What is the Half-cell potential test for concrete?
The Half-cell potential test for concrete is a standardized corrosion monitoring technique used to assess the likelihood of corrosion within the reinforcing steel of concrete structures. This non-destructive test involves measuring the electrical potential difference between the reinforcing steel and a reference electrode placed on the concrete's surface. By evaluating these potential differences, it's possible to predict areas within the concrete that are at risk of corrosion, aiding in the maintenance and longevity of the structure.
How does the Half-cell potential test work?
The test works by establishing an electrical circuit that includes the reinforcing steel, a reference electrode (commonly copper-copper sulphate), and a high-impedance voltmeter.
A stable and reproducible potential is measured by placing the reference electrode at various locations on the concrete surface. The potential values obtained are then analysed against standardized criteria to determine the probability of corrosion activity.
This allows identification of early-stage corrosion before corrosion cracks or rust stains appear, and therefore areas in need of timely repair, ensuring the durability of the concrete structure.
What is the Half-cell potential test used for?
Deterioration process | Defects | Control of repairs |
How do I carry out Half-cell potential testing?
The most common standard used for half-cell potential testing of concrete in the UK is ASTM C876:15. A simple step by step guide based on this includes the following steps:
Visually inspect the structure to identify areas of potential corrosion and to decide on measurement grid spacing based on structural geometry and expected corrosion regions.
Ensure all equipment is calibrated according to ASTM standards.
Assemble your testing apparatus: a copper-copper sulphate reference electrode connected to a high-impedance voltmeter, and ensure a stable and reproducible potential setup.
Calibrate your voltmeter and reference electrode as required, ensuring accuracy in your readings.
Clean the concrete surface at the points of measurement to ensure good electrical contact by removing debris, dust, or coatings that could affect the test results.
Establish a stable electrical connection to the reinforcing steel. This could involve direct attachment to exposed rebar or, if not possible, using an alternative documented direct connection point.
Pre-wet the surface using a conductive solution to reduce the electrical resistance of the circuit.
Conduct measurements at predetermined locations across the concrete structure. The spacing should reflect the area's complexity and the expected corrosion activity variance.
Record corrosion potential values at each location, noting any specific environmental conditions that might affect readings.
Use ASTM C876-15's criteria for interpreting corrosion potential values to determine the likelihood of corrosion activity. The standard provides thresholds for potential values that correlate with the probability of corrosion.
Create a half cell potential contour map for visual analysis, illustrating areas of similar potential and highlighting regions of concern.
Compile a comprehensive report including type of cell used, average temperature during tests, pre-wetting method, method of attaching to the reinforcing steel, an equipotential contour map, and a cumulative frequency distribution of the corrosion potentials.
What equipment and expertise are required for Half-cell potential testing?
Equipment required for half-cell potential testing includes:
A copper-copper Sulphate reference electrode to create a stable and reproducible potential reference for measurement.
A high impedance voltmeter capable of measuring DC voltage to accurately capture the potential difference without influencing the circuit.
A conductive solution such as isopropyl alcohol is used to wet the concrete surface and decrease its electrical resistance to improve measurement accuracy.
Electrical lead wires to connect the reference electrode and the reinforcing steel to the voltmeter, with minimal electrical resistance.
All in one kits for half cell potential measurements are widely available from suppliers such as Impact Test Equipment or Hammond Concrete Services.
Required expertise for the test includes an understanding of electro-chemistry, the principles of corrosion and electrochemical measurements, familiarity with concrete compositions, deterioration mechanisms and repair techniques and skills for data interpretation and presentation.
While skill requirements for execution of the tests are low, planning and data analysis require significant skill.
What are the advantages of Half-cell potential testing?
Half-cell potential testing is a simple, well stablished, and low-cost test method, making it an accessible option for assessing corrosion risk in reinforced concrete structures.
This method can provide invaluable insights into the condition of reinforcing steel without the need for invasive procedures.
A significant advantage of this technique is its ability to create corrosion probability maps, offering a visual representation of areas at risk and allowing educated maintenance and repair strategies to be established.
What are the disadvantages Half-cell potential testing?
Despite its benefits, half-cell potential testing has several drawbacks. One of the main disadvantages is its sensitivity to various factors that can affect the accuracy of the measurements. These include oxygen availability, concrete moisture content, concrete cover depth, and concrete resistivity. This means that the test results can sometimes be difficult to interpret without a thorough understanding of the structure's specific conditions and environment.
How accurate is Half-cell potential testing?
Half-cell potential measurements more negative than -0.35 volts vs. Cu/CuSO4 indicate a greater than 90% risk of active corrosion, values between -0.20 and -0.35 volts suggest uncertain corrosion risk, and readings more positive than -0.20 volts denote a low likelihood of corrosion in reinforced concrete. The difference between two half-cell readings should not exceed 10 mV when the cell is disconnected and reconnected. The difference between two half-cell readings taken at the same location with two different probes should not exceed 20 mV. As such it is evident that the test exhibits significantly higher accuracy than necessary to accurately predict corrosion potential.
What are the limits of Half-cell potential testing?
As half-cell measurements can be significantly influenced by factors such as temperature and humidity, a single series of tests may not provide a fully representative assessment of the structure's corrosion risk. Long-term monitoring or repeated measurements are often recommended to capture the effects of seasonal variability.
The method also requires complementary measurements, such as chloride content, carbonation depth, corrosion rate, and concrete resistivity, to provide a comprehensive view of the structure's condition.
Ancillary information
Maturity of test: > 10 years
Qualification & interpretation : Inspector and specialist
Service disruption: No
Preliminary works: Yes
Time consumption Medium (one day)
Cost Medium
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