Methods of Corrosion Potential Assessment of Concrete Structures
1. Cover meter survey
Ensuring sufficient cover thickness is crucial to control corrosion in structures. To assess the cover thickness, a cover thickness survey can be conducted at a specific location where damage has been identified and other locations on the same structure for comparison. The use of commercially available cover meters allows for non-destructive measurements of cover thickness and identification of the location and diameter of rebar. The COVERMASTER and PROFOMETER are two such instruments that are commonly used for this purpose.
Table 1 provides guidance on how to interpret the cover readings obtained from the cover meters for corrosion assessment. It is essential to have accurate measurements of cover thickness to determine the level of protection provided against corrosion. Conducting a cover thickness survey and interpreting the results using appropriate guidance can help prevent costly damage to structures and ensure their longevity.
Fig: Cover meter of Profometer
Table -1: Interpretation of Cover Thickness Survey
| Sl. No. | Test Results | Interpretation |
| 1 | Required cover thickness and good quality concrete | Relatively not corrosion prone |
| 2 | Required cover thickness and bad quality concrete cover | Corrosion prone |
| 3 | Very less cover thickness yet good quality cover concrete | Corrosion prone |
2. Half Cell Potential Survey
Corrosion is a process that involves the electrochemical reaction of steel wire, causing changes in its electrode potential with respect to a standard electrode. By conducting a schematic survey on grid points, useful information about the presence or probability of corrosion can be obtained. These same grid points can also be used for other measurements such as rebound hammer and ultrasonic pulse velocity to make the data more meaningful. Standard electrodes such as the Copper-Copper sulphate electrode (CSE), Silver-Silver chloride electrode (SSE), and Standard Calomel electrode (SCE) are commonly used in these measurements.
To conduct the measurement, an electrical connection is made to the rebar, and the voltage difference between the bar and a reference electrode in contact with the concrete surface is observed. Generally, as corrosion becomes more active, the voltage potential becomes increasingly negative. However, even less negative potential values may indicate the presence of corrosion activity, especially if the pH values are low. Figure 1(a) provides a visual representation of this process.
Fig-1 (a): Half Cell Potential Test
Table 2 provides the general guidelines for determining the likelihood of corrosion by analyzing half cell potential values, as suggested in the ASTM C876. This table serves as a reference for identifying the probability of corrosion based on the measured half cell potential values. The ASTM C876 provides these guidelines to assist in the evaluation of the corrosion potential of reinforced steel in concrete structures. By using the suggested guidelines, one can assess the corrosion risk of the steel reinforcement based on the measured half cell potential values. This information can be crucial in determining the need for maintenance and repair of the concrete structure.
Table-2: Corrosion Risk by Half Cell Potentiometer
The technique for identifying the corrosion state of rebars in concrete should not be used in isolation. It should be coupled with measurements of chloride content, depth of carbonation, cover to the steel, and the variation of chloride content with depth. A systematic “potential mapping survey” is considered to be a more useful method for identifying the corrosion state of rebars on-site. This survey facilitates the creation of a potential profile or potential contour that can help identify the areas of corroding steel. A typical potential contour is shown in figure 1(b) and (c).
Initially, when potential surveying was introduced as per ASTM C 876, each reading was interpreted in isolation, and the numerical value was directly correlated to the degree of corrosion. However, this approach was found to be erroneous because non-corroded steel can exhibit a wide range of potential values. Therefore, potential values should be assessed not in isolation but as a group, and the interrelationship of the potentials within a group should form the basis of interpretation.
The analysis of potential contour will generally consist of identifying the locations with accumulated potential lines indicating the corroding areas beneath. By examining the potential lines, it is possible to identify the areas of corrosion and their severity. Therefore, the potential mapping survey is a useful tool for identifying the corrosion state of rebars in concrete and can be used in conjunction with other methods to obtain a comprehensive understanding of the corrosion state of a structure.
(b) Shaded Mapping
(b) Contour Plot
Corrosion of steel reinforcement within concrete structures is a significant concern as it can compromise the structural integrity of the building. Anodic areas where corrosion is likely to occur can be identified through the use of contour plots of half cell potentials. However, there are certain parameters that must be taken into account to accurately determine whether or not a structure is actively corroding. These include the fact that the potentials measured on the surface of or within concrete may not accurately represent the values at the surface of the steel due to the physical and chemical state of the concrete, as well as the ohmic drop due to the electrical resistance of the concrete.
To determine the quality of concrete in terms of its corrosion susceptibility potential, it is important to measure the electrical resistance of the material. This parameter is expressed in terms of “Resistivity” in ohm-cm. A resistivity check is crucial for general monitoring, as concrete structures with accurately measured resistivity values below 10,000 ohm-cm are at risk of long-term corrosion. If the resistivity values fall below 5,000 ohm-cm, corrosion can be anticipated much earlier in the structure’s lifespan, possibly within five years.
To identify areas of probable corrosion risk in concrete structures, resistivity mapping can be used. Table 2 provides general guidelines for resistivity values based on which areas with probable corrosion risk can be identified. It is important to note that as the concrete cover increases, the potential values at the concrete surface over actively corroding and passing slab become similar. By taking these factors into account, it is possible to identify and address corrosion risks in concrete structures, thereby ensuring their long-term durability and safety.
Table-2: Corrosion risk from resistivity
| Resistivity ohm-cm | Corrosion Probability |
| Greater than 20,000 | Negligible |
| 10,000 – 20,000 | Low |
| 5,000 – 10000 | High |
| Less than 5,000 | Very high |
The resistivity testing principle used in concrete is comparable to that used in soil testing. However, when used in concrete, it may present certain drawbacks that should be acknowledged. The method involves utilizing a 4-probe technique, where a known current is applied between two outer probes 100 mm apart. The voltage drop between the inner two elements, spaced 50mm apart, is then measured, allowing for a direct evaluation of resistance, R. Afterward, resistivity is calculated using a mathematical conversion factor, based on the principle of four probe resistivity testing, as demonstrated in figure 2.
Fig -2: Resistivity Meter (4 Probe System)
When analyzing and interpreting resistivity values for concrete, it is important to keep in mind certain drawbacks that can affect the accuracy of the results. Firstly, the resistivity value obtained only represents an average evaluation over the depth regulated by the chosen probe spacing, which may not necessarily reflect the resistivity of concrete at the steel interface. This means that the measurement may not accurately represent the condition of the concrete in the specific area where corrosion is most likely to occur.
In addition, the resistivity of concrete can vary depending on the moisture conditions present. This means that if there are fluctuations in the moisture level of the concrete, the resistivity values obtained may not be entirely representative of the actual condition of the concrete, potentially leading to inaccurate results.
To ensure that the resistivity measurements are as accurate as possible, it is essential that the instrument used for measurement has adequate IR drop compensation. This helps to correct any errors that may occur due to resistance in the measuring circuit, ensuring that the measurement is as accurate as possible.
Table-3 provides information on the corrosion probability based on resistivity and potential mapping. By taking into account the resistivity values obtained and the potential mapping of the concrete, it is possible to determine the probability of corrosion occurring in the concrete. This information can be used to assess the overall condition of the concrete and take appropriate measures to prevent further damage or deterioration.
Measurement of corrosion rate:
Reinforced concrete structures rely on the strength and durability of the embedded steel reinforcement. However, corrosion of steel reinforcement can cause the concrete to crack and weaken the overall structure. In order to assess the corrosion rate of the steel reinforcement in concrete, it is important to determine the actual rate of corrosion in the field, since laboratory results may not accurately reflect real-world conditions.
One method used to measure the corrosion rate of steel in concrete is the Linear Polarisation Resistance (LPR) method. This method is based on the principle of linear polarization, which assumes that the polarization curve around the corrosion potential for a simple corroding system will follow a quasi-linear relationship. The slope of this curve is known as the polarization resistance.
The LPR method allows for on-site study of corrosion rates of steel in concrete. By measuring the polarization resistance, the corrosion rate of the steel reinforcement can be estimated. This method is particularly useful in assessing the effectiveness of corrosion inhibitors or other protective measures for concrete structures.
In summary, the LPR method is a technique used to measure the corrosion rate of steel reinforcement in concrete structures. It is based on the principle of linear polarization and involves measuring the polarization resistance. This method allows for on-site assessment of corrosion rates and can help inform decisions about protective measures for concrete structures.
.
From this slope, the corrosion rate can be determined using stern-Geary equation
The given context contains a mathematical formula that determines the value of a constant B based on the Tafel slopes and other variables. The constant B is a function of the Tafel slopes and the values of ba and bc. The formula used to calculate the value of B is provided in the given context.
To rewrite this in paragraphs, we can say that the constant B is determined by a formula that takes into account the Tafel slopes and the values of ba and bc. The Tafel slopes represent the rate of change in the logarithm of the current with respect to the logarithm of the potential, and they are important parameters in electrochemistry. The values of ba and bc are also important parameters that describe the reaction kinetics.
The formula used to calculate the value of B is a mathematical expression that combines the Tafel slopes and the values of ba and bc. The exact form of the formula is not given in the context, but it is clear that it is used to determine the constant B. The value of B is important in electrochemistry because it helps to describe the kinetics of electrochemical reactions.
In summary, the given context describes a formula that is used to calculate the value of a constant B in electrochemistry. The constant B is determined by the Tafel slopes and the values of ba and bc, which are important parameters in describing the kinetics of electrochemical reactions. The value of B helps to describe the rate of electrochemical reactions and is therefore an important parameter in electrochemistry.
The range of B values typically falls between 13 and 52 millivolts and varies depending on the type of corroding system, whether it is passive or active. To measure B values in the field, a testing system is used, which includes a potentiostat, a counter electrode, a reference electrode, and the reinforcement acting as a working electrode. The setup of this testing system is depicted in Figure 3.
Fig-3: Resistivity Testing for Concrete
Figure 4 depicts a typical plot of a linear polarization curve. When conducting measurements in concrete, it is imperative that the potentiostat used has electronic ohmic compensation, also known as IR drop. If the potentiostat does not have this compensation, the resulting values must be calculated or determined through separate experiments. This compensation is necessary to ensure accurate measurements in concrete.
Fig-4: Linear Polarization Curve.
| Sl. No. | Tests | Description |
| 1 | Cover Meter/ Profo-meter (in-situ Test) | Non-destructive method for measuring – Thickness of cover concrete – Reinforcement diameter – Reinforcement spacing |
| 2 | Half Cell Method (in-situ Test) | Non-destructive method for measuring/plotting corrosion potential for assessing probability of corrosion |
| 3 | Resistivity Measurement (in-situ Test) | Non-destructive method for assessing electrical resistivity of concrete. |
| 4 | Permeability a) Water b) Air | Assessment of in-situ permeability of concrete due to water and air. |
| 5 | Initial surface absorption (Lab Test) | An indicator of surface permeability |
To determine the extent of corrosion present in embedded steel, the half-cell potential measurement method is employed. This method involves measuring the electrical potential of a metal electrode, which is in contact with the corroding steel surface. By comparing the electrode’s potential with a reference electrode, the corrosion activity can be determined.
The half-cell potential measurement method is a commonly used technique in the field of corrosion monitoring. It provides a quick and non-destructive way to assess the corrosion status of embedded steel in structures such as bridges and buildings. This information is crucial for evaluating the safety and durability of these structures, as corrosion can weaken the steel and compromise their integrity.
Overall, the half-cell potential measurement method is an effective tool for detecting and monitoring corrosion activity in embedded steel. By using this technique, engineers and inspectors can identify potential corrosion issues early on and take appropriate measures to prevent further damage.