Study On Corrosion Monitoring Of Rebars In Concrete By Guard Ring Technique

The electrochemistry method is applied to evaluate the corrosion of rebars in concrete for it is an electrochemical process. The method is employed successfully to detect the corrosion state and rate of rebars in test samples and has been used to monitor and detect the corrosion of steel in field recently. However, the three-electrode system is prone to result in non-uniform current distribution on the surface of rebars in detecting the corrosion rate of on-site large reinforced concrete because of the large ratio of SWE/SCE (CE: Counter Electrode, WE: Work Electrode), and the electric signal of CE gradually decays or vanishes with its distance from CE. As a result, the polarization area of the rebars can not be confirmed in a certain location. Guard Ring Method, based on the general three-electrode system and applies a Guard Ring Electrode (GE) to surround CE, was used in order to overcome the problem. This system contained two circuits-CE and GE; the CE responding current was compensated by the polarization of GE after the rebars were polarized by CE. Thus, the potential D-value of RE2 and RE3 returned to original value before the polarization of CE. Therefore, the electric line of CE distributed mainly on the surface of WE under CE. There have been few reports about this technique in China so far. The uniform polarization measurement of rebars in concrete was employed to obtain their real corrosion rate; then, to obtain the effective polarized area of rebars, the authors studied the two-dimensional and three-dimensional current compensation in different concrete humidity and with different GE size. The results are compared with the results of uniform polarization under the same condition with the conclusions being as follows:(1) In uniform polarization measurement, active rebars in concrete (added 3%CaCl2 vs. cement weight) appear to be in a high corrosion state, while passive rebars in concrete (without CaCl2) are protected by passive films; the corrosion current density of active rebars is approximately 10 times that of passive ones.(2) Guard Ring Method could be employed to monitor and detect the corrosion of rebars in concrete, and the results were preferably compared with uniform polarization results. For active system, CE responding current was mainly under CE and seldom lateral distributed with the length of the polarized rebar approximately 3~4.5 times that of the diameter of CE; for passive system, the length of the polarized rebar is approximately 10~20 times that of the diameter of CE, indicating the current distributes laterally and can not be confined completely. For passive rebars, corrosion state could be determined preferably without serious impact by Guard Ring Electrode Method.(3) The CE current aggregated more easily by applying larger GE. Compared with two-dimensional compensation, the polarization area of rebars in three-dimensional compensation was much smaller. The more the compensation was, the smaller the CE current was, even though over-compensation might occur. In comparison with the results of uniform polarization, the corrosion current density was over-estimated without CE current compensation; the corrosion current density was underestimated when CE current was compensated completely; Only when the CE current was compensated by 30%,50%,70%,the results are satisfactory. Hence, the projecting area of CE diameter should be selected as the polarization area of rebar in greater compensation extent, and the midpoint projecting area between CE and GE should be adopted as the polarization area of rebar in narrower extent CE current compensation.