The Mini Great Dane is used to measure the half-cell potential of uncoated reinforcing steel in concrete (in accordance with ASTM C876) and to measure the electrical resistance of the cover concrete.
Typical applications include the following:
- Condition surveys of suspect reinforced concrete (RC) structures to identify areas with corrosion
- Activity for further analysis (testing for chlorides, depth of carbonation, flaws, or permeation) to establish the cause of the corrosion and estimate remaining service life
- Monitoring RC structures for changes in corrosion activity
- Checking the effect of re-alkalization or electrochemical removal of chlorides
- Measuring the corrosion activity in repaired areas
Reinforcement in concrete will not corrode if the protective film formed in the presence of highly alkaline pore fluid with a pH of about 13 is maintained. The passive layer may, however, be destroyed by the ingress of chloride ions or by a reduction in pH due to carbonation. When the passive film is destroyed, corrosion may occur in the presence of moisture and oxygen.
During the corrosion process, anodic and cathodic areas are formed on the reinforcement. At the anodes, iron dissolves and iron ions diffuse into the concrete, leaving behind electrons. At the cathodic sites, the iron ions combine with water and oxygen to form an expansive corrosion product, i.e., rust. The rate of corrosion is controlled by how easily the iron ions can move through the concrete from the anodes to the cathodes and it depends on the availability of oxygen and moisture at the cathodes.
The flow of irons ions through the concrete is associated with a potential field as shown on the right. The Mini Great Dane measures the surface potentials (relative to an Ag/AgCl reference electrode) and the electrical resistance of the cover concrete between the electrode and the reinforcement. The indicated potential, Ecorr, is in terms of a Cu/CuSO4 electrode (CSE), which are -110 mV lower in value than for the Ag/AgCl electrode. The risk of corrosion is evaluated by means of the steepness of the potential gradients measured at the concrete surface and the level of the electrical resistance of the cover concrete. A large potential gradient and a low concrete resistance will normally indicate a high corrosion rate, except in saturated concrete because of the low oxygen content.
After areas with the lowest potential, highest gradients, and lowest electrical resistance are identified, additional tests are performed to establish the cause of corrosion, e.g., testing for chlorides and carbonation. The concrete is removed at several “hot spots,” and the actual degree of corrosion is correlated to the readings. After identification of the cause of corrosion and establishment of the chloride ion profiles and depth of carbonation, the remaining service life may be estimated (e.g., using diffusion theory) or an appropriate repair strategy may be developed.