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In civil engineering, the corrosion of steel reinforcements in structural elements of concrete bares a risk of stability-reduction, mainly caused by the exposure to chlorides. 3D computed tomography (CT) reveals the inner structure of concrete and allows one to investigate the corrosion with non-destructive testing methods. To carry out such investigations, specimens with a large artificial crack and an embedded steel rebar have been manufactured. 3D CT images of those specimens were acquired in the original state. Subsequently three cycles of electrochemical pre-damaging together with CT imaging were applied. These time series have been evaluated by means of image processing algorithms to segment and quantify the corrosion products. Visualization of the results supports the understanding of how corrosion propagates into cracks and pores. Furthermore, pitting of structural elements can be seen without dismantling. In this work, several image processing and visualization techniques are presented that have turned out to be particularly effective for the visualization and segmentation of corrosion products. Their combination to a workflow for corrosion analysis is the main contribution of this work.
Literature data on the influence of concrete cracks on corrosion propagation of reinforcing steel are contradictory. This might be due to very different exposure and test conditions but also to a lack of time-resolved data in cyclic wetting–drying exposure. Here, the influence of the environmental conditions on the corrosion rates in cracked concrete is studied experimentally. The results show that the corrosion rate in cracked concrete depends on the duration of wetting and drying phases and the relative humidity (RH) during the drying phase. The lower the ambient RH in the drying phase, the faster the cracks dry, which depresses the corrosion rate in the periods between the wetting events. A model is proposed to estimate corrosion rates in cracked concrete cyclic wetting/drying exposure.
Corrosion of galvanized steel in carbonated concrete is different from the corrosion of reinforcement made of carbon steel. This is based on the corrosion mechanism of galvanized steel in dependence of the pH-value of carbonated concrete. To describe the influence of a lower pHvalue on the corrosion activity of galvanized reinforcement, different electrochemical measurements were made. The anodic and cathodic partial reaction were investigated by galvanostatic pulse measurements and potentiodynamic measurements. Especially the galvanostatic pulse measurements can describe the influence of the lower pH-value by the anodic polarization resistance with pt = 20 sec. (polarization time). Additionally, corrosion current measurements were performed, coupled with potential measurements, between galvanized steel in carbonated and non-carbonated concrete. Normally, the potential measurements are an indicator to localize the anodic and cathodic parts of a macro system. In this case the cathode was more negative than the anode before they were coupled. After the coupling of the two specimens the mixed potential was much more positive than the single potential of the galvanized steel in carbonated concrete. It increased up to the potential range of the non-carbonated specimens. This means that in case of carbonated outer areas, the embedded galvanized steel forms the cathodic part of a macro element.
In order to obtain a statement on the corrosion resistance of stainless steel against pitting corrosion, a well-tested measuring arrangement is required. For this purpose, the so-called ECISS test can be used to determine the critical chloride content. This makes it possible to assess the corrosion resistance of various stainless steels that have different chromium contents.
Alkali-activated materials such as geopolymers are currently receiving a lot of attention because of their potential to be used as binders for concrete with advantageous engineering properties and reduced manufacturing CO2 emissions. Knowledge of the durability of steel in these concretes and related properties of alkali-activated materials is a prerequisite for their application as building materials, if they are to be used for steel reinforced elements. However, to date only limited data exists on this topic. The present contribution focuses on durability-related transport properties of geopolymer-based mortars (as model systems for concrete). We report results of accelerated carbonation, rapid chloride migration (RCM) and air permeability measurements as well as porosity data for fly ash-based geopolymer mortars, including mixes containing ground granulated blast furnace slag (GGBFS). In addition, we report polarization resistance data and corrosion potential vs. time curves for carbon steel bars embedded in two of the fly ash-based geopolymer mortars and a CEM I-based mortar (as reference).
Despite comparable total porosities, the carbonation depths, the chloride migration coefficients and the air permeabilities of the mortars differed significantly. In general, the addition of GGBFS to the binders improved the performance (decreased transport coefficients); however, this was not found to be true for the air permeability in all cases. This latter effect can be explained by drying damage of the C-(A-)S-H gel in GGBFS-containing binders. On the other hand, low transport coefficients can also be achieved by optimization of the binder formulation without the addition of GGBFS, which is also reflected in the material’s air permeability. Thus, there exists no simple correlation between air permeability (of harshly dried mortars) and durability-related transport coefficients for the studied alkali-activated materials.
The corrosion potential vs. time curves in combination with polarization resistance values reveal that the steel reinforcement in geopolymer mortars assumed a passive state. However, this happened considerably later than for steel in CEM I-based mortars. The free corrosion potential of carbon steel reinforcement in the geopolymer mortars had different values than the free corrosion potential values for the CEM I-based mortar for both the active and the passive state; possible reasons for this behavior are discussed.
Carbon-based conductive coatings are complex composites, consisting of an organic or inorganic binder and conductive carbon components, for application as anodes in impressed current cathodic protection systems of reinforced concrete structures. The electrochemical properties of three coatings at different humidity and in saturated calcium hydroxide solution were studied by electrochemical methods, such as electrochemical impedance measurement, measuring of open circuit potential over time and galvanostatic polarization.
Carbon-based conductive coatings are complex composites, consisting of an organic or inorganic binder and conductive carbon components, for application as anodes in impressed current cathodic protection systems of reinforced concrete structures. The electrochemical properties of three coatings at different humidity and in saturated calcium hydroxide solution were studied by electrochemical methods, such as electrochemical impedance measurement, measuring of open circuit potential over time and galvanostatic polarization.
Carbon‐based conductive coatings are complex composites, consisting of an organic or inorganic binder and conductive carbon components, for application as anodes in impressed current cathodic protection systems of reinforced concrete structures. The electrochemical properties of three coatings at different humidity and in saturated calcium hydroxide solution were studied by electrochemical methods, such as electrochemical impedance measurement, measuring of open circuit potential over time and galvanostatic polarization. The dissolved organic and inorganic carbons in electrolyte solution were quantified by using a photometric method. The structures of the coatings were investigated before and after the electrochemical tests by microscopy and scanning electron microscope/EDX analysis. The results of the electrochemical impedance measurements show that the tested coatings all have a relatively low resistance, which is between 100 and 200V. The binder and the surface porosity influence the degradation behavior of those coatings. Especially the organic binder reacts with the strong alkaline medium under dissolving of organic carbon.
Carbon-based conductive coatings are complex composites, consisting of an organic or inorganic binder and conductive carbon components, for application as anodes in impressed current cathodic protection systems of reinforced concrete structures. The electrochemical properties of three coatings at different humidity and in saturated calcium hydroxide solution were studied by electrochemical methods, such as electrochemical impedance measurement, measuring of open circuit potential over time and galvanostatic polarization. The dissolved organic and inorganic carbons in electrolyte solution were quantified by using a photometric method. The structures of the coatings were investigated before and after the electrochemical tests by microscopy and scanning electron microscope/EDX analysis. The results of the electrochemical impedance measurements show that the tested coatings all have a relatively low resistance, which is between 100 and 200 Ω. The binder and the surface porosity influence the degradation behavior of those coatings. Especially the organic binder reacts with the strong alkaline medium under dissolving of organic carbon.