철근콘크리트 구조물의 효율적인 음극방식을 위한 전도성 모르타르 특성 연구

Abstract

The method of cathodic protection is generally divided into Impressed Current Cathodic Protection (ICCP) and Sacrificial Anode Cathodic Protection (SACP), depending on how electric current is supplied to the structure. ICCP provides current from a rectifier to a cathode (e.g., rebar for concrete structure) through an insoluble anode. In the SACP system, a current flow occurs according to the potential difference between the anode having a lower potential(e.g., aluminum, zinc, and magnesium) and the cathode. ICCP is advantageous in that current strength can be adjusted so that it can be applied to high-resistivity materials. On the other hand, SACP is difficult to be used in such a high-resistance environment because it can only deliver a small amount of current to the cathode, resulting in significant decreases in cathodic protection efficiency and throwing power. To overcome these critical limitations of SACP, the development of conductive mortar is essential to reduce concrete resistivity in tidal and splash zones and maintain its low resistivity for a long time. This study investigates conductive mortar to improve cathodic protection efficiency by lowering resistivity of mortar. The resistivity characteristics of the mortars that contain electrically conductive admixtures were first studied to reduce resistivity. As the admixtures, activated carbon, zeolite, bentonite and geopolymer that can absorb moisture were used. Activated carbon shows the lowest resistivity among the selected admixtures. As the amount of admixture increases, resistivity tends to decrease. Chemical agents—such as sodium hydroxide, calcium hydroxide, lithium hydroxide, and sodium chloride—were further mixed with admixtures to provide additional conductivity. The performance of chemical agents is highly dependent on the selection of the admixture. Among many combinations, the specimen with sodium hydroxide and activated carbon shows the lowest resistivity, and resistivity can further be reduced by adding chemical agent. Conductive mortar with activated carbon and sodium hydroxide exhibits a 2–4 times higher resistivity reduction than general mortar. Depolarization tests were conducted to evaluate the SACP system’s performance with the conductive mortars, and the highest depolarization is achieved with activated carbon. 100 mV depolarization is required to ensure sufficient cathodic protection performance, and all specimens in this study have higher depolarization than 100 mV, which demonstrates that the performance of conductive mortar is stably maintained. The current density stabilizes after a while, and higher cathodic protection currents are measured in conductive mortars than general mortars due to their low resistivity. The higher the amount of activated carbon, the higher the cathodic protection current, showing that the mixing ratio is proportional to the cathodic protection performance. Basic physical property tests were performed, and the results show that conductive mortar can have enough durability and the basic property similar to general mortar. Cathodic protection performance with conductive mortar was tested on a reinforced concrete structure exposed to salt damage. The depolarization measurements were performed at seven piers for three years. The four piers satisfy the 100 mV depolarization standard by NACE International, whereas the three piers do not. SACP performs well during the initial two years of cathodic protection system construction, whereas partial corrosion is observed two years later. However, due to characteristics of sacrificial anodes, there can be slight differences in the measured values depending on the surrounding environment (e.g., changes in tides). Considering that it is the results of the depolarization measurement in the area where variations of the high and low tides occur twice per day, cathodic protection performance is generally good. Corrosion had already been initiated when the structure was cathodically protected, which makes visual inspection difficult. Thus, steel plates were installed to visually check the corrosion progress. After three years, corrosion is hardly observed on the steel plate in columns with cathodic protection application, while considerable corrosion is observed in columns without cathodic protection system. Therefore, the cathodic protection system works well as intended. In conclusion, this study first applies the conductive mortar to an reinforced concrete structure in use, and satisfactory results are obtained through six measurements for three years. The conductive mortar developed through this study satisfies all of the electrical conductivity, durability, and other performances that the mortar should have and can be applied to a wider variety of fields. keywords : conductive mortar, activated carbon, cathodic protection, corrosion, reinforced concrete, resistivity