slab bridges are mostly designed with shear formulas which are a result of the evaluation of beam-shear tests. In a beam a weak spot may have a considerable influence on the shear capacity, whereas in a slab a weak sport is compensated by regions with a higher strength. It can therefore be expected that slabs have a higher shear bearing resistance as a result of their larger redistribution capacity.

f. Non-linear finite element programs can be very useful for the determination of the real bearing capacity. It is, however, necessary to calibrate those programs for the typical structure consid-ered, to take full advantage of these numerical tools.

6 CONCLUSIONS

1. In future codes rules for the determination of the service life of structures will be given on vari-ous levels. Deterministic criteria will be given by deem-to-satisfy-rules. Probabilistic models will be available for structures of large relevance.

2. An important task will be to assess the state of deterioration of existing structures, in order to define their remaining service life and plan the corresponding maintenance.

3. To take full profit of advanced deterioration mod-els the aspect of scatter deserves due considera-tion. Under many conditions an accurate definition of the input values is not possible. More “robust” models therefore should also be taken serious.

4. The influence of executions on durability is a major influencing factor. It is therefore useful to test the condition of a new structure and record the data in a “birth certificate”.

5. The aspect of residual bearing resistance of struc-tures should be regarded when estimating the service life of existing structures.

Figure 9. Extended service life by virtue of residual bear-ing capacity.

Figure 10. Compressive membrane action.

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REFERENCES

Beushausen, H.D., Alexander, M.G., Mackechnie, J., “Concrete durability specifications in an international context”, Betonwerk + Fertigteiltechnik, 7, 2003, pp. 22–32.

CUR Committee B82, “Durability of concrete structures in a marine environment”, Gouda, The Netherlands, 2004.

Gaal, G.C.M., “Prediction of deterioration of concrete bridges”, PhD-thesis, TU Delft, June 2004.

Li, Y., “Effect of spatial variability on maintenance and repair decisions for concrete structures”, PhD thesis, TU Delft, June 2004.

Li, Y., Vrouwenvelder, T., Wijnants, G., Walraven, J.C. “Spatial variability of concrete deterioration and repair strategies”, Structural Concrete, Journal of the fib, Vol. 5, Nr. 3, September 2004, pp. 121–129.

Meijers, S., “Computational modeling of chloride ingress in concrete”, PhD thesis, TU Delft, March 2003.

Rostam, S., “Performance based design of concrete struc-tures: the challenge of integrating strength, durability and sustainability”, Proceedings fib-Symposium “Concrete and the Environment”, Berlin, 3–5 Oct. 2001.

Sitter, W.R. de, “Costs for Service Life Optimization: the Law of Fives”, Durability of Concrete Structures”, Workshop Report, Ed. Steen Rostam, 18–20 May, 1984, Copenhagen, Denmark, pp. 131–134.