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Indirect Measurement Methods

This page describes NDT methods used for measuring rebar corrosion using embedded probes in the concrete structure.

Potentiodynamic Polarization Curves

Basic Concept: These well-known DC-type measurements can be performed on rebar steel in concrete using a three-electrode system. Typically, the working electrode is a small section of rebar, and stainless steel or graphite may be used for counter electrodes. Embedded reference electrodes need to be resistant to the highly alkaline concrete pour solution, such as manganese dioxide electrodes. External reference electrodes such as the Cu/CuSO4 variety can also be used, but it is important to be aware of possible "IR" (voltage) drop errors.

The rebar potential and current density data can provide useful information such as:

  1. Active or passive corrosion state of the rebar.
  2. Corrosion rate estimates by the LPR or Tafel extrapolation techniques.
  3. Risk of pit initiation from the pitting potential.
  4. Tendency for pit repassivation from the size of the hysteresis loop in the reverse pitting scan.
  5. Estimates of the Tafel constants for input into LPR analysis or for cathodic protection criteria.
  6. Evaluation of the effectiveness of corrosion inhibitors and the effects of concrete composition variables.
  7. Performance evaluation of alternative metallic rebar materials.

Limitations: A fundamental drawback of these measurements is that "artificial" corrosion damage and rebar surface changes can be induced at relatively high polarization levels. For steel in concrete, the potential scan rates used should be extremely low, making the tests time consuming. The data interpretation should be done by specialist personnel.

Electrochemical Impedance Spectroscopy (EIS) and Harmonic Analysis

Basic Concept: One method of obtaining corrosion rates from EIS data involves the use of equivalent circuit models. A number of different types of equivalent circuit models have been proposed for reinforcing steel in concrete. By accounting for the concrete "solution" resistance and the use of more sophisticated modeling, a more accurate corrosion rate value should theoretically be obtained compared with the more simplistic LPR analysis. However, EIS data generation and analysis generally require specialist electrochemical knowledge and can be rather lengthy, making it unsuitable for rapid evaluation of corrosion rates. To derive the corrosion rates, the Tafel constants also still have to be estimated or assumed.

Data Processing and Interpretation: A more recently evolving technique closely related to EIS measurements is harmonic analysis. One advantage of harmonic analysis is that mathematical data treatment facilitates direct computation of the Tafel constants and the corrosion rate. Since harmonic analysis is performed in a narrow frequency range, it can provide for practical and rapid rebar corrosion rate determination.

Limitations: A severe restriction of EIS and harmonic analysis is that, as in the LPR technique, the fundamental assumption of uniform rebar corrosion has to be made in the calculation of penetration rates. If localized corrosion damage is actually taking place, the data are, at best, of a qualitative nature, indicating the breakdown of passivity and the possibility of localized attack.

A number of initiatives have been launched aimed at applying EIS measurements directly to structural rebars, rather than small-scale embedded probes. These developments have involved guard ring concepts and modeling of signal transmission along the length of the rebar.

Zero Resistance Ammetry

Basic Concept: A Zero Resistance Ammeter (ZRA) is a current-to-voltage converter. This provides a voltage that is proportional to the current flowing between the input terminals while imposing a "zero" voltage drop to the external circuit. In the ZRA technique, a macrocell current is measured between two corrosion sensor elements.

Zero Resistance Ammetry is used to measure the galvanic coupling current between two dissimilar electrodes. An interesting application is when the coupling current between two nominally identical electrodes is measured. If both the electrodes were identical, then very little coupling current would flow. However, in reality, the two electrodes will be slightly different, one being more anodic or cathodic than the other, and a small coupling current will exist. Perhaps the best way to visualize this is to think of a single electrode that supports distinct anodic and cathodic regions on its surface. Then imagine this electrode chopped in half, and the two parts connected via a ZRA. The ZRA will intercept some of the current flowing between the (now separated) anodic and cathodic regions.

Data Processing and Interpretation: The macro cell current measured between embedded rebar probes can serve as an indication of the severity of corrosion. This principle is used in the ASTM G102-92 corrosion test procedure, where the current flows between rebar embedded near the surface and rebar at greater depths of cover. A similar approach has been adopted in a rebar corrosion monitoring system, with currents being measured between strategically placed carbon steel rebar probe elements.

Interestingly, the concept of measuring a macro cell current as indicator of corrosion severity can also be applied to probe elements of identical materials and exposed to the same environment. It may be somewhat surprising that a significant current will flow between nominally identical probe elements, but this principle has been used in commercial corrosion monitoring and surveillance systems for many years. It can be argued that such measurements are mainly relevant to detecting the breakdown of passivity and the early stages of corrosion damage.

Limitations: If extensive corrosion damage is occurring on both the probe elements, the macro cell current measured will not accurately reflect the severity of attack.