Depolarization Characteristics of Cathodically Protected Reinforced Concrete Structures

Zita Lourenco, PhD

ZetaCorr

Lda Portugal

INTRODUCTION

The 100 mV potential decay is the most common criterion used to assess the performance of cathodically protected reinforced concrete structures. The accepted decay time to obtain 100 mV depolarization is generally between 4 and 24 h for the atmospherically exposed part of structures. [1,2] In a marine environment structures may also be exposed to submerged, tidal and splash conditions. Each of the different zones is characterized by different concrete exposure conditions with different moisture, oxygen and chloride contents and concrete resistivities. In these conditions, the 100 mV decay criterion may not be applicable and or the decay time may have to be extended, as the rate of depolarization is dependent on the availability of oxygen at the steel interface. [3] In the submerged zone the achievement of an instant off potential more negative than -720 mV Ag/AgCl/ 0.5 M KCl is normally used. In the tidal zone, due to the influence of the tidal variation on the depolarization process, the use of a decay criteria is restricted, and, as a result, other criteria have been proposed for that area. [4]

This paper attempts to examine the depolarization characteristics of the splash and the atmospherically exposed areas of a cathodically protected semi-immersed reinforced concrete structure.

CHARACTERISTICS OF THE DEPOLARIZATION CURVES

In order to determine the long term depolarization characteristics of the different zones, the structure was left to depolarize for 7 days at 158 weeks of polarization. The potential was recorded every 10 minutes for the 7 day period. The values were plotted as a function of time and examples of the curves obtained in the different parts of the structure are presented in Figure 1. In the atmospheric zone full depolarization was obtained before the 7th day, around the 4th or 5th day. In the splash zone the structure was still slowly depolarizing at the 7th day. It was assumed, however, that the depolarization obtained at 7 days was a good approximation of the full depolarization. The percentage of depolarization at 4h, 24h, 48h and 4 days relative to the total depolarization obtained at 7 days was calculated. Results are presented in Table 1, for the 2 zones.

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