Principle
Many techniques are used to assess
the corrosion risk or activity of steel reinforcement in concrete. The most
commonly used technique is the half cell potential measurement that determines
the risk of corrosion. Even though this technique is effective in locating
regions of corrosion activity, it does not provide any indication about the
rate of corrosion. However, a low resistance path between anode and cathode
would be associated with a high
Equipment
The Wenner four probe technique is commonly
adopted for resistivity measurement of in situ concrete, even though methods resistivity
Schematic Diagram
of Wenner Four Probe Resistivity Meter
The equipment consists of four
electrodes placed in a straight line on or just below the surface of concrete at
equal spacing. Two outer electrodes act as current probes and inner electrodes
act as voltage probes. A low frequency alternating electrical current is passed
between the two outer electrodes and the voltage drop between the inner
electrodes is then measured.
The apparent resistivity, ρ (ohm-cm) is
given by the following formula:
ρ = 2πaV/I
Where,
V -is voltage drop,
I - is applied current,
a - is electrode spacing.
The calculation assumes that the
concrete is homogeneous and the inhomogeneity caused by the reinforcement
network must be allowed by the proper placement of probes to minimize its
effect.
Procedure
Resistivity measurement is an in situ
non-destructive method to obtain information related to the corrosion hazard of
embedded reinforcement bar .
The region of concrete being measured
is determined by the spacing of the four probes. The depth of the concrete zone
affecting the measurement will be equal to the electrode spacing. If the
spacing is too small, there will be high degree of scatter in the measurement
due to the presence or absence of individual aggregate particles having a very
high resistivity. Using a larger spacing may lead to increased error by the
influence of the embedded steel. Concrete sections greater than 200 mm
thickness can be measured with acceptable accuracy with small degree of scatter
by providing 50 mm spacing. The surface coupling should be efficient to
establish satisfactory electrical contact between the probes and the concrete,
which may sometimes lead to minor damages to the concrete surface.
Applications
The main application
of this test is the assessment of the corrosion rate and it is used in
conjunction with other corrosion tests such as the half-cell potential measurement.
A commonly used guide for the corrosion assessment of non-saturated concrete
where the steel is activated is given in Table 1.
Table
1. Guide for Corrosion Assessment
Resistivity (ohm cm)
|
Likely Corrosion Rate
|
< 5,000
|
Very high
|
5,000 – 10,000
|
High
|
10,000 – 20,000
|
Low / Moderate
|
>20,000
|
Negligible
|
The calibration
for this test can be done either through exposing the steel to assess its
condition, or by correlating the resistivity values with data collected using other
techniques. The values given in Table 1 can be used only when the half-cell potential measurement shows
that corrosion is possible.
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