39 research outputs found
Three-dimensional characterization of the steel-concrete interface by FIB-SEM nanotomography
While it is widely accepted that the steel-concrete interface (SCI) plays an
important role in governing the long-term durability of reinforced concrete
structures, understanding about the primary features of the SCI that influence
corrosion degradation mechanisms has remained elusive. This lack of knowledge
can be attributed, on the one hand, to the complex heterogeneous nature of the
SCI, and, on the other hand, the absence of experimental techniques suitable
for studying the relevant features of the SCI. Here, we use focused ion beam -
scanning electron microscopy (FIB-SEM) nanotomography to obtain high resolution
3D tomograms of the steel-concrete interfacial zone. Five tomograms, spanning
volumes ranging from 8,000 to 200,000 cubic micrometer, were acquired for
situations representative of both non-corroded and corroded SCIs. The achieved
voxel size falls within the range of 30-50 nm, thus providing a resolution
clearly surpassing the capabilities of computed X-ray tomography. This
resolution enables the 3D characterization of the microstructure at the
capillary scale, which is the scale at which relevant corrosion and related
mass transport processes occur. Thus, FIB-SEM nanotomography is capable of
yielding datasets of the SCI that serve as basis for the generation of digital
twins of the interfacial microstructure, thereby enabling future studies about
durability and corrosion of reinforced concrete at the pore scale
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
Transformation of 2-line ferrihydrite to goethite at alkaline pH
The transformation of 2-line ferrihydrite to goethite from supersaturated
solutions at alkaline pH >= 13.0 was studied using a combination of benchtop
and advanced synchrotron techniques such as X-ray diffraction,
thermogravimetric analysis and X-ray absorption spectroscopy. In comparison to
the transformation rates at acidic to mildly alkaline environments, the
half-life,t_1/2, of 2-line ferrihydrite reduces from several months at pH =
2.0, and approximately 15 days at pH = 10.0, to just under 5 hours at pH =
14.0. Calculated first order rate constants of transformation, k, increase
exponentially with respect to the pH and follow the progression log_10 k =
log_10 k_0 + a*pH^E3. Simultaneous monitoring of the aqueous Fe(III)
concentration via inductively coupled plasma optical emission spectroscopy
demonstrates that (i) goethite likely precipitates from solution and (ii) its
formation is rate-limited by the comparatively slow re-dissolution of 2-line
ferrihydrite. The analysis presented can be used to estimate the transformation
rate of naturally occurring 2-line ferrihydrite in aqueous electrolytes
characteristic to mine and radioactive waste tailings as well as the formation
of corrosion products in cementitious pore solutions
The Gdap1 knockout mouse mechanistically links redox control to Charcot-Marie-Tooth disease
Mutations in the mitochondrial fission factor GDAP1 are associated with severe peripheral neuropathies, but why the CNS remains unaffected is unclear. Using a Gdap1â/â mouse, Niemann et al. demonstrate that a CNS-expressed Gdap1 paralogue changes its subcellular localisation under oxidative stress conditions to also act as a mitochondrial fission facto
The steelâconcrete interface
Although the steelâconcrete interface (SCI) is widely recognized to influence the durability of reinforced concrete, a systematic overview and detailed documentation of the various aspects of the SCI are lacking. In this paper, we compiled a comprehensive list of possible local characteristics at the SCI and reviewed available information regarding their properties as well as their occurrence in engineering structures and in the laboratory. Given the complexity of the SCI, we suggested a systematic approach to describe it in terms of local characteristics and their physical and chemical properties. It was found that the SCI exhibits significant spatial inhomogeneity along and around as well as perpendicular to the reinforcing steel. The SCI can differ strongly between different engineering structures and also between different members within a structure; particular differences are expected between structures built before and after the 1970/1980s. A single SCI representing all on-site conditions does not exist. Additionally, SCIs in common laboratory-made specimens exhibit significant differences compared to engineering structures. Thus, results from laboratory studies and from practical experience should be applied to engineering structures with caution. Finally, recommendations for further research are made
Challenges and opportunities in corrosion of steel in concrete
This paper summarizes the grand societal, economic, technological, and educational challenges related to corrosion of steel in concrete, and presents the state-of-the-art of the most relevant issues in the field. The enormous financial impact of infrastructure corrosion seems to be inadequately balanced by educational and research activities. This presents a unique opportunity in many countries for maintaining or improving their competitiveness, given the major technological challenges can be solved.
The main technological challenges are 1) the ever-increasing need to cost-effectively maintain existing, ageing reinforced concrete structures, and 2) designing durable, thus sustainable new structures. The first challenge arises mainly in industrialized countries, where there is a need to abandon conservative, experience-based decision taking and instead move to innovative, knowledge-based strategies. The second challenge regards mainly emerging countries expanding their infrastructures and where thus a major beneficial environmental impact can still be made by providing long-lasting solutions. This means to be able to reliably predict the long-term corrosion performance of reinforced concrete structures in their actual environments, particularly for modern materials and in the absence of long-term experience.
During the 2nd half of the last century, civil engineers, materials scientists, and chemists have in many countries made considerable attempts towards understanding corrosion of steel in concrete, but many of the approaches got bogged down in empiricism. From reviewing the state-of-the-art one can conclude that transport modeling in concrete is relatively well-advanced, at least in comparison with understanding corrosion initiation and corrosion propagation, where many questions are still open. This presents a number of opportunities in scientific research and technological development that are discussed in this paper.ISSN:1359-5997ISSN:0025-5432ISSN:1871-687
The size effect in corrosion greatly influences the predicted life span of concrete infrastructures
Forecasting the life of concrete infrastructures in corrosive environments presents a long-standing and socially relevant challenge in science and engineering. Chloride-induced corrosion of reinforcing steel in concrete is the main cause for premature degradation of concrete infrastructures worldwide. Since the middle of the past century, this challenge has been tackled by using a conceptual approach relying on a threshold chloride concentration for corrosion initiation (Ccrit). All state-of-the-art models for forecasting chloride-induced steel corrosion in concrete are based on this concept. We present an experiment that shows that Ccrit depends strongly on the exposed steel surface area. The smaller the tested specimen is, the higher and the more variable Ccrit becomes. This size effect in the ability of reinforced concrete to withstand corrosion can be explained by the local conditions at the steel-concrete interface, which exhibit pronounced spatial variability. The size effect has major implications for the future use of the common concept of Ccrit. It questions the applicability of laboratory results to engineering structures and the reproducibility of typically small-scale laboratory testing. Finally, we show that the weakest link theory is suitable to transform Ccrit from small to large dimensions, which lays the basis for taking the size effect into account in the science and engineering of forecasting the durability of infrastructures.ISSN:2375-254
Novel sensor for non-destructive durability monitoring in reinforced concrete
Reinforced concrete is generally a durable material; however, it is also subjected to various degradation mechanisms, amongst them, corrosion of the reinforcing steel is the main one. In addition to loss in safety and serviceability, reinforcement corrosion causes high economical losses in all industrialised countries. Furthermore, costs associated with corrosion of the reinforcing steel are expected to significantly increase in the next years; this is because most of concrete structures have been long exposed to ingress of chlorides (e.g. de-icing salts, seawater) and CO2, the main aggressive agents that leads to corrosion initiation. There is currently a lack of knowledge and no data is available to determine whether the structure can be in service as this point; this forces engineers to take conservative measures, leading to the early repair of the structure.
Most of concrete structures built in industrialised countries will be reaching a critical age in the next years and will have to operate beyond their service life; based on the current conservative maintenance approaches, this would lead to a strong increase in the number of structures needing repair and the corrosion costs. Thus, there is a high need to quit conservative approaches and find cost-efficiently diagnosis.
In this work, we present a novel sensor to be embedded in concrete and to monitor the main parameters that lead to corrosion initiation, namely pH and chloride concentration, in combination with the relevant parameters associated with corrosion state and propagation: corrosion potential, electrical resistivity and temperature.
We believe this approach will significantly improve durability monitoring in reinforced concrete; as the data obtained will provide a better insight on the corrosion state of the structure at the different locations over time. Use of this data will allow postponing repair in a safe way and optimize maintenance and planning. First results from pilot projects of embedding the sensor in engineering structures will be shown
Electrochemistry and capillary condensation theory reveal the mechanism of corrosion in dense porous media
Corrosion in carbonated concrete is an example of corrosion in dense porous media of tremendous socio-economic and scientific relevance. The widespread research endeavors to develop novel, environmentally friendly cements raise questions regarding their ability to protect the embedded steel from corrosion. Here, we propose a fundamentally new approach to explain the scientific mechanism of corrosion kinetics in dense porous media. The main strength of our model lies in its simplicity and in combining the capillary condensation theory with electrochemistry. This reveals that capillary condensation in the pore structure defines the electrochemically active steel surface, whose variability upon changes in exposure relative humidity is accountable for the wide variability in measured corrosion rates. We performed experiments that quantify this effect and find good agreement with the theory. Our findings are essential to devise predictive models for the corrosion performance, needed to guarantee the safety and sustainability of traditional and future cements.ISSN:2045-232