# Corrosion Zones of Rebar in High-Volume Fly-Ash Concrete through Potentiodynamic Study in Concrete Powder Solution Extracts: A Sustainable Construction Approach

Authors: Manish Kumar; Jitu Kujur; Rajeshwari Chatterjee; Somnath Chattopadhyaya; Shubham Sharma; Shashi Prakash Dwivedi; Ambuj Saxena; S. Rajkumar; Ashutosh Anand
Journal: Advances in Civil Engineering
 (2022)

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## Abstract

This research reports the experimental outcomes of potentiodynamic analysis of the steel reinforcement in carbonated and uncarbonated high-volume fly-ash concrete powder solution extracts (CPSE). Different percentages of fly-ash content have been used to form a high-volume fly-ash concrete (0%, 20%, 40%, 50%, 60%, and 70%) with three different types of steel reinforced. Three different water-to-binder ratios (0.35, 0.40, and 0.45) have been used to form the reinforced concrete. The different zones of corrosion were observed through the anodic polarization curve, which was obtained through the potentiodynamic linear sweep test. It has indeed been demonstrated that concrete with up to 50% fly ash shows better resistance against carbonation, as compared to Ordinary Portland Cement (OPC) concrete. Fully active anodic polarization curve is obtained for carbonated concrete. Corrosion-resistant steel performed better as compared to TATA TMT and SISCON TMT types of steel. The ANOVA also verifies the experimental observation, which shows that the content of fly ash and types of steel decide the extent of corrosion in the concrete. It has also been observed that the interaction between the fly-ash content and water-binder proportion and also the interaction between fly ash and the type of steel show the strong effect on the corrosion activity, which decides the extent of different zones of corrosion.


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## Body

## 1. Introduction

Reinforced steel corrosion in concrete seems to be an electrochemical phenomenon which occurs in the presence of oxygen and moisture [1]. Passive layer formation occurs mostly on the substratum of steel reinforcement, due to higher alkalinity of concrete about pH 12.6 to 13 [2]. Concrete made with appropriate proportion of cement, fine aggregate, coarse aggregate, and water and then compacted and cured has less problems as to durability, but, in actual practice, this highly desirable condition is not achieved at the construction site. Corrosion is among the most severe issues with reinforcing concrete construction or structure. The product of corrosion occupies 6–9% higher volume than the parent material [3], which exerts tensile forces on the concrete, resulting in cracks and delamination of concrete. Globally, corrosion of steel reinforcing is viewed as a crucial longevity, reliability, and resilience issue of reinforced concrete structures. Every year, millions of dollars are spent on the rehabilitation of concrete structures worldwide.Corrosion of steel reinforcing is a result of the carbonation of concrete and by virtue of the existence of chloride ion. Carbonation of concrete occurs due to the ingress of atmospheric CO2 through the pores of concrete, which then reacts with the available moisture around the pore, resulting in the formation of carbonic acid. Further, H2CO3 reacts with Ca(OH)2 present in the concrete to form CaCO3 and hence reduces the pH value of the concrete [4]. The continuous carbonation of concrete reduces the pH to 9 or below, which results in the disintegration of the passive layer, leading to corrosion in steel. The destruction of the passive layer results in the dissolution of Fe2+ ion in the concrete. This Fe2+ ion reacts with OH− forming FeOH3 which gets deposited at the cathode. The chemical reactions which explain the corrosion of steel in concrete are shown as follows:(1)Fe⟶Fe2++2e−,(2)O2+4e−+2H2O⟶4OH−,(3)Fe2++2OH−⟶FeOH2,(4)FeOH2+O2⟶FeOH3+4H2O,Over the decade, many researchers and scientists had studied the reinforced concrete’s corrosion and its effect on the presence of alkaline solution and CPSE. Al-Amoudi and Maslehuddin (1995) revealed the effect of Cl−, SO42−, and mixed Cl− and SO42− solutions on the corrosion behavior of steel, imbedded in cement paste [5]. The experimental findings show that there was less chance to initiate reinforcement corrosion in the sulphate environment, whereas significant corrosion activity was found in samples immersed in NaCl and Na2SO4 solution. In addition, corrosion rate was increased two times with the increase in the concentration of SO42− to 2.1%. In 2002, Dehwah et al. [6] presented the lasting effect of concentration of sulphate on corrosion of reinforcement of Portland cement concretes in the presence of chloride ion. They observed that the reinforced corrosion starting time was not affected by SO42-, due to Cl−, but it increases with the increase in SO42− concentration. They also observed that the reinforced corrosion due to chloride was more in case of solution of magnesium sulphate than in solution of sodium sulphate.Al-Tayyib and Khan (1991) indicated the effect of various concentrations of sulphate ions on C steel specimens submerged in saturated solution of Ca (OH)2, by performing AC impendence and linear polarization resistance technique [7]. They observed that there was a sevenfold increase in the rate of corrosion due to the sulphate ions at the higher temperature with respect to chloride ion effect at room temperature. Moreno et al. (2004) performed a potentiodynamic linear sweeping test (PDLST) of the reinforced steel and investigated the impact of various levels of carbonation and chloride in the replicated pore solutions. The authors reported that reinforced steel remains passive in alkaline solutions [5]. They observed that the carbon steel remains inert in high alkaline solution and in higher concentration of carbonate and bicarbonate solution, even though they increased the potential to the level of oxygen evolution. They also observed generalized corrosion at all levels of potential at lower concentration of carbonate and bicarbonate.Alhozaimy et al. (2016) had investigated the impact of Cl−, replicated concrete pore solution, and temperature on the formation of unreceptive layer on steel reinforcement [8]. The study revealed that the formation of the passive film developed in a lime-saturated solution had polarization resistance 1.5 times more than that in a simulated-concrete passive-film pore solution. The concentration increases of Cl− and temperature of the pore solution help in increasing the steel passivation. Pradhan and Bhattacharjee (2007) had identified the diverse zone of corrosion by carrying potentiostatic linear sweeping test [1]. The different content of chloride effects the diverse-zones of corrosion in the steel-reinforcement.The different literature review revealed different potential ranges for various corrosion zones at different chloride concentrations. It can be summarized from the above literature survey that some researchers investigated the corrosion steel behavior in pore solutions contaminated with carbonation, chloride, or sulphates. However, the research on steel corrosion behavior reinforcement in uncarbonated and carbonated concrete powder solution extracts is rare, and no research work on corrosion behavior of steel in uncarbonated and carbonated concrete powder solution extracts has been reported.In the light of the foregoing, an investigational study was conducted to explore the steel corrosion behavior in concrete powder solution extraction by performing the PDLST on simple steel specimen. The present work aims to establish the various zones of corrosion for three different categories of reinforced steel extracts of uncarbonated and carbonated concrete power solutions. PDLST has been performed to obtain the polarization anodic curves and identification of corrosion zones on bare steel specimens. Concrete has been prepared from various fly-ash replacements with three water-to-binder ratios, that is, 0.35, 0.40, and 0.45. CPSE represents the electrolytic pore solution of concrete. ANOVA was carried out to check the key characteristics governing the various corrosion zones.


## 2. Experimental Procedure

### 2.1. Materials Used

Various test parameters have been used in this experimental study, including different types of fly-ash content in concrete, water/cement ratios, steel types, and uncarbonated and carbonated conditions. Cementitious materials such as Ordinary Portland (Grade 43) Cement (OPC) complying with Indian Standard IS:8112 [6] and Class F fly ash (FA) complying with Indian Standard IS:3812 (part-1)[7] were used. The fly ash is of light grey colour and was obtained from the National Thermal Power Corporation Limited (NTPC), Patratu, Ramgarh, Jharkhand, India. The surface areas of OPC and FA are 269 m2/kg and 392 m2/kg, respectively. Table 1 presents the chemical compositions of the cementitious materials. Table 2 depicts the mineralogical composition (%) of the cement.Table 1
Chemical compositions of cement and fly ash.
OxidesSiO2Al2O3Fe2O3CaOMgOK2ONa2OSO3Loss of ignitionOPC19.475.344.2464.271.070.780.541.862.43FA54.529.329.612.40.740.691.510.0.1.19Table 2
Mineralogical composition (%) of the cement [2].
C3SC2SC3AC4AF59.512.82.511.6As per IS:383 [9] specification, the coarse aggregate of quartzite origin and sand as fine aggregate was used. The grade III fine-aggregate sand was procured from the local river around Ranchi. Three different types of Thermomechanical Treated (TMT) bar of 12 mm diameter, namely, SISCON TMT, which is a local brand, corrosion-resistant steel (CRS-500D), and Tiscon 500D, have been used. For simplicity, we have used S1 for corrosion-resistant steel, S2 for Tiscon 500D and S3 for local TMT (nonbranded) in this article.Underground water has been used for mixing the concrete constituents, which satisfies the requirement of IS:456 [8]. The mixed percent of concrete is presented in Table 3. In order to obtain the similar workability (slump range of 90 to 120 mm) at constant water content of 165 kg/m3, a commercially available superplasticizer based on polycarboxylic ether brand named MasterGlenium ACE 30 was used.Table 3
Concrete mix proportion.
w/b ratioCement content (kg/m3)Fine aggregate (kg/m3)Coarse aggregate (kg/m3)Water content (kg/m3)0.45367.069811711650.40412.566611651650.35472.06321154165


### 2.2. Fabrication of Concrete Cube and Concrete Powder

All the concrete mixtures have been made as per Indian Standard 10262 [10]. High-volume fly-ash concrete was produced by superseding OPC with diverse percentages of fly-ash contents (i.e., 0%, 20%, 40%, 50%, 60%, and 70%) at three different water-to-binder ratios of 0.35, 0.40, and 0.45. All the mixes were tested for workability (slump cone test) before casting. They were carefully demolded after 24 ± 0.5 hours and were placed for cure in a water tank for 27 days. The concrete specimen was removed from the water tank at the test age and the extra moisture was wiped out with a cotton cloth. The compressive strength of the cubes was evaluated employing three replicates. The same cubes were used to prepare the concrete powder in a pulverizer machine, for preparing the solution extract at the age of 60 days. The pulverized specimen was filtered through the sieve with mesh size of 150 μm, which is a blend of fly ash, fine and coarse aggregate, and cement hydrates in their respective proportions. The collected powder was utilized in the PDLST on the bare steel.


### 2.3. Preconditioning and Preparation of Steel Specimens

The steel samples with length of 60 mm were used for the electrochemical cell test. The steel sample was drilled and threaded as per specification required for the electrochemical cell test. All the specimens were cleaned in order to remove rust from the steel surface by a wire brush. After that, the specimens were treated with an analytic isopropyl reagent and kept in air to dry. All the steel specimens were glazed with epoxy, leaving 4 mm from the bottom, which is shown in Figure1 [1]. All the samples were prepared with the three types of steel, six types of fly-ash content, two test conditions (uncarbonated and carbonated), and three water-to-binder ratios, with three replicates. A total of 648 steel specimens were prepared and tested using three types of steel, each type having 216 samples.Figure 1
Line diagram of steel specimen [1].


### 2.4. Accelerated Carbonation and Preparation of CPSE

The carbonated concrete powder was prepared by placing half of the concrete powder in the accelerated carbonation chamber for 70 days, with controlled environment (CO2: 4 ± 0.5%, temp.: 20 ± 2°C, and relative humidity: 55 ± 5%). The pH of the concrete powder was checked to determine the carbonating condition at the interval of every seven days. The pH value of 9.20 of concrete is required to achieve the carbonating condition.The proper mixing of concrete powder and water in equal ratio at room temperature will give the carbonated and noncarbonated concrete power solution. The dissolved solution was boiled and stirred for about half an hour with the help of magnetic stirrer [2, 4]. Then the solution was left at room temperature for 10 hours to settle down. After that the filtration of a solution was performed by Whatman Filter Paper Grade No. 1. The digital pH meter determines the pH of carbonated and noncarbonated concrete solution. This CPSE was then used on the steel specimens to perform the PDLST.


### 2.5. Different Test Methods

#### 2.5.1. Compressive Strength Test

The compressive-strength test of hardened concrete (size 150 mm cube) made of different fly-ash content was conducted in a compression-testing machine at a rate of 5.2 kN/s as per IS:516 [11]. All the concrete samples were kept in moist curing for 28 days before testing on compression-testing machine. Concrete specimens were tested with three replicates at the ages of 14, 28, 90, 180, and 365 days.


#### 2.5.2. Potentiodynamic Linear Sweeping Test

The PDLST was performed using the corrosion monitoring instrument (CMI), which consists of potentiostats/galvanostats and an electrochemical cell as per the ASTM guidelines given [12]. Electrochemical cell was a cylindrical shaped container with a movable cover, made of polypropylene. The cell was connected with a working electrode, a reference electrode, an auxiliary electrode, and a thermometer, as per the guidelines given in ASTM G59. Saturated calomel electrode (SCE) is utilized as a reference electrode [1]. The steel specimen was connected to the working electrode, the reference electrode, and auxiliary electrode in electrochemical cell, as shown in Figure 2.Figure 2
Line diagram of experimental arrangement for potentiodynamic linear sweep test.The solution extract comprises all the dissolvable materials of the carbonated and uncarbonated concrete powder. The instruments plot the polarization curves by maintaining the potential between working and auxiliary electrodes. The potential among the working electrode and auxiliary electrode has been automatically maintained on account of the appropriate amount of current passing by the instrument. The rest potential condition leads to the polarization, and the potential scan of steel specimen is in a range of 0 to 1500 mV at the sweep rate of 50 mV/minute [1].


## 2.1. Materials Used

Various test parameters have been used in this experimental study, including different types of fly-ash content in concrete, water/cement ratios, steel types, and uncarbonated and carbonated conditions. Cementitious materials such as Ordinary Portland (Grade 43) Cement (OPC) complying with Indian Standard IS:8112 [6] and Class F fly ash (FA) complying with Indian Standard IS:3812 (part-1)[7] were used. The fly ash is of light grey colour and was obtained from the National Thermal Power Corporation Limited (NTPC), Patratu, Ramgarh, Jharkhand, India. The surface areas of OPC and FA are 269 m2/kg and 392 m2/kg, respectively. Table 1 presents the chemical compositions of the cementitious materials. Table 2 depicts the mineralogical composition (%) of the cement.Table 1
Chemical compositions of cement and fly ash.
OxidesSiO2Al2O3Fe2O3CaOMgOK2ONa2OSO3Loss of ignitionOPC19.475.344.2464.271.070.780.541.862.43FA54.529.329.612.40.740.691.510.0.1.19Table 2
Mineralogical composition (%) of the cement [2].
C3SC2SC3AC4AF59.512.82.511.6As per IS:383 [9] specification, the coarse aggregate of quartzite origin and sand as fine aggregate was used. The grade III fine-aggregate sand was procured from the local river around Ranchi. Three different types of Thermomechanical Treated (TMT) bar of 12 mm diameter, namely, SISCON TMT, which is a local brand, corrosion-resistant steel (CRS-500D), and Tiscon 500D, have been used. For simplicity, we have used S1 for corrosion-resistant steel, S2 for Tiscon 500D and S3 for local TMT (nonbranded) in this article.Underground water has been used for mixing the concrete constituents, which satisfies the requirement of IS:456 [8]. The mixed percent of concrete is presented in Table 3. In order to obtain the similar workability (slump range of 90 to 120 mm) at constant water content of 165 kg/m3, a commercially available superplasticizer based on polycarboxylic ether brand named MasterGlenium ACE 30 was used.Table 3
Concrete mix proportion.
w/b ratioCement content (kg/m3)Fine aggregate (kg/m3)Coarse aggregate (kg/m3)Water content (kg/m3)0.45367.069811711650.40412.566611651650.35472.06321154165


## 2.2. Fabrication of Concrete Cube and Concrete Powder

All the concrete mixtures have been made as per Indian Standard 10262 [10]. High-volume fly-ash concrete was produced by superseding OPC with diverse percentages of fly-ash contents (i.e., 0%, 20%, 40%, 50%, 60%, and 70%) at three different water-to-binder ratios of 0.35, 0.40, and 0.45. All the mixes were tested for workability (slump cone test) before casting. They were carefully demolded after 24 ± 0.5 hours and were placed for cure in a water tank for 27 days. The concrete specimen was removed from the water tank at the test age and the extra moisture was wiped out with a cotton cloth. The compressive strength of the cubes was evaluated employing three replicates. The same cubes were used to prepare the concrete powder in a pulverizer machine, for preparing the solution extract at the age of 60 days. The pulverized specimen was filtered through the sieve with mesh size of 150 μm, which is a blend of fly ash, fine and coarse aggregate, and cement hydrates in their respective proportions. The collected powder was utilized in the PDLST on the bare steel.


## 2.3. Preconditioning and Preparation of Steel Specimens

The steel samples with length of 60 mm were used for the electrochemical cell test. The steel sample was drilled and threaded as per specification required for the electrochemical cell test. All the specimens were cleaned in order to remove rust from the steel surface by a wire brush. After that, the specimens were treated with an analytic isopropyl reagent and kept in air to dry. All the steel specimens were glazed with epoxy, leaving 4 mm from the bottom, which is shown in Figure1 [1]. All the samples were prepared with the three types of steel, six types of fly-ash content, two test conditions (uncarbonated and carbonated), and three water-to-binder ratios, with three replicates. A total of 648 steel specimens were prepared and tested using three types of steel, each type having 216 samples.Figure 1
Line diagram of steel specimen [1].


## 2.4. Accelerated Carbonation and Preparation of CPSE

The carbonated concrete powder was prepared by placing half of the concrete powder in the accelerated carbonation chamber for 70 days, with controlled environment (CO2: 4 ± 0.5%, temp.: 20 ± 2°C, and relative humidity: 55 ± 5%). The pH of the concrete powder was checked to determine the carbonating condition at the interval of every seven days. The pH value of 9.20 of concrete is required to achieve the carbonating condition.The proper mixing of concrete powder and water in equal ratio at room temperature will give the carbonated and noncarbonated concrete power solution. The dissolved solution was boiled and stirred for about half an hour with the help of magnetic stirrer [2, 4]. Then the solution was left at room temperature for 10 hours to settle down. After that the filtration of a solution was performed by Whatman Filter Paper Grade No. 1. The digital pH meter determines the pH of carbonated and noncarbonated concrete solution. This CPSE was then used on the steel specimens to perform the PDLST.


## 2.5. Different Test Methods

### 2.5.1. Compressive Strength Test

The compressive-strength test of hardened concrete (size 150 mm cube) made of different fly-ash content was conducted in a compression-testing machine at a rate of 5.2 kN/s as per IS:516 [11]. All the concrete samples were kept in moist curing for 28 days before testing on compression-testing machine. Concrete specimens were tested with three replicates at the ages of 14, 28, 90, 180, and 365 days.


### 2.5.2. Potentiodynamic Linear Sweeping Test

The PDLST was performed using the corrosion monitoring instrument (CMI), which consists of potentiostats/galvanostats and an electrochemical cell as per the ASTM guidelines given [12]. Electrochemical cell was a cylindrical shaped container with a movable cover, made of polypropylene. The cell was connected with a working electrode, a reference electrode, an auxiliary electrode, and a thermometer, as per the guidelines given in ASTM G59. Saturated calomel electrode (SCE) is utilized as a reference electrode [1]. The steel specimen was connected to the working electrode, the reference electrode, and auxiliary electrode in electrochemical cell, as shown in Figure 2.Figure 2
Line diagram of experimental arrangement for potentiodynamic linear sweep test.The solution extract comprises all the dissolvable materials of the carbonated and uncarbonated concrete powder. The instruments plot the polarization curves by maintaining the potential between working and auxiliary electrodes. The potential among the working electrode and auxiliary electrode has been automatically maintained on account of the appropriate amount of current passing by the instrument. The rest potential condition leads to the polarization, and the potential scan of steel specimen is in a range of 0 to 1500 mV at the sweep rate of 50 mV/minute [1].


## 2.5.1. Compressive Strength Test

The compressive-strength test of hardened concrete (size 150 mm cube) made of different fly-ash content was conducted in a compression-testing machine at a rate of 5.2 kN/s as per IS:516 [11]. All the concrete samples were kept in moist curing for 28 days before testing on compression-testing machine. Concrete specimens were tested with three replicates at the ages of 14, 28, 90, 180, and 365 days.


## 2.5.2. Potentiodynamic Linear Sweeping Test

The PDLST was performed using the corrosion monitoring instrument (CMI), which consists of potentiostats/galvanostats and an electrochemical cell as per the ASTM guidelines given [12]. Electrochemical cell was a cylindrical shaped container with a movable cover, made of polypropylene. The cell was connected with a working electrode, a reference electrode, an auxiliary electrode, and a thermometer, as per the guidelines given in ASTM G59. Saturated calomel electrode (SCE) is utilized as a reference electrode [1]. The steel specimen was connected to the working electrode, the reference electrode, and auxiliary electrode in electrochemical cell, as shown in Figure 2.Figure 2
Line diagram of experimental arrangement for potentiodynamic linear sweep test.The solution extract comprises all the dissolvable materials of the carbonated and uncarbonated concrete powder. The instruments plot the polarization curves by maintaining the potential between working and auxiliary electrodes. The potential among the working electrode and auxiliary electrode has been automatically maintained on account of the appropriate amount of current passing by the instrument. The rest potential condition leads to the polarization, and the potential scan of steel specimen is in a range of 0 to 1500 mV at the sweep rate of 50 mV/minute [1].


## 3. Results and Discussion

### 3.1. Compressive Strength

The testing outcomes of compressive strength are obtained at the ages of 14, 28, 90, 180, and 365 days. Results plotted between the compressive strengths and various types of fly-ash content for water-cement proportions, 0.35, 0.40, and 0.45, are shown in Figures3(a)–3(c), respectively. From Figures 3(a)–3(c), the compressive strength of concrete increases with increasing the age of concrete, irrespective of fly-ash content and water-cement proportion. It is also observed that the compressive strength of concrete made with 0% fly ash is higher in comparison with concrete made with various percentages of fly ash at 14 days. But, with increasing days of concrete curing period, the compressive strength of concrete containing 0% fly ash decreases to that of concrete made with 20% fly-ash content. Further, compressive strength of concrete made with 0% fly ash is higher than that of concrete made with 40, 50, 60, and 70% fly-ash content for higher ages. There was a reduction in the compressive strength of fly-ash concrete in early age. However, compressive strength increases with increasing ages. It was also reported that compressive strength increases continuously beyond 28 days. The enhancement in compressive strength from 28 days to 365 days was in the spans of 22% to 42%, 25% to 39%, and 24% to 43% for w/b ratios of 0.35, 0.40, and 0.45. Similarly, the percentage change in strength of 28-day concrete as compared to 180-day concrete was in the ranges of 16 to 41%, 20 to 38%, and 21 to 32%, respectively. Along the same lines, the percentage change in strength of 28-day concrete as compared to 90-day concrete was in the ranges of 3 to 32%, 8 to 25%, and 10 to 26%, respectively. This growth in compressive strength with increasing curing ages may be due to a secondary reaction with the increasing age of curing. The active improvement in the compressive strength of fly-ash-concrete was mainly because of pozzolanic reaction between Ca(OH)2 and the reactive SiO2 present in fly ash, which forms more C-S-H gels and consequently a denser matrix.Figure 3
Compressive strength versus different fly-ash content: (a) w/b, 0.35; (b) w/b, 0.40; (c) w/b, 0.45.
(a)(b)(c)


### 3.2. Corrosion Behavior of Steel in Uncarbonated Concrete

PDLST was performed on different steel samples to obtain the anodic polarization curves for different water-cement ratios and uncarbonated concrete powder (concrete made with various fly-ash content). Figure4 shows a polarization curve of corrosion-resistant steel in uncarbonated CPSE for the water-binder ratio of 0.35. Figure 4 depicts the different corrosion zones, such as passive, active, pitting, and semi-immune zones. The rest potential was −272.3 mV. It can also be observed from Figure 4 that the semi-immune zone lies below the rest potential, and the active zone is above it. In the immune zone, the steel does not undergo the anodic reaction and thus achieves the condition of immunity. The passive film steel surface does not form spontaneously in the active zone, and hence the current density increase was significantly higher, with insignificant change in potential. The potential of the active zone varies from −252.2 mV to −171.5 mV. The range for the passive zones was −171.5 mV to +558.1 mV, which is just above the active zone. This zone exhibits large change in the potential with a very slight change in the current density. This is because of the formation of a very thin inactive layer of film on the steel’s surface. Transpassive zone or pitting state zone lies at the top portion of the anodic polarization curve as shown in Figure 4 and its range is +558.1 mV (SCE). In this zone, the steel was in transpassive condition. The anodic reaction (2H2O ⟶ O2 + 4H+ + 4e) in the transpassivity zone produces O2 on the steel surface. This helps in increasing the anodic current density, which in turn reduces the pH content of concrete. The potential range of each zone can help in identifying the different stages of the corrosion process.Figure 4
Polarization curve showing different zones of corrosion for water-to-binder ratio = 0.35, uncarbonated concrete containing 50% fly ash.From Figures5(a)–5(c), it could indeed be reported that variations in rest potential values for different fly-ash content are relatively less as compared to steel types S2 and S3, respectively. Therefore, the average value of rest potential for the different type of steels was calculated and the average rest potential values obtained for steel types S1, S2, and S3 are −281 mV, −416 mV, and −446 mV, respectively. The less negative rest potential value of steel type S1 indicates less corrosion as compared to that of steel types S2 and S3. This may be due to more corrosion-resistant elements being present in S1 steel as compared to steel types S2 and S3. Similar results have been reported in the prior studies, where the chromium and manganese elements have been found in the steel-type reinforcement [13–16].Figure 5
Potential versus different fly-ash content for w/b ratio of 0.35. (a) CRS. (b) Tata TMT. (c) Local TMT.
(a)(b)(c)The rest potential, active/passive zone, and passive/transpassive zone in uncarbonated concrete for various percentages of fly-ash content in concrete with three distinct categories of steel and different water-cement ratio are depicted in Figures5–7, respectively. It is experiential from Figures 5–7 that the trend variation in rest potential values of steel type S1 are relatively less as compared to the steel types S2 and S3.Figure 6
Potential versus different fly-ash content for w/b ratio of 0.40. (a) CRS. (b) Tata TMT. (c) Local TMT.
(a)(b)(c)Figure 7
Potential versus different fly-ash content for w/b ratio of 0.45. (a) CRS (b) Tata TMT. (c) Local TMT.
(a)(b)(c)From Figures5–7, it is observed that, for steel type S1, the active/passive boundary potential and the passive/transpassive boundary potential are relatively less as compared to steel types S2 and S3. Similarly, the average rest potentials at the active/passive boundary obtained in uncarbonated condition for steel types S1, S2, and S3, are −132 mv, −185 mV, and −76 mV, respectively, for control concrete. The average boundary potential pass/transpass from Figures 5(a)–5(c) in the control concrete obtained for steel types S1, S2, and S3 are 382 mV, 411 mv, and 399 mv. JangHyun and MyeongGyu [17] observed that 1.2 M LiNO2 inhibitor reduces corrosion process in the RC structure against penetration of chloride ions. Muralidharan et al. [18] have reported that partial incorporation of 20 to 30% snail shell powder with OPC enhanced both corrosion performance of steel rebars and mechanical properties of concrete.A plot has been made between different fly ash versus rest potential, active/passive potential, and passive/transpassive potential for w/b ratio of 0.45 and steel types S1, S2, and S3 as shown in Figures5–7, respectively. From Figures 5–7, it is observed that the active zone of steel type S1 is less as compared to steel types S2 and S3. It is also observed that the passive zones of steel types S1, S2, and S3 are almost the same. It is concluded that steel type S1 attains semi-immune zone less negative rest potential having more semi-immune zone as compared to steel types S2 and S3.


### 3.3. Corrosion Behavior of Steel in Carbonated CPSE

The corrosion behavior of steel in carbonated CPSE has been studied through PDLST, which exhibits perfect Tafel’s behavior. In corrosion testing, the anodic current is able to dissolve the steel due to the cathodic reduction of oxygen, since the anodic current required to oxidize the steel comes from the cathodic reduction of O2. Under activation control, linear regions are seen on the anodic curve. Using Tafel’s plot, the potential increases exponentially with increase in the anodic current. During corrosion experiments, the gradients of lines could reveal mechanical insight. A polarization graph seems to be the observed E/I characteristics. The electrochemical kinetics connecting the potential rate to the electrochemical reaction rate are represented by the Tafel equation. The Tafel plot explains the Tafel equation in the form of a diagram or illustration. Information like rate of pitting, passivity, and corrosion susceptibility can be derived from the Tafel equation. This technique could be useful for the identifying corrosion current (Icorr). The values of corrosion potential and the corrosion current density were evaluated and plotted with the aid of a Tafel ruler and its associated software. A typical polarization curve is shown in Figure 8 for corrosion-resistant steel with w/b = 0.35.Figure 8
Anodic polarization curve of rebars in carbonated concrete powder solution and Tafel plot to obtain Icorr for F50 concrete.The passive zone range has been determined from the disparity between passivity/pitting boundary potential and active/passive boundary potential at distinct fly-ash content. It may be revealed from Figure8 that there was a reduction in the passive zone range, but there was an increase in pitting zone with the increase in fly-ash content. It is revealed that the passive film breaks with increases in the w/b ratio, which leads to a rise in corrosion current density (Icorr). The anodic polarization curve of steel is fully active in carbonated concrete, as shown in Figure 8. There is no passivation of steel in carbonated concrete for all mixing parameters in this investigation. This may be due to presence of less hydroxide ions (OH−).The points of intersection of slopes obtained by Tafel’s ruler giving the corrosion current density (Icorr) value obtained for the various fly-ash content, the three type of steels, and the three water-to-binder ratios are given in Table4. It is revealed that there is a significant decrease in corrosion current density value with decrease in the water-to-binder ratios. It is discovered that the Icorr value reduces with increasing fly-ash content up to 50%, irrespective of the type of steel and the water-to-binder ratios. When more than 50% of fly ash is used to replace OPC, an increase in the Icorr value was noted.Table 4
Icorr (μA/cm2) for carbonated CPSE for various types of steel and different fly-ash content with water-binder ratios.
Steel typeCRSTata TMTLocal TMT% FA contentw/bw/bw/b0.350.400.450.350.400.450.350.400.4501.31.72.11.81.91.92.22.83.1201.21.82.31.91.61.82.13.43.9401.21.42.31.51.52.41.62.53.6500.91.51.91.31.52.222.52.8601.82.42.82.02.53.02.222.4702.22.53.12.62.73.12.53.84.3The percentage rises in Icorr value for the concrete made with 50% fly ash in contrast to 0% fly ash of steel type CRS for water-to-binder ratios of 0.35, 0.40, and 0.45, are 44.44%, 13.33%, and 10.52%, respectively. Similarly, the percentage changes in Icorr value for the concrete made with 50% fly ash in comparison with 0% fly ash of steel type Tata TMT for water-to-binder ratios of 0.35, 0.40, and 0.45 are 38.46%, 29.80%, and 26.67%, respectively. Similarly, the percentage changes in Icorr value for the concrete made with 50% fly ash in comparison with 0% fly ash of steel type Local TMT for water-to-binder ratios of 0.35, 0.40, and 0.45 are 10%, 12%, and 10.71%, respectively. CRS has the minimum Icorr value as compared to Tata TMT and Local TMT.


### 3.4. Analysis of Corrosion Potential and Corrosion Current Density Using ANOVA

A factorial ANOVA calculation in this paper was conducted according to the guidelines offered by Hicks (1964) [19]. It is an analysis of variance that contains more than one independent variable and calculates the main effect for each independent variable as well as for the interactive effects between independent variables. This article applied ANOVA on the corrosion potential and corrosion current density values for six types of fly-ash content, three type of steels, and three water-to-binder ratios with three replicates for carbonated concrete and uncarbonated concrete powder solution extraction (Figures 5–8 and Table 4). ANOVA was carried out to check the key characteristics governing the various corrosion zones.The ANOVA also verifies the experimental observation, which shows that the content of fly ash and types of steel decide the extent of corrosion in the concrete. ANOVA further observed the interaction between the fly-ash content with water-binder proportion, and the interaction between fly ash and the type of steel shows the variation-trend influence on the corrosion activity, which will decide the extent of different zones of corrosion.The total squares were determined. The value of total squares was divided into squares (SS) amounts for different factors and the rest to the squares sum for residual random-error.The mean square of all the factors had been determined by taking the ratios of all the squares’ sum and their respective degree of freedom (df). The hypothesis of equality of variance was tested to observe the effect of individual factors on the above-mentioned engineering and durability properties level (in this study, 95% and 99% confidence levels were considered).Fstatistics was determined, which is given by equation (5). This value is compared with Fisher’s distribution (F-value).(5)Fstatistics=mean squares of factorsmean squares of residual error.It depends on the df of the individual factors, residual error, and the probability level. A modelF-value is indeed the proportion of the individual term’s mean square to a residual’s mean square. The probability of the Fstatistics was being employed to evaluate the null hypotheses, and perhaps even the probability greater than F-ratio seems to be the prob. of Fstatistics. Significant prominent factors were those that have an Fstatistics significance level of lower than 0.05.F-value more than the tabulated value indicates that the variance corresponding to the factor is more as compared to the variance of error and that, henceforth, the factor will be relevant, and its effect may not be neglected.The results of ANOVA for the rest potential for uncarbonated CPSE are presented in Table5, and w/b ratio and the types of steel have a more significant influence on governing the corrosion potential for steel as compared to other factors like water-cement ratios and their interaction with various fly-ash content. Table 5 summarizes an ANOVA result for rest potential for uncarbonated concrete, which reveals that the statistical model adequately has a 95% level of confidence. The model’s F-values of A, B, and C are 6.78, 8.81, and 17.76, respectively, and the p value is less than 0.0001. It is indicated that a relatively larger F-value of models attributed to disturbance/noise has a 0.01 percent probability of occurrence. As a result, the quadratic model is statistically significant at a 95% level of confidence. Arkadeb and Sarmila [20] have observed that steel rebars coated with Ni-P-W show minimum damage reinforced concrete structures in marine environment.Table 5
ANOVA result for rest potential for uncarbonated concrete.
SourceDegree of freedom (df)Sum of squares (SS)Mean squares (MS)F-ratioProbability >F-ratioWater-to-binder ratios (A)21304865246.780.0173FA content (B)5161679323358.810.7892Steel types (C)21303946519717.760.0515(A)∗(B)10192680192685.250(A)∗(C)486042215105.860.0002(B)∗(C)106194361941.690.0906Error1264625493671——Total1591099757———Tables6 and 7 depict interaction between water-to-binder ratios and fly ash, and interaction between fly ash and steel showed significant corrosion activity. The results showed that water-to-binder ratios and fly ash have a strong effect in defining the various corrosion zones in uncarbonated concrete, since the calculated F-value for steel type is less than the respective value obtained from the 95% confidence level, while those for other parameters are substantially higher at the same level.Table 6
ANOVA result for active to passive potential for uncarbonated concrete.
SourceDegree of freedom (df)Sum of squares (SS)Mean squares (MS)F-ratioProbability >F-ratioWater-to-binder ratios (A)216664833212.980.0327FA content (B)5231264462525.420.0001Steel types (C)2139922699618.210.0004(A)∗(B)10486304486305.70(A)∗(C)479572198932.330.0593(B)∗(C)10262000262002.330.0016Error12610743308526Total1592350724Table 7
ANOVA result for passive to transpassive potential for uncarbonated concrete.
SourceDegree of freedom (df)Sum of squares (SS)Mean squares (MS)F-ratioProbability >F-ratioWater-to-binder ratios (A)278399339199631.160.0014FA content (B)5351490702985.590.0001Steel types (C)2106158530794.220.0168(A)∗(B)10427751427753.40.006(A)∗(C)4123597308962.460.0491(B)∗(C)1010002100000.790.6338Error126158522912581Total1593443156Tables6 and 7 summarize an ANOVA result for active to passive potential and for passive to transpassive potential for uncarbonated concrete, which reveals that the statistical model adequately has a 95% level of confidence. The model’s F-values for active to passive potential for uncarbonated concrete for A, B, and C are 12.98, 5.42, and 8.21, respectively, while the model’s F-values for passive to transpassive potential for uncarbonated concrete for A, B, and C are 31.16, 5.59, and 4.22, respectively, and the p value is less than 0.0001 in both the cases. It is indicated that a relatively larger F-value of models attributed to disturbance/noise has a 0.01 percent probability of occurring. As a result, the quadratic model is statistically significant at a 95% level of confidence. Lukasz and Nikoo [13] developed an artificial neural network (ANN) combined with imperialist competitive algorithm (ICA) to predict the corrosion current density. They observed that ANN optimized with ICA owns more precision in predicting steel corrosion in concrete genetic algorithms. Ping-yi et al. [14] observed that the Taguchi particle swarm optimization provides higher computational efficiency and higher robustness when solving problems involving seven nonlinear benchmark functions: three unimodal functions, one multimodal function, two rotated functions, and one shifted function. Similar findings were reported by [1, 3, 15, 16].Similarly, the findings of ANOVA for rest potential and corrosion current density (Icorr) in carbonated CPSE are presented in Tables8 and 9, respectively. The interaction of fly ash and steel shows a significant effect in the corrosion current density for the steel reinforcement of the carbonated CPSE in comparison with other parameters, like different steel types and fly-ash content (Tables 8 and 9). The F-values obtained from the interactions of fly-ash content and steel types are less than the respective F-values at 95% confidence level.Table 8
ANOVA result for rest potential of carbonated concrete.
SourceDegree of freedom (df)Sum of squares (SS)Mean squares (MS)F-ratioProbability >F-ratioCarbonation (A)110.7 × 10610.7 × 106128.240Water-to-binder ratios (B)24091.62045.832.240.0578FA content (C)51.76 × 10535240.34.210.001Steel types (D)22.91 × 106145876.617.440(A)∗(B)21.28 × 1046446.95.770.4638(A)∗(C)51.24 × 10524820.32.970.0126(A)∗(D)21560.2780.10.090.911(B)∗(C)101.42 × 10514208.11.70.0809(B)∗(D)41.13 × 10528252.93.380.0056(C)∗(D)102.41 × 10524101.92.880.0019Error2762.30 × 1068366.7Total3194.5 × 106Table 9
ANOVA result for Icorr of carbonated concrete.
SourceDegree of freedom (df)Sum of squares (SS)Mean squares (MS)F-ratioProbability >F-ratioCarbonation (A)10.09640.09643.120.0783Water-to-binder ratios (B)20.17420.087112.820.0612FA content (C)50.44960.089932.910.014Steel types (D)20.17290.086452.80.0625(A)∗(B)20.17850.089232.890.0572(A)∗(C)50.45810.091622.970.0126(A)∗(D)20.17880.089392.90.0569(B)∗(C)100.95410.095413.090.001(B)∗(D)40.38820.097053.140.015(C)∗(D)100.93460.093463.030.0012Error2768.51980.03087Total31912.4929Tables8 and 9 summarize an ANOVA result for rest potential and Icorr for carbonated concrete, which further reveals that the statistical model adequately has a 95% level of confidence. The model’s F-values for rest potential for carbonated concrete for A, B, C, and D are 128.24, 32.24, 4.21, and 17.44, respectively, while the model’s F-values for corrosion current density for carbonated concrete for A, B, C, and D are 3.12, 2.82, 2.91, and 2.8, respectively, and the p values in both cases are less than 0.0001. It is indicated that a relatively larger F-value of models attributed to disturbance/noise has a 0.01 percent probability of occurrence. As a result, the quadratic model is statistically significant at a 95% level of confidence for both cases.From ANOVA results, it is observed that steel type has the strongest effect on rest potential for uncarbonated concrete. It can be realized that the utility of zones during cathodic protection helps to identify the shift in potential.


## 3.1. Compressive Strength

The testing outcomes of compressive strength are obtained at the ages of 14, 28, 90, 180, and 365 days. Results plotted between the compressive strengths and various types of fly-ash content for water-cement proportions, 0.35, 0.40, and 0.45, are shown in Figures3(a)–3(c), respectively. From Figures 3(a)–3(c), the compressive strength of concrete increases with increasing the age of concrete, irrespective of fly-ash content and water-cement proportion. It is also observed that the compressive strength of concrete made with 0% fly ash is higher in comparison with concrete made with various percentages of fly ash at 14 days. But, with increasing days of concrete curing period, the compressive strength of concrete containing 0% fly ash decreases to that of concrete made with 20% fly-ash content. Further, compressive strength of concrete made with 0% fly ash is higher than that of concrete made with 40, 50, 60, and 70% fly-ash content for higher ages. There was a reduction in the compressive strength of fly-ash concrete in early age. However, compressive strength increases with increasing ages. It was also reported that compressive strength increases continuously beyond 28 days. The enhancement in compressive strength from 28 days to 365 days was in the spans of 22% to 42%, 25% to 39%, and 24% to 43% for w/b ratios of 0.35, 0.40, and 0.45. Similarly, the percentage change in strength of 28-day concrete as compared to 180-day concrete was in the ranges of 16 to 41%, 20 to 38%, and 21 to 32%, respectively. Along the same lines, the percentage change in strength of 28-day concrete as compared to 90-day concrete was in the ranges of 3 to 32%, 8 to 25%, and 10 to 26%, respectively. This growth in compressive strength with increasing curing ages may be due to a secondary reaction with the increasing age of curing. The active improvement in the compressive strength of fly-ash-concrete was mainly because of pozzolanic reaction between Ca(OH)2 and the reactive SiO2 present in fly ash, which forms more C-S-H gels and consequently a denser matrix.Figure 3
Compressive strength versus different fly-ash content: (a) w/b, 0.35; (b) w/b, 0.40; (c) w/b, 0.45.
(a)(b)(c)


## 3.2. Corrosion Behavior of Steel in Uncarbonated Concrete

PDLST was performed on different steel samples to obtain the anodic polarization curves for different water-cement ratios and uncarbonated concrete powder (concrete made with various fly-ash content). Figure4 shows a polarization curve of corrosion-resistant steel in uncarbonated CPSE for the water-binder ratio of 0.35. Figure 4 depicts the different corrosion zones, such as passive, active, pitting, and semi-immune zones. The rest potential was −272.3 mV. It can also be observed from Figure 4 that the semi-immune zone lies below the rest potential, and the active zone is above it. In the immune zone, the steel does not undergo the anodic reaction and thus achieves the condition of immunity. The passive film steel surface does not form spontaneously in the active zone, and hence the current density increase was significantly higher, with insignificant change in potential. The potential of the active zone varies from −252.2 mV to −171.5 mV. The range for the passive zones was −171.5 mV to +558.1 mV, which is just above the active zone. This zone exhibits large change in the potential with a very slight change in the current density. This is because of the formation of a very thin inactive layer of film on the steel’s surface. Transpassive zone or pitting state zone lies at the top portion of the anodic polarization curve as shown in Figure 4 and its range is +558.1 mV (SCE). In this zone, the steel was in transpassive condition. The anodic reaction (2H2O ⟶ O2 + 4H+ + 4e) in the transpassivity zone produces O2 on the steel surface. This helps in increasing the anodic current density, which in turn reduces the pH content of concrete. The potential range of each zone can help in identifying the different stages of the corrosion process.Figure 4
Polarization curve showing different zones of corrosion for water-to-binder ratio = 0.35, uncarbonated concrete containing 50% fly ash.From Figures5(a)–5(c), it could indeed be reported that variations in rest potential values for different fly-ash content are relatively less as compared to steel types S2 and S3, respectively. Therefore, the average value of rest potential for the different type of steels was calculated and the average rest potential values obtained for steel types S1, S2, and S3 are −281 mV, −416 mV, and −446 mV, respectively. The less negative rest potential value of steel type S1 indicates less corrosion as compared to that of steel types S2 and S3. This may be due to more corrosion-resistant elements being present in S1 steel as compared to steel types S2 and S3. Similar results have been reported in the prior studies, where the chromium and manganese elements have been found in the steel-type reinforcement [13–16].Figure 5
Potential versus different fly-ash content for w/b ratio of 0.35. (a) CRS. (b) Tata TMT. (c) Local TMT.
(a)(b)(c)The rest potential, active/passive zone, and passive/transpassive zone in uncarbonated concrete for various percentages of fly-ash content in concrete with three distinct categories of steel and different water-cement ratio are depicted in Figures5–7, respectively. It is experiential from Figures 5–7 that the trend variation in rest potential values of steel type S1 are relatively less as compared to the steel types S2 and S3.Figure 6
Potential versus different fly-ash content for w/b ratio of 0.40. (a) CRS. (b) Tata TMT. (c) Local TMT.
(a)(b)(c)Figure 7
Potential versus different fly-ash content for w/b ratio of 0.45. (a) CRS (b) Tata TMT. (c) Local TMT.
(a)(b)(c)From Figures5–7, it is observed that, for steel type S1, the active/passive boundary potential and the passive/transpassive boundary potential are relatively less as compared to steel types S2 and S3. Similarly, the average rest potentials at the active/passive boundary obtained in uncarbonated condition for steel types S1, S2, and S3, are −132 mv, −185 mV, and −76 mV, respectively, for control concrete. The average boundary potential pass/transpass from Figures 5(a)–5(c) in the control concrete obtained for steel types S1, S2, and S3 are 382 mV, 411 mv, and 399 mv. JangHyun and MyeongGyu [17] observed that 1.2 M LiNO2 inhibitor reduces corrosion process in the RC structure against penetration of chloride ions. Muralidharan et al. [18] have reported that partial incorporation of 20 to 30% snail shell powder with OPC enhanced both corrosion performance of steel rebars and mechanical properties of concrete.A plot has been made between different fly ash versus rest potential, active/passive potential, and passive/transpassive potential for w/b ratio of 0.45 and steel types S1, S2, and S3 as shown in Figures5–7, respectively. From Figures 5–7, it is observed that the active zone of steel type S1 is less as compared to steel types S2 and S3. It is also observed that the passive zones of steel types S1, S2, and S3 are almost the same. It is concluded that steel type S1 attains semi-immune zone less negative rest potential having more semi-immune zone as compared to steel types S2 and S3.


## 3.3. Corrosion Behavior of Steel in Carbonated CPSE

The corrosion behavior of steel in carbonated CPSE has been studied through PDLST, which exhibits perfect Tafel’s behavior. In corrosion testing, the anodic current is able to dissolve the steel due to the cathodic reduction of oxygen, since the anodic current required to oxidize the steel comes from the cathodic reduction of O2. Under activation control, linear regions are seen on the anodic curve. Using Tafel’s plot, the potential increases exponentially with increase in the anodic current. During corrosion experiments, the gradients of lines could reveal mechanical insight. A polarization graph seems to be the observed E/I characteristics. The electrochemical kinetics connecting the potential rate to the electrochemical reaction rate are represented by the Tafel equation. The Tafel plot explains the Tafel equation in the form of a diagram or illustration. Information like rate of pitting, passivity, and corrosion susceptibility can be derived from the Tafel equation. This technique could be useful for the identifying corrosion current (Icorr). The values of corrosion potential and the corrosion current density were evaluated and plotted with the aid of a Tafel ruler and its associated software. A typical polarization curve is shown in Figure 8 for corrosion-resistant steel with w/b = 0.35.Figure 8
Anodic polarization curve of rebars in carbonated concrete powder solution and Tafel plot to obtain Icorr for F50 concrete.The passive zone range has been determined from the disparity between passivity/pitting boundary potential and active/passive boundary potential at distinct fly-ash content. It may be revealed from Figure8 that there was a reduction in the passive zone range, but there was an increase in pitting zone with the increase in fly-ash content. It is revealed that the passive film breaks with increases in the w/b ratio, which leads to a rise in corrosion current density (Icorr). The anodic polarization curve of steel is fully active in carbonated concrete, as shown in Figure 8. There is no passivation of steel in carbonated concrete for all mixing parameters in this investigation. This may be due to presence of less hydroxide ions (OH−).The points of intersection of slopes obtained by Tafel’s ruler giving the corrosion current density (Icorr) value obtained for the various fly-ash content, the three type of steels, and the three water-to-binder ratios are given in Table4. It is revealed that there is a significant decrease in corrosion current density value with decrease in the water-to-binder ratios. It is discovered that the Icorr value reduces with increasing fly-ash content up to 50%, irrespective of the type of steel and the water-to-binder ratios. When more than 50% of fly ash is used to replace OPC, an increase in the Icorr value was noted.Table 4
Icorr (μA/cm2) for carbonated CPSE for various types of steel and different fly-ash content with water-binder ratios.
Steel typeCRSTata TMTLocal TMT% FA contentw/bw/bw/b0.350.400.450.350.400.450.350.400.4501.31.72.11.81.91.92.22.83.1201.21.82.31.91.61.82.13.43.9401.21.42.31.51.52.41.62.53.6500.91.51.91.31.52.222.52.8601.82.42.82.02.53.02.222.4702.22.53.12.62.73.12.53.84.3The percentage rises in Icorr value for the concrete made with 50% fly ash in contrast to 0% fly ash of steel type CRS for water-to-binder ratios of 0.35, 0.40, and 0.45, are 44.44%, 13.33%, and 10.52%, respectively. Similarly, the percentage changes in Icorr value for the concrete made with 50% fly ash in comparison with 0% fly ash of steel type Tata TMT for water-to-binder ratios of 0.35, 0.40, and 0.45 are 38.46%, 29.80%, and 26.67%, respectively. Similarly, the percentage changes in Icorr value for the concrete made with 50% fly ash in comparison with 0% fly ash of steel type Local TMT for water-to-binder ratios of 0.35, 0.40, and 0.45 are 10%, 12%, and 10.71%, respectively. CRS has the minimum Icorr value as compared to Tata TMT and Local TMT.


## 3.4. Analysis of Corrosion Potential and Corrosion Current Density Using ANOVA

A factorial ANOVA calculation in this paper was conducted according to the guidelines offered by Hicks (1964) [19]. It is an analysis of variance that contains more than one independent variable and calculates the main effect for each independent variable as well as for the interactive effects between independent variables. This article applied ANOVA on the corrosion potential and corrosion current density values for six types of fly-ash content, three type of steels, and three water-to-binder ratios with three replicates for carbonated concrete and uncarbonated concrete powder solution extraction (Figures 5–8 and Table 4). ANOVA was carried out to check the key characteristics governing the various corrosion zones.The ANOVA also verifies the experimental observation, which shows that the content of fly ash and types of steel decide the extent of corrosion in the concrete. ANOVA further observed the interaction between the fly-ash content with water-binder proportion, and the interaction between fly ash and the type of steel shows the variation-trend influence on the corrosion activity, which will decide the extent of different zones of corrosion.The total squares were determined. The value of total squares was divided into squares (SS) amounts for different factors and the rest to the squares sum for residual random-error.The mean square of all the factors had been determined by taking the ratios of all the squares’ sum and their respective degree of freedom (df). The hypothesis of equality of variance was tested to observe the effect of individual factors on the above-mentioned engineering and durability properties level (in this study, 95% and 99% confidence levels were considered).Fstatistics was determined, which is given by equation (5). This value is compared with Fisher’s distribution (F-value).(5)Fstatistics=mean squares of factorsmean squares of residual error.It depends on the df of the individual factors, residual error, and the probability level. A modelF-value is indeed the proportion of the individual term’s mean square to a residual’s mean square. The probability of the Fstatistics was being employed to evaluate the null hypotheses, and perhaps even the probability greater than F-ratio seems to be the prob. of Fstatistics. Significant prominent factors were those that have an Fstatistics significance level of lower than 0.05.F-value more than the tabulated value indicates that the variance corresponding to the factor is more as compared to the variance of error and that, henceforth, the factor will be relevant, and its effect may not be neglected.The results of ANOVA for the rest potential for uncarbonated CPSE are presented in Table5, and w/b ratio and the types of steel have a more significant influence on governing the corrosion potential for steel as compared to other factors like water-cement ratios and their interaction with various fly-ash content. Table 5 summarizes an ANOVA result for rest potential for uncarbonated concrete, which reveals that the statistical model adequately has a 95% level of confidence. The model’s F-values of A, B, and C are 6.78, 8.81, and 17.76, respectively, and the p value is less than 0.0001. It is indicated that a relatively larger F-value of models attributed to disturbance/noise has a 0.01 percent probability of occurrence. As a result, the quadratic model is statistically significant at a 95% level of confidence. Arkadeb and Sarmila [20] have observed that steel rebars coated with Ni-P-W show minimum damage reinforced concrete structures in marine environment.Table 5
ANOVA result for rest potential for uncarbonated concrete.
SourceDegree of freedom (df)Sum of squares (SS)Mean squares (MS)F-ratioProbability >F-ratioWater-to-binder ratios (A)21304865246.780.0173FA content (B)5161679323358.810.7892Steel types (C)21303946519717.760.0515(A)∗(B)10192680192685.250(A)∗(C)486042215105.860.0002(B)∗(C)106194361941.690.0906Error1264625493671——Total1591099757———Tables6 and 7 depict interaction between water-to-binder ratios and fly ash, and interaction between fly ash and steel showed significant corrosion activity. The results showed that water-to-binder ratios and fly ash have a strong effect in defining the various corrosion zones in uncarbonated concrete, since the calculated F-value for steel type is less than the respective value obtained from the 95% confidence level, while those for other parameters are substantially higher at the same level.Table 6
ANOVA result for active to passive potential for uncarbonated concrete.
SourceDegree of freedom (df)Sum of squares (SS)Mean squares (MS)F-ratioProbability >F-ratioWater-to-binder ratios (A)216664833212.980.0327FA content (B)5231264462525.420.0001Steel types (C)2139922699618.210.0004(A)∗(B)10486304486305.70(A)∗(C)479572198932.330.0593(B)∗(C)10262000262002.330.0016Error12610743308526Total1592350724Table 7
ANOVA result for passive to transpassive potential for uncarbonated concrete.
SourceDegree of freedom (df)Sum of squares (SS)Mean squares (MS)F-ratioProbability >F-ratioWater-to-binder ratios (A)278399339199631.160.0014FA content (B)5351490702985.590.0001Steel types (C)2106158530794.220.0168(A)∗(B)10427751427753.40.006(A)∗(C)4123597308962.460.0491(B)∗(C)1010002100000.790.6338Error126158522912581Total1593443156Tables6 and 7 summarize an ANOVA result for active to passive potential and for passive to transpassive potential for uncarbonated concrete, which reveals that the statistical model adequately has a 95% level of confidence. The model’s F-values for active to passive potential for uncarbonated concrete for A, B, and C are 12.98, 5.42, and 8.21, respectively, while the model’s F-values for passive to transpassive potential for uncarbonated concrete for A, B, and C are 31.16, 5.59, and 4.22, respectively, and the p value is less than 0.0001 in both the cases. It is indicated that a relatively larger F-value of models attributed to disturbance/noise has a 0.01 percent probability of occurring. As a result, the quadratic model is statistically significant at a 95% level of confidence. Lukasz and Nikoo [13] developed an artificial neural network (ANN) combined with imperialist competitive algorithm (ICA) to predict the corrosion current density. They observed that ANN optimized with ICA owns more precision in predicting steel corrosion in concrete genetic algorithms. Ping-yi et al. [14] observed that the Taguchi particle swarm optimization provides higher computational efficiency and higher robustness when solving problems involving seven nonlinear benchmark functions: three unimodal functions, one multimodal function, two rotated functions, and one shifted function. Similar findings were reported by [1, 3, 15, 16].Similarly, the findings of ANOVA for rest potential and corrosion current density (Icorr) in carbonated CPSE are presented in Tables8 and 9, respectively. The interaction of fly ash and steel shows a significant effect in the corrosion current density for the steel reinforcement of the carbonated CPSE in comparison with other parameters, like different steel types and fly-ash content (Tables 8 and 9). The F-values obtained from the interactions of fly-ash content and steel types are less than the respective F-values at 95% confidence level.Table 8
ANOVA result for rest potential of carbonated concrete.
SourceDegree of freedom (df)Sum of squares (SS)Mean squares (MS)F-ratioProbability >F-ratioCarbonation (A)110.7 × 10610.7 × 106128.240Water-to-binder ratios (B)24091.62045.832.240.0578FA content (C)51.76 × 10535240.34.210.001Steel types (D)22.91 × 106145876.617.440(A)∗(B)21.28 × 1046446.95.770.4638(A)∗(C)51.24 × 10524820.32.970.0126(A)∗(D)21560.2780.10.090.911(B)∗(C)101.42 × 10514208.11.70.0809(B)∗(D)41.13 × 10528252.93.380.0056(C)∗(D)102.41 × 10524101.92.880.0019Error2762.30 × 1068366.7Total3194.5 × 106Table 9
ANOVA result for Icorr of carbonated concrete.
SourceDegree of freedom (df)Sum of squares (SS)Mean squares (MS)F-ratioProbability >F-ratioCarbonation (A)10.09640.09643.120.0783Water-to-binder ratios (B)20.17420.087112.820.0612FA content (C)50.44960.089932.910.014Steel types (D)20.17290.086452.80.0625(A)∗(B)20.17850.089232.890.0572(A)∗(C)50.45810.091622.970.0126(A)∗(D)20.17880.089392.90.0569(B)∗(C)100.95410.095413.090.001(B)∗(D)40.38820.097053.140.015(C)∗(D)100.93460.093463.030.0012Error2768.51980.03087Total31912.4929Tables8 and 9 summarize an ANOVA result for rest potential and Icorr for carbonated concrete, which further reveals that the statistical model adequately has a 95% level of confidence. The model’s F-values for rest potential for carbonated concrete for A, B, C, and D are 128.24, 32.24, 4.21, and 17.44, respectively, while the model’s F-values for corrosion current density for carbonated concrete for A, B, C, and D are 3.12, 2.82, 2.91, and 2.8, respectively, and the p values in both cases are less than 0.0001. It is indicated that a relatively larger F-value of models attributed to disturbance/noise has a 0.01 percent probability of occurrence. As a result, the quadratic model is statistically significant at a 95% level of confidence for both cases.From ANOVA results, it is observed that steel type has the strongest effect on rest potential for uncarbonated concrete. It can be realized that the utility of zones during cathodic protection helps to identify the shift in potential.


## 4. Conclusions

Few investigators have studied the corrosion steel behavior in pore solutions contaminated with carbonation, chloride, or sulphates. Although the research on steel corrosion behavior reinforcement in uncarbonated and carbonated concrete powder solution extracts is rare, so far, no research work has been conducted on corrosion behavior of steel in uncarbonated and carbonated concrete powder solution extracts in particular.Thus, this work is first encounter by conducting an investigational study to explore the steel corrosion behavior in concrete powder solution extraction by performing the PDLST on simple steel specimen. The present work aims to establish the various zones of corrosion for three different categories of reinforced steel extracts of uncarbonated and carbonated concrete power solutions. PDLST has been performed to obtain the polarization anodic curves and identification of corrosion zones on bare steel specimens. Thereby, the investigational study was conducted to evaluate the corrosion behavior of high-volume fly-ash concrete through PDLST. Various zones of corrosion in the reinforced concrete structure, such as semi-immune, active, passive, and pitting zones, were recognized through the PDLST investigation on uncarbonated concrete powder solution. It is observed that the steel types and fly ash are found to have a profound impact in describing the various zones of corrosion in the reinforced concrete structure. The polarization curve is found for steel types in a carbonated concrete powder solution, which shows Tafel’s behavior. Results indicated that concrete with up to 50% fly ash shows superior resistance against carbonation, as compared to Ordinary Portland Cement (OPC) concrete. Fully active anodic polarization curve is obtained for carbonated concrete. Corrosion-resistant steel performed better as compared to Tata TMT and SISCON TMT types of steel. From ANOVA results, it is observed that steel type has the strongest effect on rest potential for uncarbonated concrete. It can be realized that the utility of zones during cathodic protection helps to identify the shift in potential. This is essential to immunize the reinforcement and verify the extent of corrosion in rebar in the high-volume fly-ash concrete.



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*Source: 5927819-2022-03-24.xml*