The assessment of steel reinforcement corrosion in concrete is crucial in analyzing the durability and strength of a structure. Corrosion of steel not only decreases the lifespan of the structure but also results in significant expenses for inspection and maintenance. Unfortunately, there is no current instrument or technique available to directly measure the extent of corrosion of steel.
However, certain concrete properties, such as resistivity and half-cell potential, can help assess the likelihood of corrosion of reinforcing steel. By measuring these properties, it is possible to estimate the probability of corrosion occurring in the steel reinforcement. This information can then be used to inform decisions on maintenance and repair to ensure the longevity of the structure.
Basis for Corrosion Measurement of Rebars
Concrete acts as an electrolyte, allowing ions to penetrate through its pores, which can lead to the corrosion of steel bars embedded within it. As a result, an electrical potential field is generated inside the concrete over the steel bars. This electrical potential field serves as the basis for investigating the corrosion of steel bars in concrete.
Various methods have been developed to measure the corrosion of steel in concrete. Most of these techniques involve analyzing the electrochemical state of the interface between the rebar and concrete. Typically, these examinations are performed from the concrete surface.
Several techniques are used to measure the corrosion of steel in concrete, including the Resistivity meter, Half-cell potential, and iCOR®. These methods have been developed and are used to analyze the electrochemical condition of the rebar-concrete interface.
Resistivity Meter
he resistivity of concrete plays a significant role in the corrosion of steel within it. This process is electrochemical and generates a current flow, which is influenced by the concrete’s electric resistance. The lower the resistance, the higher the probability of corrosion. Therefore, resistivity is a useful indicator of the likelihood of corrosion, and a resistivity meter can be used to assess it. This technique is simple and can be easily adopted in the field.
A resistivity meter is a portable device that weighs around 2.2 kg and has two or more probes. To measure the resistivity of concrete, conductive gel is applied between the probes and the surface, and the metallic probes are placed on the concrete. A known current is passed through the outer probes, and the potential drop between the inner probes is measured. The resistance is calculated by dividing the potential drop by the current. The resistivity meter has an LCD display that shows the concrete resistivity.
Modern resistivity meters have a non-volatile memory and colored graphic displays that can transfer data to a PC. Based on the resistivity value of concrete, the probable rate of corrosion can be determined. A table is commonly used to determine this rate.
Table 1 Resistivity Level Versus Possible Corrosion Rate of Steel Reinforcements in Concrete
| Resistively level (Kilo-ohm / cm) | Possible corrosion rate |
| < 5 | Very high |
| 5 to 10 | High |
| 10 to 20 | Moderate to low |
| > 20 | Insignificant |
Hall-cell Potential Test
An electrochemical process occurs on concrete surfaces and steel, resulting in the production of an electric current. This current can be measured as an electric field on the concrete’s surface. The difference in potential between the concrete surface and steel is a reliable indicator of current flow.
To measure the potential field, a standardized electrode known as a half-cell potential test is used. This test is defined by the ASTM international standard. By taking measurements over the entire surface, it is possible to differentiate between locations that are likely to corrode and those that are not.
Table 2 provides the probability of corrosion in relation to the values of potential difference. This information is typically used to assess the likelihood of corrosion based on the potential difference values measured by the half-cell potential test.
Table 2 Value of Hall-cell Potential Test Versus the possible Rate of Corrosion of Steel bars in Concrete
| Potential Value | Possible Corrosion Rate |
| <= 0.20 V | 90% probability of no corrosion |
| 0.20 to -0.35 V | Corrosion activity uncertain |
| > 0.35 V | more than 90% probability of corrosion |
The Hall-cell potential test equipment is a lightweight and user-friendly device with a large display that can display measured values. It can store measurements in its memory and the data can be easily transferred to a PC. However, to conduct the test, the steel in a concrete structure needs to be accessible at a few locations to establish electrical connections, as shown in Fig. 2. For new structures, these locations should be determined during the design phase itself. The connections project out of the concrete, and for existing bridges, re-bars or prestressing wires need to be exposed to establish electrical connections.
Typically, bores are drilled at the desired locations to expose steel bars embedded in concrete. An electric cable is then connected to the steel bars, and after connecting them from outside, they can be plugged back in using epoxy mortar. During the test, the positive terminal of the voltmeter is connected to the exposed rebars, while the negative terminal (common) is connected to the reference half-cell.
To test the surface of the concrete, it is divided into a number of grids, and the reference electrode is moved along the nodal points to record the corresponding potentials. These potentials are known as corrosion potentials.
iCOR® Test Equipment
A new cost-effective and non-destructive test tool has been developed that can perform three tests on concrete: electrical resistivity test, hall-cell potential test, and steel bar corrosion rate. This device utilizes Connectionless Electrical Pulse Response Analysis (CEPRA) technology to operate and can measure the electrical response of the reinforcement inside the concrete without a physical connection to the rebar. This means that there is no need to drill holes in concrete to expose steel bars and connect them to corrosion measuring devices, which is required in other tests. With this device, concrete testing becomes more efficient and less invasive, allowing for accurate measurements without causing damage to the structure being tested.
Procedure
To set up the structure in the iCOR® app, you need to specify its specifications. Once this is done, you can proceed to measure a particular location on the grid by moving the device over the corresponding grid points on your tablet. The iCOR® instrument will then transmit these measurements to your app, which will process the information and provide real-time results on the level of corrosion in the concrete.
You can view this data in various formats, such as a corrosion rate map, a PDF report, or a CSV data file. This information will be useful in assessing the extent of concrete corrosion and developing strategies to mitigate its impact. With the iCOR® app, you can easily and efficiently monitor concrete corrosion levels and make informed decisions to ensure the longevity and durability of your structures.
Fig.3: iCOR Instrument Test