Instrumentation and monitoring of structures - Concrete Repair - page 139

are that they are durable when cast in concrete and exposed to the
environment, the readings are meaningful and that readings do not drift
with time as recalibration is difficult or impossible. By embedding probes
and monitoring remotely we remove the access requirement and minimise
the manpower needed. We also know that data were collected in exactly
the same way and at exactly the same location for each successive set of
data.
7.2.1 Permanent reference electrodes
The simplest probe is the reference electrode. These are designed for
exposure in concrete for installation with cathodic protection systems
( Chapter 4). However, it has been shown that once embedded in concrete,
they cannot be recalibrated if they drift (Ansuini and Diamond 1994) and a
very large number are required if a useful 'potential map' ( Chapter 1) is to
be produced. Reference electrodes are incorporated into LPR probes (next
section) but are rarely used on their own except in a cathodic protection
system where a potential shift is recorded so the absolute calibration of the
electrode is not required.
7.2.2 Corrosion rate by polarisation resistance sensors
Corrosion rates can be measured by linear polarisation or by galvanic or
macrocell techniques. Both have their merits and limitations as discussed in
the next two sections.
The LPR probe is ideally suited to permanent installation, both in new
construction and as a retrofit into existing structures. Probes consist of a
reference electrode, an auxiliary electrode and a working electrode. In the
case of a 'new build' probe, the working electrode can be a piece of steel
of known surface area so that an accurate corrosion rate measurement is
made on a working electrode in the same environment as the rest of the
reinforcement. An electrical connection to the reinforcement means that
measurements can also be taken on the reinforcement itself. Figure 7.1 is
an example of such a probe installed in a reinforcing cage prior to casting.
Such a design cannot be used in existing structures. In such cases the
working electrode must be the reinforcement in an undisturbed area of
concrete. This means that the probe consists of a reference electrode and
auxiliary electrode with a connection to the reinforcement at a suitable
location. The probe assembly must be installed with minimum disturbance
of the reinforcement, as shown in Figure 7.2.
7.2.3 Corrosion rate by galvanic sensors
A galvanic ladder probe for installation in new structures is shown in Figure
7.4. This consists of a series of mild steel anode rungs connected to a