By different testing methods (electrochemical techniques, potential shift monitoring, and Environmental Scanning Electron Microscope), this research evaluates the stray current corrosion of steel rebar in different layouts. The more significant corrosion state is observed when the steel bar is parallel to stray current flow, compared to the situation as a steel bar is vertical to the stray current. These outcomes are further clarified by the recorded level of stray current picked-up by steel rebar. It is found that the level of current actually picked-up by the steel rebar is decreasing. At the instant when the stray current supply is just turned off, an opposite current flow (back flow) is recorded. Besides an expansion of the database for monitoring stray current interference on reinforced concrete structures, the recorded results can be the basis for better understanding the process of stray current interference.

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The focus of this work is to present test results on the bond of steel-mortar interface undergoing stray current. The bond strength, derived by pull-out tests, is correlated to the electrochemical response of the steel rebar and the properties of the mortar bulk matrix. The effects of curing regimes (in terms of duration of curing) and starting point of stray current are also investigated. It is found that stray current exerts bond degradation of the steel-mortar interface in all investigated cases, irrespective of the presence or absence of a corrodent (Cl⁻) in the external medium. For the ease of operation in lab tests, the stray current is generally simulated by anodic polarization, although fundamentally, the stray current effect on the steel surface is composed of both anodic and cathodic polarizations. Hence this work also differentiates the effects of stray current on steel-mortar bond, versus the effects of anodic polarization.

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Different concrete structures (viaducts, bridges, or tunnels) in the neighborhoods of railways may be subject to the stray current leaking from the rails. In these cases, the reinforcing rebars embedded in concrete act as conductors, “pick up” the stray current, and can corrode. For simulating the stray current-induced corrosion of metals, most researchers just supplied anodic polarization on samples. However, stray current induces both cathodic polarization and anodic polarization. This work experimentally justifies the different effects of stray current and anodic polarization on reinforcing steel embedded in mortar. A comparison between stray current and anodic polarization effects on the corrosion behavior of embedded steel is performed for both fresh (24 hour-cured) and hardened matrix (28 day-cured) in chloride-free (Cl-free) and chloride-containing (Cl-containing) environments. It is found that in all studied conditions, anodic polarization leads to a significantly different electrochemical performance of the steel rebar compared to the stray current. Hence, anodic polarization cannot reflect all the effects of stray current, and therefore, it has limited significance for simulating stray current. It is also clarified that the curing regimes and starting time of the stray current play significant roles in the formation of a corrosion product layer on the steel surface.@en

In this paper the potentiometric response of a Ag/AgCl electrode as a chloride sensor in cementitious materials of different mix design was studied. The chloride sensor’s response was discussed with respect to the presence of hydration products around the sensor. The free chloride content inferred from the sensor’s response was compared to the one obtained from destructive water and acid soluble chlorides. The measured free chloride content, obtained via sensor’s reading, was lower than the obtained water and acid soluble chlorides. The results indicated the influence of the cementitious mix design on the correlation between the free chloride content obtained via sensor’s reading, water and acid soluble chlorides.@en

The addition of nitrogen doped mesoporous carbon spheres (NMCSs) to Portland cement-based materials was studied as an approach to produce materials of desired properties. Lower electrical resistivity and higher compressive strength were recorded in the presence of NMCSs, while the addition of a dispersion agent (Pluronic F127), caused an increase in electrical resistivity and strength reduction. This performance was related to the variation in zeta potential and hydrodynamic radius of NMCSs in alkaline medium. The zeta-potential of NMCSs was defined by the non-ionic surfactant F127. This led to a “poisoning” of the active NMCSs surface, consequently hindering the interaction of NMCSs with the cement-based material. Therefore, F127 is a non-suitable dispersing agent. In contrast, the presence of NMCSs alone enhances the desired cement-based materials properties, where reduced electrical resistivity and increased compressive strength were achieved, while cement hydration and pore network development were maintained close to NMCSs-free specimens.@en

In this work, the different techniques for non-destructive in situ measurement of chloride ion concentration are presented. Non-destructive (ND) in situ measurement is crucial for reliable and continuous determination of chloride ion concentration in concrete. Over the last 20 years, several studies have been performed on ND measurements. These were mainly focused on the application of electrochemical and electromagnetic techniques. Each technique has its advantages and disadvantages. Depending on the requirement of assets managers and constructors and considering the limitations, these techniques can be well applied. The main concepts and comparative analysis, in view of possibilities and limitations, of these non-destructive techniques are presented in this paper.@en

Corrosion of steel reinforcement is the main focus of many studies on condition assessment of road infrastructure. The major uncertainty involves the behavior of steel rebar during dynamic loading imposed by traffic. Especially in countries that use deicing salts during winter, a combined loading situation emerges in which stress, frequency, and chlorides are present at the same time. Laboratory tests are conducted to evaluate the performance of single steel rebars simultaneously exposed to different model media (alkaline and chloridecontaining solutions), different frequencies, and different initial stress levels. These so-called chloride-exposed fatigue tests show the impact of chloride-induced corrosion on the performance of dynamically loaded rebar. Despite the well-known low susceptibility of construction steel to enhanced stress-induced damage in a corrosive medium, the recorded behavior indicates altered electrochemical performance under dynamic load. The results allow for an alternative view of the assessment of service-life design of infrastructure.

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The stability and reproducibility of an Ag/AgCl sensors’ response in an alkaline medium are important for the application of these sensors in cementitious materials. The sensors’ response, or their open circuit potential (OCP), reflects a dynamic equilibrium at the sensor/environment interface. The OCP response in an alkaline medium is affected by the presence of hydroxide ions. The interference of hydroxide ions leads to inaccuracies or a delay in the sensors’ response to a certain chloride content. In this article, the potentiometric response (or OCP evolution) of the chloride sensors is measured in model solutions, resembling the concrete pore water. The scatter of the sensors’ OCP is discussed with respect to the interference of hydroxide ions at varying chloride concentration in the medium. The deviation of the sensor’s response from its ideal performance (determined by the Nernst law) is attributed to dechlorination of the AgCl layer and the formation of Ag2O on the sensor’s surface. Results from the surface XPS analysis of the AgCl layer before and after treatment in alkaline medium confirm these observations in view of chemical transformation of AgCl to Ag2O.@en

The corrosion of reinforced steel, and subsequent reinforced concrete degradation, is a major concern for infrastructure durability. New materials with specific, tailor-made properties or the establishment of optimum construction regimes are among the many approaches to improving civil structure performance. Ideally, novel materials would carry self-repairing or self-healing capacities, triggered in the event of detrimental influence and/or damage. Controlling or altering a material's behavior at the nano-level would result in traditional materials with radically enhanced properties. Nevertheless, nanotechnology applications are still rare in construction, and would break new ground in engineering practice. An approach to controlling the corrosion-related degradation of reinforced concrete was designed as a synergetic action of electrochemistry, cement chemistry and nanotechnology. This contribution presents the concept of the approach, namely to simultaneously achieve steel corrosion resistance and improved bulk matrix properties. The technical background and challenges for the application of polymeric nanomaterials in the field are briefly outlined in view of this concept, which has the added value of self-healing. The credibility of the approach is discussed with reference to previously reported outcomes, and is illustrated via the results of the steel electrochemical responses and microscopic evaluations of the discussed materials.

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Characterization of the Ag/AgCl electrode is a necessary step toward its
application as a chloride sensor in a highly alkaline medium, such as concrete.
The nucleation and growth of AgCl on Ag in 0.1 M HCl was verified through
cyclic voltammetry. Ag anodization was performed at current densities, determined
by potentiodynamic polarization in the same (0.1 M HCl) medium. The
morphology and microstructure of the AgCl layers were evaluated via electron
microscopy, while surface chemistry was studied through energy-dispersive
spectroscopy and X-ray photoelectron spectroscopy. At current density above
2 mA/cm2, the thickness and heterogeneity of the AgCl layer increased. In this
condition, small AgCl particles formed in the immediate vicinity of the Ag
substrate, subsequently weakening the bond strength of the Ag/AgCl interface.
Silver oxide-based or carbon-based impurities were present on the surface of the
sensor in amounts proportional to the thickness and heterogeneity of the AgCl
layer. It is concluded that a well-defined link exists between the properties of the
AgCl layer, the applied current density and the recorded overpotential during
Ag anodization. The results can be used as a recommendation for preparation of
chloride sensors with stable performance in cementitious materials.@en

Determination of the free chloride content in a concrete structure is important for evaluation of the risk of corrosion of reinforcing steel. The Ag/AgCl sensor is sensitive to chloride ions and exhibits an open circuit potential (OCP) that depends on the chloride ions activity in the environment. The interference of hydroxide and sulfide ions in the pore solution of cementitious materials with the sensor affects the stability of the sensor’s OCP. In this paper, the sensor’s OCP (i.e. sensor reading) in Portland and slag cement pastes is monitored over 300 days of immersion in solutions with different chloride concentration. The results show the applicability of the chloride sensor for continuous and non-destructive determination of the free chloride content in Portland cement paste. The significantly different OCP of the sensor in slag cement paste was attributed to the interference of sulfide ions and formation of Ag2S on the sensor’s surface. Hence, the content of free chloride ions in slag cement paste could not be determined from the sensor’s OCP. The acid-soluble chloride and water-soluble chloride in the cement pastes were also determined after 300 days of immersion of specimens in the solutions. In case of Portland cement paste, the measured acid and water-soluble chlorides were compared to the sensor reading. The acid and water-soluble chlorides were higher than sensor reading. The release of physically and chemically bound chlorides into the acid and water solvents were the main cause for this trend. The results show the importance of cement type and chloride binding ability of hydration products for the sensor’s response and the acid and water-soluble chloride contents. @en

This work reports on the development of microstructural and mechanical properties of mortar cubes under the synergetic action of stray current and various environmental/curing conditions. The study refers to specimens cured for 24h only, followed by a 112 days period of partial or full submersion in water or alkaline medium. Additionally, equally prepared mortar specimens were tested in sealed conditions. The outcomes for submerged and saturated conditions were compared to sealed conditions. Three current density regimes were employed i.e. 1 A/m2, 100 mA/m2, and 10 mA/m2, simulating different levels of stray (DC) current environment. The highest level of 1A/m2 was also comparable to stray current densities, as measured in field conditions. The tests were designed in a way, so that the effects of diffu-sion-controlled transport (ions leaching due to concentration gradients), were distinguished from migration-controlled ones (ion/water transport in stray current conditions). Mechanical, microstructural and electrical properties were moni-tored throughout the test. For water-conditioned specimens, the stray current was found to accelerate degradation pro-cesses. This was reflected by decreased compressive strength, reduced electrical resistivity and increased porosity of the matrix. The results were attributed to leaching-out of alkali ions due to concentration gradients, where except diffusion, migration took place i.e. the leaching-out effect was accelerated by water and ions migration in conditions of stray cur-rent flow. In contrast, stray current flowing through mortar in sealed conditions (as well as through mortar in alkaline medium) resulted in increased compressive strength and electrical resistivity. These were accompanied by densification of the bulk matrix and reduced porosity. It can be concluded that for a cement-based material at early hydration age, both positive and negative effects of stray current flow can be expected. The level and direction of these effects are dependent on the external environment and the current density levels, where stray currents above 100 mA/m2 and in conditions of concentration gradients with the external medium, would lead to more pronounced negative effects on microstructural and micromechanical performance. @en

Reinforced concrete deterioration due to acidification of the environment from microbial activity in view of steel performance is seldom reported and still a debate. An initial scrutiny of several inhibitors indicated methylene blue dye and trisodium-phosphate as the most promising candidates for mild steel protection in diluted H₂SO₄. Such compounds were combined together into two organic/inorganic hybrids composed of hydroxyapatite (HAP) or vaterite porous matrixes impregnated with methylene blue dye. The novel hybrid systems were characterized by means of scanning electron microscopy, X-ray diffraction, and Brunauer-Emmett-Teller analysis. The electrochemical response of steel specimens in a simulated environment containing loaded and empty HAP host was monitored by means of linear polarization resistance and electrochemical impedance spectroscopy. The results confirmed the inhibitive properties of the chosen compounds in acidic medium, pointing out a synergistic effect resulting from the release of the organic compound and the dissolution of the inorganic matrix.

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Stray current is an electrical current “leakage” from metal conductors and electrical installations. When it flows through cement-based materials, electrical energy is converted to thermal energy that causes increasing temperature due to Joule heating phenomena. The aim of this paper is to shed light on the influence of electrical current flow on cement hydration, thermal properties and pore structure changes of cement-based materials. Calorimetry tests show that degree of cement hydration increases as a results of temperature increase due to electrical current flow through cement-based materials. To evaluate the influence of electrical current on the thermal properties of cement paste, the specific heat of cement paste was calculated based on the degree of cement hydration and temperature development during the hydration process. MIP tests were carried out to quantify changes in the pore structure due to electrical current flow. The results shows that if no other factors are present, leaching is avoided and for relatively early cement hydration age, the electrical current flow accelerates cement hydration, leading to an initial decrease in porosity of the cement paste.@en

This paper reports the results of microstructural analysis based on image analysis subjected to electrical current as a simulation of stray current effect. The purpose is to investigate the influence of electrical current flow on the development of microstructural properties in reinforced cement-based materials. In view of the significant contribution to material performance, the characterization of cement-based microstructure in an economical and reliable way is of high relevance to permeability prediction and durability studies of cement-based materials. In this study, taking the cement paste submerged in Ca(OH)₂ conditions as specimens, the pore size distribution and percolation was derived from image analysis of ESEM micrographs. The electrical properties of mortars were measured and their microstructural characteristics were investigated using quantitative image analysis techniques. Moreover this approach is compared with other general methods such as mercury intrusion porosimetry (MIP) and the comparison shows good consistency in development of parameters characterizing the materials' microstructure.

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Metallic corrosion can cause substantial damage at various levels and in almost all types of infrastructure. For metallic corrosion to occur, a certain external environment and the presence of corrodents are the prerequisites. Stray current-induced corrosion, however, is a rather underestimated issue in the field of corrosion and civil engineering. Stray current arising from power sources and then circulating in metal structures may initiate corrosion or even accelerate existing corrosion processes. The most frequent sources of stray current are light rail transits and subways, which are also main traffic tools with continuously accelerating urbanization all over the world. Stray currents from these systems may easily flow into nearby metallic structures, making stray current-induced corrosion the most severe form of damage of buried structures, such as tunnels, pipelines, and various underground reinforced concrete structures. The objective of this paper is to critically review stray current-induced steel corrosion in infrastructure with regard to sources of stray current and the characteristics and mechanism of stray current corrosion in view of electrochemical aspects. The methods and techniques for the evaluation, monitoring, and control of stray current-induced corrosion for steel and reinforced concrete structures are also presented and discussed.@en

This work reports on the synergetic effect of water-to-cement ratio, curing conditions, varying external environment and stray current on the microstructural (porosity and pore size), electrical (resistivity) and mechanical (compressive strength) properties of 28 days-cured cement-based materials. The influence of curing on porosity and pore size, in stray current conditions, was assessed by correlating the performance of 28 days cured mortar with that of fresh (24h-cured only) mortar specimens in identical environmental medium. Three different levels of electrical current density (i.e. 10mA/m2, 100 mA/m2 and 1 A/m2) were applied to simulate stray current flow through hardened mortar specimens with water-to-cement ratio of 0.5 and 0.35. Different environmental conditions were employed i.e. sealed conditions, partly immersed, and fully submerged in water and calcium hydroxide medium. Microstructural, mechanical and electrical properties were monitored in the course of 140 days. The outcomes suggest a potentially positive effect of the stray current, where water and/or humidity exchange with the external environment is restricted. The potential for this positive effect was experimentally supported through the recorded matrix densification and increased compressive strength of mortar specimens, subjected to stray current and treated in calcium hydroxide and/or sealed conditions, compared to equally handled and treated control cases. In contrast, for water submerged mortar specimens, subjected to stray current, coarsening of the bulk matrix and reduced compressive strength were observed. The outcomes were irrespective of w/c ratio and curing conditions. The effect of stray current was found to be predominantly determined by the current density level and increased at values > 100mA/m2. This would result in compromised mechanical properties and potentially reduced performance of cement-based materials within service life. Therefore, concrete curing and conditioning on site need to include considerations for the potential effect of stray currents. @en

The adhesion mechanisms between C[sbnd]S[sbnd]H and fillers are of great significance when fillers are employed as a cement replacement. The affinity of a filler's surface towards ions in the pore solution of cement paste is reported in this work from the view point of the governing mechanisms. The discussion on various interactions is justified by results from zeta-potential measurements, and further supported by microscopic observations of hydration products on the filler's surfaces. The bond strength between the filler and hydrates is also evaluated. The C[sbnd]S[sbnd]H/filler adhesion appears due to the interactions between a filler's surface and calcium ions. In the case of calcite, the interactions between a filler's surface and calcium ions are predominantly determined by acid-base interactions which lead to the formation of a strong bond (most likely ionic-covalent bond). In the case of silica, the adhesion is found to be governed by an attractive ion-ion correlation force.

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