The measurement of the resistivity of concrete gives an indication of the possibility of risk of the existence of significant levels of reinforcement corrosion.
Corrosion of steel reinforcement within a structure will occur if sufficient chloride is able to penetrate through the concrete and depassivate the steel. If this occurs the reinforcement potential will decrease to approximately -350mV and lower as corrosion progresses. The rate of corrosion then becomes increasingly related to both the resistivity of the concrete and the rate at which oxygen can reach the steel.
In general it has been shown that the lower the resistivity the greater the risk of corrosion, and when the value of the half cell potential indicates that corrosion is occurring this may be stated empirically as follows:
Values greater than 12000 ohm.cm indicates that a significant rate of corrosion is unlikely.
5000 to 12000 ohm cm indicates that a significant rate of corrosion is almost certain.
The equipment used consists of four 6mm diameter steel probes which are pressed onto the surface of the concrete using colloidal graphite as a contact medium. Each probe is provided with a terminal connection from which wires lead to an earth resistance meter.
A current is passed between the two outer probes and the voltage between the two inner probes is measured by the meter which calculates and displays the resistance directly.
Method of Test
The test procedure is adapted from the Wenner Method for the measurement of the resistivity of soils.
A check is made using a cover meter to ensure that no reinforcement is in the immediate vicinity (nearer than about 20mm to the surface). The four 6mm diameter steel pins are then coated with colloidal graphite, pressed onto the surface and connected to the earth resistance meter. Readings of resistance are directly read from the instrument.
The resistivity of the concrete (r) is related to the measured resistance (R) by the formula
r = R 2 π a
where r is the resistivity in ohm cm, R is resistance in ohms and a is the separation of electrodes in cm.
Wenner F. "A Method of Measuring Earth Resistivity" Bulletin of the US Bureau of Standards Science Paper 12 (1916) No. 3.
Kalous K. and Bartale W. "Practical Evaluation of Grounding Measurements" Microtecnic 5/66, Volume XX, Scriptar S. A. Lausanne.