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Keywords:

  • concrete;
  • corrosion;
  • economy;
  • maintenance;
  • repair;
  • survey

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Stages in development of corrosion damage
  5. 3 Statistical distribution of corrosion damage
  6. 4 Strategies for management and corrosion control
  7. 5 Economic consequences of strategies
  8. 6 Starting points for survey
  9. 7 Discussion
  10. 8 Conclusions
  11. Acknowledgements

This paper investigates the economic effects of full corrosion surveys of concrete structures. The background is that the existing concrete infrastructure is aging, while being exposed to aggressive influences, which increases the occurrence of corrosion and related concrete damage over time. The central proposition is that solely relying on visual inspection for interventions (repair) may result in unnecessarily high costs and associated risks. The reason is that visual inspection can only signal deterioration that is in a relatively advanced stage of development. Consequently, heavy and costly repairs are needed, while undetected degradations still go on developing, presenting future risks. On the other hand, carrying out full surface corrosion surveys may be considerably more economic. This is because using detailed survey information, degradation can be detected at an early stage. Prevention of corrosion is generally less costly than correction. Consequently, an optimal mix of preventive and corrective measures can be applied at the right time and at the right places. These alternative approaches to inspection may be considered elements in so-called reactive and proactive strategies for maintenance of infrastructure, respectively.


1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Stages in development of corrosion damage
  5. 3 Statistical distribution of corrosion damage
  6. 4 Strategies for management and corrosion control
  7. 5 Economic consequences of strategies
  8. 6 Starting points for survey
  9. 7 Discussion
  10. 8 Conclusions
  11. Acknowledgements

Reinforced concrete is a cost effective construction material, used for a large part of the world's physical infrastructure. The durable combination of steel and concrete is essential for its safe and serviceable functioning. Normally, the physical and chemical nature of concrete protects the embedded reinforcing steel against corrosion. However, in the course of time, this protection can be reduced or completely lost due to the ingress of aggressive substances from the environment, for example chloride ions from seawater and de-icing salts and carbon dioxide from the atmosphere, resulting in reinforcement corrosion. Corrosion frequently creates expansive corrosion products, which crack and subsequently spall-off the concrete cover; eventually corrosion will reduce bar diameters to unsafe values, upon which collapse cannot be excluded. In very wet conditions, heavy pitting may produce so-called black rust without obvious external signs.

Corrosion of reinforcement is the main cause of damage to concrete structures. The European infrastructure has reached an age where maintenance costs have increased to such an extent that they constitute the major part of the total costs.

The occurrence of corrosion in concrete structures shows an extremely wide distribution. Some structures will not develop corrosion over a period of 100 years or more, and some already show damage after some years of service 1. This wide distribution is due to strong variations in cover depth, concrete quality, environmental aggressiveness, level of routine maintenance and several other factors. This means that it is impossible to evaluate the durability of a particular structure exposed to aggressive conditions a priori, without specific information from it. Considering that in Europe probably hundreds of thousands of structures are exposed to aggressive conditions, a statistical approach to corrosion management is required. Some relevant information is given in the following section.

National (road, railway) and local authorities and many private stockowners have management systems in place (e.g. for bridges) 2, 3. They generally are based on routine visual inspections of structures, which are supposed to signal the appearance of damage. Such an approach may allow taking countermeasures in the right place, but not necessarily at the right time. In fact, repairing damage that has developed until a visually detectable stage may be quite costly.

More sensitive detection methods exist that can signal corrosion in an earlier stage. With such information an owner could take remedial actions well before damage appears and his spending on repairs would be reduced.

The central proposition of this paper is that carrying out full surface corrosion surveys instead of only relying on routine visual inspection, will save considerable amounts of money. Using instrumented monitoring systems with embedded sensors may be an alternative approach whose economic aspects have been considered elsewhere 4 and is treated in the paper on Monitoring in this special issue 5.

2 Stages in development of corrosion damage

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Stages in development of corrosion damage
  5. 3 Statistical distribution of corrosion damage
  6. 4 Strategies for management and corrosion control
  7. 5 Economic consequences of strategies
  8. 6 Starting points for survey
  9. 7 Discussion
  10. 8 Conclusions
  11. Acknowledgements

The development of corrosion in concrete over time is generally seen as a multi-stage process as shown in Fig. 1 6. In the first period, aggressive substances such as chloride ions or carbon dioxide penetrate the concrete cover to the reinforcement and ultimately reach the steel, which causes the onset of corrosion (mark 1), called depassivation or corrosion initiation. In the second period, actual corrosion takes place (its rate depends on availability of moisture and oxygen). Several sub-stages can be distinguished related to damage phenomena. First, the expansion due to corrosion products being formed builds up tensile stresses in the concrete cover until it cracks (mark 2). This cracking produces the first visible signs of corrosion, although in some cases, rust stains at the concrete surface may show before cracking. In the next sub-stage, expansion due to corrosion proceeds until parts of the concrete cover completely detach and spall-off (mark 3), constituting a potential danger for users of the structure or the general public. In the fourth and final sub-stage, reinforcing bar diameter loss becomes so severe as to approach the minimum required for structural capacity; eventually, collapse cannot be ruled out (mark 4). The period from t = 0 until mark 1 is called the initiation stage. The period from mark 1 until mark 4 is generally called the propagation stage. In civil engineering terms, each of the marks can be seen as a limit state. Service life design methods generally only model the development until mark 1, although attempts are being made to include the next stages 2 and 3 7 and more recently even stage 4. Indicatively, the initiation period may last between 10 and 100 years (see below). The subsequent propagation sub-stages from mark 1 to 2 and from 2 to 3 may last a few to about 10 years.

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Figure 1. Various stages in the development of reinforcement corrosion in concrete

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3 Statistical distribution of corrosion damage

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Stages in development of corrosion damage
  5. 3 Statistical distribution of corrosion damage
  6. 4 Strategies for management and corrosion control
  7. 5 Economic consequences of strategies
  8. 6 Starting points for survey
  9. 7 Discussion
  10. 8 Conclusions
  11. Acknowledgements

To illustrate the uncertainties associated with corrosion and large amounts of structures, data from a recent PhD study are summarized 1. Gaal collected literature and field data on state-managed motorway bridges in The Netherlands and made a prediction of corrosion in concrete bridges by probabilistic modelling. He found that:

  • the service life of bridges determined by corrosion related damage has a very wide distribution; that is, a relatively large number of structures have a much shorter life than the average structure (large standard deviation)

  • the first bridges that show spalling (mark 3 in Fig. 1) are as young as 15 years old

  • at an age of 40 years, 5% of the bridges have spalling over 2.5% of their surface; 2.5% is the amount that owners see as necessitating repairs

  • 50% of the bridges exceed the spalling criterion at an age of 70 years (“average service life”)

  • the time between initiation of corrosion (mark 1) and spalling (mark 3) can be typically as short as about 5 years.

These generalized observations apply to concrete motorway bridges in The Netherlands, where bridges are relatively young (typically 40–45 years age), concrete quality and cover depth are considered to be good and the climate is relatively mild. Further analysis in order to predict future maintenance is underway 8. Gaal also analyzed results from the USA Bridge Inventory database that was installed in 1968 (after 45 people were killed in a bridge collapse). It shows that in the US (with supposedly lower concrete quality, older bridges and more aggressive climate), at 40 years age:

  • 10% of the bridges are “structurally deficient” (supposedly stage 3, possibly approaching stage 4 in Fig. 1

  • 15% of the bridges have already been (heavily) repaired after having been found to be structurally deficient.

Recent information from the U.S. indicates that future costs for rehabilitating infrastructure are systematically underestimated. Practical experience 4 suggests that over Europe these statistics will be the same as in The Netherlands or worse (older, lower quality concrete, more aggressive environment). No statistical overview is available for car parks. It is thought that car parks develop corrosion earlier than bridges, because until recently their internal climate was not considered aggressive. Experience in the UK with many older car parks (including some collapses!) and increasingly so in Germany, shows that a rapidly increasing number of these structures develops corrosion and needs repairs. For marine quay walls statistical information is scarce. Harbour quays in Norway seem to have relatively short lives; in The Netherlands the condition of several marine structures after 20 to 40 years was found to be good with one exception 9. In general the condition of quay walls in Europe is probably similar to that of bridges.

The life of repairs to concrete structures also has a wide distribution. In a European survey, average repair life was found to be about 10 years 10, 11; a later survey in The Netherlands has generally confirmed these results. Cathodic protection is generally more durable than most conventional repair methods; working lives between 15 and 25 or more years are common 12, although in some cases minor maintenance is necessary in that period.

4 Strategies for management and corrosion control

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Stages in development of corrosion damage
  5. 3 Statistical distribution of corrosion damage
  6. 4 Strategies for management and corrosion control
  7. 5 Economic consequences of strategies
  8. 6 Starting points for survey
  9. 7 Discussion
  10. 8 Conclusions
  11. Acknowledgements

For management of structures in aggressive environments two basic strategies are available, which might be called popularly “wait & see” and “monitor the invisible”. In the fib Guide to Good Ownership 13, these strategies are called “reactive” and “pro-active” approaches, illustrated in Fig. 2, respectively. The “wait & see” or “reactive” strategy employs regular visual inspections of rather superficial (routine) nature, e.g. with 5-year intervals. Visual inspection will not observe anything until a significant amount of damage has developed (mark 2 in Fig. 1, possibly approaching mark 3), which causes a high cost of repair in the short term (see below) and potentially the need to decommission the structure from use for a considerable period of time. The pro-active strategy comprises two sub-strategies: (a) detailed, full surface coverage corrosion survey and (b) monitoring using embedded sensors, which could be operated as alternatives or in parallel.

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Figure 2. Performance (top) and cost (bottom) versus time diagram indicating the differences between reactive (thick dotted lines) and pro-active (solid lines) approaches to structural durability management; IL intervention levels (thin dotted lines)

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Economic benefits of detailed surveys are the subject of this paper. A detailed survey in principle implies: a full surface cover depth survey; full surface potential mapping; local chloride and carbonation sampling; and possibly additional survey by e.g. resistivity or corrosion rate mapping. An overview, various technical aspects and case studies are provided in other papers in this special issue 14–18. Sensor-based monitoring allows following the (invisible) ingress of aggressive substances continuously (in practice on a monthly to annual basis), which is elaborated in Ref. 5. The schematic in Fig. 2 may illustrate the principles. In the upper part, the performance over time is depicted, which decreases due to degradations. Following the reactive strategy, that is, relying on visual inspection only, the intervention level (denoted IL visual) is dictated by the visibility of corrosion damage (cracking etc.). The related amount of damage necessitates corrective and consequently heavy repair interventions, which have a high cost level. The proactive strategy involves (full surface) corrosion surveys, with an associated intervention level at a smaller loss of performance (denoted IL survey). Consequently, interventions may have a preventive character and are less costly. The overall cost of both strategies differs as a result of the differences of the associated costs of corrective versus preventive interventions. The total costs at the end of the service period of the preventive strategy are lower than those of the reactive strategy.

5 Economic consequences of strategies

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Stages in development of corrosion damage
  5. 3 Statistical distribution of corrosion damage
  6. 4 Strategies for management and corrosion control
  7. 5 Economic consequences of strategies
  8. 6 Starting points for survey
  9. 7 Discussion
  10. 8 Conclusions
  11. Acknowledgements

A basic assumption is that it is more economical to prevent corrosion than to repair it. This assumption is justified because preventive measures are relatively low-cost, as they only have to delay the onset of corrosion; they shift mark 1 in Fig. 1 to later. Applying preventive measures will obviously delay subsequent propagation and thus postpone subsequent heavy and costly repair measures. That means that their effectiveness depends on the present state of corrosion. However, such cheap interventions can only prevent corrosion from starting, but they cannot stop corrosion that is already ongoing. So, choosing the right (effective) type of intervention strongly relies on knowing the stage of degradation.

This is tentatively quantified below, based on estimated average industry costs of interventions (that may vary between countries), which are given here excluding additional cost for access, etc. 4:

  • Stage 1 hydrophobic treatment or simple coating € 25/m2.

  • Stage 2 local repair (with up to 20 mm of old concrete removed) and high performance coating or preventive cathodic protection (CP) € 100/m2.

  • Stage 3 extensive repair (with up to 50 mm removal of old concrete) or local repair plus cathodic protection € 300/m2.

  • Stage 4 replacement of the structure (rebuilding) € 1000/m2.

Then, if the amount of corrosion (corroding area) tends to increase with time, a detailed survey now may save money if the cost of later repair (with increased amount) is higher than taking preventive measures now. In contrast, relying on visual inspection only will allow corrosion to develop over larger areas and thus cause the need for larger repairs. Applying a visual inspection strategy will be termed option 0 (zero), carrying out a detailed corrosion survey is called option 1. What a detailed survey comprises will be described in other papers in this volume. Here it suffices that the complete surface is surveyed with non-destructive methods and samples for analysis are taken as necessary.

It should be added that additional or indirect costs can be significant and must be taken into account, including additional costs for access. Costs for access heavily depend on the type and location of the structure and the type of surface to be investigated (top of deck, overhead beams). Indirect costs may be the loss of parking revenues during the period for carrying out interventions. Extensive repairs cause a longer period of closure than simple preventive measures. For simplicity, this issue is further ignored. It should be stressed that all cost levels given here are indicative and for purpose of the systematics only. This particularly applies to the cost of full surface surveys, which may considerably depend on the size of the structure to be surveyed, with larger surveys tending to have a lower cost per square meter of surveyed area. Apart from the issues mentioned they may vary widely from country to country. In real cases, cost information should be gathered based on the individual structure and the prevailing boundary conditions.

6 Starting points for survey

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Stages in development of corrosion damage
  5. 3 Statistical distribution of corrosion damage
  6. 4 Strategies for management and corrosion control
  7. 5 Economic consequences of strategies
  8. 6 Starting points for survey
  9. 7 Discussion
  10. 8 Conclusions
  11. Acknowledgements

Depending on the state of the structure at the point in time when a detailed survey is considered to be carried out, several situations are possible. A hypothetical case is considered, for a parking deck of 25 years of age with 1000 m2 deck surface, exposed to chlorides from de-icing salts that are brought in by vehicles. Visual inspection is estimated to cost about € 400 and full surface investigation (visual inspection, potential mapping, and concrete cover depth plus chloride profiles, carbonation depths and inspection openings) costs about € 6000 (again, these costs may vary between countries).

Case (1): this structure has corroding bars and concrete damage in several spots with a total area of 50 m2 due to heavy chloride load over the complete surface that has rather poor resistance against chloride penetration. Presently, only the areas with the lowest cover depth are corroding, but the corroding area will increase significantly with time. However, this fact nor the future amount of damage are known beforehand.

Case (2): this structure has the same total area but 250 m2 of corrosion due to the same issues as in case 1, but more advanced (later in time). The spots that are corroding are scattered over various parts of the surface. Moreover, the structure will develop further corrosion over the remaining 750 m2 in 10 years.

Case 1

For case (1), visual inspection (option 0) identifies the (50 m2) corrosion-damaged area, which will be repaired. However, the structure will develop further corrosion over 250 m2 in 10 years that eventually has to be repaired. Carrying out a detailed survey (option 1) will reveal the same corroding area and will predict future corrosion over the larger area, thus allowing to take preventive measures for this 250 m2.

The repair costs with option 0 will simply be 50 m2 multiplied by 100–300 Euro/m2 (assumedly producing a repair life of 10 and 25 years, respectively), is 5000 to 15 000 Euro. However, over the next 10 years, 250 m2 of corroded area will develop, necessitating repairs of 25 000 to 75 000 euro.

Following option 1, the costs are the sum of repair, preventive measures and survey. This is 5000 to 15 000 Euro for 50 m2 repair plus 6250 Euro for preventive application to the endangered area, plus the cost of the survey, 6000 Euro. If this option is chosen, no further corrosion will develop.

It is obvious that in this case a survey can save a significant amount of money, here about € 15 000 to 65 000 (cost of option 0 minus cost of option 1).

Case 2

For case (2), visual inspection (option 0) identifies the (250 m2) corrosion-damaged area, which needs to be repaired. However, the structure will develop further corrosion over the remaining 750 m2 in 10 years. Carrying out a detailed survey (option 1) will reveal the same corroding area and will predict future corrosion over the remaining area, thus allowing to take measures at the right time and in the right places. It is probably too late to effectively apply preventive measures. Looking at the serious overall weaknesses of this structure, it may be wiser to apply CP to the complete surface. The unit price of CP in the area that is not yet corroding (basically of preventive nature, without the need for concrete removal and repairs, if applied in the present, undamaged, stage) may be as low as about 100 Euro/m2. CP will guarantee durability and absence of corrosion for a long time, with minor maintenance to the CP system up to 25 years 12.

The repair costs with option 0 will be 250 m2 multiplied by 100–300 Euro/m2 (for a repair life of 10 and 25 years, respectively), is 25 000 to 75 000 Euro. Corrosion damage will develop in the remaining 750 m2 over the next 10 years, possibly gradually, necessitating repairs of 75 000 to 225 000 Euro. The total cost over 10 years, intended to maintain the structure, may vary from at least 125 000 Euro up to 300 000 Euro.

Following option 1, the full CP scenario would cost 100 Euro/m2 for simple repair of 250 m2 and 100 Euro/m2 for (preventive) CP for the total 1000 m2, plus 6000 for the survey, adding up to 131 000 Euro. This is nearly equal to the cheapest scenario following option 0. However, following that cheapest scenario involves the risk of losing rebar cross-section and ending up with an unsafe situation. Heavy repairs also would cause more loss of parking revenues and their timing would be rather unpredictable. Nevertheless, CP should include maintenance (checks) of the CP system (over 10 years say 10,000 Euro). Finally, all the options considered should be compared to the cost of new construction; here 1 million Euro.

An overview of the costs associated with the two cases and application of the two options is provided in Table 1.

Table 1. Overview of cost estimates in Euro for two cases, each 1000 m2 parking deck surface area, starting with different amounts of corrosion and degradation rates; for visual inspection only and for full corrosion survey see text; lower cost levels (denoted min) are for 10 year added life by repairs, higher cost levels (denoted max) are for 25 year added life
caseOption 0: Visual inspection onlyOption 1: Full corrosion survey
Repair nowFuture repairRepair nowFuture repair
MinMaxMinMaxMinMax 
1 medium corrosion, increase f(t)500015 00025 00075 00017 25027 250None
1 whole life cost (10 year)Min 30 000, max 90 000Min 17 250, max 27 250 
2 large corrosion, increase f(t)25 00075 00075 000225 000131 000 for CP10 000 for maintenance of the CP system
2 whole life cost (10 year)Min 100 000, max 300 000 risk unknown (non-negligible)141 000 

The effect of carrying out a full surface corrosion survey and taking appropriate preventive measures on a structure that is essentially still in the initiation phase is shown in Fig. 3.

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Figure 3. Effect of early warning from full surface corrosion survey and applying preventive measures at a point in time indicated by the diagonal arrow on the delay of marks 1 and 2

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7 Discussion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Stages in development of corrosion damage
  5. 3 Statistical distribution of corrosion damage
  6. 4 Strategies for management and corrosion control
  7. 5 Economic consequences of strategies
  8. 6 Starting points for survey
  9. 7 Discussion
  10. 8 Conclusions
  11. Acknowledgements

The example shows that if the owner knows that mark 1 (corrosion initiation) is soon to be reached, which would only be possible if a full corrosion survey is carried out, he can take (relatively cheap) preventive measures that postpone the development of damage as illustrated in Fig. 3.

Carrying out a full corrosion survey on a structure at a rather early age following the pro-active strategy, would allow for the results to indicate that corrosion initiation (mark 1) is coming up, say in 10 years. The decision on whether or not to take preventive measures can be made well in advance. A full corrosion survey at a later stage, say when some corrosion has developed, may be even more useful. Taking appropriate measures following the survey will offer the opportunity to make large financial savings, because it will enable avoidance of entering stage 2 on the damage continuum over large parts of the surface. This will be impossible if the reactive strategy based on visual inspection is followed: damage must develop before it may be noted and consequently, heavy repairs cannot be avoided. Corrosion surveys in structures with heavily developed corrosion may not save immediate repair costs (because serious measures such as CP are necessary), but will reduce the risk of doing too little now and having to spend much more later.

It should be noted that this example is a gross simplification. The costs of various actions could vary strongly, depending on the local situation. The effect of discount rate should also be taken into account (life cycle costing), as well as the cost of e.g. access. Experience shows, however, that the bigger picture presented by this example is generally correct.

8 Conclusions

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Stages in development of corrosion damage
  5. 3 Statistical distribution of corrosion damage
  6. 4 Strategies for management and corrosion control
  7. 5 Economic consequences of strategies
  8. 6 Starting points for survey
  9. 7 Discussion
  10. 8 Conclusions
  11. Acknowledgements

For owners of concrete structures, corrosion of reinforcement is difficult to handle because visual inspection schemes do not provide accurate information about the corrosion state and its development in time, that is, before a serious amount of damage is already present. Corrosion development is a multi-stage process; the first part (ingress of aggressive substances) is invisible to the naked eye. This prevents it being detected by commonly applied visual inspection methods. Consequently, corrosion will not be detected before it has significantly developed into the second stage (damage to concrete and steel cross section loss). It follows that measures for protection and repair will be necessarily heavy and costly.

As an alternative, full surface corrosion survey is aimed at signaling corrosion well before it is in the fully developed stage. With proper information about the future development of corrosion, owners would be in a position to initiate remedial actions well before the development of actual damage. The cost of preventive interventions that can be applied effectively prior to the occurrence of damage is considerably less than the cost of corrective actions needed at a later stage. Moreover, such information from a corrosion survey will reduce the risk of “unseen” deterioration becoming a major (economical and structural) problem. This paper quantifies the benefits of full surface corrosion survey for an example case that is necessarily a simplification, but nevertheless is thought to be representative for many structures.

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Stages in development of corrosion damage
  5. 3 Statistical distribution of corrosion damage
  6. 4 Strategies for management and corrosion control
  7. 5 Economic consequences of strategies
  8. 6 Starting points for survey
  9. 7 Discussion
  10. 8 Conclusions
  11. Acknowledgements

Comments to earlier drafts of this paper provided by John Broomfield, Michael Grantham, and Ulrich Schneck are much appreciated. However, this does not imply that they would agree with everything stated in the final version.

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