PACE 2021- Ataturk University, Engineering Faculty, Department of Civil Engineering, Erzurum, 25030, TURKEY 20-23 June 2021 1

Keynote 20-23 June 2021

Enhancing The Corrosion Resistance of Reinforced Concrete Rebar by The Implementation of Anti Corrosive Coatings with Using TVA System

İlker Bekir Topçu1, Arda Uzunömeroğlu*2, Suat Pat3 1Professor, Eskişehir Osmangazi University, Faculty of Engineering and Architecture, Civil Engineering Department, Eskisehir, Turkey. 2Eskişehir Osmangazi University, Faculty of Engineering and Architecture, Civil Engineering Department, Eskisehir, Turkey. 3Eskişehir Osmangazi University, Faculty of Science and Letter, Department of Physics, Eskişehir, Turkey. Corresponding Author E-mail: ardauzunomeroglu@gmail.com Corresponding Author ORCID: 0000-0003-4204-0564

Keywords Abstract Corrosion, Reinforcement, Structure, Concrete, Accelerated Corrosion Test.

Corrosion results cracking in the concrete and it reduces the bearing capacity. Corrosion affects the durability of rebar; therefore, usage would not be economical and complete its lifetime if this situation persists. Iron and steel materials lose their physical, chemical and electrical properties by being affected by these different atmospheric conditions. In this study, the corrosion resistance of reinforcement in concrete structures was increased. For this purpose, the reinforcements are coated with combinations of zinc and boron elements by using a different principle, the thermionic vacuum arc system. The coated reinforcements are embedded in the concrete mixture in the mold. After the curing stage, the samples were taken out of the molds and subjected to accelerated corrosion test. The results show that the rebars covered with the TVA system were damaged later than the uncoated rebars. Thanks to the coatings, the damage occurrence time is extended and the corrosion resistance of the reinforcement is increased. It is believed that with this study, it will contribute to the prolongation of the life of the structures by eliminating the corrosion reactions that occur as a result of negative effects in the reinforcement.

1.Introduction

Researchers indicated by years that corrosion of reinforcement threatens most of the structures. In addition, the corroded reinforcement occurs a volume increase of 2 to 6 times, which forms an internal tension in the concrete. Consequently, concrete-steel adherence decreases [1-5]. The bearing capacity of the reinforcement decreases because of corrosion, which causes cracks and collapses to the concrete by time.

The "durability" properties of reinforced concrete mean, not to lose its strength and quality for years without being adversely affected came from external influences or the components of the concrete. It can be also defined as, maintaining its original features and design function, under environmental and service conditions [6]. Concrete loses its properties under adverse conditions such as environmental condition, material properties or design errors. Among all these factors, the most effective and concealed deterioration was corrosion of reinforcement. Iron and steel materials lose their physical, chemical and electrical properties by being affected by these different atmospheric conditions [7]. Generally, moisture, oxygen and chlorides in the vicinity of reinforcement are major factors responsible for the corrosion of reinforcement. Their presence in concrete depends upon the temperature, humidity, chemical contents in the surrounding environment, and the quality and thickness of the concrete cover [8-14].

The most rational and definitive solution for the protection of iron and steel against corrosion is the coating of the material with hot dip galvanizing method. Hot dipped galvanized zinc coating is a standard, effective, economical and easy applicable method of protection when steel is exposed to corrosion in air, soil, or water [15]. In terms of the construction sector, galvanized rebars comply well with concrete and it can increase the bond strength of steel up to two-fold [16]. Galvanizing process is generally done with zinc. This method experimented on many industrial components and nanomaterials, and widely used in both academic and industrial environments [17-19].

Corrosion is the result of a series of chemical reactions. These reactions are divided into two as anode and cathode reactions. Corrosion reactions on the reinforcement are given in the Equations 1-6.

Anode reactions on reinforcement:

𝐹𝐹𝐹𝐹 → 𝐹𝐹𝐹𝐹+2 + 2𝐹𝐹− (1) 2𝐹𝐹𝐹𝐹 + 3𝐻𝐻2𝑂𝑂 → 𝐹𝐹𝐹𝐹2𝑂𝑂3 + 6𝐻𝐻+ + 6𝐹𝐹− (2) Cathode reactions on reinforcement: 2𝐻𝐻+ + 2𝐹𝐹− → 𝐻𝐻2 (3)

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PACE 2021- Ataturk University, Engineering Faculty, Department of Civil Engineering, Erzurum, 25030, TURKEY 20-23 June 2021 2

2H2O+ O2+4e- → 4OH- (4)

𝐹𝐹𝐹𝐹+2 + 2𝑂𝑂𝐻𝐻− → 𝐹𝐹𝐹𝐹(𝑂𝑂𝐻𝐻)2 (5) 2𝐹𝐹𝐹𝐹(𝑂𝑂𝐻𝐻)3 → 𝐹𝐹𝐹𝐹2𝑂𝑂3 + 3𝐻𝐻2𝑂𝑂 (6)

Figure 1. Corrosion reactions on structural rebar

In low carbon steels, surface oxidation and pitting type corrosion can occur and surfaces have the appearance of red-brown rust. By creating a coating on the surface, the corrosion resistance of steel can be improved. Many methods have been used to protect the rebar from corrosion, but these methods are either very expensive (stainless steel) or reduce the adherence between concrete and reinforcement (epoxy coatings) [20-22].

The method used in this study is a unique and unprecedented technique. The TVA system is used in this article in order to enhance the corrosion resistance of steels. Unlike other corrosion protection methods, this system is based on covering the reinforcement surface. Thus, chemical liquids such as corrosion inhibitors and epoxy coatings that reduce adherence are avoided. The system provides cheap and fast application. It was previously used in industrial applications to protect metal from corrosion.

2. Testing process

2.1. Test specimens and material properties

Ordinary Portland cement (CEM I) produced by Eskişehir cement plant was added in the concrete as binder. The chemical and physical properties of cement are listed in Table 1. Crushed limestone and river sand were added in the concrete as aggregates. The specific gravity of the cement, the coarse aggregate, the fine aggregate was 3.15, 2.71 and 2.58 g/cm3 respectively.

Table 1. Chemical properties of portland cement

Chemical Components of Cement Value

SiO2 21.97%

Al2O3 8.59%

Fe2O3 3.54%

CaO 50.2%

MgO 3.45%

SO3 1.27%

Cl 0.048%

Ignition loss 2.14%

Specific gravity (g/cm3) 3.15

Specific surface(cm3/g) 3280

Table 2. Proportions of concrete mixtures

w/c ratio

Water (kg)

Cement (kg)

Fine agg. (kg)

Coarse agg. (kg)

Superbly. (kg)

0.50 175 350 683 1119 2.65

Figure 2. Prepared concrete lollipop specimens

The materials used and their proportions are given in Table 2. In this study, 0.5 water / cement ratio with 350 kg cement dosage. Cube (15 cm) and cylinder (100x200 cm) samples were cast for the experiments. Ribbed rebar with a length of 20 cm was placed inside the cylinders. Samples were cured in water for 28 days. The samples were kept in curing until the time of the experiment, when the time came, they were taken out of the pool and subjected to the experiment.

2.2. Test Methods

2.2.1. TVA System

In this study, construction reinforcements are covered with thin films in a system called TVA. TVA can be considered a PVD system. As a result of recent studies, it has been observed that such thin films protect the reinforcement from corrosion. In this study, the effect of coated rebars on corrosion, which is one of the most important structural defects, has been investigated.

Figure 3. a) Photo from outside the TVA system b) Inside the vacuum chamber

TVA technique is used in many new technological applications. The most important of these is the ion-supported coating technique. Coatings produced by bombarding the anode material with ions; These are extremely smooth, low roughness, dense and high cohesion coatings. In the carbon and boron coatings made with TVA, the structures were seen to be in nano-size and this caused the TVA technique to be used in nano technology applications. The representation of TVA is as shown in Figure 3. TVA system consists of 9 main parts. These are;

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PACE 2021- Ataturk University, Engineering Faculty, Department of Civil Engineering, Erzurum, 25030, TURKEY 20-23 June 2021 3

• Vacuum Chamber

• Directly heated cathode

• Anode material holder

• Cathode current and anode potential power supply

• Pumping system

• Vacuum measurement systems

• Ammeters and Voltmeters system

• Thin film thickness measurement systems

• Cooling systems [22-29].

After the reinforced concrete samples are covered with nanostructures, the durability of these coatings on the rebar surface against corrosion was ready to experiment. The method and coating material that we use is unique system. Table 3 was showed the parameters of the coating that used in this study.

Table 3. TVA Parameters

Parameter unit Zn Zn-B Zn-BN

Filament Current ampere 16 16 21

Discharge Current ampere 200 200 100

Ignition Voltage volt 300 300 400

Pressure torr 6x10-5 1x10-4 1x10-4

Time min 7 7 4

Figure 4. Samples of rebar coated with different elements

Figure 4 shows the rebar samples covered with 3 different coatings. Each coating reacted differently to the corrosion effect and reduced the corrosion rate differently. Since the concrete mixes are the same for all the samples poured, the effects of the coatings in preventing corrosion could be compared. Concrete casting was carried out immediately after the coating process was completed.

Figure 5. Rebar samples sent for laboratory analysis

The Figure 5 shows the sample pieces sent for laboratory analysis. The rebars were cut in 1 cm length, their surfaces were polished in the polishing device and made ready for analysis. Separate samples were prepared for each coating series.

Figure 6. Results of the EDX analysis of elements

As a result of EDS analysis in Figure 6, we can observe that a thin film with a nano-crystalline structure formed on the surface. This crystalline structure is the coating itself, and all reinforcements are covered in this way. The elements coated on the reinforcement completely envelop the surface and protect the reinforcement from corrosion.

Table 4. Proportions of elements in surface

Elements Wt % Wt% Sigma

B 0.00 8.79

N 0.26 0.44

O 1.40 0.25

Zn 98.34 0.50

Total 100.00

The Table 4 shows the proportions of elements in surface of the rebars. As can be seen, a high rate of Zn spectrum was detected on the reinforcement surface. As a result of the analysis in Figure 12, 98.34% Zn compound was found on the surface. This means that the thin film coating is formed properly on the reinforcement surface. These coatings protected the reinforced concrete reinforcement from harmful effects such as chloride by preventing corrosion.

2.2.2. Compressive Strength and Electrical Resistivity

Although the reinforcement corrosion that occurs in reinforced concrete structures is dependent on environmental effects and reinforcement, if the quality of the concrete is high, the corrosion event occurs either too late or not at all [30]. Therefore, the compression strength, placement, quality and cement ratio of concrete are very important parameters affecting the reinforcement corrosion [31-33].

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PACE 2021- Ataturk University, Engineering Faculty, Department of Civil Engineering, Erzurum, 25030, TURKEY 20-23 June 2021 4

Figure 7. Performing the compressive strength test

In this study, the mechanical properties of unreinforced concrete specimens were investigated. The reason for this is to have information about the quality of concrete. In the experiments shown in Figure 7 and Figure 8, compressive strength, electrical resistance values of unreinforced concretes were determined. These values were made separately for cube and cylinder samples, and the endurance values on the 28th and 90th days are shown on the graphics in result and discussion section.

Figure 8. a.) Applying gel to the samples b.) Execution of the test

2.2.3. Accelerated corrosion tests Method

The aim of the impressed current tests was examining the corrosion performance of reinforced concrete specimens [33-35]. During the impressed current technique, accelerated corrosion was applied on the concrete lollipop specimens shown in the Figure 7. In this test, the embedded rebar in lollipop specimens acted as an anode and stainless-steel plates act as a cathode. The electrolyte was %4 sodium chloride solution (NaCI). A constant voltage of 20V was applied from the external direct current power supply source between anode and cathode. The electrical current was recorded by time.

Figure 9. Execution of accelerated corrosion test

In the Figure 9 above, the application of the accelerated corrosion test on reinforced concrete specimens is shown. The voltage supplied from the power supply is transmitted to the samples. The voltage is read as

current on the data logger after passing through the samples. The data logger records each data during the 16-day test period. With the help of these data transferred to the computer, the corrosion flow chart given in Figure 10 was drawn.

3. Result and Discussion

In Figure 10, severe corrosion damage can be seen in samples caused by high current and chloride effect. While some samples have pitted corrosion, some have corrosion damage in the form of general mass loss.

Figure 10. Images of the concrete samples after the accelerated corrosion test

While the reinforced concrete samples were being prepared, each of the rebars placed inside were covered with separate materials with the help of TVA system. When we look at the damage caused by the accelerated corrosion test, the coatings reduced the corrosion damage by preventing the corrosion current. Each coating provided a different degree of corrosion protection. Highly current absorbing samples were damaged more, samples with less current impeded by the coatings were damaged less.

Figure 11. Corrosion current graph of coated samples after accelerated corrosion test

As seen in Figure 11, the sample with the highest initial corrosion current absorbed by the system is the uncoated sample. Since the current applied by the accelerated corrosion test system is directly proportional to the corrosion potential, the samples that absorb less current are less corroded.

Damage occurrence times are also seen on the same graph. While damage occurred earlier in uncoated reinforcement, damage occurrence times increased according to the coating type. In the

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control, Zn-coated and Zn-B-coated samples, crack formation started and damage occurred earlier. However, the Zn-BN coated sample remained uncracked for approximately 15 days. Since the concrete quality is the same for all samples, the effectiveness of the coating is clearly visible in the samples that crack lately.

Figure 12. Compressive strength results of unreinforced cylinder and cubic samples

In Figure 12, when we look at the compressive strength results of the concrete specimens poured without reinforcement, we see that the 90th day compressive strength of the cube specimens is 40.7 MPa. This compressive strength meets the standards, considering the dosage and type of cement used. The compressive strength results of the cylindrical specimens were slightly lower due to the shape effect. On the 90th day, their compressive strength is 32.5 MPa.

Figure 13. Electrical resistivity results of unreinforced cylinder and cubic samples

The electrical resistance properties of concrete specimens poured without reinforcement are given in Figure 11. Since the concrete mixes are the same, the electrical resistance results are close to each other regardless of the shape effect. The conclusion to be drawn from this is that as the curing time increases, the hydration reactions within the concrete develop better, the compressive strength and electrical resistance results of the concrete samples have increased positively at further ages.

4. Conclusion

In this experimental study, the importance of rebar coatings against the damage caused by corrosion to buildings has been revealed. The damage caused by corrosion to reinforced concrete structures has been revealed by many researchers before. Methods developed for

protection from corrosion are either ineffective or require high costs in terms of implementation. Some methods are difficult to implement in terms of integration into the construction field. The point that distinguishes this study from other methods is that the TVA method, which is used for different purposes, is used for the first time in the literature to cover the rebar used in reinforced concrete structures. Zn, Zn-B and Zn-BN coatings were applied to the rebars using TVA system and successful results were obtained. Thin films formed in reinforcement bars protected the reinforcement from corrosion reactions. As a result of this study, best result observed by Zn-BN sample with lower corrosion current and longer damage occurrence time. It is thought that this study will shed light on future applications. The most effective way to avoid the destructive effects of corrosion is to avoid the occurrence of corrosion reactions from the very beginning. The cost of preventing corrosion at the beginning of the construction process is much less compared to the costs after corrosion occurs. In addition, the loss of life and property that may cause corrosion should be kept in mind. By using this system, reinforcements can be produced as resistant to corrosion and there is no need for additional methods and costs to protect the structure from the harmful effects of corrosion.

Declaration of Conflict of Interests

The authors declare that there is no conflict of interest. They have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments

The authors would like to thank the financial support from Eskişehir University Research Fund (BAP) (Project Code Number: 202015066).

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