Potential field measurement

Max Hartmann, summer semester 2018

The potential field measurement is a method for the detection of actively occurring corrosion in reinforced concrete. It is counted as a non-destructive-testing Method, even though it causes minimal damage to the structure. The potential field measurement records the corrosion with the help of reference electrodes and outputs the results in the form of an equipotential plot. It is a robust technique for the first assessment of structures for damaged reinforcement, helping to narrow down the affected area.

Fig. 1: Measurement equipment with wheel-reference-electrode

Source: Mrowietz F., 2016, Master Thesis - Anwendung des Potential- und Radarmessverfahrens an einem Stahlbetonkörper, Technische Universität München, 2016

Basics

Reinforcing steel is usually protected in the concrete by a so-called passive layer against corrosion. The passive Layer is formed by electrochemical reactions due to the high pH-value of concrete. The protective cladding of the passive layer can be destroyed by mainly two mechanisms. On the one hand the entry of chlorides, for example by de-icing salt, or by lowering the pH-value through carbonation. The two destructive mechanisms differ mainly in their propagation. While the chloride-induced damage tends to concentrate locally, carbonation destroys the passive layer over a larger area.

Once the protective layer is destroyed the main corrosion process starts under the availability of oxygen and water. The corrosion is divided into two partial reactions, the anodic and the cathodic.

Anodic partial reaction (iron dissolution):

$\begin{array}{l}Fe \longrightarrow Fe^{2+} + 2e^−\end{array}$

Cathodic partial reaction:

$\begin{array}{l}O_2 + 2H_20 + 4e^− \longrightarrow 4OH^−\end{array}$

The two reactions are running simultaneously, which guaranties the electron neutrality.

Due to the potential difference of the anode and cathode, an electrical current in short-circuited galvanic elements occurs. This electric current can be measured during a potential field measurement.^[1]


Fig. 2: Modell of corrosion mechanism

Method

For correct chemical potential measurement, the equipment as well as the procedure are very important. All steps must be carefully documented so similar outcomes can be accomplished during a repeated measurement.

Equipment

There are mainly two different methods to measure the potential difference.

For the first one the measurement is made between the reinforcing steel and a reference electrode of the measuring instrument. The electrode can be moved freely over the surface of the concrete, which allows an individual measurement grid. Nevertheless, a direct contact to the reinforcement bar is needed to put it under voltage. Therefore, it is necessary to remove some concrete over the rebar which shall be examined.

The other method uses two reference electrodes. In this case no direct contact to the rebar is needed.

The reference electrode has a wetted sponge on the bottom to ensure a good conductivity. The electrode usually consists of copper or copper sulphate but silver or silver sulphate is also used.

The measured potential values can give direct conclusions to the active corrosion of the rebars.^[2]


Fig. 3: Modell of a potential measurement with one reference electrode	Fig. 4: Modell of a potential measurement with two reference electrodes	Fig. 5: Modell of a reference electrode

Procedure

The measurement can either be obtained punctually, which can be problematic because one mistake can lead directly to wrong conclusions, or extensive. For the extensive method usually, several singular or wheel electrodes are used parallelly.

The measurement can be influenced by many different factors: For that reason, it is important to document all the environmental conditions such as relative humidity and temperature.

To get better conductivity the concrete can be wetted before the measurement. It is important to establish an evenly wetted concrete surface or else the results will vary. If the wetted concrete dries out, due to wetter related causes, the measurement results can differ vastly. Beside the moisture, there are also other factors which can influence the measurement, such as oxygen content, pH-value, degree of carbonation or chloride content. Additionally, the temperature, the cover thickness of the concrete, the concrete composition and age can make several influences. The temperature must have over 5 degrees Celsius or else the error frequency is increasing. Likewise, honeycombing, air pores, cracks or joints make it harder to evaluate the measured results. Also, the chosen grid plays an important role when recording the data. It can be chosen in between 10x10 to 50x50 cm. The smaller the grid the higher the resolution and the easer it can get to detect the corroding rebars precisely.^[3]

From the measured values obtained, actively occurring corrosion processes can be detected, which form as potential funnels in the component. The values are predominantly negative potential values in the [mV] range.

Evaluation

The Data can be visualized in an equipotential plot which uses different colours to highlight the different measuring results. In general, it can be said that more negative potential differences suggest higher corrosion. Due to the many influencing factors and the output in potential differences it is essential to have specific knowledge in this field of study to make proper conclusions. Nevertheless; to verify the results it is inevitable to remove the concrete over the affected rebars and make a visual inspection.^[4]

Fig. 6: Exemplary formation of a potential funnel over a corrosion site in 3D