influence of reducibility environment around steel ...

INFLUENCE OF REDUCIBILITY ENVIRONMENT AROUND STEEL BARS ON STEEL CORROSION IN CONCRETE Takahiro NISHIDA1*, Nobuaki OTSUKI2, Atsushi SAITO3, Keiyu KAWAAI4 ¹ Department of Civil & Earth Resources Engineering, Kyoto University, Kyoto, Japan ² Office of Education and International Cooperation, Tokyo Institute of Technology,, Tokyo, Japan 3 Research Center, HAZAMA ANDO Corporation, Ibaraki, Japan 4 Department of Civil and Environmental Engineering, Ehime University, Ehime, Japan *Corresponding author: [email protected]

Abstract : Steel corrosion in concrete is one of the major concerns around the world. Especially this corrosion phenomenon cannot be avoided in port and harbor port structures because a corrosion inducing material, chloride ion, easily penetrate in concrete in such environment. On the other hand, it is considered that Blast Furnace Slag (BFS) is one of the effective byproducts against steel corrosion in concrete. In present paper, the corrosion protection effect of BFS was discussed from the viewpoint of reducibility environment around steel bar derived from BFS power. As a result of this study, the dissolved oxygen reduction of BFS powder was confirmed and the cathodic polarization curve of steel bar in chloride contaminated concrete with BFS decreased. It was considered that low Oxygen environment as well as Chloride immobilization dew to BFS hydrates resulted in the high durability of reinforced concrete against chloride induced corrosion. Key words : steel corrosion, seawater mixing, reducibility, BFS

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INTRODUCTION

Steel corrosion in concrete is one of the major concerns around the world. Especially this corrosion phenomenon cannot be avoided in port and harbor port structures because a corrosion inducing material, chloride ion, easily penetrates into concrete in such environment. On the other hand, it is considered that Blast Furnace Slag (BFS) is one of the effective by-products against steel corrosion in concrete. BFS is a by-product material produced from steel mill. The usage of BFS is increasing in Japan year-by-year from the viewpoint of its performance as mineral admixture as well as the usage of waste materials (Nippon Slag Association, 2012). An advantage of concrete with BFS against chloride attack is well known as the resistivity of chloride ion penetration, which flyash also has this advantage. On the other hand, the color of inside concrete with BFS tends to become blue indigo because of the reducibility environment, as the BFS is produced under quite high reducibility environment. The reactivity of BFS with Oxygen make the concrete low Oxygen environment. The steel bars in concrete are protected from steel corrosion by the high alkalinity in concrete derived from cement hydrates. The high alkalinity environment makes passive film around steel bar in concrete although chloride ions attack and break it. Even if the passive film disappears around steel bar due to chloride ions, the corrosion speed is very low under low oxygen/ reducibility environment, which can be explained by electrochemical phenomena of steel corrosion process as shown in Figure 1 (Fontana and Greene). Figure 1 shows the schematic diagram of polarization curve of anode and cathode of steel material. In the case of sound concrete, the anodic and cathodic polarization curves are represented as “Anode A” and “Cathode D” and the corrosion current is decided by the intersection of Anodic/Cathodic polarization curves, point X. After the chloride ion penetrate around steel bar, the anodic polarization curve changes to “Anode B” and the corrosion is decided at point Y. Although the amount of chloride ion is high (in the case of “Anode B”), the cathodic polarization curve is small (in the case of “Cathode C”) the corrosion current keeps low level as shown in Point Z. As the result, it is considered that the speed of steel corrosion in concrete is kept at low level under low Oxygen environment even much chloride ion exists around steel bars. Considering above backgrounds, it is considered that the steel bars in concrete with BFS are protected from steel co rrosion not only by the resistivity of concrete against chloride ion, but also by the Oxygen situation around steel bar. In the present study, the Oxygen reduction effect of BFS was confirmed and the cathodic polarization and corrosio n rate of steel bars in mortar with BFS were evaluated.

Proceedings of the WOW CONCRETE 2017 @ DLSU

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Figure 1 Schematic figure of Anodic and Cathodic polarization curves in concrete and effects of Chloride and Oxygen.

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CONFIRMATION OF OXYGEN REDUCTION EFFECT OF BFS

2.1 Methodology The fundamental experiments to confirm Oxygen reduction effect of BFS power were conducted in laboratory. The solutions with several mixing ratios of power and distilled water were prepared. These solutions were mixed for 30 minutes by magnetic stirrer. The solid/liquid ratio prepared were 62.5, 125, 250, 500 750 and 1000 g/L respectively, and amount of distilled water was 400 g. The amounts of dissolved Oxygen in mixed solution were measured by Oxygen meter under 23 degrees centigrade. The material propertied of powers is shown in Table 1. Three kinds of construction materials, BFS, ordinary Portland cement (OPC) and fly ash (FA) were used as powder. Table 1 Physical properties of materials. Powder BFS OPC FA

Density (g/cm3) 2.91 3.16 2.23

Specific surface area (cm2/g) 4310 3210 3630

2.2 Relationship between Dissolved Oxygen and Solid/liquid Ratio The relationship between dissolved Oxygen and solid/liquid ratio is shown in Figure 2. From this result, it was confirmed that the amount of dissolved Oxygen in solution with BFS decreases as the solid/liquid ration increasing. Particularly the amount of dissolved Oxygen resulted in the half of initial concentration in solution (dissolved water) in the case of 1000 g/L of solid/liquid ratio. On the other hand, the concentration of dissolved Oxygen in solution is almost same in OPC and FA solution regardless of solid/liquid ratio. As mentioned before, the inside of concrete with BFS powder is reducibility environment and it is supposed that this effect is made by the Oxygen reduction phenomena of BFS powder.


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Figure 2 Relationship between Dissolved Oxygen and Solid/liquid Ratio.

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STEEL CORROSION IN MORTAL WITH BFS

3.1 Experimental procedures The resistance of steel corrosion in concrete mixed with BFS due to chloride attack was evaluated using the experimental data. The cathodic/anodic polarization behavior and the time dependent change of corrosion current density of steel bars in concrete were investigated to understand the resistivity to corrosion of steel bars in concrete mixed with BFS. The specimen used in this investigation was mortar. The water cement ratio of specimen was 0.5. The mix proportions of mortar specimens are shown in Table 2. In order to evaluate the effect of BFS on the corrosion of steel bars in concrete, a part of the ordinary Portland cement was replaced by BFS, at 0, 40, 55 and 70 % ratio. The specimens were mixed with fresh water or artificial seawater. The chemical composition of artificial seawater is given in Table 3. Table 2 Mix proportion of mortar specimen. (W/C=0.5)

Unit of weight (kg/m3)

BFS No.

Mixing

S/C

water

(%)

Ratio W

C

BFS

S

311

622

0

1245

305

366

244

1220

303

272

333

1210

296

178

328

1200

311

622

0

1245

305

366

244

1220

(%) OPC- F

0

B40- F

40 Fresh

B55- F

50

B70- F

70 2.0

OPC-S

0

B40-S

40 Sea

B55-S

50

303

272

333

1210

B70-S

70

296

178

328

1200

W: water (fresh water or seawater) C: ordinary Portland cement (density: 3.14 g/cm3, surface area: 4660 cm2/g) BFS: blast furnace slag (density: 2.89 g/cm3, surface area: 4200 cm2/g, activity index (28 days): 94 %) S: natural river sand (density (SSD): 2.60 g/cm3, water adsorption ratio: 2.20 %, F.M. 2.59) AW: Air entraining water reducing agent AE: Air entraining agent Seawater: artificial seawater mixed with chemicals shown in Table 3.


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Table 3 Chemical composition of artificial seawater. Chemicals

NaCl

MgCl2-6H2O

Na2SO4

CaCl2

KCl

NaHCO3

Weight (g/L)

24.54

11.10

4.09

1.16

0.69

0.20

The mortar specimens, with 3 round steel bars (Φ13mm，cover depth: 10mm) embedded, were exposed to sprayed environment of 3.0% NaCl solution at 50°C. Here the cathodic/anodic polarization curve and polarization resistance were electrically measured as shown in Figure 3. Electric current was applied between steel bar (WE: working electrode) and stainless plate (CE: counter electrode) in the measurement of the cathodic and anodic polarization curve with 1.0mV/sec of the sweep speed. On the other hand, the polarization resistance was measured by AC impedance method using high frequency (10kHz) and low frequency (10mHz) alternative current and corrosion current density was calculated with 0.0209V of SternGeary constant (Stern, 1957). Reference electrode (RE, Ag/AgCl) Stainless plate (CE) Corrosion monitor Epoxy coating except the exposure surface

Steel bar (WE) (Φ13, cover:10mm) Mortar

Figure 3. Measurement of polarization curves and polarization resistance. 3.2 Cathodic and Anodic Polarization Phenomena The corrosion progresses based on the anodic and cathodic reactions and the rate of corrosion is decided by the balance of these reactions. Therefore, cathodic and anodic polarization phenomena are also important to evaluate the corrosion of steel bars in concrete mixed with seawater. The cathodic and anodic polarization curves about 140 days of exposure to acceleration environment, when the steel corrosion had already started due to external Cl penetration in OPC concrete mixed with fresh water, were measured as shown below. Figure 4 (a) shows the cathodic polarization curves of steel bars in mortar with different replacement ratio of BFS. From this, it was confirmed that the cathodic polarization curve went to left-down side of the graph with the increase of the replacement ratio, which indicated that the oxygen content around steel bar decreased when the higher replacement of BFS got higher. It was considered that this phenomenon was derived from the low pore volume of cement matrix in BFS mortar and the reduction action of BFS powder in concrete. Especially BFS power was produced under reductive atmosphere and oxidation number of Fe or Mn in BFS tended to low (Nippon Slag Association, 2012). Then, it was considered that the amount of oxygen around the steel bar in BFS decreased. Thus, from the viewpoint of cathodic polarization phenomena, it was estimated that the higher replacement of BFS was effective against steel corrosion in concrete. On the other hand, Figure 4 (b) shows the anodic polarization curves of steel bars in mortar. From this, the current density of steel bar with OPC in anodic polarization tended to be larger than that with BFS. Especially the steel bars in concrete with more than 55 % replacement ratio of BFS seemed to have better passive film even if concrete was mixed with sea water. Considering the basic cathodic and anodic polarization phenomena, it seems that the higher replacement ratio of BFS has better corrosion resistance against chloride attack.


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Figure 4 Cathodic polarization and oxygen diffusivity of steel bar in mortar. 3.3 Corrosion Rate of Steel Bar in Concrete The effect of BFS on the corrosion behavior of steel bars in concrete was discussed. The time dependent changes of steel bars in concrete under accelerated condition of chloride attack (50°C, spray of 3.0 % NaCl solution) is shown in Figure 5. In this graph, 0.2 µA/cm2 of the corrosion current density (dashed line) indicates the judgment criteria of corrosion proposed by CEB. In the case of OPC, the corrosion current density increased with the time and this tendency of seawater mixing was much faster than that of fresh water mixing. However, the periods over the criteria of CEB became longer as the BFS replacement ratio got higher. Also, the difference between seawater and fresh water became smaller with the increase of replacement ratio especially at longer duration. It was considered that the higher resistivity to corrosion of steel bars in mortar was due to the properties of BFS concrete, such as the Cl immobilization, low porosity and the low oxygen effect. Especially the tendency of corrosion resistivity of steel bar corresponded with the result of cathodic polarization phenomenon. Therefore, it can be assumed that BFS was one of the effective additives for concrete mixed with seawater from the viewpoint of corrosion protection, and this effect derives from the low oxygen effect in the cement matrix as well as Cl immobilization.


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dashed line：Judgment criteria of corrosion proposed by CEB, larger than 0.2μA/cm2 : Corrosive state, lower than 0.2μA/cm2 : Passive state

Figure 5 Time dependent change of corrosion current density of steel bar under 3 % NaCl sol. sprayed condition at 50 ºC

4 CONCLUSIONS In the present study, the steel corrosion in concrete with blast furnace slag powder was discussed. Following conclusions were derived. 1. The results of solution test, it was confirmed that blast furnace slag powder show the reduction effect of Oxygen regardless ordinary Portland cement and fly ash did not confirmed it. 2. The cathodic polarization curve decreased with the amount of blast furnace slag powder increased. 3. The results of long term exposure test indicated the high possibility of utilization of blast furnace slag and seawater as a material of reinforced concrete. Moreover, the experimental data indicated that the introduction of BFS might contribute significantly to the corrosion resistance of steel bar due to low oxygen environment around steel bar as well as Cl immobilization phenomena. Thus, it could be concluded that the steel corrosion in concrete mixed with BFS was remarkably resist against steel corrosion in concrete even if the concrete mixed with seawater.

REFERENCES Fontana and Greene (1978), Corrosion Engineering, McGraw-Hill. Nippon Slag Association. (2012). “Utilization of blast furnace slag in cement”, FS-127, p.42 (in Japanese). Stern, M. and Geary, A. L. (1957).“Electrochemical Polarization: I. A Theoretical Analysis of the Shape of Polarization Curves”, Journals of the Electrochemical Society, Vol.104,No.1,pp.56-63.


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