ABSTRACT This paper constitutes a literature review of the electrical resistivity of concrete. Electrical resistivity is an important consideration in such applications as cathodic protection systems and hospital operating room floors, where low resistivities are required and electrically powered rapid transit lines, where high resistivities are needed. Electrical resistivity is also important where reinforced concrete will be exposed to corrosive conditions, as corrosion currents will flow more easily in concrete having low resistivity. Information on the effects of concrete materials and mix designs on electrical resistivity are presented in this paper. The effects of such variables as temperature, and moisture content are also discussed. Data taken from the literature is presented in tabular and graphical form. Over 50 references to the published literature are also included.
INTRODUCTION Electrical resistivity is an important physical property of portland cement concrete, that affects a variety of applications. Electrical resistivity (or its inverse, conductivity) is important as a measure of the ability of concrete to resist the passage of electrical current. This has direct relevance to such applications as cathodic protection systems and hospital operating room floors, where low resistivities are required and electrically powered rapid transit lines, where high resistivities are needed.
The electrical resistivity of concrete is an important component of reinforcing steel corrosion cells, as high resistivity of the electrolyte (in this case concrete) will reduce corrosion currents and slow the rate of corrosion. Electrical resistivity is fundamentally related to the permeability of fluids and diffusivity of ions through porous materials such as concrete. Therefore, electrical resistivity can also be used as an indirect measure of the ability of concrete to prevent penetration of chloride salt solutions that may cause corrosion of the reinforcing steel.
Electrical resistivity of concrete is an important consideration with respect to the corrosion of steel in concrete. Corrosion occurs due to the formation of an electrochemical corrosion cell. A corrosion cell must have four elements' in order to function: 1) an anode where the metal is oxidized; 2) a cathode where a reduction process, such as hydrogen evolution or oxygen reduction, occurs; 3) an electrical connection between the anode and cathode; and 4) an ionic conduction path provided by an electrolyte. In the case of metals embedded in concrete the electrolyte for the corrosion cell is the concrete itself. A resistivity of less than 5,000 ohm. cm can support very rapid corrosion of steel 2. If the electrolyte has high resistance to the passage of current, or if the electrolyte is dry and unable to support ionic flow, then corrosion will occur only at a very low rate, if at all. Various researchers ~'~~ have determined that corrosion can be limited by increasing resistivity. A table of suggested values ~ is shown as Table 1. When resistivities exceed a value of 20,000 ohm. cm, the risk of corrosion is low. Where steel is actively corroding, however, Broomfield et al. ~ state that resistivity must exceed 50,000 ohm. cm to reduce corrosion to an acceptable rate, and that resistivity must exceed 100,000 ohm.cm to stop corrosion entirely.
This paper discusses the fundamentals of electrical resistivity. Comparisons of resistivities for a range of material types are shown. Various theoretical relationships between electrical resistivity and the properties and amounts of constituents in a composite material are presented in this chapter. In spite of the many theories which have been developed, it is not yet possible to