문헌연구를 통한 콘크리트 중의 철근부식 전류밀도 산정을 위한 스테른-기어리 상수 B(mV)값 분포에 관한 고찰

Abstract

본 연구에서는 미선형최소간등법을 이용하여 타펠상수 결정과정의 오차를 줄여 측정된 국내연구결과를 이용하여 콘크리트 중의 철근부식 전류밀도를 산정시 배합인자들의 영향에 의한 B상수값 분포 및 변화를 분석하였다. 자연전위와 분극저항이 측정되고 압축강도 및 염화물 이온 혼입률에 따라 분류된 데이터를 추출하였으며, 압축강도에 따라 분류된 데이터의 정규분포곡선을 도출하여 Andrade와 Gonzales가 제시한 B=26mV의 타당성 검토 및 염화물 혼입률에 따른 B값의 변화를 분석하였다. 자연전위, 분극저항, 부식전류밀도, 스테른-기어리 상수B간의 관계를 각 배합인자의 영향에 따라 비교하여, 비파괴적으로 철근부식상태의 진단시 고려할 수 있는 B값의 영향을 분석하여 제시하였다.; Polarization resistance technique is measuring the change in the open-circuit potential of the short-circuited electrolytic cell when an external current is applied to the cell and it is possible to assess the quantitative status of steel corrosion. To calculate the corrosion current density in concrete, the stern-geary constant B(㎷) is determinated from anodic-cathodic tafel slope and it influences calculating the corrosion current density. Values of B used to calculate Icorr from Rp are published by each foreign researcher due to the difficult in determination of Tafel slope. In this study, domestic research was evaluated to determine the effect of B(㎷) constant distribution on corrosion current for reinforcing bar from various mixing factors. Classified data from compressive strength and chloride ion concentration for half-cell potential and polarization resistance was extracted. Feasibility study on suggested B(㎷) value, 26㎷, by Andrade and Gonzales and change in B(㎷) value was analyzed by deduction of normal distribution curve from compressive strengths. Relationship between half-cell potential, polarization resistance, corrosion current density, and Stern-Geary constant, B(㎷), was compared, and the effect of B(㎷) value on the non-destructive evaluation of corrosion status for reinforcing bar was suggested. This study consists of five chapters and contents are as follows. The first chapter describes the background, purpose, method, and range of this study. The second chapter describes the documentary records study on corrosion of steel in concrete and nondestructive methods. The third chapter describes the theoretical background of stern-geary constant B(㎷) and published values of B(㎷) used to calculate Icorr from Rp measurements. In the fourth chapter, the appropriate data for this study using the documentary records was extracted and it was rearranged. The influence of constant B(㎷) value was analyzed when using the polarization resistance method. The fifth chapter shows the conclusion which summarized analysis results of data and contents are as follows. 1. From 160 data by changing compressive strength and cover thickness of concrete, distribution of Stern-Geary constant B(㎷), showed considerable amount of sample with less than 13㎷. Therefore, there might be an error in corrosion current, which is more than 2 times larger than the published value, B=26㎷, by Andrade and Gonzales. 2. Compressive strength (18, 21, 24, 27㎫) and cover thickness of concrete did not affect on half-cell potential and polarization resistance, but the tafel slope of anode was increased due to reduced chloride ion by increased cover thickness. As a result, the higher value of B(㎷) was achieved. From the tafel slope of anode, the qualitative evaluation on the concentration distribution of chloride ion might be possible. Measurement of AC impedance and comparison on actual corroded amount should be succeeded in future study. 3. From the previous literature, diffusion rate of chloride ion is reported to be reduced from density of internal matrix of concrete by increased compressive strength (reduced W/C), but there is no significant change in Tafel slope of anode, polarization resistance, half-cell potential. On the measurement of corrosion current and half-cell potential, the value was estimated to be less than 0.2㎂/㎠ and between -100㎷ and -200㎷, respectively. Therefore, reinforcing bar was thought to be in passive state in overall due to the short corrosion promotion period. 4. By the increased cover thickness of concrete (20, 30, 40, 50㎜), there was no significant change in polarization resistance, but corrosion current was increased from higher B(㎷) value. Therefore, difference in corrosion current from the change in B(㎷) value from the slope of polarization curve for anode and cathode might exist even at the same polarization resistance. CEB standard, which qualitatively determines corrosion current only from polarization resistance, is thought to be unreasonable. 5. Tafel slope was measured by changing the amount of chloride ion in specimen with 24㎫ of compressive strength. Higher concentration of chloride ion increased the slope of anode in overall, and it was contradictory result from tafel slope theory. Therefore, further study should be conducted. 6. With the increment of chloride ion concentration (0.5%, 1.0%, 1.5%), half-cell potential and polarization resistance was significantly dropped, and showed the facilitated corrosion. Therefore, not only the effect of compressive strength and cover thickness, but also the quantitative regulation of chloride amount should be implemented at the concrete mixing.