The 4 point electrical resistivity measurement device (wenner array probe) is used to measure the electrical resistivity of concrete for analyzing the corrosion potential and offers an indication of its permeability.

Method

Corrosion is an electro-chemical process. The rate of flow of the ion

Ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s between the anode and cathode areas, and therefore the rate at which corrosion can occur, is affected by the resistivity

Resistivity

Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...

 of the concrete. To measure the electrical resistivity of the concrete a current is applied to the two outer probes and the potential difference is measured between the two inner probes.
Empirical tests have arrived at the following threshold values which can be used to determine the likelihood of corrosion.

When ρ ≥ 120 Ω-m	corrosion is unlikely
When ρ = 80 to 120 Ω-m	corrosion is possible
When ρ ≤ 80 Ω-m	corrosion is fairly certain

The electrical resistivity of the concrete cover layer decreases due to:

• Increasing concrete water content
• Increasing concrete porosity
• Increasing temperature
• Increasing chloride content
• Decreasing carbonation depth

When the electrical resistivity of the concrete is low, the rate of corrosion increases.
When the electrical resistivity is high, e.g. in case of dry and carbonated concrete, the rate of corrosion decreases.

Transformer method

In this method a transformer is used to measure resistivity without any direct contact with the specimen. The transformer consists of a primary coil which energises the circuit with an AC voltage and a secondary which is formed by a toroid of the concrete sample. The current in the sample is detected by a current coil wound around a section of the toroid (a current transformer

Current transformer

In electrical engineering, a current transformer is used for measurement of electric currents. Current transformers, together with voltage transformers , are known as instrument transformers...

). This method is good for measuring the setting properties of concrete, its hydration and strength. Wet concrete has a resistivity of around which progressively increases as the cement sets.

Standards

• ASTM Standard C1202-10: Standard Test Method for Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration

Application

The presence of rebars disturbs electrical resistivity measurement as they conduct current much better than the surrounding concrete. This is particularly the case when the cover depth is less than 30 mm. In order to minimize the effect, none of the electrodes should be placed above a rebar when measuring, or if this is unavoidable, then perpendicular to the rebar.