Evolution of rebar potential, Rs, and Rc for SL & FA samples
In the figures in this section and next section, day zero does not correspond to the age of the specimen but rather to the day when solution was poured into the reservoirs. The dashed lines are followed (to the right of them) by an arrow mark, which designates the after-migration period. If there are two black dashed lines, the range represents the overall amount of time that the samples were subjected to electromigration process, and the blue prisms inside represent the approximately measured amount of time that the electric field was applied. As some of the samples went through slightly accelerated corrosion mechanism by means of a modest applied current, a blue solid line was used to indicate the beginning of this process and the gray columns indicate each instance where applied current took place. For ease of comparison specimens with similar reservoir lengths are compared together. Figure 2 shows the comparison in rebar potential, Rs and Rc value trend for various samples according to their reservoir size respectively.
Figure 2 shows four plots describing the rebar potential, Rs and Rc evolution for four different samples with rebars under 5 cm solution reservoir. The SL-7, SL-8, FA-1, and FA-3 all had a reservoir size of 5 cm. It is interesting to note the variety of potential ranges observed. The rebar potential values for the rebar in SL-7 dropped significantly after removing electromigration, reaching a value of around − 0.435 Vsce at day 390, tended to drift towards more positive values thereafter. Rc values for the rebar in SL-7 were mostly kept below 3 kΩ throughout the monitored period. Regarding rebar embedded in sample SL-8, after removing electromigration, there was a substantial decrease in both the rebar potential and Rc values. The rebar potential values began to shift towards more positive values from day 490 onwards, while the Rc values exhibited an oscillating pattern subsequently. Although the rebar in FA-1 exhibited a more negative trend for its rebar potential during the initial stages of electromigration, the rebar potential reached − 0.155 Vsce after electromigration was eliminated on day 295, with a Rc value that exceed 2 kΩ. Subsequently, the rebar potential showed a monotonic increase from day 340 onwards to the entire monitoring period thereafter, while the Rc values tended to exhibit an oscillating pattern. The sample FA-1 was subjected to modest anodic polarization around day 330, which persisted until approximately day 690. The rebar in FA-3 required a comparatively shorter period of electromigration when compared to other samples with similar reservoir sizes. The rebar potential values for the FA-3 sample were recorded at -0.468 Vsce on day 81, then showed a monotonic increase until day 220, followed by potential value dropped to -0.502 Vsce on day 619, and tended to shift towards more positive values thereafter. Throughout the monitoring period, the Rc values for the rebar in FA-3 remained mostly below 2 kΩ. For all these samples, the rebar potential values demonstrated a tendency to drift towards more positive values as the days progressed. The rebar potential values measured at day 1069 were − 0.070 Vsce (SL-7), -0.114 Vsce (SL-8), -0.011 Vsce (FA-1), and − 0.163 Vsce (FA-3).
Evolution of corrosion current
The following section shows how corrosion current (Icorr) changed over time as determined by LPR measurements for various samples. For SL and FA samples, the Icorr plots correspond to the values measured for around 1100 days using the LPR method.
The evolution of corrosion current (Icorr) with time obtained from LPR method for SL single rebar samples under different size solution reservoirs are shown in Fig. 3. In the case of samples with 17.5 cm solution reservoir length, it is observed that all the samples (SL-1, SL-2, and SL-3) showed oscillating trend in terms of corrosion current with time. Icorr values ranged from 24.6–84.2 µA for SL-1 sample, 16.0-43.7 µA for SL-2 sample, and 20.5–44.5 µA for SL-3 sample. It is to be noted that SL-1 and SL-2 samples were terminated after approximately 600 days. When looking at the Icorr plots for samples with 10 cm solution reservoir length, it is noted that SL-9, SL-10, and SL-11 samples showed oscillating trend with time as well. Icorr values ranged from 3.8–93.5 µA for SL-9 sample, 10.2-116.4 µA for SL-10 sample, and 13.3–70.5 µA for SL-11 sample. Most Icorr values were less than 40 µA. In the case of 5 cm solution reservoir length, it is found that SL-6 sample showed a decreasing trend initially, followed by an oscillating trend throughout the monitored period and Icorr values ranged from 1.8–35.6 µA. A significant decrease of corrosion current was observed for SL-7 sample until day 180, therefore represented an oscillating pattern with time, having only one excursion to a value of 110.6 µA at day 585, with most other values being less than 20 µA. For SL-8 sample, the corrosion current values were mostly less than 20 µA throughout the monitored period and ranged from 2.3–43.0 µA in terms of Icorr values. While observing Icorr plots for 2.5 cm solution reservoir length, it is observed that for SL-4 sample, Icorr value dropped significantly around day 160, showed slight fluctuations over time but the values were under 10 µA for most of the monitored periods, and Icorr ranged from 0.6–47.5 µA. In case of SL-5 sample, corrosion current values showed a plateau trend from 190–390 days, showed slight fluctuations with time but the values were under 10 µA for most of the periods, and Icorr ranged from 0.5–36.2 µA.
Figure 4 displays the evolution of corrosion current (Icorr) with time for FA single rebar samples under various size solution reservoirs as determined by the LPR method. For 17.5 cm solution reservoir length, it is found that all the samples (FA-7, FA-8, and FA-9) showed oscillating trend in terms of corrosion current with time. Icorr values ranged from 4.8–69.8 µA for FA-7 sample, 5.1–90.0 µA for FA-8 sample, and 7.0-42.8 µA for FA-9 sample. It is to be mentioned that FA-8 and FA-9 samples were terminated after approximately 600 days. It is noticed when examining the Icorr plots for 7.5 cm solution reservoir length that the FA-4, FA-5, and FA-6 samples also displayed an oscillating pattern over time. Icorr values for FA-4 sample, FA-5 sample, and FA-6 sample varied from 7.6–45.5 µA, 8.6–45.4 µA, and 8.3–81.8 µA, respectively. While observing the 5 cm solution reservoir length, it is interesting to note that FA-1 and FA-2 samples showed certain fluctuations in terms of Icorr over time, and the values were less than 20 µA throughout most of the monitored period. Icorr values ranged from 3.1–20.0 µA and 2.0-65.9 µA for FA-1 and FA-2 samples, respectively. The rebar embedded in FA-3 sample showed an oscillating Icorr trend over time and corrosion current values ranged from 6.0-45.7 µA. In case of 2.5 cm solution reservoir length, it is observed that FA-10 sample showed a decreasing trend initially, reached a peak value at day 480, and Icorr values ranged from 2.2–31.6 µA. FA-11 sample showed a significant decrease initially, therefore followed slight fluctuations with time, and Icorr ranged from 1.3–73.5 µA.
Table 4 and Table 5 shows average Rs, average Rc, and average Icorr values obtained from the measurements performed from day 750 to day 1100 (last 10 sets of measurements reported in here) using LPR/EIS method. The goal was to acquire a comprehensive assessment of the corrosion occurring in the rebar embedded within the concrete specimens during this period of time.
Table 4 indicates that the rebars embedded in specimens with the smaller solution reservoir of 2.5 cm had the highest Rs(average) and highest Rc(average) values during the indicated period. However, for the rebars embedded in specimens with the longer solution reservoir of 17.5 cm, the lowest Rs(average) and lowest Rc(average) values were obtained. It was interesting to note that the rebar with the largest average Icorr was the rebar embedded in sample SL-1 with an average value of 59.8 µA, which had the longer solution reservoir of 17.5 cm. The rebar with the lowest average Icorr was for the rebar embedded in SL-4 sample, that had the smaller solution reservoir of 2.5 cm. Icorr average value for SL-4 sample was found to be 6.0 µA.
Table 4
Average: Rs, Rc, and Icorr obtained from LPR/EIS readings – SL specimens
Sample Number | Reservoir Length (cm) | Average values from LPR/EIS |
Rs (kΩ) | Rc (kΩ) | Icorr (µA) |
SL-1 | 17.5 | 0.9 | 0.5 | 59.8 |
SL-2 | 1.2 | 0.9 | 38.8 |
SL-3 | 1.8 | 0.9 | 33.1 |
SL-4 | 2.5 | 16.9 | 8.6 | 6.0 |
SL-5 | 14.6 | 4.2 | 6.5 |
SL-6 | 5 | 6.2 | 2.1 | 13.7 |
SL-7 | 5.6 | 2.9 | 11.9 |
SL-8 | 5.6 | 3.5 | 10.2 |
SL-9 | 10 | 2.0 | 1.1 | 25.8 |
SL-10 | 2.3 | 1.7 | 16.4 |
SL-11 | 1.9 | 1.2 | 22.9 |
Table 5
Average: Rs, Rc, and Icorr obtained from LPR/EIS readings – FA specimens
Sample Number | Reservoir Length (cm) | Average values from LPR/EIS |
Rs (kΩ) | Rc (kΩ) | Icorr (µA) |
FA-1 | 5 | 5.4 | 3.2 | 10.4 |
FA-2 | 6.3 | 3.6 | 9.9 |
FA-3 | 5.2 | 2.3 | 13.2 |
FA-4 | 7.5 | 3.0 | 1.8 | 16.4 |
FA-5 | 2.3 | 1.3 | 21.5 |
FA-6 | 3.3 | 1.7 | 18.4 |
FA-7 | 17.5 | 1.8 | 1.4 | 22.6 |
FA-8 | 1.1 | 1.1 | 35.2 |
FA-9 | 1.3 | 1.1 | 25.9 |
FA-10 | 2.5 | 11.8 | 7.7 | 3.8 |
FA-11 | 16.5 | 7.7 | 3.5 |
From Table 5, it was found that the rebars embedded in specimens with the smaller solution reservoir of 2.5 cm had the highest Rs(average) and highest Rc(average) values during the indicated period. On the other hand, the rebars embedded in specimens with the solution reservoir of 17.5 cm had the lowest Rs(average) and lowest Rc(average) values. It was noted that the rebar with the largest average Icorr was the one embedded in sample FA-8, which had a 17.5 cm solution reservoir and an average value of 35.2 µA. The rebar embedded in the FA-11 sample, which had a smaller solution reservoir of 2.5 cm, had the lowest average Icorr value. The FA-11 sample's Icorr average value was found to be 3.5 µA.
During Otieno et al.'s laboratory experiment, they observed significant disparities in the corrosion characteristics between SL (50% GGBS) and FA (30% fly ash) concrete specimens [3]. The study examined concrete specimens with a w/cm ratio of 0.40 and two different cover depths, 40 mm, and 20 mm. The specimens were subjected to an accelerated laboratory corrosion (i.e., cyclic 3 days wetting with 5% NaCl solution followed by 4 days air-drying). These experiments were conducted in a controlled laboratory environment with a temperature of 25 ± 2°C and a RH of 50 ± 5%. In Otieno's study, an anodic impressed current (IC) was applied to induce an active corrosion state in the specimens. It was assumed that the entire exposed steel surface area of 86 cm2 (approximately 27.5 cm long circumferential steel surface) was undergoing corrosion. The SL concrete specimens with a 40 mm cover depth had an average Icorr value of 32.7 µA, while their counterparts with a 20 mm cover depth showed an average Icorr of 44.7 µA [3]. The FA concrete specimens with a 40 mm cover depth exhibited a significantly higher average Icorr value of 91.2 µA, whereas those with a 20 mm cover depth had an average Icorr of 49.9 µA [3]. The age of the specimens at the time of data collection was approximately 854 days, equivalent to 122 weeks. Otieno's findings indicated that all the examined specimens were in a state of high active corrosion.
Hope and Ip conducted experiments on SL samples composed of 50% slag and 50% PC, featuring a w/cm ratio of 0.45 and a cover depth of 56 mm [23]. These specimens underwent multiple wetting and drying cycles, involving immersion in a 3.5% NaCl solution followed by exposure to laboratory air to allow chloride penetration into the concrete. It was assumed that the entire exposed steel surface area of 102 cm2 was corroding. Initially, the average Icorr values for these concrete specimens were approximately 51 µA during the first 200 days of exposure [23]. Subsequently, as the specimens went through the process of oven-drying, followed by wetting and drying cycles, the average Icorr values displayed an upward trend, reaching around 61.2 µA within the exposure period of 270 to 315 days [23]. The results of Hope and Ip’s experiment indicated that all the specimens were experiencing a state of high active corrosion.
O'Reilly et al. conducted laboratory experiments involving various types of concrete specimens, including Slag (20) and Slag (40) with 20% and 40% by volume of Grade 100 slag cement, as well as FA (20) and FA (40) with 20% and 40% by volume of Class C fly ash [24]. These specimens all had a w/cm ratio of 0.45 and a cover depth of 25 mm. In O'Reilly's study, the specimens were subjected to alternating exposure cycles, comprising 12 weeks of wet-dry cycles followed by 12 weeks of continuous wet cycles. During the wet-dry cycles, the specimens were ponded with 300 mL of a 15% NaCl solution and maintained at room temperature for 4 days. After this period, corrosion measurements were taken, the salt solution was removed, and the specimens were placed under a heat tent at 100 ± 3°F (38 ± 2°C) for 3 days. This cycle was repeated for a total of 12 weeks. Subsequently, the specimens entered a continuously wet cycle, during which they were continuously ponded with a 15% NaCl solution and kept covered at room temperature for another 12 weeks. In O'Reilly's experiment, it was assumed that an exposed steel surface area of 152 cm2 was undergoing corrosion. For the Slag samples, the average Icorr values were 21 µA for SL (20) and 13.8 µA for SL (40) specimens [24]. Conversely, for the FA specimens, the average Icorr values were 38.5 µA for FA (20) and 17.6 µA for FA (40) specimens, respectively [24]. The age of these specimens at the time of data collection was approximately 672 days (96 weeks), during which it was observed that most of the specimens were in a state of high active corrosion. It is important to mention that in the investigations carried out by Otieno et al., Hope and Ip, as well as O'Reilly et al., the corrosion current density values reported were converted into Icorr values [3, 23, 24].
It is important to note that any rebar section outside of the concrete as well as any section of the rebar not exactly below the reservoir influenced the corrosion current and other readings. In some instances, the moisture level was so high that the rebar that was exposed to the atmosphere outside the concrete corroded. A corroding site (embedded or not) could act as cathodic protection (prevention) on the rest of the rebar surface, even if chlorides at the rebar surface were high. Current reduction could be indicative that corroding site was not as active or that some area might have repassivated as suggested by the mix potential measured at later times. A summary of the variation of average corrosion current with length of solution reservoir and concrete mixes are shown graphically in Fig. 5. The average corrosion current values were calculated from the measurements taken from day 750 to day 1100 using LPR/EIS method (last 10 sets of readings). An intriguing observation revealed that, with the increase in the length of the solution reservoir, the corrosion current values also showed a corresponding increase for rebars embedded in both SL and FA concrete mixes. This might be attributed to the increasing length of the solution reservoir, which would result in greater exposure of the rebar surface to chlorides. Consequently, the likelihood of rebar corrosion occurrence could also increase. Furthermore, it is worth noting that, when comparing similar reservoir lengths, the corrosion current values for rebars embedded in SL concrete mixes were larger in comparison to those of rebars embedded in FA concrete mixes.
Theoretical (Faradaic) calculation of mass loss
As no samples in this investigation had any apparent cracks, a theoretical mass loss method was taken into consideration. Rc values were obtained periodically using the LPR and EIS techniques. Corrosion current was generated using the Rc values obtained using the LPR/EIS techniques. Two successive Rc measurements were used to compute the average corrosion current (the average of the two consecutive values for that period). The total amount of charge was then computed by multiplying it by the time difference between each measurement, and each rebar's estimated charge values were added as given in Eq. (1). The apparent mass loss was calculated using Faraday's equation as illustrated in Eq. (2).
$$C=\sum _{N=1}^{n}\left(\frac{{I}_{N }+{I}_{N-1}}{2}\right){t}_{N}$$
1
Where C is in coulombs and t is in seconds.
Calculated mass loss by using Faraday`s law is given by-
$$Mass Loss=C*Atomic Mass/nF$$
2
where Atomic Mass is 55.85g (for Fe), n is 2 (# of electrons), and F is 96,500 C (Faraday’s constant).
Table 6
Estimated Mass loss in grams obtained from LPR readings for SL single rebar samples.
Sample Number | Reservoir Length (cm) | Mass Loss (grams) |
---|
SL-1 | 17.5 | 0.719 |
SL-2 | 0.690 |
SL-3 | 1.186 |
SL-4 | 2.5 | 0.181 |
SL-5 | 0.150 |
SL-6 | 5 | 0.326 |
SL-7 | 0.528 |
SL-8 | 0.344 |
SL-9 | 10 | 0.740 |
SL-10 | 0.739 |
SL-11 | 0.918 |
Table 7
Estimated Mass loss in grams obtained from LPR readings for FA single rebar samples.
Sample Number | Reservoir Length (cm) | Mass Loss (grams) |
---|
FA-1 | 5 | 0.254 |
FA-2 | 0.278 |
FA-3 | 0.582 |
FA-4 | 7.5 | 0.676 |
FA-5 | 0.603 |
FA-6 | 1.002 |
FA-7 | 17.5 | 0.565 |
FA-8 | 0.312 |
FA-9 | 0.306 |
FA-10 | 2.5 | 0.183 |
FA-11 | 0.221 |
The estimated mass loss in grams for SL and FA single rebar samples obtained using the LPR technique is highlighted in Tables 6 and Table 7. The presented values are grouped based on the length of the installed solution reservoir. The measurements performed during the monitored period, which lasted around 1100 days, were used to determine the mass loss values. It is noted that SL-1, SL-2, FA-8, and FA-9 samples (17.5 cm reservoir lengths) were terminated after approximately 600 days. The computed mass loss for rebars in SL samples with a 17.5 cm solution reservoir size varied from 0.69 to 1.19 grams, while it varied from 0.31 to 0.57 grams for rebars in FA samples. Rebars in SL samples exhibited a mass loss ranging between 0.74 and 0.92 grams for rebars under 10 cm solution reservoir size, while it ranged between 0.60 and 1.01 grams for FA samples with a reservoir length of 7.5 cm. It was observed that the mass loss values were quite similar and comparable for samples with reservoir lengths of 2.5 cm when comparing the mass loss for SL and FA single rebar samples. For SL samples, longer reservoir length samples (17.5 cm and 10 cm) showed larger mass loss values. For FA samples, it was found that samples with reservoir length of 7.5 cm showed larger mass loss value.
A summary of the variation of average mass loss values with length of solution reservoir and concrete mixes cast with single rebar are shown in Fig. 6. The average mass loss values for different concrete mixes (per reservoir size) were calculated via LPR method from the readings taken throughout the monitored period of approximately 1100 days. An intriguing observation is that as the length of the solution reservoir increases, the average mass loss value also increases for rebars embedded in both SL and FA concrete mixes. However, there was an exception to this trend for FA samples with 7.5 cm reservoir lengths, where the average mass loss value was the largest.
Concrete beam specimens and corroding cylindrical specimens reinforced with carbon steel rebar were the subjects of experimental research by Torres-Acosta [25]. During preparation, chlorides were added to the concrete mix of these specimens. All specimens were exposed to a RH of 75%. During Torres-Acosta’s study, an impressed current of 100 µA/cm² was utilized to induce corrosion. In the case of concrete beam samples, the observed mass loss values ranged from 0.3 to 14.4 grams based on forensic analysis, and from 0.3 to 12.5 grams through Faradaic calculations. For concrete cylindrical specimens undergoing corrosion, the mass loss values ranged from 0.7 to 5.1 grams as determined by gravimetric analysis, and from 0.6 to 5.8 grams using Faradaic calculation. It was reported that the calculated mass loss values (amount of corrosion products) caused cracks in all the specimens [25], but no cracks were observed by visual inspection in case of current investigation, as it will be shown below that the corrosion in the present study did not take place in the whole rebar, whereas in Torres-Acosta it did.
An experimental investigation was performed on reinforced concrete pipe sections having two different types of concrete compositions [26]. One composition contains fly ash (20% cement replacement), and the other composition contains ordinary Portland cement as the cementitious material. During preparation, chlorides were not added to the concrete mix of these specimens. These specimens were exposed to different environmental conditions. In this investigation, an electromigration technique was employed to initiate corrosion [26]. In the case of compositions containing fly ash exposed to high humidity environment (95% RH, and 21ºC), the mass loss values ranged from 2.0 to 10.3 grams based on forensic examination, and from 1.9 to 11.3 grams by Faradaic calculation. In scenarios where compositions containing ordinary Portland cement were subjected to high humidity environments, the observed mass loss values ranged between 0.6 to 3.2 grams based on forensic analysis, and between 1.0 to 3.9 grams through Faradaic calculation. In the case of vertically exposed (water) fly ash specimens, the mass loss values ranged from 0.6 to 1.2 grams through forensic analysis, and from 2.0 to 5.9 grams through Faradaic calculation. Conversely, in the context of horizontally exposed (water) fly ash samples, the mass loss values ranged from 0.1 to 0.3 grams based on forensic examination, and from 1.9 to 2.0 grams through Faradaic calculation. In the current study, the actual size of the corroding sites was unknown as most of the specimens were not terminated for forensic analysis. 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 [26]. 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 [26]. 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.
Forensic analysis
Figure 7 shows that only a small corrosion spot is present on the rebar embedded in specimen SL-1. A modest amount of corrosion is observed in the region after cleaning. The length of the corroding spot is about 3 to 4 mm [18, 22]. Figure 8 shows that there were two corrosion spots on the top surface of the rebar embedded in specimen SL-2. The corroding sites were somewhat deeper on the rebar from SL-2 specimen than the rebar embedded in SL-1 specimen. The solution reservoir was about 17.5 cm long; thus, it appears that corrosion initiated and propagated on a small fraction of the surface subjected to electromigration [18, 22].
Figure 9 and Fig. 10 show the rebar surface before and after cleaning on rebars embedded in specimens FA-8 and FA-9, respectively [18, 22]. Figure 9 shows a small corroding site before cleaning on the rebar embedded on specimen FA-8; the cross-section loss is visible but not significant. Figure 10 shows that three corrosion spots developed on the rebar embedded in sample FA-9. The smallest spot is a little hard to see on the picture after cleaning due to the angle of the picture (an arrow indicates the location). The larger corroding spots show a modest but significant cross-section loss; the rebar in specimens FA-9 was subjected to modest anodic current to accelerate corrosion for a short period of time. Both rebars had a solution reservoir of about 17.5 cm long, hence just a small fraction of the area corroded. The opposite side of both rebars showed no corrosion.
General discussion
Accelerated steel corrosion in concrete is needed for timely findings to be obtained in laboratory settings. To examine the corrosion propagation phase in RC structures, particularly in the marine environment, the electromigration method was introduced to initiate corrosion of the embedded rebar. An electric field then drove the chlorides in the solution above each rebar via electromigration method into the concrete and towards the embedded rebar. After a short period of time (weeks/months), corrosion of the embedded rebar started. The rebar potential and Rc values were used to identify the initiation of the corrosion (Rc values were converted to Icorr values). Shortly after the electromigration was stopped, further rebar potential drops were observed. The rebar potential values became more negative than − 0.200 Vsce for most of the samples, indicating that the accelerated chloride transport method was successful in initiating corrosion (immediately or after a certain period from suspending electromigration) of the rebars embedded in high-performance concrete specimens.
It may be said that the EIS and LPR measurements were effective in monitoring the corrosion potential, Rs, Rc, and Icorr values. During the corrosion propagation stage, the values sometimes fluctuated. In some instances, the rebar potential and accompanying Rc value shifted toward more positive potentials and larger Rc values, which are both indicative of lower Icorr values. This phenomenon may occur if corrosion progressed more slowly. In other instances, it is hypothesized that the rebar repasivated or that over time, the corroding sites (corroding at slower rates) were polarized to a more positive value by the non-corroding portion of the rebar.
Several types of rebar potential transients were observed throughout the monitored period. The transient might sometimes be influenced by the concrete's composition, the size of the solution reservoir, total ampere-hour applied, and other factors such as the moisture content at rebar depth, oxygen availability, and relative humidity around the samples.
The samples were kept inside in a lab setting with a reasonably high humidity environment. All the samples had a section of the rebar that was exposed outside of the concrete that was originally uncovered. In a few instances, the rebar was found to have droplets of moisture that formed due to the high humidity environment, which in some cases led to the outer rebar portion corroding. In other cases, the solution that had filled the reservoir spilled outside, reaching the rebar rather than the concrete. This could lead to corrosion in the extended rebar section, which might potentially have an impact on the measured rebar potential values and corrosion rates. After around 500 days of corrosion propagation period, the rebar section extending outside of the concrete was wire-brushed, sanded, and then covered with shrinkage wrap to limit any corrosion of the rebar surface. As compared to earlier readings, electrochemical measurements beyond this stage were found to be more consistent.
It has been reported that in some cases, in which chloride-induced corrosion took place rust‐stains or cracks were not visible and upon forensic examination a significant cross-sectional loss was observed at the corroding site [27]. It was described that for a purely chloride‐induced (pitting) corrosion, the usually assumed model (i.e., corrosion taking place around the whole rebar) appears not to be suitable to describe the occurring processes for several reasons [27]. First, iron dissolution only takes place locally rather than uniformly around and along the entire rebar. Second, the corrosion products are much more soluble, and precipitation is unlikely, near the corroding spot (due to presence of chlorides which enhance solubility). Finally, initiation of chloride‐induced corrosion is often associated with local weaknesses in the steel/concrete interface, viz. higher local porosity, which would-for the case of precipitation, additionally prevent formation of expansive pressure. Angst paper documented cases in which corrosion initiated by chlorides and that there were little signs (or no) of corrosion stains at the surface during inspections. It was found that upon terminating, a significant cross-sectional loss was found. Similar observations could be found upon terminating some of the samples from this research work.