Civil Engineering Reference
In-Depth Information
or post-tensioned steel reinforcement, where only a mechanical connection
such as a crocodile clip can be permitted). The reference connection is
connected to the positive terminal of a high impedance digital millivoltmeter.
A reference cell such as a copper/copper sulfate half cell is attached to the
negative terminal of the meter. This cell has a porous plug at one end, which
permits an electrical connection. A grid is marked on the surface of the
concrete at suitable intervals, usually between 0.5 to 1 m spacings in a square
grid pattern. The surface of the concrete to be tested is wetted with a wetting
agent at each node to be measured on the grid intersections. The meter is
touched to the concrete surface and the millivoltmeter reading noted. The
cell is then removed and touched again and a second reading taken. The
readings should be within 10 mV of each other. Caution should be exercised
if readings drift - stable values should be obtained.
Factors affecting the potential field
When surface potentials are taken, they are measured remote from the
reinforcement due to the concrete cover. The potentials measured are
therefore affected by the ohmic potential drop in the concrete. Several
factors have a significant effect on the potentials measured:
C
ONCRETE
COVER
DEPTH
With increasing concrete cover, the potential values at the concrete surface
may indicate an average of local active and passive steel, making the location
of corroding anodic areas more difficult.
C
ONCRETE
RESISTIVITY
The concrete humidity and the presence of ions in the pore solution affect
the electrical resistivity of the concrete. The resistivity may change both
across the structure and with time as the local moisture and salt content
vary. This may create an error of ±50 mV in the measured potentials (John
et al., 1987).
H
IGHLY
RESISTIVE
SURFACE
LAYERS
The macrocell currents tend to avoid highly resistive concrete. The measured
potentials at the surface become more positive and corroding areas may be
undetected.
P
OLARISATION
EFFECTS
Steel in concrete structures immersed in water or buried in the earth often
have a very negative potential due to restricted oxygen access (Popovics et
al., 1983). In the transition region of the structure (splash zone or above
ground), negative potentials can be measured due to galvanic coupling with
immersed rebars. These negative potentials are not related to corrosion of the
