International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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BEM ANALYSIS ON HALF-CELL POTENTIAL MEASUREMENT FOR CORROSION ESTIMATIONVeerachai Leelalerkiet; Kumamoto University, Kumamoto, Japan
Je-Woon Kyung; Kumamoto University, Kumamoto, Japan
Masaru Yokota; Shikoku Research Institute Inc., Takamatsu, Japan
Masayasu Ohtsu; Kumamoto University, Kumamoto, Japan
Corrosion of reinforcement in reinforced concrete occurs due to an electro-chemical reaction. Accordingly, half-cell potential measurement is employed to estimate corrosion of reinforcing steel-bars (rebars). Applying the three-dimensional boundary element method (3D-BEM), potential distribution and current flow from the anodic region to the cathodic region on rebars are analyzed. The analysis simulates the half-cell potential measurement by a double-electrode probe. Thus, the influence of voids on the potential distribution and the current flow is investigated. Results of current flows reveal the presence of macro-cell mechanism and the generation of micro-cell mechanism. These mechanisms between the cathodic and the anodic regions could lead to the fact that the half-cell potential measurement is not readily applicable to estimate the corrosion near voids.
In concrete structures, reinforcing steel bars (rebars) in concrete normally do not corrode because of a passive oxide film (with high pH) on the surface of the steel. However, the corrosion may occur when these structures are subjected to salt attack. Chloride ions penetrate from the surface of concrete, reach to rebars and break the passive film. As a result, the corrosion of rebar nucleates and develops cracks in reinforced concrete (RC) structures. Thus, early monitoring is necessary to detect the corrosion before cracking becomes visible and critical. To this end, nondestructive evaluation (NDE) methods are recently applied to estimate the corrosion at early stage. So far, a half-cell potential measurement is widely applied to obtain the information on the probability of corrosion [1, 2, 3]. However, the shortcoming of the measurement results from the fact that the potentials are measured not near rebars but on concrete surface . Thus, compensation is unavoidable to get more reliable results. In the present paper, three-dimensional boundary element method (3D-BEM) is applied to study the results obtained from the half-cell potential measurement. Potential distribution and current flow are analyzed for identifying the corroded areas. In order to compare with analytical results, tests of RC slabs subjected to chloride penetration under the wet and dry conditions are conducted.
2. EXPERIMENT AND ANALYSIS
2.2 Boundary element method
where y is also located on the boundary S surrounding the concrete. G(x,y) is the fundamental solution. In order to simulate the macro-cell phenomenon, each concrete slab is referred to electrically as two regions. The left-hand side corresponds to the cathode and the right-hand side to the anode. Thus, the potentials on rebars are prescribed as - 0.1 volts for the cathodic region and - 0.3 volts for the anodic region, respectively. By applying BEM solution, the unknown potentials and currents are calculated.
2.3 Test procedure
2.4 Visual inspection and levels of corrosion
3. RESULTS AND DISCUSSION
3.1 Potential distribution by BEM
In the intact model, the potentials in either the cathodic region or the anodic region show almost common values for entire area of each part, except the position of the electrode. In contrast, in the void model, the potentials around voids become higher than the areas right over the voids in both the cathode and the anode regions. This implies that the micro-cell mechanism is generated around a void. Locally, the area of the rebar under the void becomes the cathode, while the rebar around the void becomes the anode. Thus, it is roughly concluded that only the macro-cell phenomenon is found in the intact case, while the micro-cell phenomenon is observed due to the presence of voids in addition to the macro-cell.
The potential distributions inside concrete at elevation views of rebars 1 and 6 in the case of the electrode on mesh No.26 are shown in Fig.7. The effect of the electrode is apparent on rebar 6, because the electrode is placed right over. In the intact model, the affected areas by the electrode in the depth are slightly larger than the void model, while those are almost the same in the reinforcing direction. Similarly to the potentials on concrete surface, the potential distribution is almost uniform in the left-hand and the right-hand sides. In the void model, the potentials are varied at void areas in which potentials are negatively high around the voids as the same as the potentials on concrete surface. Again, it is observed that the macro-cell phenomenon is found in intact model, while both macro-cell and micro-cell mechanisms are present in the void model.
3.2 Current flow by BEM
As mentioned before, the potentials both on the surface of concrete and inside the concrete are influenced by the electrode (see Figs.6 and 7). Hence, the effective length should be taken into account as illustrated in Figs.9 and 10. The elective lengths of the electrode on meshes No.11 and No.23 are referred to as the shaded lengths.
The electrode places on mesh No.11 exactly over the cross-over point of two rebars as illustrated in Fig.9. The currents on the effective areas of rebars 3 and 1 are illustrated in Fig. 11, where the size and direction of arrows represent the amount and direction of current. It is clearly seen that all current values on rebars are negative, and the current on segments of the side near the electrode is more negative than that of the opposite side. In other words, the electrode has great influence on the current at the near-side of the rebar. Especially in rebar 3 (upper rebar), the upper segments of the rebar have more negative current ratios than those of the lower segments. In the case of the void model, it is observed that the presence of voids causes the decrease in the upper current ratio (UR) of both rebars 3 and 1.
The case of the electrode on mesh No.23 is illustrated in Fig.10 and the results are given in Fig.12. It is observed that the currents on segments S1 and S4 of rebar 4 are positive. Due to the influence of electrode, the currents on segments S2 and S3 of rebar 4 become negative. In rebar 5, all current values are negative. The currents on rebars 1 and 2 are quite the same. The positive currents are noticed in the cathodic region, while the negative currents are observed in the anodic region. Accordingly, the current flow are suggested from rebar 4 and rebars 1 and 2 in cathodic region to rebar 5 and rebars 1 and 2 in anode region.
Current flows of the entire rebars are represented by the signs of current values as shown in Fig.13. Compared to the results of visual inspection in Fig.14, it is found that the current flows by BEM analysis identifies the corroded areas. In the intact model, the negative currents on rebars in the anode region correspond to the corroded portions in the right-hand side of the specimen in Fig.14 (a). Although the negative currents are observed in the rebar 3 in the cathodic region, the current values are actually low compared with the others in the anode region. In the void model, the current flows elucidate the micro-cell mechanism around the void areas. High negative currents are found around the voids, while in the rebar areas under the voids the positive currents are observed. Results are also conformable to the actual corroded areas in the void specimens as illustrated in Fig.14 (b). Thus, it is concluded that current flow analyzed show good agreement with corroded areas by visual inspection.
3.3 Half-cell potentials measured
The potentials and current flows of rebars inside concrete are quantitatively studied by BEM. Results obtained are concluded as follows: