Understanding defects, testing and inspection - Concrete Repair - page 52

Half-Cell Contour Plot St. John's Level 4 E xt en sion

-mV

4

8

350-400
300-350
250-300
200-250
150-200
100-150
50-100
0-50

12

16

20

24

28

32

Figure 1.33 Half cell potential plot of a car park deck on a 1 m grid.

a is the electrode spacing

I is the current passed between outer probes (amps)

V is the potential measured between inner probes (volts).

Interpretation
Interpretation is empirical. The following interpretations of resistivity
measurements have been cited when referring to depassivated steel:

> 20 kcm low corrosion rate
10-20 kcm low to moderate corrosion rate
5-10 kcm

high corrosion rate

< 5 kcm very high corrosion rate.

Limitations
The resistivity measurement is a useful additional measurement to aid in
identifying problem areas or confirming concerns about poor-quality concrete.
Readings can only be considered when used with other measurements, such
as chloride content, depth of carbonation, cover to reinforcement and half
cell potential.
It should be understood that the resistivity does not tell you how fast the
corrosion is or how much corrosion there is, it simply indicates the capacity of
the concrete to support a corrosion cell if one exists . For example, a concrete
showing a low half cell potential but also a low resistivity is not actively
corroding, but could do if the conditions for corrosion existed. Conversely,
a concrete showing a high half cell potential and a high resistivity is unlikely
to show a significant rate of corrosion.
There are devices which can actually measure corrosion rate directly
(Broomfield, 2007) such as the Gecor 8, although these have seen only
limited use in the field.