Civil Engineering Reference
In-Depth Information
stainless steel cathode via an ammeter. When chlorides or carbonation reach
successive rungs the current between anode and cathode rises.
The ladder probe is designed for installation in new structures and has
been installed in major bridges, tunnels and other structures thoughout
Europe. Variations on this design are supplied by different manufacturers.
A more recent development is a 'washer probe' that can be fitted tightly
into a cored hole to provide similar data on existing structures, as shown in
7.2.4 Corrosion rate by electrical resistance sensors
In an electrical resistance probe (ER), a strip, wire or tube sensor of
known cross-section is exposed to the environment. In the case of concrete
structures, it must be embedded at the time of construction otherwise it is not
in the same environment as the reinforcement it is supposed to simulate. The
sensor metal must be similar to the structure metal. The electrical resistance
of the sensor is measured after initial installation and at subsequent time
periods. As metal is lost by corrosion of the exposed surface of the sensor,
the measured electrical resistance will increase which allows the amount of
loss to be quantified. A proprietary automated corrosion probe reader is
used to report the thickness loss directly. This uses a Wheatstone bridge
fitted into a cored hole in the concrete cover so that successive washers depassivate
and a galvanic current starts to flow between the anode and a noble metal cathode
(photograph courtesy Dr Ing. M. Raupach).
