Civil Engineering Reference
In-Depth Information
372.0
in situ from the use of a calcium chloride additive) can destroy the passive
film. In the presence of oxygen and humidity in the concrete, corrosion of
the steel starts. A characteristic feature for the corrosion of steel in concrete
is the development of macrocells - the co-existence of passive and corroding
areas on the same reinforcing bar with the corroding area as the anode and
the passive surface as the cathode. The voltage of such a cell can reach as high
as 0.5 V or more, especially where chloride ions are present. The resulting
current flow (which is directly proportional to the mass lost by the steel) is
determined by the electrical resistance of the concrete and the anodic and
cathodic reaction resistance.
The current flow in the concrete is accompanied by an electrical field
which can be measured at the concrete surface, resulting in equipotential
lines that allow the location of the most corroding zones at the most negative
values. This is the basis of potential mapping, the principal electrochemical
technique applied to the routine inspection of reinforced concrete structures
(Broomfield, 2007).
The use of the technique is described in an American Standard, ASTM
C876-09, Standard Test Method for Half Cell Potentials of Reinforcing Steel
in Concrete (ASTM, 2009). The standard has changed considerably from its
earlier incarnation in 1980 and now contains numerous caveats and more
guidance on factors influencing half cell potential values.
In use, a reference connection is made by exposing a reinforcing bar and
connecting by brazing or by drilling a small hole and inserting a self-tapping
screw into the bar (CAUTION - this method is not suitable for prestressed
