Critical chloride content in reinforced concrete — A review
Introduction
After it was recognised in the second half of the last century that chloride may induce steel corrosion in reinforced concrete structures, great research efforts have been made in this regard: over the last fifty years, a considerable amount of papers has been published presenting values for critical chloride content (Ccrit) in reinforced concrete [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53]. Considering marine exposure conditions and the extensive use of de-icing salts in many countries, chloride induced corrosion is one of the most common causes of degradation of reinforced concrete structures. Both for the design of new structures and for condition assessment of existing structures, knowledge of reliable Ccrit values is important as the remaining service life is often considered as the time required to reach the chloride threshold value at the depth of the reinforcement. In probabilistic service life modelling, Ccrit has been identified to be one of the most decisive input parameters [54], [55].
Despite the multitude of studies undertaken, many aspects of chloride induced reinforcement corrosion in concrete are still incompletely understood and no general agreement on a Ccrit value has been achieved. Results reported in the literature scatter over a large range [56], [57], [58]. This is not only the result of different definitions, measuring techniques and testing conditions, but also owing to the stochastic nature and complexity of initiation of pitting corrosion. Thus, often conservative values are nowadays used as critical chloride content: In European countries as well as in North America it has become common practice to limit the tolerable chloride content to or around 0.4% by weight of cement [59]. In probabilistic modelling the critical chloride content is a stochastic variable as e.g. in the fib model code for service life design[60], where Ccrit is defined by a beta-distribution with a lower boundary of 0.2% chloride by weight of cement and a mean value of 0.6% by weight of cement. Although there is a strong need for reliable Ccrit values, an accepted or standardised test method to measure critical chloride does at present not exist.
The present review summarises the state of the art regarding critical chloride content in reinforced concrete. It is not only aimed at collecting Ccrit values reported in the literature, but also all the relevant details about experimental procedures are collated. The data is analysed with regard to factors that have an influence on Ccrit, thereby focussing on experimental setups and measurement techniques. By highlighting advantages and drawbacks of experimental parameters, a basis for developing a test setup will be provided. In addition, the literature evaluation will reveal certain aspects of chloride induced reinforcement corrosion in concrete, that are currently not well understood and where researchers working in this field need to focus on.
Only carbon steel is considered, although a limited number of publications on corrosion resistant reinforcement in connection with critical chloride content can be found in the literature, e.g. [19], [61], [62], [63]. Minor parts of this review were presented in [64].
Section snippets
Definitions
Reinforcement corrosion in non-carbonated, alkaline concrete can only start once the chloride content at the steel surface has reached a certain threshold value [65]. In the literature, this value is often referred to as critical chloride content or chloride threshold value. In the present work, both terms — or simply the abbreviation Ccrit — are used.
Two different ways of defining Ccrit are common [22], [66]: From a scientific point of view, the critical chloride content can be defined as the
Critical chloride contents in the literature
Numerous publications in connection with Ccrit can be found in the literature. Occasionally, Ccrit values have also been reported based on calculations and results from others: For example in Refs. [17], [28], the authors determined the pH after pore liquid expression, and used Cl−/OH− ratios according to Hausmann [6] and Gouda [8] to estimate the tolerable chloride concentration in the pore solution; a method to detect depassivation, however, was not included in the experiments and thus the
Influencing parameters
From an electrochemical point of view, it is the potential of the steel, Ecorr, relative to the pitting potential, Epit, that determines whether corrosion will start or not. The pitting potential depends on both environmental influences (chloride content) and on properties of the metal such as the degree of alloying (e.g. stainless steel). The open circuit potential of the passive steel, on the other hand, only depends on the environment (pH and oxygen content). Whereas parts of the steel
General considerations regarding the determination of Ccrit values
Apart from a suggestion for a method to determine Ccrit in the field [107], an accepted testing procedure does at present not exist. Experimental setups are thus developed individually. To measure Ccrit values in the laboratory or on a real structure, such a setup has to include the following [64]:
- 1.
A steel electrode embedded in a cement based material (cement paste, mortar, concrete) or immersed in a solution that simulates the concrete.
- 2.
Chloride ions at the steel surface.
- 3.
A method to detect
Evaluation with regard to chloride binding
In the literature review in Ref. [70] Glass and Buenfeld evaluated data by Lambert et al. [25] and pointed out that Cl−/OH− threshold ratios span a larger range (from 3 to 20) in comparison with corresponding total chloride contents (from 1.5 to 2.5%). More publications are now available reporting Ccrit values in terms of total and free chloride or Cl−/OH− ratios and they confirm this finding without exception as illustrated in Fig. 8. For example, Pettersson [26] reported total chloride
Conclusions
From the present literature review, the following major conclusions are drawn:
- 1.
The critical chloride content in reinforced concrete can and has been studied by many different experimental setups. When evaluating the entirety of reported results in the literature, it appears that certain parameters inherent to the test procedure (such as the application of an electric field to accelerate chloride ingress or potentiostatic control of the rebar) can have a more dominant influence on the result than
Acknowledgements
The authors acknowledge the support of COIN (www.sintef.no/coin).
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