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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Critical chloride content in reinforced concrete State of the art

U. Angst & . VenneslandFaculty of Engineering Science and Technology, Department of Structural Engineering,Norwegian University of Science and Technology, Trondheim, Norway

ABSTRACT: In the present work, a large number of publications in connection with critical chloride content in reinforced concrete has been evaluated. The reported results scatter over more than two orders of magnitude when expressed as total chloride content by cement weight and over three orders of magnitude when expressed as Cl/OH ratio. This large scatter is partly the result of different definitions and experimental techniques, but also due to many factors affecting critical chloride content. Dominating influences have been identified as fol-lows: 1) The steel-concrete interface, 2) the pH value of the pore solution and 3) the electrochemical potential of the steel. Experimental investigation of the issue of critical chloride content is possible in a wide variety of procedures. At present, there exists no generally accepted or standardized procedure for the determination of the critical chloride content.

1 INTRODUCTION

It is well recognized that the presence of chloride in reinforced concrete can lead to corrosion of the reinforcement by destroying the passive layer on the steel surface (Page & Treadaway 1982). While mod-ern standards impose restrictions on the amount of chloride that may be introduced to the fresh concrete mix, penetration of chloride into hardened concrete is nowadays the major cause for pitting corrosion in concrete structures.

The concept of critical chloride content in rein-forced concrete is based on the general agreement that corrosion in non-carbonated, alkaline concrete can only start once the chloride content at the steel surface has reached a certain threshold value. In lit-erature, this value is normally referred to as critical chloride content or chloride threshold value. The knowledge of such values is of importance for serv-ice life design or service life predictions when pitting corrosion is the likely failure mechanism. In service life modeling, chloride threshold values are required as input parameters. Whereas more and more, sophis-ticated mathematical models are developed, there is still a lack of reliable chloride threshold values and consequently often conservative values are used (Schiessl & Lay 2005).

A lot of studies have been undertaken in order to find chloride threshold values in cement based materials and the reported results scatter over several orders of magnitude. This large span of results might be due to several reasons: First, different definitions

for critical chloride content have been used; second, various techniques to find critical chloride contents have been applied by different researchers and, last but not least, critical chloride content is a complex matter that depends on a variety of influencing fac-tors. Major parameters are the pH of the pore solu-tion, the electrochemical potential of the steel and the quality of the steel-concrete interface (COST 521). The inhibiting effect of high pH levels was recog-nized early (Hausmann 1967, Gouda 1970). Since the presence of chloride ions at the steel surface modifies the anodic polarization curve, primarily by shifting the pitting potential to more negative values, it is evi-dent that the potential of the steel is of importance with regard to pitting corrosion: Higher chloride contents can be tolerated for steel with more nega-tive potentials. This was confirmed experimentally: The chloride threshold was found to be independent of the potential for values higher than 200 mV vs. SCE, whereas below that value the chloride thresh-old increased with decreasing potential (Alonso et al. 2002). With regard to the steel-concrete interface, it is a dense and lime-rich layer of hydration products that protects the steel by acting as a physical barrier and buffering the pH in the pore solution (Page 1975). Lower chloride threshold values have been reported when the formation of this layer on the steel surface was restricted (Yonezawa et al. 1988). It has to be noted that the condition of the interface cannot be quantified.

Given the numerous investigations undertaken on critical chloride content in reinforced concrete,

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literature reviews were published towards the end of the 1990s (Glass & Buenfeld 1997a, b, Breit 1998b, Alonso et al. 2000). In the present article, these reviews are brought up to date by includ-ing newer publications. The available literature is evaluated with regard to definitions, techniques and results.

2 DEFINITIONS

2.1 Critical chloride content

The critical chloride content can be defined in two ways (Schiessl & Lay 2005): From a scientific point of view, critical chloride content is usually defined as the chloride content required for depassivation of the steel (definition 1). However, it has to be kept in mind that depassivation may not necessarily lead to deterio-ration, e.g. in a dry concrete the corrosion rate is kept low due to ohmic control of the current flow. Thus, from a practical, engineering point of view, critical chloride content is often defined as the chloride con-tent associated with visible or acceptable deteriora-tion of the structure (definition 2). It has to be noted that the term acceptable deterioration is somewhat imprecise and confusing.

Figure 1 illustrates these two different defini-tions by combining Tuuttis corrosion model (Tuutti 1982) with an assumed constant chloride ingress leading to a linear increase in chloride concentra-tion at the steel reinforcement. The figure clearly

shows that different chloride threshold values are obtained by using different definitions. Definition 2 leads to higher values, which is the result of a longer time passing until the chloride content is determined.

2.2 Expression of critical chloride content

The critical chloride content is most commonly expressed as total chloride content relative to cement weight. The main reason for this is the fact that the determination of total chloride content is relatively simple and well documented in standards. Since the binder content is not always known, it is sometimes preferred to relate the total chloride content to the weight of concrete.

Since only the free chlorides are generally con-sidered to be of importance for corrosion initiation, the critical chloride content is sometimes expressed by use of free chloride contents, either related to the binder or concrete weight or as a concentration in mol/l in the pore solution.

The form of relating the chloride concentration to the hydroxide ion concentration, i.e. expressing critical chloride content as Cl/OH ratio, is based on work conducted in solutions (Hausmann 1967, Gouda 1970). It was however noted that the Cl/OH ratio depends on the pH and increases with higher pH, i.e. the inhibiting effect of OH is stronger at higher pH levels (Li & Sags 2001). This form was considered as the most accurate to express critical chloride contents, but Glass and Buenfeld argue that this is not supported by analysis of available data in literature (Glass & Buenfeld 1997b). They suggested that presenting critical chloride contents is best done in the form of total chloride by weight of binder.

The various forms to express critical chloride contents reflect both the destructive species and the inhibitive properties of the concrete in differ-ent ways. Table 1 sums up the available expression forms.

Figure 1. Different definitions for critical chloride content based on Tuuttis model.

Table 1. Different forms to express critical chloride content.

Aggressive species Inhibitive property Expressed as

Total chloride by binder weight % by weightby concrete weight % by weight

Free chloride by binder weight % by weightby concrete weight % by weight

Free Cl mol/l

concentration by OH concentration Cl/OH

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3 EXPERIMENTAL APPROACH TO CRITICAL CHLORIDE CONTENT IN CONCRETE

3.1 Test setups

In order to experimentally investigate the issue of critical chloride contents in reinforced concrete the experimental setup has to include the following:

1. A steel electrode of interest embedded in a cement based material (cement paste, mortar, concrete) or immersed in a solution that simulates the concrete (synthetic pore solution).

2. Chloride ions present at the steel surface, either at a constant concentration or increasing over time.

3. Detection of the depassivation of the steel elec-trode (or acceptable degree of corrosion when sticking to definition 2).

4. Quantification of the corresponding chloride con-tent at the steel surface.

To achieve a setup that fulfils these four require-ments, a lot of possibilities exist. A variety of options are available only for selecting a steel electrode: Dif-ferent steel types exist (normal carbon steel, stainless steel, galvanized steel, etc) in different shapes such as smooth or ribbed bars and they can be prepared in several ways, e.g. polished, sandblasted, pre-rusted, etc. Also with regard to the chlorides, several possi-bilities are available: The type of chloride salt (NaCl, CaCl

2, etc) and a method by which the chloride is

introduced at the steel surface must be selected. In case of cement paste, mortar or concrete, chloride might be added directly to the mix or introduced later into the hardened material by capillary suction and/or diffusion or accelerated by migration; in solution experiments, the dynamic increase of chloride con-centration with time is easily done by adding chlo-ride to the solution. Also for point 3, the detection of depassivation or acceptable degree of corrosion, several techniques are available. Last but not least,

Figure 2. Experimental possibilities to investigate critical chloride content in reinforced concrete.

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the chloride content associated with corrosion can be determined and expressed in different ways. An over-view of these experimental possibilities is depicted in Figure 2. Please note that in the present work, as also in most of the evaluated literature, it is only focused on normal carbon steel.

Not surprisingly, the multiplicity of possible test setups leads to poor comparability of the results. This has to be kept in mind when evaluating reported criti-cal chloride contents.

3.2 Methods to determine the chloride content

3.2.1 Total chloride contentUsually cores are drilled from hardened samples, cut in slices of a certain thickness, crushed and powdered and subsequently analyzed in order to obtain a chlo-ride profile. Normally, the acid soluble chloride con-tent is determined; it is also possible to determine total chloride contents in powder samples by X-ray fluorescence spectrometry.

3.2.2 Free chloride contentThe determination of the free chloride content is rather easy in solutions, but becomes more compli-cated when dealing with paste, mortar or concrete. A few techniques are known to determine the amount of chloride freely available in the pore solution.

The most accepted method is the pore solution expression technique: A solid sample is subjected to pressure and the expressed pore solution is collected and analyzed. The method is regarded as accurate and good reproducibility was reported (Tritthart 1989). However, the application is limited in case of lower w/c ratios, coarse aggregate particles or dry speci-mens. In addition, it has to be kept in mind that this technique results in average values of the sample volume under investigation; in case of concentration gradients, no local values can be determined. It was also noted that under the pressure, part of the bound chloride is released and increases the chloride content in the pore solution (Glass et al. 1996).

Leaching techniques are based on mixing crushed or ground samples with a solvent and measuring the amount of chloride passing into solution. A variety of methods have been investigated using different solvents such as water, methanol or ethyl alcohol and differ-ent procedures with regard to leaching time and tem-perature (Arya et al. 1987). The amount of extracted chloride was shown to depend strongly on the selected procedure. In a later study, it was found that also other parameters such as cement type or source of chloride affect the results (Arya 1990). It was concluded that leaching techniques are not practical for determining the free chloride content. Castellote et al. suggested a leaching technique based on an alkaline solvent to

extract the free chloride (Castellote et al. 2001). Good accuracy was reported in comparison with pore solu-tion expression; however, it has to be noted that rather high chloride concentration levels were investigated (>2% total chloride by cement weight).

Another technique to determine the free chloride content is the use of silver/silver chloride electrodes embedded in the matrix (Molina 1993, Elsener et al. 2003). The potential of these sensors follows Nernsts law and depends on the activity of the chloride ions in the pore solution. With these sensors it is thus possible to measure the free chloride content non-destructively.

4 LITERATURE EVALUATION

In the present article, nearly 40 references reporting critical chloride contents from laboratory or field studies have been evaluated. Not all the available data can be presented here due to limited space.

Table 2. Reported chloride threshold values in total chloride content per weight of binder.

Reference Cl Cement typeReported value

Richartz 1969 A OPC 0.4Gouda et al.

1970A OPC, GGBS 1.03.0

Stratfull et al. 1975

D various 0.21.4

Locke et al. 1980 A OPC 0.40.8Elsener et al.

1986A OPC 0.250.5

Hope et al. 1987 A OOPC 0.10.19Hansson et al.

1990D various cements 0.41.37

Schiessl et al. 1990

A/D various cements 0.52.0

Lambert et al. 1991

A/D OPC, SRPC 1.52.5*

Thomas 1996 D OPC, FA 0.20.7Alonso et al.

2000A OPC 1.243.08*

Alonso et al. 2002

D various cements 0.73

Castellote et al. 2002

D/M SRPC 0.150.23

Trejo et al. 2003 M OPC 0.020.24Manera et al.

2008A OPC, SF 0.62.0

minmax 0.023.08

A = chloride added to the mix; D/M = chloride introduced into hardened samples by diffusion/capillary suction (D) or migration (M); *Steel potential below 200 mV vs. SCE.

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4.1 Publications reporting total chloride contents

Table 2 shows a selection of published chloride thresh-old values reported in the form of total chloride con-tent per weight of binder. Generally, a large scatter was found with results from 0.02 to 3.08% chloride by binder weight (over two orders of magnitude). Details about the experimental procedures (according to Fig-ure 2) were evaluated but no systematic trends with regard to characteristics such as cement type, rebar type (smooth, ribbed) or chloride introduction method were identified from the totality of the data. Obviously, the effect of a single parameter on the critical chloride content is not pronounced enough to be globally appar-ent. The only factor that appears to have an overall effect is the electrochemical potential of the steel: In many studies, this was higher than 200 mV vs. SCE; in some publications, no potentials were measured or reported, but in most cases the described exposure conditions indicate that the potentials presumably were in the same range. Only in two references (Lambert et al. 1991, Alonso et al. 2000) steel potentials below 200 mV vs. SCE were reported. The corresponding critical chloride contents are on a clearly higher level than in the majority of the other publications.

4.2 Publications reporting Cl/OH ratios

Figure 3 shows a selection of published chloride threshold values expressed in the form of Cl/OH ratios. Also this data presents a high overall scatter, ranging from 0.03 to 45 and thus over three orders of

magnitude. It is evident from Figure 3 that the values obtained from studies dealing with mortar or con-crete samples scatter much more compared to those from experiments performed in solutions. In addi-tion, remarkably higher chloride threshold values were reported when investigating mortar or concrete specimens in comparison with solution experiments. This might be explained by the inhibiting effects of the interface of steel embedded in a cement matrix due to formation of a Portlandite layer at the steel surface (Page 1975). This is also apparent from the data by Yonezawa et al. (1988), where the formation of this layer at the interface was hindered intentionally. Most authors have not measured or reported the pH and thus it is not possible to see an overall effect of the alkalin-ity on critical chloride content. However, the results of two studies (Breit 1998a, Li & Sags 2001) show that the Cl/OH threshold ratio increases with increasing pH of the pore solution, i.e. it is no constant value.

4.3 Experimental procedures

In Figure 2, the components of possible test setups are schematically depicted; the numbers given under each box represent the percentage of studies in which the cor-responding component was used. For example, in 31% of the evaluated references chlorides were admixed initially, whereas in 69% the chlorides were introduced later. In only 12% of the studies, steel bars were inves-tigated as-received; in the other studies they were prepared (sandblasted, polished, etc) or no details were

Figure 3. Reported chloride threshold values expressed as Cl/OH ratios. As the values span a large range, they are divided into two separate plots with different scaling on the ordinate.

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given. In only 25% of the studies the use of ribbed steel was reported. In practice, chloride is usually not present at the steel surface initially, but penetrates into the hardened concrete during service life. Also the use of prepared and smooth steel does not give realistic con-ditions with regard to the steel-concrete interface. The results of many studies are thus not practice-related.

5 CONCLUSIONS

From this literature review on critical chloride con-tent, the following conclusions can be drawn:

The results available in literature scatter in a wide range. The reported values span from 0.02 to 3.08% total chloride by binder weight and thus over two orders of magnitude. Published Cl/OH threshold ratios even range from 0.03 to 45, which is over three orders of magnitude.

No unique chloride threshold value exists. It depends on various factors which are intercon-nected and variable with time. Major parameters have been identified to be 1) the quality of the steel-concrete interface, 2) the pH of the pore solu-tion and 3) the electrochemical potential of the steel. The quality of the steel-concrete interface depends on compaction (voids) and on the pres-ence of a lime rich layer of hydration products; however, it cannot be quantified. Other influenc-ing factors are the moisture content in the concrete, type of cement, w/c ratio, temperature, etc.

Experimental investigation of critical chloride con-tents can be performed in a wide variety of pos-sible test setups. The multiplicity of parameters includes rebar type (smooth or ribbed), steel sur-face condition (polished, sandblasted, etc), matrix (cement paste, concrete, solutions, etc), chloride introduction techniques (mixed-in, diffusion, etc), depassivation detection techniques, etc.

The numerous experimental possibilities lead to poor comparability of the reported results. At present, no accepted or standardized procedure for the determi-nation of the critical chloride content exists.

Many used test setups are not practice-related. Although the steel-concrete interface is recognized as a major influencing factor, smooth and/or pre-pared rebars (polished, sandblasted, etc) have been used in many studies. Also, often chlorides have been mixed-in.

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