Chloride-induced corrosion of steel in concrete -- insights from bimodal neutron and X-ray microtomography combined with ex-situ microscopy

The steel-concrete interface (SCI) is known to play a major role in corrosion of steel in concrete, but a fundamental understanding is still lacking. One reason is that concrete's opacity complicates the study of internal processes. Here, we report on the application of bimodal X-ray and neutron microtomography as in-situ imaging techniques to elucidate the mechanism of steel corrosion in concrete. The study demonstrates that the segmentation of the specimen components of relevance - steel, cementitious matrix, aggregates, voids, corrosion products - obtained through bimodal X-ray and neutron imaging is more reliable than that based on the results of each of the two techniques separately. Further, we suggest the combination of tomographic in-situ imaging with ex-situ SEM analysis of targeted sections, selected on the basis of the segmented tomograms. These in-situ and ex-situ characterization techniques were applied to study localized corrosion in a very early stage, on reinforced concrete cores retrieved from a concrete bridge. A number of interesting observations were made. First, the acquired images revealed the formation of several corrosion sites close to each other. Second, the morphology of the corrosion pits was relatively shallow. Finally, only about half of the total 31 corrosion initiation spots were in close proximity to interfacial macroscopic air voids, and above 90 percent of the more than 160 interfacial macroscopic air voids were free from corrosion. The findings have implications for the mechanistic understanding of corrosion of steel in concrete and suggest that multimodal in-situ imaging is a valuable technique for further related studies

Service life cost of selected design and repair strategies for concrete structures in chloride exposure: Particular consideration of 12% chromium steel

Calculation of service life costs allows for a holistic assessment of the cost-effectiveness of options in design regarding materials and intervention strategies for an infrastructure object. A method is presented in which probabilistic service life modeling and cost calculations are combined. This method is exemplified by two cases of reinforced concrete structures exposed to a chloride-bearing environment. Two different types of reinforcing steel and several intervention strategies are analyzed. The results show that the method is suitable for decision-making on the most economical solution. The solution with the lowest initial construction costs does not necessarily result into the lowest service life costs, depending on chloride exposure conditions and the anticipated interventions.ISSN:1464-4177ISSN:1751-764

Modelling life cycle costs for edge beams of three Norwegian bridges

For the planning of cost-effective infrastructure maintenance, it is important to assess the condition of structures and to estimate the optimum time, the extent and the type of repair needed to continue operating the structures safely. In this study, a novel approach is presented, which combines data from condition assessment of reinforced concrete structures (e.g. chloride profiles), predictive modelling (future chloride ingress) and experimentally determined chloride threshold values (Ccrit) on cores retrieved from the structures with a method recently developed at ETH Zurich, Switzerland. This approach is applied to three bridges in Norway.ISSN:1893-116

Critical chloride content in concrete: realistic determination and influence of air voids

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

Towards understanding corrosion initiation in concrete – influence of local concrete properties in the steel-concrete interfacial zone

Chloride-induced corrosion is the most common deterioration process for reinforced infrastructure objects. Improving the understanding of the conditions for initiation of localized corrosion is urgently needed. Research is focused on the influence of “defects” at the steel-concrete interface (SCI), as these weak points might be responsible for corrosion initiation. In contrast to numerous studies with “lab concrete”, this study reports results from reinforced concrete cores drilled from old infrastructure objects containing a non-corroding rebar. In contrast to laboratory studies, this guarantees real conditions at the SCI comprising also irregularities such as air voids, plastic settlement voids, cracks, etc. This allows to study chloride-induced corrosion in real conditions and to determine the so-called “critical chloride content” Ccrit. Visual inspection of the SCI enables to establish (or not) influences of the local conditions at the SCI and Ccrit. It was found that Ccrit strongly decreased with the carbonation depth, even if the carbonation front had not reached the steel. Moreover, coarse air voids and cracks were in this study not particularly susceptible sites for corrosion initiation

Towards understanding corrosion initiation in concrete – influence of local concrete properties in the steel-concrete interfacial zone

Chloride-induced corrosion is the most common deterioration process for reinforced infrastructure objects. Improving the understanding of the conditions for initiation of localized corrosion is urgently needed. Research is focused on the influence of “defects” at the steel-concrete interface (SCI), as these weak points might be responsible for corrosion initiation. In contrast to numerous studies with “lab concrete”, this study reports results from reinforced concrete cores drilled from old infrastructure objects containing a non-corroding rebar. In contrast to laboratory studies, this guarantees real conditions at the SCI comprising also irregularities such as air voids, plastic settlement voids, cracks, etc. This allows to study chloride-induced corrosion in real conditions and to determine the so-called “critical chloride content” Ccrit. Visual inspection of the SCI enables to establish (or not) influences of the local conditions at the SCI and Ccrit. It was found that Ccrit strongly decreased with the carbonation depth, even if the carbonation front had not reached the steel. Moreover, coarse air voids and cracks were in this study not particularly susceptible sites for corrosion initiation

A systematic data collection on chloride-induced steel corrosion in concrete to improve service life modelling and towards understanding corrosion initiation

While the critical chloride content for corrosion initiation in concrete, Ccrit, is a vital parameter for (probabilistic) service life modelling, little information is available on its statistical distribution. To close this gap, we present a comprehensive data collection of Ccrit measured on samples retrieved from various engineering structures. Additional detailed information about the structures (steel type, concrete properties, age, etc.) is systematically documented. This provides guidance to engineers performing service life modelling to select the statistical distribution of Ccrit as model input. Finally, the systematic documentation will allow identifying the dominant parameters influencing chloride-induced corrosion initiation in concrete

Investigations of accelerated methods for determination of chloride threshold values for reinforcement corrosion in concrete

Testing the critical chloride content (Ccrit) in laboratory samples or in samples retrieved from structures enhances the understanding of rebar corrosion initiation in concrete and can result in a significant improvement of predicting the residual service life of ageing structures. A possible drawback of test methods for Ccrit is that they can be time-consuming. In this work, different accelerated methods for Ccrit testing are studied and compared. It is found that the acceleration method influences the test results. The results are discussed with respect to evaluation of existing structures and service life modelling.ISSN:2378-9697ISSN:2378-968