ABSTRACT
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
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 Figure 7.5. 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
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).
