Accelerated Electromigration Approach to Evaluate Chloride-Induced Corrosion of Steel Rebar Embedded in Concrete

Hoque, Kazi Naimul; Presuel-Moreno, Francisco; Nazim, Manzurul

doi:https://doi.org/10.1155/2023/6686519

Advances in Materials Science and Engineering

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Abstract Introduction Results and Discussion Conclusion Data Availability Disclosure Conflicts of Interest Acknowledgments Supplementary Materials References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 6686519 | https://doi.org/10.1155/2023/6686519

Accelerated Electromigration Approach to Evaluate Chloride-Induced Corrosion of Steel Rebar Embedded in Concrete

Kazi Naimul Hoque,¹Francisco Presuel-Moreno,²and Manzurul Nazim²

Academic Editor: Robert Černý

Received04 Aug 2023

Revised12 Oct 2023

Accepted13 Nov 2023

Published05 Dec 2023

Abstract

Two distinct binary blended concrete mixes were prepared for the study. The first mix involved a cement replacement of 50% slag, denoted as SL. The second mix incorporated a cement replacement of 20% fly ash, referred to as FA. No chlorides were added during the preparation of these concrete specimens. To accelerate chloride transport, electromigration was employed by placing specimens with varying reservoir lengths (ranging from 2.5 cm to 17.5 cm) on their top surfaces. These reservoirs were subsequently filled with a 10% NaCl solution. In this paper, corrosion propagation was monitored over a period of approximately 650 days using electrochemical measurements such as open circuit potential, linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS). The evolution of rebar potential, polarization resistance, solution resistance, and corrosion current were analyzed to understand the corrosion behavior. This paper focuses on how the length of the solution reservoirs influences the corrosion-related parameters such as polarization resistance, solution resistance, rebar potential, and corrosion current. During the monitored propagation period, the corrosion current values (last 7 sets of readings) exhibited higher magnitudes for the embedded rebars in specimens made with SL mix in comparison to those made with FA mix. Corrosion current measurements likewise showed an increasing trend as the reservoir lengths increased. None of the specimens had any visible cracks or corroded products that could reach the concrete surface throughout the monitored period. The experimental results provide insights into the corrosion mechanisms and the effectiveness of accelerated corrosion techniques in simulating real-life conditions.

1. Introduction

The degradation of structures reinforced with concrete (RC) in marine and high humidity environments is predominantly attributed to the steel reinforcement corrosion. Failure to deal with this problem expedites the degradation of RC constructions and gives rise to a range of interconnected negative effects. These consequences encompass a reduction in the RC structure’s service lifespan and a diminution in the steel’s cross-sectional surface. In RC constructions, steel corrosion may be dramatically caused by the ingress of chlorides (chloride-induced corrosion). The corrosion rate caused by the ingress of chlorides typically progresses slowly, posing challenges in acquiring pertinent information for decision-making purposes. Moreover, the dearth of research and data concerning the initiation and progression of corrosion further compounds the issue [1–10]. These studies emphasize that substantial damage resulting from corrosion in RC structures experiencing natural corrosion requires prolonged durations to manifest.

Accelerated steel corrosion in concrete signifies the occurrence of corrosion at a heightened speed compared to the natural process. Unlike natural corrosion, accelerated corrosion exhibits noticeable effects, such as depassivation and corrosion-induced damage, in a relatively short time. Both anodic and cathodic reactions occur during natural as well as expedited corrosion, demonstrating a common characteristic [11–17]. In earlier research, accelerated corrosion methodologies were employed to assess the commencement of corrosion, damage induced by corrosion, and their effects on factors such as characteristics of deformation, toughness, bonding strength, and processes of failure in RC structures [18–21]. Nevertheless, given the current requirement to quantitatively assess and account for the propagation phase of corrosion to ensure the operational efficiency of corrosion-affected RC structures [22–24], it becomes imperative to explore and develop methodologies that can mimic the natural progression of corrosion while eliminating the initiation phase without introducing a substantial increase in corrosion-induced damage.

A range of techniques exists to expedite chloride-induced corrosion, employing mixed chlorides, cyclic wetting and drying using a solution of chlorides, and an impressed current (IC) application using either an anodic galvanostatic or potentiostatic voltage across a distinct cathode and the steel reinforcements (anode), such as stainless-steel, or combining these techniques. In the case of IC, an artificial polarization is induced in the steel, subjecting the steel surface to an applied electric field that accelerates corrosion [25]. When IC or voltage is employed, chloride ions (Cl-) penetration towards the concrete primarily occurs through migration, diverging from the natural diffusion-based ingress of chlorides. Anodic IC triggers widespread corrosion across the entire exposed steel surface, as opposed to the localized pitting corrosion that naturally arises when discrete anodes and cathodes are present (with a high cathode-to-anode area ratio). In addition, when IC is applied, it induces variations in the chemical composition of its concrete pore solution, causing modifications of the distribution of ions. This technique is suggested for evaluating the magnitude of concrete’s ability to corrode steel and their impacts on parameters such as flexural capability and durability since it enables the monitoring of corrosion levels. The extent of corrosion serves a vital part in determining the remaining lifespan of corroding RC constructions [18].

To expedite the corrosion initiation phase in accelerated corrosion investigations, admixed chlorides are commonly utilized, typically ranging from 1% to 5% of the cement’s mass [18, 26]. By employing this approach, the development of a passive protective coating on the steel is effectively hindered until the chloride threshold is reached. In addition, it eliminates the natural chloride binding effects and can potentially modify the alkalinity of the concrete pore solution. By facilitating rapid chloride penetration through capillary suction and diffusion [19–21], the cyclic wetting and drying technique accelerates steel corrosion in concrete. This technique allows dissolved oxygen to replenish at the steel surface during the drying cycle, thereby supporting the cathodic reaction mechanism.

In this research, two separate binary blended concrete mixes were prepared, as well as the transport of chloride was accelerated using electromigration, based on previous research findings [27]. Corrosion of the rebar typically begins after a few weeks or months, and the corrosion propagation stage was monitored by means of open circuit potential, linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS) measures. Parameters such as polarization resistance, solution resistance, rebar potential, and corrosion current were continuously observed for approximately 650 days. This paper focuses on how the length of the solution reservoirs influences the corrosion-related parameters such as polarization resistance, solution resistance, rebar potential, and corrosion current. There are very few studies which conducted accelerated chloride transport tests in a laboratory environment, considering the influence of the length of the solution reservoirs on the corrosion-related parameters of reinforced concrete specimens. This aspect is the main novelty of this research work.

2. Experimental Details

2.1. Concrete Mixes, Casting, and Curing of Specimens

Concrete samples measuring 7.6 cm tall, 12.7 cm wide, and 30.5 cm long (direction of the rebar) were meticulously constructed using a water-to-cement ratio of 0.41. Two distinct types of binders, namely, 80/20 PC/FA (where PC represents Portland cement and FA stands for fly ash) and 50/50 PC/SL (with SL denoting slag), were utilized. The mixture ratios are outlined in Table 1, while additional information regarding each concrete mixture can be found in Supplementary Materials (available here) [28]. Each specimen incorporated a high yield steel reinforcing bar, 0.94 cm in diameter, embedded within a cover depth of 0.75 cm. A total of 22 specimens, consisting of eleven single rebar pieces per batch, were meticulously prepared.

The rebar segments were precisely cut to the required dimensions, and wire brushes were utilized to clean them. To ensure the absence of any grease on the reinforcement, hexane was employed before the casting process. Prior to the concrete casting, each rebar underwent drilling on one end, followed by the installation of a stainless-steel screw. This facilitated electrical contact, enabling corrosion monitoring. During casting, a stainless-steel mesh (alternatively, a titanium mix metal oxide known as “TiMMO”) was placed on the upper surface of each specimen, which served as the bottom surface throughout the duration of the experiment. The mesh played a role in accelerating chloride transfer and varied in length from 2.5 cm to 17.5 cm. The reservoir lengths were taken along the length of the rebar. SL and FA specimens shared three identical reservoir lengths, measuring 2.5 cm, 5 cm, and 17.5 cm. In addition, SL and FA specimens featured distinct reservoir lengths of 10 cm and 7.5 cm, respectively. The reason for the different reservoir length was to attempt to have different anode lengths for testing (once corrosion of the rebar initiated). Positioned in the center of the rebar, the mesh was approximately 3 cm wide. The specimen preparation took place at the FDOT-SMO. The mold had been removed after one day and placed in a fog chamber to be cured for a minimum of 28 days.

2.2. Specimen Setup Preparation for Electromigration

Following the casting process, the specimens were transferred to the FAU SeaTech campus, specifically to the Marine Materials & Corrosion Laboratory, for the experiment’s next stage. All these samples remained stored there until the setup of the solution reservoir in a high humidity chamber. Once the solution reservoir for ponding was installed, the samples were moved to a lab setting with an ambient temperature of 21°C and a RH of 65%.

For attaching the plastic reservoir to the top surface (which served as the casting mold surface), marine adhesives were used. The reservoir was installed, not later than forty days following the casting. NaCl solution (10% NaCl by weight) was poured into the reservoir. While multiple corroding lengths were intended, the length of the reservoir somewhat limited the area where corrosion could initiate. Prior to installing the reservoirs, the specimens were stored in a very humid environment for a period of 3–7 days. This was done to acclimate the specimens to a specific humidity level or to prevent dimensional changes in materials that were sensitive to humidity fluctuations. This could help ensure that the specimens were stable and would not be adversely affected by changes in humidity once the reservoirs were placed. For SL samples, the chloride concentration was 37.38 ppm (0.14 lb/yd³). On the other hand, the chloride concentration for FA samples was 29.63 ppm (0.109 lb/yd³).

Electrodes, either made of stainless-steel mesh or TiMMO mesh with dimensions like those comprised within the specimens, were set on the top surface of the solution reservoir. The samples were stored in transparent plastic containers, with about one centimeter of the submerged concrete piece in a saturated calcium hydroxide solution. A white acrylic perforated plastic mesh was placed on top of each specimen, serving to minimize concrete leaching during the electromigration process.

2.3. Electromigration

The experimental setup involved the use of a power source to create a voltage variation in the upper and lower mesh, generating an electric field. This electric field induced the movement of chlorides present in the solution above each rebar, guiding them approaching the rebar that is embedded within the concrete. The negative end of the power source was attached to the electrode submerged in the NaCl solution, while each specimen’s embedded mesh was attached to the positive end. To minimize a direct connection of the TiMMO mesh and the surface of the concrete, an acrylic mesh was introduced into the solution reservoir. Figure 1 provides a visual representation of the electromigration experimentation setup.

Tables 2 and 3 present the assigned labels for each sample along with relevant information such as the sample name/ID, reservoir length, and the duration of electromigration. In addition, these tables feature a column that indicates the calculated ampere-hour applied, showing the integrated values. For a given specimen, a higher ampere-hour value suggests that it will take a longer period of electromigration to initiate corrosion. Conversely, a specimen with a lower ampere-hour value implies that corrosion initiation will occur in a shorter electromigration period. Each specimen underwent multiple electromigration periods during the initial phase of the experiment.

Each specimen was subjected to an electromigration process with a predetermined protocol. Initially, a potential of 9 V was applied, and while the electric field was active, a saturated calomel reference electrode (SCE) was used to measure the rebar potential. A potential exceeding +2 V was observed, indicating the influence of the applied voltage. After a duration of seven days, the employed potential was decreased to 3 V. By measuring the change in potential across a 100-ohm resistor, the magnitude of the current applied for specific voltages over multiple days was determined. The potential of the rebar was assessed when the system was deactivated. Despite not being directly connected during the electromigration process, the applied electric field generated an ionic current that polarized the rebars. After disconnecting the system, the potential of the rebar was continuously monitored for a certain period, typically up to two hours. If the most recent measurement of the rebar potential indicated the absence of corrosion initiation, the potential employed was continued. The process of electromigration proceeded until the sample exhibited an off-rebar potential, reaching or exceeding −0.200 Vsce, which suggested the possible onset of corrosion in the embedded rebar. Previous studies [16] have indicated that corrosion initiation typically occurs at a potential value of −0.150 Vsce, approximately −0.220 V vs. CSE (copper sulfate electrode).

Some of the selected single rebar samples (after stopping electromigration and corrosion propagation had been monitored for more than 100 days) were chosen for a modest anodic polarization via an applied current. The selected samples were SL-2, SL-5, SL-6, SL-9, FA-1, FA-5, FA-8, and FA-11. A modest (small) current was applied between the embedded rebar and an electrode in the reservoir. This anodic polarization tends to accelerate any ongoing corrosion. Selected specimens underwent accelerated corrosion using a galvanostatic pulse (i.e., applying a small constant anodic current up to a maximum of 125 μA). Samples were chosen by comparing the rebar potential and Rc values for those specimens with the same length of reservoir. SL and FA specimens had more than one sample for a given concrete reservoir size. A comparison of rebar potential and Rc values was used to select the samples in which it was to be applied. The samples were selected based on a criterion (in most instances, the specimen with the most negative rebar open-circuit potential and smallest Rc values for a given reservoir size sample of the same concrete mixture was selected; alternatively, the sample with the most negative rebar potential was selected if the difference in Rc was small).

2.4. Electrochemical Measurements for Monitoring Corrosion

During the corrosion propagation phase, the rebar half-cell potential (OCP) was continuously monitored utilizing a SCE [29]. For the selected samples that underwent modest anodic polarization, the same type of electrode used during the electromigration stage was employed to assess the rebar potential. Measurements of solution resistance (Rs) and polarization resistance (Rc) were conducted for a minimum of two days following disconnecting the system.

The EIS test was conducted between the frequencies of 10 kHz and 1 Hz, with the impedance magnitude set at 54.51 Hz for Rs [30]. This was done before the LPR measurement [30]. The LPR test was conducted from 10 mV below OCP to 1 mV above it. After approximately six months, the LPR measurements were carried out from 8 mV below OCP to the OCP using either a 0.1 mV/s or 0.05 mV/s scan rate. From LPR, apparent polarization resistance (Rpapp) values were collected. Rs (solution resistance) values were measured via EIS. Rc (polarization resistance) was calculated by subtracting Rs from the measured Rpapp, i.e., Rc = Rpapp–Rs.

Rebar half-cell potential, EIS, and LPR measurements were taken during the period of electromigration but a minimum of two days once the system is turned off. These measurements were conducted monthly throughout the corrosion propagation period. The values of Rc derived from LPR/EIS readings were turned into corrosion current (Icorr) in the absence of information about the corroding area. The Stern–Geary equation, Icorr = B/Rp, was utilized to determine Icorr, where Rp represents the polarization resistance (previously defined as Rc) and B is the Stern–Geary coefficient. For actively corroding steel in concrete, previous research predominantly used a reading of 26 mV for B, while 52 mV was used for passive steel [31, 32]. Hence, a reading of 26 mV was chosen for this study.

3. Results and Discussion

3.1. Evolution of Rebar Potential, Polarization Resistance (Rc), and Solution Resistance (Rs)

In the following figures, “day zero” specifically refers to the day when the solution was being poured into the attached solution reservoirs, not indicating the specimen’s age. The progression of corrosion is typically depicted as the area of the plot located on the right side of the dotted line. The period after migration is indicated by an arrow symbol following (to the right) the dotted lines. In cases where two black dotted lines are present, the range represents the overall duration during which the accelerated chloride transport approach was applied to the samples. The blue prisms within the range represent an approximation of the time when the electric field was activated or when the system switched on. The initiation of the accelerated corrosion mechanism in certain samples, resulting from a moderately applied current, is shown using a continuous blue line, while the occurrences of this mechanism are indicated by gray columns.

Figure 2 illustrates the measured Rs, Rc, and rebar potential for specimens SL-1 and SL-2. These specimens had 17.5 cm of solution reservoirs. Following the cessation of electromigration, specimen SL-1 experienced a decrease in the rebar potential. Rc exhibited a slight decline and maintained a consistently low value throughout the monitoring period. During the corrosion propagation stage, i.e., after suspending electromigration, the Rs values remained below 1 kΩ. From day 180 onwards, the rebar potential in SL-1 displayed oscillations and tended to drift towards more positive values. Electromigration durations on the rebar in SL-2 were extended compared to SL-1 although the total ampere-hour remained comparable. On day 230, the rebar potential dropped and remained at −0.250 Vsce or more negative values. There was a minor decline in Rc values. Both Rc and Rs demonstrated a monotonic increase during electromigration. Starting on day 280, a modest anodic current application was performed in order to accelerate corrosion in the rebar of sample SL-2, but it was suspended by day 520. The duration of each modest anodic polarization is approximately represented by the gray bars. All values presented are at least two days after discontinuing the modest anodic polarization. During the subsequent corrosion monitoring period, Rs appeared to remain unchanged although exhibiting slight oscillations. The rebar potential in specimen SL-2 remained more negative than −0.250 Vsce. Rc also showed fluctuations but remained below 1 kΩ after suspending electromigration. In some instances, the rebar potential switched to a more positive value, suggesting the possibility of corrosion continuing at a reduced rate. It is speculated that the rebar repassivated in certain cases or that over time, the corroding sites became more positive as the noncorroding rebar polarized them.

Figure 3 depicts the measured Rs, Rc, and rebar potential for specimens FA-8 and FA-9. The solution reservoir in these specimens was 17.5 cm. Additional electromigration durations were conducted on the rebar of FA-8. Following the suspension of electromigration, the rebar potential exhibited a sharp decline, reaching a value more negative than −0.350 Vsce around day 275. Rc values also experienced a significant drop. During the accelerated chloride transport procedure, Rs displayed a monotonic increase, while Rc showed a substantial increase. On day 280, a modest anodic current application was performed in order to accelerate corrosion in the rebar of sample FA-8, but it was suspended by day 530. The duration of each modest anodic polarization is approximately represented by the gray bars. All values presented are at least two days after discontinuing the modest anodic polarization. During the subsequent corrosion monitoring period, Rc exhibited oscillations, whereas Rs displayed a monotonic increase. Although the rebar potential of the specimen FA-8 demonstrated slight oscillations, it remained within the approximate range of −0.250 Vsce. When electromigration was suspended, there was a significant potential drop in the rebar of the specimen FA-9, reaching −0.495 Vsce by day 360. During electromigration, Rs showed a monotonic increase, while Rc experienced a slight increase. Starting on day 360, the rebar potential oscillated and tended to drift towards more positive values. Rc sharply declined after electromigration and exhibited oscillations during the monitored period. The Rs values remained below 2 kΩ after suspending the electromigration during the propagation stage. The measured values of rebar potential and Rc suggest the initiation of corrosion in the mentioned specimens [29, 33]. Similarly, the evolution of the rebar potential and Rc values in the other SL and FA specimens, documented in the provided references [28, 34–37], indicates the initiation of corrosion as well. Figures 4–6 represent the measured Rs, Rc, and rebar potential on selected SL and FA specimens, having different size reservoir lengths.

Tables 4 and 5 present the average values of Rs, Rc, and rebar potential for SL and FA specimens derived from LPR/EIS measurements. These averages were calculated based on measurements taken over approximately 650 days. Analysis of Table 4 revealed that rebars embedded in specimens with smaller solution reservoirs of 2.5 cm exhibited the highest average Rs and Rc readings. Conversely, rebars in specimens with larger solution reservoirs of 17.5 cm displayed the smallest average Rs and Rc values. In most cases, the average rebar potential readings were less than or equal to −0.150 Vsce, except for rebar SL-11, which had an average rebar potential of −0.120 Vsce. Examining Table 5, it was observed that rebars in specimens with smaller solution reservoirs of 2.5 cm had the highest average Rs and Rc readings. On the other hand, rebars in specimens with 7.5 cm and 17.5 cm reservoirs had the lowest average Rs and Rc values, respectively. For all specimens in Table 5, the average rebar potential values were less than or equal to −0.150 Vsce. Interestingly, the rebars under a 7.5 cm reservoir length exhibited average rebar potential values that were more negative than −0.325 Vsce, indicating a notable influence of reservoir length on the rebar potential. It is worth noting that the measurements of rebar potential and other readings were affected by factors such as the rebar portion that was exposed outside of the concrete and not directly below the reservoir. In addition, in certain cases, high moisture levels lead to corrosion of the exposed rebar outside the concrete. These external influences likely had an impact on the Rc values and rebar potential measurements. Corrosion potential fluctuations might arise from variations in the corrosion activity of different regions within the reinforced concrete structure. Factors such as variations in concrete mixes, oxygen availability, moisture content, and chloride concentration could lead to localized corrosion and subsequent potential fluctuations. Corrosion potential might vary because of changes in the corrosion environment as well. Factors such as temperature and humidity could influence the electrochemical reactions occurring at the steel reinforcement, leading to potential fluctuations.

3.2. Evolution of Corrosion Current (Icorr)

The subsequent figure presents the evolution of Icorr over time, as determined through LPR/EIS measurements. It is important to note that “day zero” does not indicate the specimen’s age in any of the depicted plots. Instead, it refers to the day when the solution was being introduced to the attached solution reservoirs. Since the solution reservoirs were not all filled simultaneously, the days since the initial fill varied. For SL and FA mixes, the Icorr plots represent the measured values for approximately 650 days via using the LPR/EIS method.

Figure 7 illustrates the progression of Icorr over time using electrochemical measurements for SL and FA samples. The reservoir length of each of these samples was 17.5 cm. The SL samples, SL-1, SL-2, and SL-3, are notable for showing an oscillating pattern in terms of Icorr as time passed. For SL-1, SL-2, and SL-3, respectively, the Icorr values were between 24.6 and 84.2 μA, 10.7 and 69.4 μA, and 9.8 and 68.4 μA. It was noted that the Icorr values for all three FA samples (FA-7, FA-8, and FA-9) were at 20.0 μA or lower until day 370. However, beyond that point, an oscillating trend in the Icorr was seen throughout the entire monitoring period. Icorr values for the FA-7, FA-8, and FA-9 samples varied from 4.8 to 51.8 μA, 5.1 to 66.9 μA, and 7.0 to 42.8 μA, respectively. These findings highlight the varying corrosion behavior exhibited by the SL and FA samples over time. The SL samples revealed a consistent oscillating trend in Icorr, whereas the FA samples initially demonstrated lower Icorr values, which later exhibited similar oscillatory behavior. Figures 8–10 represent the progression of Icorr over time using electrochemical measurements for SL and FA samples, having different size reservoir lengths.

For SL and FA concrete mixes, Tables 6 and 7 provide the average Icorr and standard deviation (STD) values obtained from measurements carried out using the LPR/EIS technique. These tables specifically showcase the data collected from day 480 until the end date, encompassing the last 7 sets of measurements.

Based on Table 6, it is worth mentioning that the rebar embedded in sample SL-1 exhibited the highest average Icorr value, measuring 59.8 μA. Notably, SL-1 had a longer solution reservoir length of 17.5 cm. The rebar embedded in the sample SL-5, on the other hand, had the lowest average Icorr value of 3.4 μA due to its shorter 2.5 cm solution reservoir length. These findings highlight the correlation between the reservoir length and average Icorr values for the respective samples. Table 7 shows that the rebar embedded in the sample FA-6, which had a 7.5 cm length for the solution reservoir, had the highest average Icorr value of 54.8 μA. In contrast, the rebar in the FA-11 sample had the lowest average Icorr value of 2.1 μA due to its shorter 2.5 cm solution reservoir length. Insights into the variability of Icorr measurements within each sample are gained by examining the standard deviations. For the analyzed samples, Tables 6 and 7 show intriguing correlations between the reservoir length and average Icorr values. The data help in understanding the influence of the reservoir length on corrosion behavior in the respective reinforced concrete mixes. In addition, the standard deviations offer valuable information about the variability of Icorr measurements within each sample.

The Icorr average values of the rebars that were embedded in concrete test specimens, derived from both current and prior studies [3, 38, 39], are summarized in Table 8. In Table 8, the studies referenced as [3, 38, 39] implemented a methodology where the complete steel surface area was polarized to determine the corroding area. However, in the current study, the actual corroding area remained unknown. It is notable that the Icorr values obtained from various researchers exhibit variations. The observed variations can be ascribed to a range of factors encompassing diverse experimental setups, variations in the size of the specimens (both small and large), discrepancies in the total surface area being exposed to the corrosive environment, variations in the methods employed for chloride transport, discrepancies in the kinds of admixtures and raw ingredients utilized, variations in cement mixes, inconsistencies in the mixed design, variations in the microstructure of the concrete, differences in the steel potential, discrepancies in the total ampere-hour applied, and variations in oxygen availability, moisture content, as well as environmental temperature [40, 41]. These diverse experimental parameters and conditions contribute to the observed discrepancies in Icorr values among different studies, emphasizing the need to consider and account for these factors when comparing results across different research endeavors.

The findings provided in this study enhance the comprehension of the corrosion behavior exhibited by reinforced concrete structures, a critical aspect for accurately assessing their remaining lifespan. However, to delve deeper into the underlying factors influencing the observed corrosion trends, further analysis and investigation are important. These additional studies will contribute to a more comprehensive understanding of the complex dynamics involved in corrosion processes and aid in the development of effective strategies for mitigating corrosion in reinforced concrete structures.

4. Rebar Corrosion in Terminated Specimens

For inspecting the state of the surface and sites of corrosion, the specimens SL-1, SL-2, FA-8, and FA-9 were terminated and cut open. Figure 11 shows that the rebar in the specimen SL-1 had a modest area of corrosion on it. After cleaning, a modest degree of corrosion is observed in a localized region, measuring approximately 3–4 mm in length. On the other hand, Figure 12 reveals the existence of two corrosion spots on the top surface of the rebar that was embedded in the specimen SL-2. These corroding sites appear to be slightly deeper compared to the rebar taken from the specimen SL-1. Considering that the solution reservoir had a length of approximately 17.5 cm, it indicates that corrosion initiated and propagated only on a small portion of the surface exposed to electromigration.

Figures 13 and 14 display the condition of rebar surfaces before and after cleaning in specimens FA-8 and FA-9, respectively. In Figure 13, a small, corroded area is evident on the rebar embedded in the specimen FA-8, with a noticeable but not substantial cross section loss. Figure 14 reveals the development of three corrosion spots on the rebar within the sample FA-9. The smallest spot is somewhat challenging to identify in the postcleaning image due to the angle of the picture (indicated by an arrow to highlight its location). The larger corroded spots exhibit a modest yet significant reduction in the cross section. It should be noted that the rebar in the specimen FA-8 underwent the application of an anodic current for a brief duration to accelerate corrosion. Despite having a 17.5 cm long solution reservoir, only a small fraction of the area experienced corrosion. Notably, the opposite side of both rebars did not show any signs of corrosion.

The quantity of corrosion products necessary to cause the concrete to crack may vary depending on the size of the corroding sites. Under the reservoir, there may be corroding regions and the corrosion products may have penetrated the concrete for a few millimeters [42]. It was found that corrosion was localized on most of the specimens upon forensic analysis. It was also found that the exposed area of the rebar was larger than the corroding area, and no cracks were observed [42]. It can be speculated that smaller corroding sites require greater amount of mass loss to cause concrete-cover cracking. The high moisture state of the concrete raises the possibility that the corrosion products may disperse over a wider area rather than concentrating the bursting force in one place. There were no visible cracks or corrosion bleed outs on the specimens because the corrosion products may have moved through the concrete pore structures and may have found a place distant from the reinforcing surface in the concrete cover. Very small corroding sites and relatively moderate cross-sectional loss were observed in the few specimens that were terminated.

5. Conclusion

The use of electromigration in this investigation proved to be a successful method for accelerating the transportation of chlorides, leading to the initiation of corrosion on all the specimens within a short period of time (between a few weeks and several months).

The Icorr values derived from experiments for various concrete mixtures were significantly influenced by the length of the solution reservoirs. In most cases, the corrosion rate (Icorr) of steel reinforcement rebars within concrete samples exhibited an upward trend as the length of the solution reservoirs increased. Nevertheless, a notable anomaly was observed in the case of rebars embedded in FA specimens equipped with a 7.5 cm solution reservoir. So, it can be said that Icorr is a direct function of the length of solution reservoir. By analyzing the Icorr values derived from the most recent seven sets of readings, it was noted that the rebars embedded in SL mix exhibited the highest values, while those in specimens prepared with FA mix had slightly lower values. The range of values associated with Icorr for SL samples were between 0.5 and 84.3 μA, while for FA samples, it ranged from 1.3 to 66.9 μA.

No cracks or corroded products were being observed on the surface of the terminated concrete sample. The quantity of corrosion products necessary to produce cracking in the concrete could depend on the corroding sites’ size. It is hypothesized that the high content of moisture of the concrete enabled the corroded products to penetrate the pore structure in the liquid form, preventing the concentration of bursting force in a single location. No obvious cracks or corrosion bleed outs were noticed throughout the monitoring period of around 650 days.

A noticeable variation in corrosion-related parameters (rebar potential, Rs, Rc, and Icorr) was also observed obtained from different electrochemical measurements. This was because of changing parameters such as concrete compositions, concrete-cover thickness, and reservoir size, as well as other factors such as temperature, moisture content, RH, exposure conditions, and oxygen availability. Therefore, it can be said that the natural corrosion process will also be affected by these parameters.

In summary, this study provides valuable insights into the accelerated corrosion of steel embedded in concrete structures and demonstrates the effectiveness of electromigration as a technique for accelerating chloride-induced corrosion.

Data Availability

The data used to support the findings of this study are included within the article.

Disclosure

The opinions expressed in this paper are those of the authors and not necessarily those of FDOT, USA.

Conflicts of Interest

The authors declare that there are no conflicts of interest.

Acknowledgments

The authors are indebted to the Florida Department of Transportation (FDOT) for preparing the samples.

Supplementary Materials

Table A1: Composition of the slag (SL) mix [28]. Table A2: Composition of the fly ash (FA) mix [28]. (Supplementary Materials)

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Copyright © 2023 Kazi Naimul Hoque et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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