What is Corrosion rate testing of concrete?
Corrosion rate testing of concrete is a sophisticated diagnostic technique for measuring the rate of degradation for steel reinforcement within concrete structures.
Electrochemical methods, such as polarization resistance, are used to identify the corrosion current density and estimate a quantifiable rate of metal loss in mm/year; essential for evaluating structural health and durability, predicting service life, identifying necessary repairs, and pinpointing areas of accelerated corrosion before significant damage occurs.
How does Corrosion rate testing work?
Corrosion rate testing in concrete structures involves measuring the corrosion current density (icorr) of steel reinforcement using polarization resistance (Rp) methods.
This technique, grounded in Faraday’s law, quantifies the volumetric metal loss per unit area and time, translating icorr into corrosion rates (Vcorr) expressed in mm/year.
To measure corrosion rate (µA/cm2) in concrete, a small voltage is applied near the steel's natural electrical level, and the resulting electrical flow is observed. This setup helps to instantly gauge the corrosion rate. The corrosion speed is then determined by dividing a predefined constant (B) by the observed electrical resistance (Rp), providing a precise measurement of how fast the steel corrodes.
This method provides a non-destructive means to estimate the corrosion rate, crucial for assessing structural health and predicting service life.
What is Corrosion rate testing used for?
Deterioration process | Defects | Control of repairs |
How do I carry out Corrosion rate testing?
Several standards and technical guides have been developed for corrosion rate testing such as ASTM G59-97 and Rilem TC 154-EMC. These both follow a similar process which is shown below.
Remove cover concrete to expose a small area of the steel reinforcement and attach a probe, ensuring electrical contact.
Attach the steel to the corrosion meter as the working electrode, with reference and counter electrodes to form a circuit.
Apply a minor voltage near the steel's corrosion potential.
Record the current to calculate polarization resistance (Rp) and add this into the Stern--Geary equation.
Equation 1. Stern-Geary equation
Calculate B based on Tafel slopes representing the relationship between the electrical current and voltage for surface anodic (oxidation) and cathodic (reduction) reactions. These measure how quickly current changes with voltage with a figure of 26mV often used for reinforced concrete.
Solve the Stern-Geary equation to determine the corrosion current density (icorr).
Calculate the corrosion rate (vcorr) in mm/year using the Equation 2.
Equation 2. Corrosion current density to corrosion
What equipment and expertise are required for Corrosion rate testing?
Additionally, software packages like Webcore provide advanced analytics, representing data collected through graphical interfaces and detailed reports, aiding in the interpretation and management of corrosion data. These tools combined offer a comprehensive solution for monitoring and analysing corrosion in concrete structures.
Conducting corrosion rate tests and analysing the results requires expertise in civil engineering, materials science, and electrochemistry.
Professionals need to understand the principles of corrosion, the use of specialized testing equipment, and the interpretation of data through software.
Analytical skills are crucial for evaluating the health of concrete structures and making maintenance recommendations. Training in specific testing methods and software usage is often provided by equipment manufacturers or through specialized courses.
What are the advantages of Corrosion rate testing?
Identifies the corrosion rate, allowing for timely maintenance and repair, thus extending the lifespan of structures.
Quantitative information about rebar cross section loss by extrapolation.
Helps prioritize areas needing repair, reducing unnecessary expenditures on sound structures.
Allows for the early identification of corrosion problems before they lead to significant structural damage, facilitating timely interventions.
Provides quantitative data on corrosion rates, offering a clear metric for assessing the severity of corrosion and its potential impact on structural integrity.
High upfront costs can be offset by the long-term savings achieved by preventing major repairs or failures.
What are the disadvantages Corrosion rate testing?
Requires access to sophisticated devices and significant expertise to carry out the test and interpret the results.
Expensive and time-consuming.
Measurement accuracy can be affected by environmental conditions such humidity and temperature.
May not be able to asses corrosion accurately deep inside the concrete.
Mainly suitable for structures exposed to atmosphere.
How accurate is Corrosion rate testing?
The lowest rate of corrosion detectable by corrosion rate testing in concrete structures depends on the sensitivity of the specific testing equipment used and the methodological approach. Generally, modern electrochemical methods such as linear polarization resistance (LPR) can detect very low corrosion rates, often as low as a few microamperes per square centimetre (µA/cm²), which can correspond to metal loss rates of less than 0.1 mm/year. However, the exact detectable limit can vary based on the testing setup, calibration, environmental conditions, and the specific characteristics of the concrete and reinforcement material.
The accuracy of corrosion rate testing in concrete can be influenced by environmental factors such as temperature and humidity, the test device sensitivity and resolution, and its applicable range. Additionally, challenges in detecting localized corrosion, the necessity for regular instrument calibration, operator expertise, and structural complexities, like varying compositions and embedded materials, also impact accuracy. Comparable environmental conditions should be such that 90 % of the measurements should not differ more than 4 times with the minimum values recorded.
What are the limits of Corrosion rate testing?
Corrosion rate testing in concrete structures is essential for assessing structural health, yet it faces limitations. Accurate measurements require knowledge of the polarized area beneath the counter electrode, challenging in complex structures. The cost of the system, the time it takes to carry out the test and the availability of the required expertise may limit to potential applications for which the test could be used.
The method may underestimate localized corrosion and is sensitive to environmental factors like temperature and humidity, necessitating repeated measurements over time to capture seasonal variability and provide an accurate corrosion rate.
Primarily effective for surface analysis, it struggles with deep internal corrosion and can be skewed by nearby metallic objects or dense reinforcement. The maximum depth at which corrosion rate can be detected depends on the testing method and equipment used. Traditional techniques primarily assess surface-near corrosion, typically effective up to 10cm below the surface.
Consequently, while invaluable, corrosion rate testing should be part of a broader assessment strategy, complemented by other methods to ensure comprehensive structural evaluation.
Ancillary information
Maturity of test: > 10 years
Qualification & interpretation : Inspector and specialist
Service disruption: No
Preliminary works: Yes
Time consumption High (> one day)
Cost Medium
Access to element 1 face