What Is the Half-Cell Potential Test?

Example of Concrete Corrosion
Example of Concrete Corrosion

Corrosion is a natural process that occurs when a structure is exposed to elements like CO2 or chloride, which can penetrate the concrete all the way to the steel reinforcement. This can have serious durability and safety consequences, which is why it is important to monitor corrosion using an accurate and trusted method.

Safely Pour Concrete Under Any Weather Conditions

The leading concrete sensor for real-time temperature and strength monitoring

The half-cell potential test is the only corrosion monitoring technique standardized in ASTM C876 – 15: Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete. It is used to determine the probability of corrosion within the rebar in reinforced concrete structures. This blog dives into the specifics of concrete corrosion, the half-cell potential technique for testing concrete corrosion, and the ways in which the data from the half-cell potential test can be interpreted.

The Basics of Concrete Corrosion

In reinforced concrete structures, there is a natural protective film that forms on the surface and prevents the bar from corroding. With time, chlorides (from de-icing salts or marine exposure) and/or CO2 penetrate the concrete and breakdown that protective layer. Chlorides destabilize the passive film leading to its localized breakdown, while CO2 lowers the pH of the concrete below the level of stability of the passive film. In the presence of oxygen and water, an electrochemical reaction initiates the process of corrosion.

Corrosion can be illustrated as shown in Figure 1, where the metal (rebar) reacts in the solution (available in the concrete pores) and gives away electrons from the anode (where oxidization occurs) to the cathode (where reduction occurs). The positive ions formed at the surface of the anode will react and create corrosion by-products. This electrochemical reaction creates a potential difference, and consequently a corrosion current, between the anodic and cathodic areas at the surface of the steel reinforcement. This current, or the potential distribution on the reinforcement surface, is what is of interest when measuring half-cell potential.

Figure 1: Reinforced concrete corrosion reaction
Figure 1: Reinforced concrete corrosion reaction

The Half-Cell Potential Technique

The schematic in Figure 2a represents a cell where each side is referred to as a half-cell. Each half-cell is represented by an electrode in a solution (electrolyte) and both half-cells are connected together. Since one of the electrodes has a higher tendency to corrode compared to the other, that electrode (anode) will oxidize and in turn will donate electrons.

To keep the system in equilibrium and balance the charges in the electrolytes, there will be an exchange of ions through the salt bridge. The voltmeter will measure the potential difference (voltage) between both electrodes, which indicates the rate of dissolution of the anode.

Interested in detecting corrosion in reinforced concrete? Learn more here!

To apply this concept to concrete and to interpret the results of corrosion potential, a reference electrode with a known potential is needed. Typically, for reinforced concrete applications, a copper/copper sulfate electrode (Cu/CuSO4) or silver/silver chloride electrode (Ag/AgCl) is used for the half-cell reference. This reference electrode is connected to the other half-cell represented by the embedded rebar (Figure 2b). By connecting that reference electrode to the reinforcing steel and placing the reference electrode on the surface of the concrete, it is possible to measure the potential difference between the two half-cells.

half-cell potential measurement
Figure 2: Half-cell potential measurement

The Cell Potential

ASTM C876 provides a guideline on how this measurement can be undertaken, and the relationship between the measured potential values and the corrosion probability. Interpretation of the result is qualitative and is based on the copper sulfate electrode (CSE). Table 1 shows the general interpretation guideline proposed by ASTM, where the measured potential ranges are categorized in three categories; more than 90% chance, less than 10% chance or an uncertain chance of corrosion.

Table 1: Relationship between the potential values and corrosion probability
(adapted from ASTM C876)

Measured Potential(mV CSE)Probability of steelcorrosion activity
>-200
Less than 10%
-200 to -350
Uncertain
<-350
More than 90%

Interpreting Half-Cell Potential Data

At first glance, this test method seems very simple and comprises of the following steps:

  1. Identify rebar location
  2. Make a connection with the reinforcement (more than one connection can be required if there is a discontinuity between reinforcements)
  3. Prepare concrete surface through wetting

Measurements are quick as potential values only take a few seconds to stabilize before the next measurement can be taken. However, there are important limitations in terms of data interpretation that need to be taken into consideration.

The effect of the concrete condition (dry or wet), presence of chloride, absence of oxygen at the rebar surface (due to saturation), cover thickness, concrete resistivity, and temperature are all factors that can influence the results by shifting their potential reading towards a more positive or negative value as shown in Table 2. This can make the data interpretation challenging when using the guidelines given in the ASTM C876 (Table 1), especially around the uncertain measurement ranges.

Table 2: Typical ranges of half-cell potentials of rebar in concrete (adapted from RILEM TC-154, 2003)

ConditionsPotential values (mV/CSE*)
Humid, chloride free concrete-200 to +100
Wet, chloride contaminated concrete-600 to -400
Water saturated concrete without oxygen-1000 to -900
Humid, carbonated concrete-400 to +100
Dry, carbonated concrete0 to +200
Dry concrete0 to +200

In addition, half-cell measurement is considered to be a zonal measurement as it will take an average potential measurement of the surroundings. An example is illustrated in Figure 3, where the measured potential will show sort of an average over a certain distance and where the actual location of the corroded bar can be challenging to distinguish, even with corrosion potential mapping.

Figure3 - Actual vs. measured half-cell potential results
Figure3 – Actual vs. measured half-cell potential results

Nevertheless, this technique is being widely used since it is the only corrosion monitoring technique standardized by ASTM. Giatec’s XCell™ device uses a silver/silver chloride electrode, making this NDT device more stable and accurate than those half-cell devices which use a copper/copper sulfate electrode. It can be very useful in doing a quick assessment and identifying the regions where there might be a relatively higher corrosion activity. It remains that the corrosion potential technique’s output is qualitative as it provides information only on the chance of corrosion activity, not quantitative information such as the rate at which the rebar is corroding; this is more useful information when it comes to determining the plan of action for repair or corrosion mitigation.

Related Readings: Obtaining Effective Half-Cell Potential Measurements in Reinforced Concrete Structures
Half-Cell Potential Measurements – Potential Mapping on Reinforced Concrete Structures

What Is the Half-Cell Potential Test?

**Editor’s Note: This post was originally published in May 2018 and has been updated for accuracy and comprehensiveness

13 Responses

    1. Hi there, usually, either a copper/copper sulfate electrode (Cu/CuSO4) or a silver/silver chloride electrode (Ag/AgCl) are used as half-cell reference in the case of concrete reinforcement. However, Giatec’s XCell device uses the latter (Ag/AgCl) to increase stability and accuracy in the measurements.

  1. Hi
    Can we use two reference electrodes (half cells) on the surface and measure the potential difference? If not why?

    1. With this specific instrumentation, there can only be one reference electrode on the surface of the concrete. The half-cell potential test measures the potential difference in the rebar, which is caused by the corrosion of that rebar. By not connecting the voltmeter to the steel reinforcement, there is no way to measure the electrochemical reaction as the rebar (where the corrosion occurs) is not one of the half-cell.

      There is also a non-standardized methods which, instead of giving potential values, will give differential value with a point in the structure where corrosion is unknown. However, the XCell would not be appropriate for this uncommon method, and going with a standardize method is more recommendable.

      If you do not have access to the steel reinforcement, look into a non-destructive test method, like the iCOR, which will easily map the corrosion of rebar: https://www.giatecscientific.com/education/how-to-assess-corrosion-in-concrete-with-an-ndt-method/

  2. The potential values on concrete in which chlorides are added during concrete preparation, shown more negative(-600) initially and tend towards positive direction like+20 to +10. What it means and please let me know the reason. ..

    1. Hi Raghu, thank you for your comment. Regarding the negative values obtained (i.e. -600mV), in the case where chlorides ions are added in the concrete due to the use of calcium chloride as an accelerator for example, half-cell potential (HCP) values will usually be around -400mV to more negative values. Such readings can be seen at very early age of reinforced concrete when the passive layer may not be formed yet within the matrix. As the concrete starts to harden and free water evaporates from the system and the concrete dries, HCP values tend to shift to higher readings (positive range). Furthermore, it is also good to note that HCP values increase to be less negative due to increasing resistance of carbonated concrete (i.e. presence of carbonation on the concrete surface). Please feel free to reach out to support@giatec.ca for further discussion in details.

  3. Hi
    I am interested in conducting half cell potential test on a concrete wall which is 10 m wide and 14 m high.
    How many half cell test shall be conducted on the wall to understand corrosion situation in the entire wall.
    Is it that I have to test the full surface area of the wall or test at discrete location are enough?
    Thanks

    1. Thank you for your comment Damodar. There is no predefined maximum or minimum spacing between measurements but ASTM C876 Section 7 notes that a spacing of 4ft (1.2m) is usually satisfactory for a quick assessment of the structure. The larger the spacing the more chances of missing localized corrosion. Based on the results you obtain at different locations it would be up to the engineer to decide if more measurement points are required. Please contact support@giatecscientific.com with any other questions.

  4. Excellent, i wood like to know what´s the price?

    1. Hi Victor, thank you for your comment. One of our sales representatives will be in contact with you shortly

Leave a Reply

Your email address will not be published. Required fields are marked *

Related Articles

Use These Tech Tools for More Efficient Cold Weather Concreting

Let’s dive into our sensors in detail.   Safely Pour Concrete Under Any Weather Conditions The leading concrete sensor for real-time temperature and strength monitoring Learn More SmartRock Sensors:    SmartRock is a wireless concrete sensor designed for temperature and strength monitoring with the ability to record real-time temperature data for every 15-minute interval for 2 months and comes with remote-monitoring capabilities. It is fast, simple, designed rugged and waterproof, and can be activated and installed hassle-free. The sensor contains two points of temperature measurements located in the sensor cable and body, and comes with an extended temperature cable and probe for mass…

Everything You Need to Know About Pouring Concrete in Winter

Winter is coming! Worried about the cold weather concreting that comes with it in the construction industry? We’ve got it covered with Giatec’s SmartRock sensors.  Keep reading to learn more.  Safely Pour Concrete Under Any Weather Conditions The leading concrete sensor for real-time temperature and strength monitoring Learn More The temperature is dropping, the days start to get shorter, and frost covers the ground. While it may be exciting to imagine a festive winter season, that is not what comes to mind when working in the construction industry. Especially, when you have a project to complete, a schedule to maintain, and a desired concrete temperature and…

Giatec’s Unique Concrete Knowledge Resources

The Giatec Scientific website is a treasure trove of concrete knowledge, ranging from blogs and case studies to podcasts and videos! Find a few of our exclusive resources below: Safely Pour Concrete Under Any Weather Conditions The leading concrete sensor for real-time temperature and strength monitoring Learn More Giatec’s Concrete Terminology Search Bar Need a one-stop destination for searching definitions for a plethora of terms related to concrete knowledge? Make sure to check out our concrete terminology search bar! To search for a term related to…

We use cookies to provide you with a better experience, analyze site traffic and assist in our marketing efforts. By continuing to use this website, you consent to the use of cookies in accordance with our Privacy Policy Page.