The steel reinforcement in concrete structures (rebar) is susceptible to corrosion when chloride ions enter into the concrete. If chlorides are present in sufficient quantity, they disrupt the passive film on the rebar resulting in corrosion. Oxygen content, moisture content, and temperature also affect the corrosion rate. Corroding rebar in concrete can weaken its structural strength, creating cracking, delamination, and spalling. Rebar corrosion may also affect bonding of the rebar to the surrounding concrete, caused by changes in the diameter of the rebar and from the friable iron oxide, which may accompany the corrosion.

A comprehensive search did not find any geophysical techniques that would indicate the amount of corrosion that had taken place and, hence, the quality of the rebar. However, there are numerous methods used to find if corrosion is currently active; these will be discussed in this section. Presumably, if corrosion activity is monitored and found to be active for a significant period of time, then the "quality" of the rebar will become degraded.

One non-geophysical method was found for determining the degree of bonding of the rebar to the concrete. The test is done using an instrument called POWER, manufactured by Germann Instruments. POWER measures the pullout force of embedded anchors, either in situ or in the laboratory on specimens of rebar cast into concrete. Figures 1 a-b show the Power equipment.

Figure 38. Power equipment to test rebar bonding: (a) Preparation Kit, and (b) Power equipment. (Germann Instruments)

The following section briefly describes the methods used to detect corroding rebar in concrete. These methods are classified as direct and indirect methods as listed below.

Direct Methods

These methods measure rebar corrosion directly from an exposed surface of a concrete structure, and include:

1. Half-cell Potential.
2. Linear Polarization Resistance.
3. Galvanostatic Pulse Technique.
4. Electrochemical Impedance Spectroscopy/Harmonic Analysis.
5. Acoustic Emissions.

Indirect Methods

These methods measure rebar corrsion using embedded probes in the concrete structure, and include:

1. Potentiodynamic Polarization Curves
2. Zero Voltage Ammetry/Zero Resistance Ammetry
3. Electrochemical Noise

Generally, there are two types of devices for measuring corrosion of steel in concrete. The first uses measurements on the surface of the concrete such as Rebar Potential Measurements (Half-cell Potential), and those that take measurements using embedded probes. Examples of methods using embedded probes include Electrochemical Impedance Spectroscopy (EIS) and Zero Resistance Ammetry.

The corrosion of steel in concrete is an electrochemical process that produces an electric current, similar to that of a battery. This electric current spreads out from the rebar into the surrounding concrete, and the resulting voltage is measurable at the surface of the concrete. A characteristic feature of rebar corrosion in concrete is the development of macrocells. These result from corroding areas of the rebar being adjacent to non-corroding areas. These sites may be macroscopically adjacent or they may be some distance apart. However, an electrically conductive path must exist between the sites. The metal rebar will conduct using electrons and ions through the electrolyte in the concrete. The corroding area is an anode and the non-corroding area is a cathode. These form a galvanic cell and can produce voltages between the anode and cathode of 0.5 volts or more, especially when chloride ions are present. The electrical resistance of the concrete and the anodic and cathodic reaction resistance determine the resulting current flow. The amount of current flow is directly proportional to the rate of loss of the steel mass.

Measurements of the rate of corrosion are usually made using sensors on the surface of the concrete. However, sensors are also available that can be attached to the rebar and can be used to measure the corrosion rate in the concrete environment using two electrodes on the sensor. Single corrosion rate readings have limited value, since the measurement is dependent on many factors, and rebar corrosion rates can change considerably with time. Such variations are the result of fluctuations in temperature, humidity, degree of aeration, microstructural changes in the concrete, and development of cracks. Much more value can be placed on corrosion readings taken over a period of time.

Listed below are three tables giving the steel corrosion rate in concrete (table 3), the relationship between corrosion rate and the remaining service life (table 4), and the likelihood of corrosion damage based on corrosion potential (table 5).

Table 3. Corrosion rates of steel in concrete.

Table 4. Proposed relationship between corrosion rate and remaining service life.

Table 5. Likelihood of corrosion damage as a function of the corrosion potential.

General Discussion and Conclusions

A number of electrochemical rebar corrosion measurement techniques are available presently, each with certain advantages and limitations. The complexity and required specialist expertise in applying these techniques also varies significantly. To obtain maximum information about the corrosion state of rebar in a particular structure, a combination of measuring techniques is recommended. Furthermore, decisions should not be based on isolated corrosion readings.

Many measuring instruments will display a certain rate of corrosion penetration, but caution is required in interpreting these values, since they are often based on many simplifying (in some cases unrealistic) assumptions. Although the electrochemical corrosion measurements are usually qualitative, or at best semi quantitative, significant benefits can be derived from them.