The electrical resistivity of concrete is one of the main controlling factors in connection with corrosion of embedded steel. Reliable information about the electrical resistivity is also very important for evaluation of galvanic couplings between freely exposed steel and embedded steel and for design and efficiency of cathodic protection of the freely exposed steel in such galvanic couplings.
A number of experiments were undertaken in order to provide data and information on the electrical resistivity of concrete in general and for concrete exposed to ocean environments in particular.
By successively drying out the concrete from 100- to 20-percent water saturation, the electrical resistivity increased from about 7 × 103 ohm cm to about 6,000 × 103 ohm cm, indicating that the degree of water saturation is the main controlling factor for the electrical resistivity of concrete.
The results also indicate that those factors controlling the permeability of the concrete also affect the electrical resistivity. Thus, by reducing the water-cement ratio from 0.70 to 0.50, the resistivity of mortar was more than doubled, and for a water-cement ratio of 0.50, the resistivity of mortar was more than three times higher than that of concrete. Cracking of the concrete due to drying shrinkage did also affect the resistivity significantly.
Contamination of concrete with chlorides in amounts of up to about 1-percent CaCl2 based on the cement weight, slightly increased the resistivity, while larger amounts of up to about 8-percent CaCl2 reduced the resistivity with up to about 50 percent. Chemical action between concrete and sea water formed a dense layer on the concrete surface, which increased the electrical resistance distinctly. Hence, this is also an effect that must be taken into account for concrete structures exposed to ocean environments.
Concrete normally provides embedded steel with a high degree of protection against corrosion. This is because the water phase in concrete normally has a pH within the range of 12.5 to 13.2, and in this alkaline environment a thin protective film of ? - Fe2O3 is formed on the steel surface. Under certain circumstances, however, this protective film can be disrupted either by a lowering of pH of the water phase in concrete (e.g., by carbonation) or by presence of aggressive ions (e.g., chlorides) that may penetrate the film.
Once the protective film is disrupted, the availability of oxygen at the steel surface and the electrical resistivity of the concrete will be the main controlling factors to further steel corrosion.
Not only will the electrical resistivity be one of the main controlling factors to corrosion of embedded steel, most of the large offshore concrete structures built so far have a variety of freely exposed steel components, e.g., skirts and pipeline systems in metallic connection with embedded steel.
