Half cell potential mapping

Only calcium nitrite, introduced more than thirty years ago, has a long and proven track record as a corrosion inhibitor for reinforced concrete [1,4]. MFP and alkano-lamine-based organic inhibitor blends are increasingly used but unfortunately most of the commercial apphcations lack rigorous control of the inhibitor effect. One of the very few comparative field tests on chloride-contaminated concrete studied MFP and a proprietary alkanolamine inhibitor added in the side-walls of a tunnel (16). The inhibitors were applied by the producers. The measurements of macrocell currents and half-cell potential mapping revealed that both inhibitors were virtually ineffective at the chloride concentrations of 1-2 % by mass of cement present [16]. 

B. Elsener, Half-cell potential mapping to assess repair work on RC structures . Construction and Budding Materials, 2001, 15, 133-139. 

Half-cell potential mapping has provided a very useful, non-destructive means to locate areas of corrosion for monitoring and condition assessment as well as in determining the effectiveness of repair work [4]. As an early-warning system, corrosion is detected long before it become visible at the concrete surface. Based on potential mapping, other destructive and laboratory analysis (e. g. cores to determine chloride content) and corrosion-rate measurements can be performed more rationally [3]. In addition, the amount of concrete removal in repair works can be minimized because the corrosion sites can be located precisely. 

Figure 16.7 Examples of half-cell potential maps (riding deck In the tunnel San Bernardino). Data representation colour plot (a) and equicontour line plot (fc). DF dilatation joint (every 25 m)...

Other types of resistance measurements, especially also involving the rebar network, have been apphed. Commercially available instruments combine half-cell potential mapping with resistance measurements between the electrode and the rebars. This results in resistance maps, however, conversion to true resistivity is much more difficult because the cell constant is also influenced by the cover depth to the steel bars and the size of the external electrode. 

Chloride profile based on cores or powder drilling. The location of the cores to be taken should be based on a problem-oriented approach, e. g. on a half-cell potential map. For cores dry drilling is preferable, in any case excess of water should be avoided due to the risk of washing out chlorides. The cores shall be identified and stored in plastic bags until analysis in the laboratory. 

Summarizing, half cell potential mapping has provided a very useful, non-destructive means of delineating areas of corrosion sites for monitoring and condition assessment as... 

The goals of the electrochemical repair methods is to check ongoing corrosion of the rebars by increasing the alkalinity of the pore solution and to extract chlorides from the concrete cover. No official standard for the acceptance of the treatments exists, but at the end of the treatment the effectiveness should be checked. This can be done by direct measurements (half cell potential mapping) or by indirect means (chloride content, sodium profiles, total charge flow, etc.). 

ASTM C876-91 (1991) Standard on half cell potential mapping, Philadelphia American Society for Testing and Materials. 

Interpretation. The presence of an above-critical amount of chloride ions at the rebars leads to depassivation and in the presence of oxygen and water to corrosion attack. From chloride profiles information on the transport of chlorides into the concrete (Chapter 6) can be obtained. In combination with results from potential mapping, the critical chloride content for the specific structure can be obtained. On chloride-contaminated structures an empirical correlation between chloride content and half-cell potential could be established, thus the chloride distribution can be roughly obtained from the potential map. 

Today potential mapping is the only commonly recognized and standardized non-destructive method for assessing the corrosion state of the rebars in concrete structures. In addition to the existing standard (ASTM C876-91), a RILEM recommendation is about to be published (RILEM, 1999a). Several national guidelines (as e.g. the Swiss SIA 2006, 1996) describe the use and interpretation of half cell potential measurements. 

When large areas are to be surveyed the huge number of potential readings can be examined statistically. Half cell potential data represented as cumulative probability (Fig. 8-19) are especially well suited for comparative purposes. The different bridge decks represented in Fig. 8-19 show similar curves but shifted on the potential axis. This shift is due to the influence of concrete temperature and humidity. The San Bernardino curve corresponds to the map in Fig. 8-17 and it is interesting to note that in the... 

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