Chloride-induced corrosion is the main cause for the degradation of reinforced concrete structures, in particular structures of the road infrastructure, such as bridges and tunnels. An essential parameter in the condition assessment of existing structures exposed to chlorides is the chloride content in the concrete at the level of the reinforcing steel. To assess the risk of corrosion, this chloride content is generally compared to the so-called critical chloride content Ccrit. It is common practice to rely on a fixed, universal Ccrit as given in codes or standards. In Switzerland, as in many other countries, this Ccrit is around the value 0.4% chloride by mass of cement. It is known, however, that this value is generally not applicable to different structures and different conditions.
For the determination of Ccrit for a specific structure, a test method was recently developed in the ASTRA project AGB 2012/010. This method is here referred to as “ETH- method”. The general idea of this method is to drill reinforced concrete cores from structures that are still not corroding and to subject these samples to a laboratory corrosion test to determine Ccrit. The main advantage of this approach is to ensure that the sample under test is real. This is important because it is virtually impossible to mimic realistic steel-concrete interfacial conditions in samples produced in the laboratory, which significantly impairs the applicability of the related test results to engineering conditions.
In this project, the ETH method was applied to 10 structures, from which 119 samples were retrieved. Additionally, the samples were characterized in detail with respect to various material properties. To study the influence of macroscopic concrete voids (air voids) at the steel-concrete interface, selected samples were studied by means of X-ray computed microtomography. Moreover, a series of reproducible laboratory samples was exposed to chlorides in two different exposure regimes (continuously immersed vs. cyclic wetting and drying) to study the influence of the exposure moisture conditions on the test results.
It was found that the ETH method to determine Ccrit is robust and capable of reliably delivering test results for reinforced concrete cores retrieved from engineering structures. The obtained Ccrit were highly structure-dependent, which means that Ccrit can strongly differ from one structure to another. It was also observed that the Ccrit in different structures may be both lower and higher than the commonly assumed value of value 0.4% chloride by mass of cement. This means that using this universal Ccrit value given in standards can lead to both an under- or overestimation of the corrosion risk in an actual structure. Additionally, this work confirmed that Ccrit can be variable within one structure and thus needs to be statistically described.
It was further found that macroscopic concrete voids in the steel-concrete interfacial are not necessarily weak spots favouring corrosion initiation. This observation may be explained by the fact that macroscopic voids were not water-filled under the here studied conditions. The exposure conditions during the laboratory test were found to have an effect on the test result: When the samples were continuously immersed in chloride solution, the average Ccrit was approx. 25% higher compared to samples exposed to wetting/drying (daily cycles of 6 h wet and 18 h dry).
Regarding the implementation of the ETH method in engineering practice, the following recommendations can be made. For condition assessment, there are opportunities to improve the quantification of the corrosion risk, namely by estimating the residual time until corrosion initiation for an inspected structure by means of chloride ingress modelling. For a reliable model prediction, it is, however, crucial to have reliable input data for the model, in particular Ccrit. The results of this study have shown that for such model calculations, the structure-specific determination of Ccrit is important. Thus, it is recommended to standardize a laboratory test method. The ETH method can serve as basis for such a standardized test method. Such an approach can deliver a quantitative basis for the maintenance planning, which is considered as an improvement with respect to the current practice.
The experimental findings indicate that further research to understand chloride-induced corrosion of steel in concrete is needed, particularly concerning the effect of properties related to the steel (metallurgy, surface condition, and rebar geometry) and the effect of macroscopic interfacial concrete voids and their moisture state
