Understanding defects, testing and inspection - Concrete Repair - page 51

reinforcement. Corrosion rate measurements can be useful in assessing the
significance of these high potential values, as can resistivity measurements.

Results and interpretation
According to the ASTM method, corrosion can only be identified with 90%
certainty at potentials more negative than -350 mV. Experience has shown,
however, that passive structures tend to show values more positive than
-200 mV and often positive potentials. Potentials more negative than -200
mV may be an indicator of the onset of corrosion. The patterns formed by
the contours can often be a better guide in these cases.
In any case, the technique should never be used in isolation, but should
be coupled with measurement of the chloride content of the concrete and
its variation with depth and also the cover to the steel and the depth of
carbonation. The ASTM document also suggests a potential difference
technique where differences in potential are plotted over relatively small
areas to locate corroding steel. This can be helpful in dry weather, for
example, when corrosion cells become less active and may not always follow
the ASTM guideline values. However, peaks of potential at corroding areas
may well still exist. It has to be remembered, when evaluating results, that
the technique measures what is happening on the day. Considerable variation

in numeric values can be encountered over different seasons and weather
conditions.
Figure 1.33 shows a survey of a car park deck slab using a 1 m grid. The
corroding areas can be readily identified.

1.9.2 Resistivity

The electrical resistivity is an indication of the amount of moisture in the
pores, and the size and tortuosity of the pore system. Resistivity is strongly
affected by concrete quality, i.e. cement content, water/cement ratio, curing
and additives used.

Equipment and use
The main device in use is the four-probe resistivity meter. These have been
modified from soils applications and are used by pushing pins directly onto
the concrete with moisture or gels to enhance the electrical contact. Millard
et al. (1991) described two versions of the equipment. Some variations
use drilled-in probes or a simpler, less accurate two-probe system. Further
information can be found in Concrete Society Technical Report No 60
(Concrete Society, 2004).

Resistivity = 2 ʌ a V / I (cm)

where: R is the resistance by the 'IR drop' from a pulse between a surface
electrode and the rebar network measured by a half cell reference electrode