Instrumentation and monitoring of structures - Concrete Repair - page 143

electrical circuit and uses an embedded thermometer to compensate for the
effect of temperature on readings.
This technique measures losses from corrosion due to ineffective cathodic
protection and is the only technique that can measure in-situ corrosion loss
under cathodic protection. Valid measurements are possible even in non-
conductive environments.
The main disadvantage of ER probes is that they only give valid data
when the corrosion mode is uniform. These instruments are not suitable
when corrosion is localised (pitting, cracking). This is its major limitation in
concrete where corrosion initiates with pitting, particularly when chlorides
are present. There is a trade-off between the sensitivity of the probe and its
usable lifetime. The thinner the probe the more sensitive it is but the more
quickly it is consumed.
Probes are permanently installed and measurements are made on a
periodic basis. As a guide, the probe should be monitored at least once a
month for the first 12 months. Thereafter, the monitoring frequency can be
reduced to once every 3 months.
A graph is made of metal loss vs. time. The interval corrosion rate would
be the slope of the graph between any two data points while the average
corrosion rate would be the slope of the trendline as calculated by the least
squares method. Many graphing programs, such as MS Excel®, have built-in
capability to determine trendline slopes.
When used to demonstrate the effectiveness of a cathodic protection
system, an average corrosion rate (trendline) of less than 0.1 mils/year (2.5
microns/year) over a 12-month monitoring period indicates that cathodic
protection is effective at the location of the sensor.
Corrosion probes of the electrical resistance type are rarely used in
concrete. They can suffer from localised pitting around the ends of the
corrodible steel leading to rapid failure once corrosion initiates in this
environment. They have been used to show that corrosion is under control,
e.g. in impressed current cathodic protection systems, but in other cases have
been found to be poor indicators of rates of corrosion.
7.2.5 Concrete resistivity sensors
Resistivity probes can be embedded in concrete. The design shown in Figure
7.6 was installed in the Dartford Tunnels on the River Thames estuary
(Broomfield 2000). Data collected is shown in Figure 7.7.
7.2.6 Humidity monitoring
Commercially available relative humidity probes are available that are
durable and are suitable for embedding in concrete section. The effect of
relative humidity on corrosion rate is discussed in (Broomfield 2007).