# Chloride Levels That Initiated Corrosion of Duplex Stainless Steel Embedded in Mortar

Authors: Yu-You Wu; Francisco Presuel-Moreno
Journal: Advances in Materials Science and Engineering
 (2019)

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## Abstract

Chloride-induced corrosion of carbon steel has been widely recognized as one of the main causes of premature failure on the reinforced concrete structures. Various strategies and measures such as employing stainless steel reinforcements have been developed to address this problem. Past studies have been concerned with the identification and characterization of chloride threshold since corrosion would not initiate as long as the chloride concentration values at the reinforcing stainless steel depth remains below this threshold value. It is therefore a critical parameter for the design of new stainless steel reinforced concrete structures and the assessment of existing concrete structures. This study presents the finding on the chloride threshold of stainless steel UNS32304 embedded in mortar with two different mixes. Reinforced mortar specimens were subjected to ponding exposure and wet/dry cycle exposure with a sodium chloride solution. The specimens were monitored by using the measurements of the open circuit potential, electrochemical impedance spectroscopy, and linear polarization resistance. The paper also discusses the chloride threshold values of such stainless steel embedded in mortar and concrete with other mixes reported by other researchers and the factors that may affect these values.


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## Body

## 1. Introduction

Chloride-induced corrosion of carbon steel has been widely recognized as one of the prominent causes of premature failure on the reinforced concrete structures [1]. Various strategies and measures have been developed to address this problem [2], for instance, improving concrete quality, increasing concrete cover of steel rebars, adding corrosion inhibitors, installing cathodic prevention system, and employing zinc-coated steel, using epoxy-coated steel reinforcements and using corrosion-resistant alloy reinforcements. For the latter, it is reported that a concrete pier with stainless steel bars built from 1937 to 1941 in Mexico’s marine environment is still in good condition without the structural damages due to the corrosion of stainless steel rebar [3]. Today, it has become a normal practice to use the corrosion-resistant alloy rebars such as stainless steel in the construction of new bridges exposed to chloride environments as to achieve bridge repair-free within the designed service life in Virginia and Oregon, USA [4].The chloride threshold is one of the critical parameters for the durability analysis of reinforced concrete structures [5]. Many studies have been conducted to identify the chloride threshold values of the corrosion-resistant alloy rebars such as austenitic stainless steel, duplex stainless steel, and low alloy content stainless steel (e.g., UNS S 30400, UNS S30403, UNS S31600, UNS31603, UNS S31653, UNS S32101, UNS S31803) [6–8]. It has been reported that, for a given reinforcing steel rebar, the chloride threshold value is affected by many different factors, which include specimen size, specimen total surface area exposed, chloride transport method [5] (e.g., dry/wet cycles, electric field application, and chlorides cast during mix preparation), the raw materials used, cement composition, steel potential, and environmental temperature. But there is insufficient information reporting the effect of mortar or concrete mix on the chloride critical concentration.This paper presents the finding on the chloride threshold of stainless steel UNS32304 embedded in mortar with the water-to-cement ratios of 0.5 and 0.41. The surface condition of the rebar was as-received (pickled). Reinforced mortar specimens were subjected to ponding exposure and wet/dry cycle exposure with a 15% sodium chloride solution. The specimens were monitored by using the measurements of the open-circuit potential, electrochemical impedance spectroscopy, and linear polarization resistance. The paper also discusses the chloride threshold values of such stainless steel embedded in mortar and concrete with other mixes reported by other researchers and the factors that may affect these values.


## 2. Materials and Methods

### 2.1. Stainless Steel Rebar and Mortar

The chemical compositions of stainless steel UNS32304 are shown in Table1. The rebar was segmented into 16 cm long pieces for the mortar mix 1 specimens and 32 cm long for the mix 2 mortar specimens. The surface condition of the rebar was as-received (pickled). The mortar mixes employed to fabricate specimens with the materials and proportions are listed in Table 2. Type I/II cement was used. Silica sand from Florida was used in saturated surface dry condition before adding it to the mix. Mix 1 had a target water-to-cement ratio (w/c) of 0.5, and the proportion of the mortar mix was 2.7/1 for sand/cement. Mix 2 had a w/c of 0.41, and the proportion of the mortar mix was 2.78/1 for sand/cement.Table 1
Compositions of stainless steel UNS32304.
Component
Mo
Cu
Ni
Fe
Mn
Cr
Content (%)
0.27
0.30
4.09
71.60
1.25
21.90Table 2
Details of mortar mixes.
Materials
Type I/II cement (density: kg/m3)
Water (density: kg/m3)
Fine aggregate (density: kg/m3)
Water-to-cement ratio (w/c)
Sand/cement
Mix 1
2142
1000
1682
0.50
2.70/1
Mix 2
2142
1000
1682
0.41
2.78/1


### 2.2. Specimen Preparation and Experimental Setup

The Mix 1 mortar specimens were 7.5 cm tall, 12.5 cm wide, and 13.5 cm long (direction of the rebar). A schematic illustration of the specimen is shown in Figure1. A couple of mortar specimens (front view) are shown in Figure 2. The Mix 2 mortar specimens were 7.5 cm tall, 12.5 cm wide, and 30 cm long (direction of the rebar), with 25.4 cm of each rebar exposed under a solution reservoir. There are four specimens (32304-C1, 32304-C2, 32304-C3, and 32304-C4) in Mix 1 mortar and three (32304-D1, 32304-D2, and 32304-D3) in Mix 2 mortar as shown in Table 3.Figure 1
Schematic illustrations of the specimen: (a) cross section in parallel with the rebar direction; (b) front view (units: mm).
(a)
(b)Figure 2
A couple of mortar specimens with CRA rebars (front view).Table 3
Details of specimen series.
Mixes
Specimen series
Water-to-cement ratio (w/c)
Cover depth (cm)
Rebar size
Rebar surface condition
Mix 1
32304-C1
0.50
0.91
#5
Pickled
32304-C2
0.50
0.91
#5
Pickled
32304-C3
0.50
0.91
#5
Pickled
32304-C4
0.50
0.91
#5
Pickled
Mix 2
32304-D1
0.41
0.91
#5
Pickled
32304-D2
0.41
0.91
#5
Pickled
32304-D3
0.41
0.91
#5
PickledEach specimen consisted of one rebar. The rebar was centered along the 12.5 cm × 7.5 cm faces. Each rebar was drilled and tapped on one end to accommodate machine screws that were then used as electrical contacts. The average mortar cover was 0.91 cm (#5 Rebar). The rebars were placed at the bottom of the mold. The specimens were horizontally cast in three layers for each batch and were properly compacted by using a vibration table. Subsequent to casting, the specimens were covered with plastic wrap at room temperature for 24 hours. The molds were removed after 24 hours, and then the specimens were cured in a high-humidity environment for seven (7) days. The specimens were then moved to laboratory humidity and temperature (65–70% RH and 22°C). Once these were dry enough, the specimens were inverted to make the bottom surface at cast to the top side during exposure.A solution reservoir was installed by attaching a plastic cylinder with 2.5 cm tall, 9.5 cm internal diameter, and 11.2 cm outside diameter with a marine contact adhesive and sealant on the top face of each specimen. The samples remained with no solution in the reservoir for the rest of the second week. For the mortar samples with a target water-to-cement ratio (w/c) of 0.5 (Mix 1), thereafter a 15% NaCl solution was placed in the reservoir. Wet/dry cycles began on day 165. The specimens with a target water-to-cement ratio (w/c) of 0.41 (Mix 2) were prepared a few years back (currently ∼1600 days of exposure) and also had a solution reservoir installed and 15% NaCl solution. Continuous ponding took place for ∼1000 days before starting an alternate wet/dry schedule. The wet/dry schedule was 4 days wet and 3 days dry. The details of all specimen series are listed in Table3.


### 2.3. Specimen Monitoring

Specimen monitoring involved the measurements of the half-cell potential (OCP), electrochemical impedance spectroscopy (EIS), and linear polarization resistance (LPR) test during the whole period of exposure. The OCP was measured at least once a week using a saturated calomel reference electrode (SCE) [9], and it was carried out more frequently during the early exposure period. The solution resistance (Rs) was measured by employing electrochemical impedance spectroscopy (EIS) [10] with an impedance magnitude at a frequency of 56 Hz assumed. This was performed prior to the linear polarization resistance (LPR) test [10]. The LPR was performed every week at the beginning, and after ∼60 days, it was done every two weeks or at other frequencies. The LPR test was carried out from 15 mV below OCP to the OCP at a scan rate of 0.1 mV/s. During the alternate wet/dry period, these tests were performed the last wet day in the cycle. The LPR values (Rc) in the results sections are reported with the solution resistance (Rs) subtracted but no rebar area corrected.


### 2.4. Visual Inspection and Chloride Analysis

Two mortar specimens prepared with w/c = 0.5 and two specimens prepared with w/c = 0.41 were chosen for forensic analysis. Specimens were terminated at different ages on the basis of corrosion initiation. These specimens were then autopsied for visually spotting corrosion sites. In parallel with the work above, 1∼2 mm millings were taken from the mortar rebar-trace away from any observed corrosion sites in contact with the mortar. Corresponding chloride concentration was obtained by following the chloride titration procedure, which was performed on the milled powder as per the FDOT wet chemistry technique (modified to accommodate smaller mass) [11]. This method measures total chloride concentration.


## 2.1. Stainless Steel Rebar and Mortar

The chemical compositions of stainless steel UNS32304 are shown in Table1. The rebar was segmented into 16 cm long pieces for the mortar mix 1 specimens and 32 cm long for the mix 2 mortar specimens. The surface condition of the rebar was as-received (pickled). The mortar mixes employed to fabricate specimens with the materials and proportions are listed in Table 2. Type I/II cement was used. Silica sand from Florida was used in saturated surface dry condition before adding it to the mix. Mix 1 had a target water-to-cement ratio (w/c) of 0.5, and the proportion of the mortar mix was 2.7/1 for sand/cement. Mix 2 had a w/c of 0.41, and the proportion of the mortar mix was 2.78/1 for sand/cement.Table 1
Compositions of stainless steel UNS32304.
Component
Mo
Cu
Ni
Fe
Mn
Cr
Content (%)
0.27
0.30
4.09
71.60
1.25
21.90Table 2
Details of mortar mixes.
Materials
Type I/II cement (density: kg/m3)
Water (density: kg/m3)
Fine aggregate (density: kg/m3)
Water-to-cement ratio (w/c)
Sand/cement
Mix 1
2142
1000
1682
0.50
2.70/1
Mix 2
2142
1000
1682
0.41
2.78/1


## 2.2. Specimen Preparation and Experimental Setup

The Mix 1 mortar specimens were 7.5 cm tall, 12.5 cm wide, and 13.5 cm long (direction of the rebar). A schematic illustration of the specimen is shown in Figure1. A couple of mortar specimens (front view) are shown in Figure 2. The Mix 2 mortar specimens were 7.5 cm tall, 12.5 cm wide, and 30 cm long (direction of the rebar), with 25.4 cm of each rebar exposed under a solution reservoir. There are four specimens (32304-C1, 32304-C2, 32304-C3, and 32304-C4) in Mix 1 mortar and three (32304-D1, 32304-D2, and 32304-D3) in Mix 2 mortar as shown in Table 3.Figure 1
Schematic illustrations of the specimen: (a) cross section in parallel with the rebar direction; (b) front view (units: mm).
(a)
(b)Figure 2
A couple of mortar specimens with CRA rebars (front view).Table 3
Details of specimen series.
Mixes
Specimen series
Water-to-cement ratio (w/c)
Cover depth (cm)
Rebar size
Rebar surface condition
Mix 1
32304-C1
0.50
0.91
#5
Pickled
32304-C2
0.50
0.91
#5
Pickled
32304-C3
0.50
0.91
#5
Pickled
32304-C4
0.50
0.91
#5
Pickled
Mix 2
32304-D1
0.41
0.91
#5
Pickled
32304-D2
0.41
0.91
#5
Pickled
32304-D3
0.41
0.91
#5
PickledEach specimen consisted of one rebar. The rebar was centered along the 12.5 cm × 7.5 cm faces. Each rebar was drilled and tapped on one end to accommodate machine screws that were then used as electrical contacts. The average mortar cover was 0.91 cm (#5 Rebar). The rebars were placed at the bottom of the mold. The specimens were horizontally cast in three layers for each batch and were properly compacted by using a vibration table. Subsequent to casting, the specimens were covered with plastic wrap at room temperature for 24 hours. The molds were removed after 24 hours, and then the specimens were cured in a high-humidity environment for seven (7) days. The specimens were then moved to laboratory humidity and temperature (65–70% RH and 22°C). Once these were dry enough, the specimens were inverted to make the bottom surface at cast to the top side during exposure.A solution reservoir was installed by attaching a plastic cylinder with 2.5 cm tall, 9.5 cm internal diameter, and 11.2 cm outside diameter with a marine contact adhesive and sealant on the top face of each specimen. The samples remained with no solution in the reservoir for the rest of the second week. For the mortar samples with a target water-to-cement ratio (w/c) of 0.5 (Mix 1), thereafter a 15% NaCl solution was placed in the reservoir. Wet/dry cycles began on day 165. The specimens with a target water-to-cement ratio (w/c) of 0.41 (Mix 2) were prepared a few years back (currently ∼1600 days of exposure) and also had a solution reservoir installed and 15% NaCl solution. Continuous ponding took place for ∼1000 days before starting an alternate wet/dry schedule. The wet/dry schedule was 4 days wet and 3 days dry. The details of all specimen series are listed in Table3.


## 2.3. Specimen Monitoring

Specimen monitoring involved the measurements of the half-cell potential (OCP), electrochemical impedance spectroscopy (EIS), and linear polarization resistance (LPR) test during the whole period of exposure. The OCP was measured at least once a week using a saturated calomel reference electrode (SCE) [9], and it was carried out more frequently during the early exposure period. The solution resistance (Rs) was measured by employing electrochemical impedance spectroscopy (EIS) [10] with an impedance magnitude at a frequency of 56 Hz assumed. This was performed prior to the linear polarization resistance (LPR) test [10]. The LPR was performed every week at the beginning, and after ∼60 days, it was done every two weeks or at other frequencies. The LPR test was carried out from 15 mV below OCP to the OCP at a scan rate of 0.1 mV/s. During the alternate wet/dry period, these tests were performed the last wet day in the cycle. The LPR values (Rc) in the results sections are reported with the solution resistance (Rs) subtracted but no rebar area corrected.


## 2.4. Visual Inspection and Chloride Analysis

Two mortar specimens prepared with w/c = 0.5 and two specimens prepared with w/c = 0.41 were chosen for forensic analysis. Specimens were terminated at different ages on the basis of corrosion initiation. These specimens were then autopsied for visually spotting corrosion sites. In parallel with the work above, 1∼2 mm millings were taken from the mortar rebar-trace away from any observed corrosion sites in contact with the mortar. Corresponding chloride concentration was obtained by following the chloride titration procedure, which was performed on the milled powder as per the FDOT wet chemistry technique (modified to accommodate smaller mass) [11]. This method measures total chloride concentration.


## 3. Results and Discussion

### 3.1. Evolution of Potential and Linear Polarization Resistance

Figure3 shows the evolution of potential and linear polarization resistance values on mortar specimens S32304-C with a w/c = 0.5.Figure 3
Evolution of potential and linear polarization resistance on mortar specimens S32304-C (w/c = 0.5, wet/dry cycles began on the day indicated by the vertical dash line).
(a)
(b)It is observed that the rebar potential values of the UNS S32304-C1 specimens shifted to more positive and then varied in a range −165 mV to −143 mV after 400 days exposure. Similarly, the potential values of the UNS S32304-C3 rebar on mortar specimen also moved toward more positive and then varied in a range −204 mV to −114 mV after the same exposure duration. For the UNS S32304-C2 specimen, the rebar potential values shifted to more positive by day 338, but then experienced a potential drop from −242 mV on day 406 and to −307.5 mV on day 546, which was followed by a movement toward more positive values. For the UNS S32304-C4 specimen, the rebar potential values also shifted to more positive by day 493, then experienced a drop to more negative (−305 mV) values on day 504, and then shifted toward positive values in a range between −188.5 mV and −168.5 mV. It is also observed that theRc values of the mortar specimens S32304-C2 and S32304-C4 increased before the wet/dry cycle exposure and then experienced a decrease in a range of between 2.5 kΩ and 25 kΩ after around day 500. Based on the values of both Ecorr and Rc, it could be argued that corrosion had onset on the specimens S32304-C2 and S32304-C4 [9, 12]. In addition, corrosion had onset on the specimens S32304-D1 and S32304-D3 with a w/c = 0.41 in terms of the evolution of potential and linear polarization resistance values, which are not shown here.


### 3.2. Corrosion of Rebars in Terminated Specimens

The specimens S32304-C2, S32304-C4, S32304-D1, and S32304-D3 were terminated and cut open to inspect the surface condition and corrosion sites. It was observed that the rebar embedded in the specimen S32304-C2 had a very small site on the top side (Figure4), whereas there was no corrosion spot observed on the rebar embedded in specimen S32304-C4 (Figure 5) even though the potential transient negatively took place. However, corrosion might have initiated on the rebar embedded in specimen S32304-C4 during the negative potential excursion and re-passivated shortly after. Additionally, no corrosion was observed on the exposed bottom sides of both specimens’ rebars. For the rebars embedded in the specimen S32304-D with a w/c = 0.41, the corrosion spots were significantly smaller than those observed on rebars of specimens S32304-C with a w/c = 0.5. A corrosion spot on the rebar within the specimen S32304-D1 was observed close to the center on the top side and another at the center on the bottoms side, plus a third spot on the bottom side close to the specimen edge. A corrosion site on the rebar in the specimen S32304-D3 was observed but was not quite on the top side.Figure 4
Corrosion spot on the top side of rebar embedded in mortar specimens S32304-C2.Figure 5
Visual examination after exposing rebar embedded in mortar specimens S32304-C4.


### 3.3. Chloride Concentration at Rebar Trace and Chloride Threshold

Mortar milling was carried out at two locations both right and left the corrosion sites along the rebar trace to a depth of 1 to 2 mm maximum on specimens 32304-C2 and 32304-C4 (Mix 1) and specimens 32304-D1 and 32304-D3 (Mix 2), respectively. The chloride concentration on each specimen was measured at two locations, and at each location, the measurements were measured in duplicate. The corresponding chloride concentration was obtained by chloride titrations procedure, which was performed on the milled powder as per the FDOT wet chemistry technique (modified to accommodate smaller mass) [11]. Table 4 lists the chloride concentrations (total chloride concentrations) obtained from each specimen. Each value on each specimen presented in Table 4 is the average value of the test results. The rebar trace herein is the mortar surface in contact with the top side of the stainless steel bar.Table 4
Chloride concentration at rebar trace.
Mixes
Specimen series
Average chloride concentrations (kg/m3)
Average chloride concentrations (%, by weight of cement)
Mix 1
32304-C2
25.0
4.7
32304-C4
31.1
5.9
Mix 2
32304-D1
37.4
7.1
32304-D3
32.8
6.2It can be seen from Table4 that the chloride concentration ranged from 4.7 to more than 7.1 by weight of cement. No mortar was milled on those specimens in which the corrosion products covered most of the mortar surface above the rebar trace. It is possible to suggest that the value of the chloride threshold for the rebars embedded in the specimen S32304-C with the w/c = 0.5 is greater or equal to 4.7% by weight of cement, while the value of the rebar embedded in the specimen S32304-D with the w/c = 0.41 is approx. 6.2 % by weight of cement. It is therefore evident that the chloride threshold value of the rebar UNS S32304 embedded in the mortar specimen with a lower water-to-cement ratio is higher than that of the same rebar embedded in the mortar specimen with a higher water-to-cement ratio.Table5 summarizes the chloride threshold values of the rebars UNS S32304 embedded in mortar and concrete, which were obtained from present and previous studies [7, 13–15].Table 5
Chloride threshold of the rebar UNS S32304 embedded in mortar and concrete.
Authors
Water-to-cement ratio (w/c)
Corroded (yes/no)
Chloride threshold (%)∗
Note
Schönning et al. [13]
0.5
No
>4.0
(1) Rebar in mortar; (2) NaCl was added to the mixing water; (3) specimens were placed in saturated Ca (OH)2 solution at lab
Berolini and Gastaldi [14]
0.5, 0.65
Yes
3.0
(1)Rebar in concrete; (2) chloride was added to the mixing water; (3) specimens without mixed in chloride were subjected to ponding; (4) all specimens were placed in a climatic chamber at 20°C and 90% RH
Gastaldi and Bertolini [7]
0.5, 0.65
Yes
2.5 at 40°C
(1)Rebar in concrete; (2)Chloride was added to the mixing water; (3) specimens without mixed in chloride were subjected to ponding; (4) all specimens were placed in a climatic chamber at 20°C to 60°C and 90% RH
Paredes et al. [15]
0.5
Yes
6.5
(1)Rebar in concrete; (2) specimens were subjected to wet/dry cycle with NaCI solution at outdoor environment
Investigators of present study
0.5
Yes
4.7
(1)Rebar in mortar; (2) specimens were subjected to wet/dry cycle with NaCI solution at lab
0.41
Yes
6.2
∗Chloride threshold is based on the weight of cement or cementitious materials.All rebars in Table5 were in the as-received (pickled) condition. In one instance [13], corrosion did not initiate and their chloride threshold values shown are lower bound values, thus the actual the chloride threshold values are greater than those listed. It is observed that the chloride threshold value obtained from different researchers may vary. This may be attributed to the different experiments that were performed with different specimen sizes (small and large specimens), different total surface area exposed, and different methods of chloride transport (i.e., dry/wet cycles, electric field application, and chlorides cast during mix preparation), the types of raw materials and admixtures used, cement composition, mix design, concrete microstructure, potential of the steel, and environmental temperature [5, 7]. It should be noted that the chloride threshold value is not an individual value. Instead, it is variable or distributed over a range for identically designed and exposed specimens [16, 17].


## 3.1. Evolution of Potential and Linear Polarization Resistance

Figure3 shows the evolution of potential and linear polarization resistance values on mortar specimens S32304-C with a w/c = 0.5.Figure 3
Evolution of potential and linear polarization resistance on mortar specimens S32304-C (w/c = 0.5, wet/dry cycles began on the day indicated by the vertical dash line).
(a)
(b)It is observed that the rebar potential values of the UNS S32304-C1 specimens shifted to more positive and then varied in a range −165 mV to −143 mV after 400 days exposure. Similarly, the potential values of the UNS S32304-C3 rebar on mortar specimen also moved toward more positive and then varied in a range −204 mV to −114 mV after the same exposure duration. For the UNS S32304-C2 specimen, the rebar potential values shifted to more positive by day 338, but then experienced a potential drop from −242 mV on day 406 and to −307.5 mV on day 546, which was followed by a movement toward more positive values. For the UNS S32304-C4 specimen, the rebar potential values also shifted to more positive by day 493, then experienced a drop to more negative (−305 mV) values on day 504, and then shifted toward positive values in a range between −188.5 mV and −168.5 mV. It is also observed that theRc values of the mortar specimens S32304-C2 and S32304-C4 increased before the wet/dry cycle exposure and then experienced a decrease in a range of between 2.5 kΩ and 25 kΩ after around day 500. Based on the values of both Ecorr and Rc, it could be argued that corrosion had onset on the specimens S32304-C2 and S32304-C4 [9, 12]. In addition, corrosion had onset on the specimens S32304-D1 and S32304-D3 with a w/c = 0.41 in terms of the evolution of potential and linear polarization resistance values, which are not shown here.


## 3.2. Corrosion of Rebars in Terminated Specimens

The specimens S32304-C2, S32304-C4, S32304-D1, and S32304-D3 were terminated and cut open to inspect the surface condition and corrosion sites. It was observed that the rebar embedded in the specimen S32304-C2 had a very small site on the top side (Figure4), whereas there was no corrosion spot observed on the rebar embedded in specimen S32304-C4 (Figure 5) even though the potential transient negatively took place. However, corrosion might have initiated on the rebar embedded in specimen S32304-C4 during the negative potential excursion and re-passivated shortly after. Additionally, no corrosion was observed on the exposed bottom sides of both specimens’ rebars. For the rebars embedded in the specimen S32304-D with a w/c = 0.41, the corrosion spots were significantly smaller than those observed on rebars of specimens S32304-C with a w/c = 0.5. A corrosion spot on the rebar within the specimen S32304-D1 was observed close to the center on the top side and another at the center on the bottoms side, plus a third spot on the bottom side close to the specimen edge. A corrosion site on the rebar in the specimen S32304-D3 was observed but was not quite on the top side.Figure 4
Corrosion spot on the top side of rebar embedded in mortar specimens S32304-C2.Figure 5
Visual examination after exposing rebar embedded in mortar specimens S32304-C4.


## 3.3. Chloride Concentration at Rebar Trace and Chloride Threshold

Mortar milling was carried out at two locations both right and left the corrosion sites along the rebar trace to a depth of 1 to 2 mm maximum on specimens 32304-C2 and 32304-C4 (Mix 1) and specimens 32304-D1 and 32304-D3 (Mix 2), respectively. The chloride concentration on each specimen was measured at two locations, and at each location, the measurements were measured in duplicate. The corresponding chloride concentration was obtained by chloride titrations procedure, which was performed on the milled powder as per the FDOT wet chemistry technique (modified to accommodate smaller mass) [11]. Table 4 lists the chloride concentrations (total chloride concentrations) obtained from each specimen. Each value on each specimen presented in Table 4 is the average value of the test results. The rebar trace herein is the mortar surface in contact with the top side of the stainless steel bar.Table 4
Chloride concentration at rebar trace.
Mixes
Specimen series
Average chloride concentrations (kg/m3)
Average chloride concentrations (%, by weight of cement)
Mix 1
32304-C2
25.0
4.7
32304-C4
31.1
5.9
Mix 2
32304-D1
37.4
7.1
32304-D3
32.8
6.2It can be seen from Table4 that the chloride concentration ranged from 4.7 to more than 7.1 by weight of cement. No mortar was milled on those specimens in which the corrosion products covered most of the mortar surface above the rebar trace. It is possible to suggest that the value of the chloride threshold for the rebars embedded in the specimen S32304-C with the w/c = 0.5 is greater or equal to 4.7% by weight of cement, while the value of the rebar embedded in the specimen S32304-D with the w/c = 0.41 is approx. 6.2 % by weight of cement. It is therefore evident that the chloride threshold value of the rebar UNS S32304 embedded in the mortar specimen with a lower water-to-cement ratio is higher than that of the same rebar embedded in the mortar specimen with a higher water-to-cement ratio.Table5 summarizes the chloride threshold values of the rebars UNS S32304 embedded in mortar and concrete, which were obtained from present and previous studies [7, 13–15].Table 5
Chloride threshold of the rebar UNS S32304 embedded in mortar and concrete.
Authors
Water-to-cement ratio (w/c)
Corroded (yes/no)
Chloride threshold (%)∗
Note
Schönning et al. [13]
0.5
No
>4.0
(1) Rebar in mortar; (2) NaCl was added to the mixing water; (3) specimens were placed in saturated Ca (OH)2 solution at lab
Berolini and Gastaldi [14]
0.5, 0.65
Yes
3.0
(1)Rebar in concrete; (2) chloride was added to the mixing water; (3) specimens without mixed in chloride were subjected to ponding; (4) all specimens were placed in a climatic chamber at 20°C and 90% RH
Gastaldi and Bertolini [7]
0.5, 0.65
Yes
2.5 at 40°C
(1)Rebar in concrete; (2)Chloride was added to the mixing water; (3) specimens without mixed in chloride were subjected to ponding; (4) all specimens were placed in a climatic chamber at 20°C to 60°C and 90% RH
Paredes et al. [15]
0.5
Yes
6.5
(1)Rebar in concrete; (2) specimens were subjected to wet/dry cycle with NaCI solution at outdoor environment
Investigators of present study
0.5
Yes
4.7
(1)Rebar in mortar; (2) specimens were subjected to wet/dry cycle with NaCI solution at lab
0.41
Yes
6.2
∗Chloride threshold is based on the weight of cement or cementitious materials.All rebars in Table5 were in the as-received (pickled) condition. In one instance [13], corrosion did not initiate and their chloride threshold values shown are lower bound values, thus the actual the chloride threshold values are greater than those listed. It is observed that the chloride threshold value obtained from different researchers may vary. This may be attributed to the different experiments that were performed with different specimen sizes (small and large specimens), different total surface area exposed, and different methods of chloride transport (i.e., dry/wet cycles, electric field application, and chlorides cast during mix preparation), the types of raw materials and admixtures used, cement composition, mix design, concrete microstructure, potential of the steel, and environmental temperature [5, 7]. It should be noted that the chloride threshold value is not an individual value. Instead, it is variable or distributed over a range for identically designed and exposed specimens [16, 17].


## 4. Conclusions

The value of the chloride threshold for the rebar UNS S32304 embedded in the specimen with the w/c = 0.5 is greater or equal to 4.7% by weight of cement while that of rebar UNS S32304 embedded in the specimen with the w/c = 0.41 is approx. 6.2 % by weight of cement, suggesting the chloride threshold value of the rebar UNS S32304 embedded in the mortar specimen with a lower water-to-cement ratio is higher than that of the same rebar embedded in the mortar specimen with a higher water-to-cement ratio.It is, however, important to recognize that the chloride threshold value of UNS S32304 can be affected by many factors such as the different specimen sizes, different total surface area exposed, and different methods of chloride transport, the types of raw materials and admixtures used, cement composition, mix design, concrete microstructure, potential of the steel, and environmental temperature.



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*Source: 6949176-2019-03-03.xml*