Strategy for protection of marine concrete structures against corrosion of steel reinforcement

by

Prasert Suwanvitaya Prinya. Chindaprasirt and H. Trinh Cao Dept. of Civil Engineering

Faculty of Engineering Kasetsart University Bangkok, Thailand

Khon Kaen University Faculty of Engineering Khon Kaen , Thailand

CSIRO, Building Construction&Engineering

North Ryde, NSW, Australia

Abstract Life cycle costing is an important consideration for new concrete structures. It is also an integral part of asset management and in achieving sustainable construction. Cost effectiveness is the key driver. This paper presents an overall approach to formulate a strategy for protection of reinforced concrete structures against steel corrosion. The objectives of the protection are to ensure the design service life and to manage the risk of failure due to steel corrosion. The advantages and disadvantages or doubtful points of each protection measures are discussed. Introduction In recent years, more emphasis has been given to creating more durable and cost effective concrete structures. This is due to the increase cost of repair and maintenance, which may be similar to the cost of building a new structure in many cases. “Cost optimal design” of new concrete structures is no longer a strange terminology to owners, specifiers and engineers Ensuring durability of concrete structure is definitely not a simple and static process. It requires clear definitions, understanding of responsibilities and collaborations. Design for durability of concrete structure needs to be considered on case-by-case basis. A design, which is cost effective in one case, may not be so in another case. In this paper, additional protection of marine concrete structures, which is an important aspect of “cost optimal design” is discussed. The scope of this discussion is limited to damage caused by chloride induced corrosion of steel reinforcement, which is the principal cause of loss of durability of reinforced concrete in marine environment. Service Life of Concrete Structures It is very difficult to discuss durability of concrete structure without reference to service life prediction and modelling. An easy means of illustrating design/service life is through the consideration of extend of damage as shown in Figure 1. From this

figure, the service life is equal to the period of service during which the damage level is less than an acceptable benchmark level. The case-to-case basis of service life and durability is apparent because of different patterns of damage vs time and different acceptable damage levels. The difference patterns of damage can be caused by local conditions, design configurations, degree of maintenance and others. Different acceptable damage levels can be caused by different applications/expectations, safety issues and others.

Figure 1: Definition of Design/service life For marine reinforced concrete structures, the modelling of service life is usually based on a criterion related to chloride-induced corrosion of steel reinforcement. Acceptable chloride level (based on an acceptable corrosion rate of steel leading to acceptable damage to concrete cover) has been suggested as a criterion 1. The referred chloride level is that at the steel surface after a given exposure period to marine environment. Other notable approach to modelling of service life involves summation of the initiation period t I and a proportion of the propagation period t P of the corrosion process of steel reinforcement 2,3. In this approach, the end of initiation period is determined when the onset of steel corrosion occurs. This is signalled when chloride level reaches a threshold level. The determination of the propagation period to be included in the service life requires the consideration of the cracking of concrete cover due to the built-up of corrosion product. With some assumptions, this proportion is often assumed to follow an empirical formulae such as t P = K.σ t.ρ where K is an empirical constant; σ t is the tensile strength and ρ is the resistivity of concrete. While the first approach is simpler to be used, the second approach is also valid. The critical issue in both approaches is the selection of appropriate experimental data or empirical parameters for modelling. Regardless of the modelling approach, the

common feature is the prediction of chloride penetration into concrete with exposure time. The chloride penetration into concrete is practically modelled by the following equation based on Fick’s 2nd law:

( )

−=

tDxerfCtxC S .2

1, (1)

where C(x,t) = Chloride concentration at depth x at time t; CS = Chloride concentration on the exposed surface; t = Exposure time; x = Depth; erf = error function; and D = Diffusion coefficient Based on such a relationship, concrete’s resistance to chloride penetration is characterised by CS and D. These are time-dependent, binder-dependent, exposure-dependent and concrete mix dependent parameters. The combination of the concrete cover and concrete quality required for a given design life can be estimated from a model incorporating equation (1). Specification for concrete for durability can be formulated from such approach. A paper dealing with specification of marine concrete for service life requirement is also presented in this conference 4. Distribution of service life and why additional protection measures for marine concrete are needed With the above consideration, it appears initially that service life of marine concrete can reasonably be assured with adequate concrete cover and concrete quality. This is only true in idealised cases. In practice, there are several situations where earlier-than-expected failure of marine concretes can occur. These can be caused by incomplete consideration of deterioration mechanisms and kinetics of steel corrosion in modelling of service life of marine concrete. For example, neutralisation of concrete (carbonation or leaching), penetration of sulfate and Magnesium attack are prevalent deterioration mechanisms in marine environments. In the context of chloride-induced corrosion of steel reinforcement, these phenomena can lead to reducing concrete quality such as chloride binding capacity, resistivity and chloride threshold level. This results in higher corrosion damage. Erosion of concrete can effectively reduce concrete cover. Furthermore, in most idealised cases, concrete is assumed to be crack-free. This is not true in practice. The influence of cracking and microcracking of concrete on chloride-induced corrosion of steel reinforcement is a complex issue. It is still not completely or at least not effectively dealt with. Beside affecting transportation of chloride bearing water, cracking of concrete, depending on crack density, orientation, width and depth, can lead to formation different corrosion cells on steel surface 5.

The damage caused by steel corrosion in cracked concrete is therefore different to crack-free concrete. This will at least influence the acceptable damage level. It should be noted further that although self-healing of crack is possible in moist concrete 6, predicting this ability is very difficult especially if it is to be incorporated in a service life model. Microcracking of concrete especially in the tension zone can lead to increase in chloride penetration. A empirical relationship 7 reflecting the effect of microcracking has been given as:

( ) ( )[ ])(110 tVLDD SS ++= σσ where D(σS) = Diffusion coefficient in loaded state; D0 = Diffusion coefficient in loaded state; L(σS) = Function of the load; σS = max. tensile stress in rebar at cracking and V(t) = Aging function denoting change in concrete characteristics with time and damage. Adding to the complexity of ensuring service life of marine concrete strictures, there are other chloride driving mechanisms such as influence of hydraulic head (permeability-type) and absorption (“wicking” effect/capillary suction) 8. These are also time-dependent, exposure dependent, binder dependent and concrete dependent phenomena. For example, permeability of concrete is given as:

( )n

refref

tt

Zk

Ttk

= .,

where k = permeability coefficient at time t and temperature T k ref = permeability coefficient at reference time t ref Z = viscosity temperature correction factor n = porosity With the above discussion, it can be seen that there are high degrees of uncertainty in predicting and ensuring service life of marine concrete structures and there are high degrees of use of empirical relationships which need to be tailored to suit local materials and conditions. Regardless of the modelling approach adopted, the results are as good as the appropriateness of inputs. One of the major uncertainties in ensuring service life of marine concrete is the variation of concrete cover and concrete quality. This is partly due to the inherent inhomogeneous nature of concrete and mostly due to inconsistency during construction. These variations alone can make the real service life vastly different to the estimated one 8. In summary, the estimated service life of marine concrete structures should be best described as a statistical parameter. A designer/specifier would need to make a

reasonable estimate of its distribution using the most appropriate model and data available. Different distributions of service life as shown in Figure 2 can have critical impact on cost effectiveness in design for durability and protection strategy.

Figure 2: Distribution of service life for design service life of 100 years with different variations

Factors such as local construction technology, local QA/QC, configuration and local materials affect the distribution of service life. In Figure 2, β is the reliability index. The use of reliability index in design for structural performances is generally defined and given in many building codes. This concept has not filtered to design for durability until recently 9. This is an important concept needed for risk management and cost optimal design for durability of concrete structures. In adopting β for design for durability, factors such as ease of inspection/repair, possible change in use and environment should be considered in conjunction with the usual consideration of the loss of serviceability, cost of repair/replacement, safety and consequence of loss of durability on structural performances. Figure 3 illustrates the different risks of failure due to steel corrosion at different periods resulting from different distributions of service life of similar mean service life of 150 years. The establishment of the level of risk of failure (due to loss of durability) at different period during service is the task of the specifier. From such an evaluation, a strategy for protection of marine concrete structures against-chloride induced corrosion of steel reinforcement can be formulated. This is needed to satisfy the acceptable risk levels provided or agreed by owner of the structures. The cost equation will dictate the adopted protection measures.

Figure 3: Risk of failure at different periods during service Why marine concrete structures require additional protection? The main reason is to ensure that the designed service life is achieved at manageable risks in a cost effective manner. It must be re-emphasised that design for service life of marine concrete structures starts with specifying suitable combination of concrete cover and concrete’s quality, especially resistance to chloride ion penetration. There are cases where this is not achieved in practice due to design constraints or construction defects. Lack of required concrete cover is a common occurrence, which can only be detected by post-construction inspection. Similar situation is applicable to inadequate quality of in-situ concrete. The important point is that protection of concrete should always be considered in design for service life to cater for “mistakes” as well as uncertainties discussed previously even when the concrete specification is well formulated. Strategies for protection of marine concrete against chloride attack While this paper focus on the additional protection measures, the two most important aspects of achieving design life of concrete structures are suitable concrete quality and adequate cover. These has been dealt with by Chindaprasirt et al 4. A sensible strategy for protection of marine concrete structures is the use of “best available” concrete technology (with appropriate model and relevant data) to determine the required concrete cover and concrete quality necessary to obtain the design service with acceptable risk levels at different periods during service. Additional protection measures are incorporated into the design as “insurance” and to cater for unexpected damage. This strategy should be adopted for important concrete structures, structures with long design life or part of the structures where inspection and repair are difficult or impossible. When the additional protection measures are selected carefully and not overused, this approach can be the most cost-effective means of achieving design life.

The other strategy involves the consideration of the effectiveness of the additional protection measures into the estimation of service life and risk evaluation process. This approach requires a thorough understanding of the performance and application range of the relevant protection measure and long-term data/experience of its use. Relying on untried additional measure is not recommended with this strategy. Furthermore, with this approach, the QA/QC and compliance of the application of the protection becomes more critical in comparison to that needed using the previous strategy. For example, it is quite wrong to rely on a coating for retardation of chloride penetration into concrete when the completeness of the coating can not be checked/evaluated on site. As in the previous one, this can be very effective achieving the specifier’s objective. The objective is to achieve the design life with manageable risk levels in a cost-effective way. Additional protection measures for marine concrete available commercially are wide ranging. While considering the process of chloride-induced corrosion of steel reinforcement, it appears that there are two main groups, which will be termed “front-end” additional protection measures and “back-end” additional protection measures. “Front-end” protection measures are referred to those whose major functions are to retard or to inhibit the ingress of chloride ions. “Back-end” protection measures are referred to those whose primary functions are to lessen or to inhibit the corrosion tendency/corrosion rate of steel once chloride reaches steel surface. In each group, subdivisions based on timing of application can be made depending on whether the protection measures are incorporated into the concrete member after construction or during construction (in ingredients or construction process). Practically, protection measures, which can be incorporated during the construction process, are often preferred. It is not possible to generally discuss the whole range of additional protection measures for marine concretes within the limit of this paper. Hence only the “major and popular” protection measures are briefly discussed. They include the following: “Front-end” protection measures: • Controlled permeability formwork; • Concrete coatings/surface treatment; “Back-end” protection measures: • “corrosion resisting” reinforcement; • coated/protected reinforcement; • corrosion inhibitors; • cathodic protection/prevention These are the major items listed in the BRE Digest 444 part 1 10 as the factors significantly affect the achievement of corrosion protection and concrete durability. Controlled permeability formwork (CPF) The quality of concrete is rarely uniform. Unfortunately, the worst region is that near the surface of the concrete, ie the concrete cover over reinforcement. This is due the

vulnerability of the cover to curing, compaction, trapped air and excess water (driven to the form face by internal pore pressure). Controlled permeability formwork is essentially normal formwork lined with a custom design polypropylene fabric made to tight specified maximum pore size (eg. 0.07 mm). Under compaction, the network of fine pores allows the trapped air and excess water to pass away while retaining the fine in the mix. The use of CPF leads to decrease of the near surface water-to-cement ratio. This results in denser pore structure at and near the surface. The depth of concrete favourably influenced by CPF appears to be in the range of 5 to 20 mm depending on position and orientation of the formwork as well as the concrete grade 11,12,13. The effect of CPF on concrete resistance to chloride ion penetration is shown in Figure 4. Extraordinary claims of extension of service life of marine concrete structures have been made in commercial brochures based on reduction in chloride diffusion coefficients observed with short-term data (eg. Figure 4). For short-term exposure, the surface characteristics of the concrete will be the dominating feature as expected. In the context of service life, which is long-term exposure, the improved performance of concrete cast with CPF is not anticipated to be as drastic as portrayed. Nevertheless, some improvement of long-term concrete’s resistance to chloride ion penetration is expected with CPF. This can indeed provide a cost-effective insurance measure for a well specified and well made concrete for the intended design life. It should be noted that in situations where multiple use of CPF is considered for cost saving purpose, the gradual reduction in effectiveness should be evaluated to establish the optimal re-use number.

Figure 4: Influence of CPF on short-term chloride penetration (28 d ponding) into concrete –5 to 20 mm depth 13

Concrete surface treatment/coatings Coatings and surface treatments of concrete against the ingress of chloride ion are effective means of achieving the design life and controlling concrete degradation. It would not be a surprise to most practicing engineers to state that most of the commercial systems/materials of concrete surface treatments or coatings work in one way or another. The key issues are how well, for how long, under what conditions, can it be applied on site and how much. It is apparent that the protection strategy will influence the selection of the concrete surface treatment systems. With a strategy where additional protection measures are used as “insurance”, the permanence of the surface treatments/coating is important but not critical. With a strategy where additional protection measures are integral parts in durability risk evaluation, the surface treatments/coatings may require to last the design life of the structure. In selection of surface treatments/coatings for concrete durability, focus should be paid to field data/experience. It has been noted that despite showing large difference in laboratory evaluation, the differences in field performances of several organic coating systems are not obvious 14. Field data are invaluable in making economical choice of additional protection measures. Nevertheless, laboratory data are valid starting points for determining the effectiveness of a given system in achieving the objective of lessening chloride ingress and/or providing barrier to water. The effectiveness of a penetrating sealant (trimethoxy isobutyl silane) in reducing chloride ingress is shown in Figure 5. Comparative/laboratory evaluation of different surface treatments by a DC driven rapid chloride permeability test is shown in Figure 6.

Figure 5: Field evaluation of chloride penetration into untreated concrete (top) and silane treated concrete (right) 15

Figure 6: Comparative laboratory evaluation of different surface treatments by a

version of rapid chloride permeability test (Leeds Uni.) 16 Coated/protected reinforcement Two common types of providing protection to chloride attack on reinforcements are epoxy coated bars and zinc coated bars. Epoxy layer on reinforcement acts as a barrier between steel and chloride while the zinc layer acts as a sacrificial anode. There are also other less common or newer breeds of protected reinforcement including inorganic silicate based coated bars, nylon-/ vinyl-coated, stainless-/copper-/nickel-cladded bars and ceramic-coated bars 17,18. The general concern with coated/protected steel reinforcements is their cost effectiveness, especially in structures with long design life. For example, zinc would dissolve (slowly) in water with pH similar to that of concrete pore water ~ 13. With the requirement of long service life such as 100 years, it is not certain that the residual zinc layer would be sufficient to provide the expected protection to steel in presence of chloride ions. Available data 19 appear to suggest that zinc coated bars are more suitable as an insurance in the protection strategy. The bonding of zinc-coated bars and epoxy coated bars to concrete is also of concern. The particular issues of epoxy coated bars in marine environment include: • Lost of long term adhesion of epoxy to steel due to water penetrating the coating.

This can render the protection useless and creates different mode of corrosion in presence of chloride ion.

• Once corrosion is progressing under the epoxy layer, it is difficult (if not impossible) to detect.

• Rehabilitation technique such as cathodic protection is highly improbable. In conjunction bad experience with epoxy coated bars 20,21, excellent experience has also been reported 22.

Control

Sodium Silicate

2 coats Silane/acrylic blend

Silane

Acrylic solvent based

2 coats Silane/methacrylate

2 coats modified epoxy

Corrosion resisting reinforcement Stainless steel (eg. type 304 and 316) bar is undoubtedly more resistance than normal black steel bar against chloride induced corrosion of reinforcement 23. The selection of stainless steel reinforcement or normal steel reinforcement depends on cost and protection strategy. Stainless steel reinforcement is well suited to the second strategy. It is wasteful to consider stainless steel reinforcement as an insurance type protection. Stainless steel reinforcement can be used in conjunction with black steel reinforcement. Practically there has been no reported problem of galvanic corrosion due to stainless steel/black steel connection in chloride-free concrete (high resistivity with relatively small potential difference). In the presence of chloride, this can be a serious issue. Therefore, the specifier must ensure that in mixed usage mode, the black steel is situated beyond the estimated zone of chloride affected area in concrete during the design life. Exotic non-metallic reinforcements are alternative solution to solving the problem of managing chloride attack. These include glass, carbon or aramid set in a suitable resin to form a rod or a grid. Currently the use of these non-metallic reinforcements are at the experimental/development stage. Cost and lack of long term structural performance (eg. creep issue) have limited their wider trials in real engineering scale. Corrosion inhibitors Corrosion inhibitors are not new to concrete technologists. They are used as admixtures, ie. added to fresh concrete. Both inorganic and organic corrosion inhibitors are available to combat chloride attack. The major inorganic corrosion inhibitor is based on calcium nitrite. There is considerable experience with the use of calcium nitrite based corrosion inhibitor in concrete. Calcium nitrite is an anodic corrosion inhibitor. The effectiveness of calcium nitrite in reducing the chloride-induced corrosion of steel reinforcement is shown in Figure 7.

Figure 7: Influence of a calcium nitrite based corrosion inhibitor on steel corrosion 24

In order to calcium nitrite to work, sufficient amount must be present in the concrete pore solution at the time when chloride is available at steel surface. This roughly means that if the specifier wish to extent the service life of a reinforced concrete structure beyond 50 years, for example, calcium nitrite must be in the pore solution at that time. Unfortunately, leaching of inhibitor is a known phenomenon 25. For this reason, the dosage of calcium nitrite should be work out with allowance for loss due to leaching in conjunction with the estimated chloride pattern at the steel surface. In another word, the dosage of the inhibitor should be based on the relevant concrete and cover. Recommended dosage is not a efficient or effective means of using inhibitor. The specifier should note further that calcium nitrite is a set accelerator. This should be guarded against in hot weather or situations where the delivery of concrete is delayed. Care should also be paid to drying shrinkage of concrete containing calcium nitrite based corrosion inhibitor. Organic corrosion inhibitors are relatively new. They are mostly based on amine or alkanolamine compounds. It has been noted that organic corrosion inhibitors can be very effective (in comparison to calcium nitrite) in controlling chloride induced corrosion of steel reinforcement 26. The working mechanism of amine based corrosion inhibitor is very different to that of calcium nitrite. No adverse influence to either fresh or hardened concrete properties has been reported 27. Cathodic prevention and protection Cathodic protection is a well-known protection measure with proven efficiency. Criteria and other considerations for application of cathodic protection to concrete structures can be found in the literature 28. Cathodic prevention is a term invented to describe a similar process applied to new structures. The term cathodic protection is reserved for rehabilitation of structures already containing chloride or suffering from steel corrosion. Due to different electrochemical states of steel surface, the operating criteria and parameters of cathodic prevention are quite different to those used in cathodic protection 29. For example, the current density is mush lower and the potential range is much wider than those commonly used in cathodic protection. The reinforcement is polarised throughout the design life of the structure. In principle, cathodic prevention can be the most effective protection measure for marine concrete structures. However, (as in the case of cathodic protection) while maintenance cost can be low, installation of this measure needs high capital investment. The high capital cost may be “justifiable” for new structures of high importance (disruption in service can result in high economic loss) or repair and maintenance is difficult or impossible due to limited access. In most situations, its use in new marine concrete structure may not be adequately justified based on cost comparison to other protection measures. Two features of cathodic protection, which can be usefully adopted in formulating protection measures for marine concrete, are continuous monitoring of steel condition and preparation of reinforcement for electrical continuity. These are simple procedures involving embedding reference electrode (or corrosion sensor or even some indirect means such as embedded studs for resistivity measurement) in concrete at areas of high corrosion risk and spot welding of reinforcement. If implemented, the

cost of cathodic protection will be much less and it will likely to be provided at appropriate time. Continuous/regular monitoring of the corrosion condition of the reinforcement can provide critical inputs into the decision making process of budgeting for asset management. Concluding remarks Design for service life of marine concrete is a difficult task to be achieved in a cost efficient manner. A clear and structured strategy is needed to manage the risk involved. For a given set of requirements, configurations and constraints, there are usual more than one solution. The common goal is lowest cost. Appropriate scientific/technical data and field experience are needed to formulate the most effective strategy. In this paper, only aspects related to protection of marine concrete structures are discussed in abbreviated and general fashion. Achieving design life or durability of marine concrete structures should not be considered, as a stand-alone item to be performed in isolated manner after structural drawing is prepared. Life cycle costing can be optimised with durability considered from onset of the project with inputs from all principal parties including the owner, the user and the specifiers. From the specifier’s point of view, ensuring design life of a marine structure involves a number of related steps (not subsequent). Two mentioned in this conference are specification of concrete cover and quality and formulation of protection measures. These two steps must be interlinked for cost effectiveness. In fact, the other steps such as construction compliance, acceptance, inspection and maintenance strategy should also be linked to each other and the previously mentioned steps. Cost optimal design for service life definitely not a top down, one direction process. As a final remark, the authors wish to point out that while the principles of design for durability may be universally applicable, strategies for corrosion protection are localised. References 1. Cao, H.T., Moorehead, D. and Potter, R.J., ‘Predicting of Service Life of

Reinforced Concrete Structures in Marine Environment and AS 3600’, Concrete Institute of Australia 19th Biennial Conference Proceedings, Sydney, May 1999, pp 131-137.

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