Stainless Steel Voltammetric Sensor to Monitor Variations in Oxygen and Humidity Availability in Reinforcement Concrete Structures
Abstract
:1. Introduction
2. Materials and Methods
- Phase (1) Study in solution. In this preliminary phase, the processes that took place on the SS electrode surface when immersed in solution were studied, which represent the different conditions that could come into play in concrete. The obtained results were compared with those found in the literature.
- Phase (2) Studies performed with the SS sensor embedded in conventional concrete. SS voltammetric sensors were embedded in conventional concrete samples at different water/cement ratios (w/c). Samples were studied under distinct O2 availability conditions.
2.1. Studies in Solution
- In the first situation, O2 availability in solution was reduced by argon bubbling for 30 min before testing
- In the second situation, KOH solution was O2-saturated by bubbling synthetic air for 30 min before testing. Synthetic air was used to prevent an atmospheric CO2 solution and to avoid the employed solution’s carbonation.
- Solutions 0.1 m KOH (pH = 13) and 0.1 m NaOH (pH = 12.7) to simulate non-carbonated concrete [32]
- 0.1 m Na2CO3 solution (pH = 11.45) to simulate the initial degree of carbonation [32]
- 0.1 m NaHCO3 solution (pH = 8.35) to simulate carbonated concrete [32]
- Solution 0.1 m NaOH +Na2CO3 Ci m (pH = 12.7), where Ci came in these concentrations: 0.02 m, 0.05 m and 0.1 m. The purpose was to evaluate how the presence of carbonates influenced the obtained results.
- Solution 0.1 m NaOH (pH = 12.7) by adding NaCl at 0.5 m
- Solution 0.1 m NaOH (pH = 13) to simulate non-carbonated concrete by adding H2O2 at 0.02 m, and to assess the possible appearance or stabilization of this intermediate product in reducing O2, as suggested in the literature
The Applied Electro-Analytical Techniques
2.2. Studies Performed with the Sensor Embedded in Concrete
- Concrete type. Samples made of three different concrete types and, therefore, with distinct porosity, were used, which would imply differences in O2 availability in the vicinity of the electrode when samples were under similar environmental conditions
- O2 availability. Samples were left under four different environmental O2 availability conditions:
- ∘
- Atmospheric conditions (ATM), O2 molar fraction (xO2) 0.21
- ∘
- Partial air pressure condition, 500 mbar (500 mbar), O2 molar fraction (xO2) 0.105
- ∘
- Vacuum conditions (VAC), the O2 molar fraction (xO2) came very close to 0
- ∘
- H2O saturation conditions (SAT), the available O2 in the concrete matrix was limited by the gas solution capacity in the concrete pore solution.
2.2.1. Study in the Non-Saturated State
- Vacuum conditions: To homogenize the initial behavior of the sensors embedded in the different samples, the studied samples were subjected to a vacuum for 12 h. After finishing the adaptation period, the aforementioned electro-chemical studies were carried out.
- Atmospheric conditions: Once the tests under the vacuum conditions ended, synthetic air was allowed to enter the desiccator. When a 1-bar pressure was achieved inside, 60 min were allowed for the system to stabilize before testing began.
- Partial 500-mbar air pressure: when the tests under atmospheric conditions ended, the pressure inside the desiccator was lowered until vacuum conditions were achieved. At this point, 30 min were allowed before letting air in until a sTable 500-mbar pressure was accomplished. After a 15-min wait, argon was injected until the 1-bar pressure was recovered. In this way, the O2 molar fraction inside the desiccator diminished to half in relation to the working atmospheric conditions. Measurements were taken on sensors after 1 h under 500 mbar conditions.
2.2.2. Study in the Saturated State
2.2.3. Applied Electro-Analytical Techniques
2.2.4. Concrete Characterization Tests
- Hardened concrete tests, Part 3: determining sample strength to compression (UNE 12390-3:2009). Resistance to compression was determined at 28 days (fck). One cylindrical sample (10 cm diameter, 20 cm high) was prepared for each mass per dosage, with nine samples in all. The equipment used was Ibertest MEH-3000.
- Determining H2O absorption, density, and H2O accessible porosity (UNE 83980:2014). One cylindrical sample (10 cm diameter, 5 cm high) was prepared for each mass per dosage. Nine samples in all.
- Determining H2O penetration depth under pressure (UNE 83-309-90). One sample (15 cm diameter, 30 cm high) was produced for this test for each mass per dosage. Nine samples in all.
- Determining air permeability (UNE 83981:2008). One cylindrical sample (15 cm diameter, 5 cm high) was prepared for each mass per dosage. To perform the test, the sides of the samples were covered with sealing paint. The air permeability coefficient was obtained (k). Nine samples in all.
- Determining electrical resistivity (ρ): direct method (reference method) (UNE 83988-1:2008). A prismatic sample (4 × 4 × 16 cm3) was prepared for each mass per dosage. Nine samples in all.
- Mercury injection porosity (MIP) test (ASTM-D4404-10). This test allows information to be acquired about the volume of interconnected pores and their size distribution. These tests were performed by the ITC of the Universitat Jaume I (Spain). The MIP method consists of injecting mercury at different pressures. The volume of absorbed mercury at each pressure is recorded so that the volume of absorbed mercury within a certain range of pressures is associated with a given access size range.
3. Results
3.1. Phase 1: Studies Performed in Solution
3.1.1. Reducing O2 on the SS Electrode’s Surface
- The open circuit potential (OCP) shifts to the left if the atmosphere is aired.
- In this representation, two straight-lined sections appear. These lines are associated with O2, H2O2, and H2O reduction processes. The following can be deduced from the study:
- ∘
- The first straight line is that which varies the most with changes in O2 availability, and the value of its slope in the airless atmosphere goes from −2.630 V−1 to −4.272 V−1 (Table 5). The variation in the intercept is slight. This variation in the representation of potentials versus the SCE is 6.8% and 4.5% versus the OCP (Table 5). These minor variations do not enable us to establish significant differences between both states.
- ∘
- The second line is practically horizontal. The value of this line’s slope seems to be independent of the solution’s airing (Table 5). This plateau aspect indicates that it is due to a limitation in the speed of the material’s transport process.
3.1.2. Influence of pH on the SS Electrode
- Normal conditions (pH 13 and pH 12.72).
- The initial carbonation state (pH of 11.45).
- Carbonated concrete (pH 8.35).
3.1.3. Influence on the Electro-Chemical Behavior of Other Chemical Species
- Influence of carbonates: Solution 0.1 m NaOH + Na2CO3 Ci m (pH = 12.7), with Ci concentrations of 0.02 m, 0.05 m and 0.1 m.
- Influence of chlorides: Solution 0.1 m NaOH (pH = 12.7) by adding NaCl at 0.5 m.
- Influence of H2O2: Solution 0.1 m NaOH (pH = 12.7) by adding H2O2 at 0.02 m.
3.2. Phase 2: Studies Performed with the Sensor Embedded in Concrete
4. Discussion
5. Conclusions
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Acknowledgments
Conflicts of Interest
Appendix A
References
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Materials | kg/m3concrete | ||
---|---|---|---|
w/c = 0.6 | w/c = 0.5 | w/c = 0.4 | |
Cement I 42.5 R-SR5 Water Superplastificiser | 315 | 385 | 490 |
189 | 193 | 196 | |
2.2 | 2.7 | 34 | |
Silica sand Coarse aggregate | 1212 | 1179 | 1115 |
653 | 635 | 601 |
w/c | Number of Masses | Number of Samples/Mass | Total No. Samples |
---|---|---|---|
0.6 | 3 | 2 | 6 |
0.5 | 3 | 2 | 6 |
0.4 | 3 | 2 | 6 |
TOTAL | 18 |
NON-SATURATED STATE | SEQUENCE 1: samples dosage w/c = 0.6 | Samples were tested per dosage under the ATM, 500 mbar, and VAC conditions. Each sample was tested 3 times in each state. |
SEQUENCE 2: samples dosage w/c = 0.5 | ||
SEQUENCE 3: samples dosage w/c = 0.4 | ||
SATURATED STATE | SEQUENCE 4: samples dosage w/c = 0.6, w/c = 0.5 and w/c = 0.4 | All the samples were tested under SAT conditions. Each sample was tested 3 times in this state |
fcK28days (MPa) | CV | %W.A.P | CV | %Abs. H2O | CV | W.P.D (mm) | CV | ||
w/c = 0.6 | 34.5 | 6.03% | 19.19% | 13.79% | 7.59% | 6.81% | 47 | 17.71% | |
w/c = 0.5 | 42.9 | 5.93% | 17.21% | 6.04% | 7.47% | 7.32% | 26 | 3.10% | |
w/c = 0.4 | 56.8 | 4.50% | 14.85% | 2.59% | 6.65% | 4.68% | 10 | 17.63% | |
(a) | |||||||||
fcK28days (MPa) | k (×10−18 m2) | CV | ρ (Ωm) | CV | %Total Pores MIP | ||||
w/c = 0.6 | 34.5 | 667.76 | 18.82% | 50.68 | 12.88% | 13.2% | |||
w/c = 0.5 | 42.9 | 521.37 | 3.45% | 52.38 | 11.37% | 12.9% | |||
w/c = 0.4 | 56.8 | 413.56 | 12.64% | 62.56 | 9.88% | 10.6% | |||
(b) |
Line 1 | Line 2 | |||||
---|---|---|---|---|---|---|
Line Range | m1 | n1 | Line Range | m2 | n2 | |
Airless atmosphere (Ref.SCE) | −0.34 V to −0.870 V | −2.630 V−1 | −6.920 | −0.940 V to −1.220 V | −0.006 V−1 | −4.468 |
Airless atmosphere (Ref.OCP) | −0.110 V to −0.61 V | −2.630 V−1 | −6.336 | −0.770 V to −0.880 V | −0.006 V−1 | −4.467 |
Aired atmosphere (Ref. SCE) | −0.400 V to −0.800 V | −4.272 V−1 | −7.388 | −1.210 V to −0.880 V | −0.010 V−1 | −3.879 |
Aired atmosphere (Ref.OCP) | −0.100 V to −0.460 V | −4.272 V−1 | −6.052 | −0.630 V to −0.920 V | −0.010 V−1 | −3.876 |
pH | LINE 1 | LINE 2 | ||||
---|---|---|---|---|---|---|
Range | m1 (V−1) | n1 (log|A/cm2|) | Range | m2 (V−1) | n2 (log|A/cm2|) | |
13 | −0.46 V a −0.79 V | −6.0452 | −8.6277 | −0.82 V a −1.1 V | −0.2934 | −3.9496 |
12.72 | −0.38 V a −0.78 V | −5.4329 | −8.07202 | −0.82 V a −1 V | −0.5868 | −4.1606 |
11.35 | −0.43 V a −0.68 V | −7.0652 | −8.5235 | −0.74 V a −1.12 V | 0.1537 | −3.5363 |
8.35 | −0.24 V a −0.40 V | −7.3822 | −7.1177 | −0.46 V a −0.9 V | −0.1207 | −3.992 |
σ | - | 0.9 | 0.69 | - | 0.31 | 0.26 |
Whithout H2O2 | 0.02 m H2O2 | |||
---|---|---|---|---|
m | n | m | n | |
LINE 1 | −5.388 | −8.041 | −7.037 | −8.183 |
LINE 2 | −0.608 | −4.180 | 0.0149 | −3.412 |
LINE 3 | −3.438 | −6.471 | ||
LINE 4 | −2.112 | −5.012 |
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Martínez-Ibernón, A.; Lliso-Ferrando, J.; Gandía-Romero, J.M.; Soto, J. Stainless Steel Voltammetric Sensor to Monitor Variations in Oxygen and Humidity Availability in Reinforcement Concrete Structures. Sensors 2021, 21, 2851. https://doi.org/10.3390/s21082851
Martínez-Ibernón A, Lliso-Ferrando J, Gandía-Romero JM, Soto J. Stainless Steel Voltammetric Sensor to Monitor Variations in Oxygen and Humidity Availability in Reinforcement Concrete Structures. Sensors. 2021; 21(8):2851. https://doi.org/10.3390/s21082851
Chicago/Turabian StyleMartínez-Ibernón, Ana, Josep Lliso-Ferrando, José M. Gandía-Romero, and Juan Soto. 2021. "Stainless Steel Voltammetric Sensor to Monitor Variations in Oxygen and Humidity Availability in Reinforcement Concrete Structures" Sensors 21, no. 8: 2851. https://doi.org/10.3390/s21082851