Resistivity of concrete and mortars

Corrosion of reinforcing steel is an electrochemical process. For corrosion of the steel to occur a current must pass between the anodic and cathodic regions of the concrete. The electrical resistivity of the concrete affects the flow of ions and the rate at which corrosion can occur. A higher concrete resistivity decreases the flow; an empirical relationship between the corrosion rate and resistivity has been determined from measurements on actual structures. Various approaches for measuring resistivity are available but the four-probe device is the most suitable. 

Resistivity is affected by a number of factors including cement content, mix design, the use of polymers and microsilica, hence permeability, but is very dependant on the moisture content of the concrete. 

In concrete cured under water (100% RH), most of the pores and capillaries are filled with water which is a good conductor of electric current and therefore the resistivity is low. Various approaches for measuring resistivity are available but the Wenner four-probe device is the most suitable. Modern devices are spring-loaded and are applied directly to the surface. A current is applied between the two outer probes and the potential difference measured between the two inner probes. Resistivity measurement is useful for identifying areas of reinforced concrete at risk from corrosion. It should not be considered in isolation but used in conjunction with other techniques such as half-cell potential.

The Highways Agency Specification for Roads and Bridges in Clause 1700: structural concrete : 1770 AR for Repair concrete - class 29F, specifies the following method :

Electrical resistivity shall be measured on three 150mm cubes stored at 20°C under saturated conditions. A probe shall be cast into each cube. The probe shall consist of four pins of stainless steel, spaced at 20mm centres. Each pin shall have a diameter of 5mm with a 10mm length embedded into the cube to a depth of 40mm as shown in Fig. 17/74AR.

Resistivity shall be measured by the 4 -point Wenner system using a resistance meter with an AC frequency of 105 to 495 Hz. Measurements shall be taken at 14, 21 and 28 days, immediately after removing the cube from the waterbath and drying off the surface with absorbent paper. The resistivity shall be calculated from the following formula :

Resistivity (in ohm cm) = 2 π a (V/l)

a = pin spacing

V = voltage measured on inner pins
I = current flowing between outer pins

It should be noted that because resistivity varies with moisture content, the accepted standard of measurement of repair products in laboratory conditions is in a fully saturated (but surface dry) condition at 28 days. But when measurements are taken on site using portable probes, on dry or damp or wet concrete, these readings of resistivity on concrete will be quite different to lab-measured values. Therefore this must be taken into consideration when choosing appropriate repair products.

For example, an average resistivity value of 50 kΩ.cm of parent concrete at an unspecified age and a moisture content of perhaps 75%, might have a corresponding resistivity of say 25  kΩ.cm when measured at 100% RH.

We have examined the resistivity of several Weber products that are used for concrete repair.

The table to the left shows the difference in resistivities between products that have been specially developed to have low resistivity, ie high conductivity for applications such as embedment of anode mesh, encapsulation of anodes or to comply with the requirements of the Highways Agency Specification BD 27/86 for low resistivity repair mortars. The resistivities in dry conditions will be greater than the included figures. 

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