Corrosion of steel bars is one cause of deterioration in reinforced concrete structures. The state of corrosion of rebar in concrete structures can be measured using the AC impedance method. The typical three-electrode AC impedance method requires electrical continuity between the rebar and the measurement device, which may be inconvenient because the structure requires to be partly damaged by processes such as drilling. The present study proposes a method to obtain the same impedance spectra as the three-electrode method by modifying the electrode configuration of the four-electrode method. The procedures for such a measurement are as follows: first, place the counter electrode and the measurement reference electrode at the measurement point; place another counter electrode at a sufficient distance from the measurement point; place another reference electrode at a sufficient distance from both the measurement point and the applied counter electrode; and connect WE_I to the applied counter electrode, WE_V to the reference electrode, CE to the applied counter electrode, and RE to the applied counter electrode, and measure the impedance. In this paper, the physical experiments and the numerical experiments demonstrate the validity of the above method. Under the conditions of this physical experiment (concrete W/C = 54.2%, cover 50 mm), the distance between the measurement point and the application counter electrode was required to be about 1.3 m. The distance between the measurement point and the reference matching electrode was also required to be about 1.3 m. Graphical abstract

桟橋鉄筋コンクリート (RC) 上部工は， 点検結果に基づき鉄筋位置の塩化物イオン (Cl-) 濃度が 2.0kg/m 3 となる時期を予測し，対策実施時期を設定しているが，腐食理論に基づけば，環境や材料・配合等に応 じて鉄筋腐食の発生条件は種々変化するはずである．本研究では，塩害劣化予測の高精度化に向けた基 礎検討として，海水中に含まれるイオンが鉄筋腐食速度，および Cl-の浸透性状と細孔溶液の pH に及ぼ す影響を実験的に検討した．その結果，健全な RC 構造物中の pH では，鉄筋表面の Cl-濃度が 3%になっ ても腐食電流密度は極めて小さいが，pH が低下し海水中の多種イオンが存在すると低 Cl-濃度でも腐食 電流密度が大きくなった．また，海水中の多種イオンの影響により Cl-の拡散と移流は抑制されることが 明らかとなった．

Corrosion of steel bars is one cause of deterioration in reinforced concrete structures. The state of corrosion of rebar in concrete structures can be measured using the AC impedance method. The typical three-electrode AC impedance method requires electrical continuity between the rebar and the measurement device, which may be inconvenient because the structure requires to be partly damaged by processes such as drilling. The present study proposes a method to obtain the same impedance spectra as the three-electrode method by modifying the electrode configuration of the four-electrode method. The procedure for such a measurement is as follows. First, the counter electrode and the measurement reference electrode place at the measurement point. Next, another counter electrode at a sufficient distance from the measurement point place. Another reference electrode place at a sufficient distance from both the measurement point and the applied counter electrode. And Connect WE_I to the applied counter electrode, WE_V to the reference electrode, CE to the applied counter electrode, and RE to the applied counter electrode, and measure the impedance. In this paper, the physical experiments and the numerical experiments demonstrate the validity of the above method.

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