What is Capacitive measure testing?
Capacitive measure testing is a non-destructive test that uses electrical capacitance to assess the condition of concrete structures. By applying an alternating current through electrodes, it evaluates changes in the electromagnetic properties of concrete, enabling the detection of moisture content, cracks, and other defects.
This technique provides critical insights into the concrete's integrity, aiding in the diagnosis of deterioration processes and facilitating targeted repair strategies, ensuring the safety and longevity of concrete infrastructures.
How does Capacitive measure testing work?
Capacitive measurement testing of concrete utilizes electrodes to apply an alternating current across the surface, acting like a capacitor. This technique probes the concrete's electromagnetic properties by measuring capacitance changes, enabling moisture and defect detection for material condition analysis.
What is Capacitive measure testing used for?
Deterioration process | Defects | Control of repairs |
All deterioration processes affected by the presence of water | Moisture heterogeneities Early age cracks | - |
How do I carry out Capacitive measure testing?
While no specific standards documents have been created for this type of testing a typical procedure based on the test principal and industry best practice is presented.
Clean and prepare the concrete surface to ensure good contact between it and the electrodes/sensors used.
Identify and mark specific locations where measurements must be taken.
Attach capacitive sensors/electrodes at prescribed locations or if using “sensing skin” approach evenly across the entire surface.
Ensure secure connections and good electrical contact using adhesive if required.
Connect sensors/electrodes to measurement equipment based on manufacturer’s instructions and test functionality.
Calibrate measurement equipment for maximum accuracy.
Take baseline measurements for benchmarking changes in capacitance.
Apply AC current between the sensors/electrodes with a frequency dictated by sensors specifications and the objective of the test.
Record capacitance values of the concrete.
Asses changes in capacitance to identify defects such as cracks, voids, or moisture.
What equipment and expertise are required for Capacitive measure testing?
While simple surface moisture meters based on this principle are relatively widespread, capacitive measurement systems for testing concrete structures are less so and require specialized sensors or electrode arrays, data acquisition systems, and software for data analysis.
Expertise in electrical engineering, materials science, and structural engineering along with practical experience of working with NDT equipment and data analysis software is crucial for accurate collection and interpretation of data.
What are the advantages of Capacitive measure testing?
Leave no marks on blemishes on concrete surface.
Simple and fast test with rapid results.
Efficient for scanning large areas or regular monitoring.
Highly sensitive to changes in moisture content.
Versatile test method which can identify other defects such as cracks or voids through assessing changes in the dielectric properties.
Portable, hand-held, battery-operated test devices are available.
What are the disadvantages Capacitive measure testing?
Primarily measures surface moisture so might not accurately reflect internal moisture.
Presence of reinforcement bar can interfere with readings.
Accurate interpretation of readings requires significant expertise and calibration specific to the concrete mix and conditions.
The effective penetration depth of capacitive sensors may not be sufficient for thick concrete elements, potentially missing defects or moisture deep inside.
Environmental temperature and humidity can influence readings
How accurate is Capacitive measure testing?
Moisture probes that use capacitive measurement have commonly exhibited accuracies ranging from 0.05-0.5%.
The resolution of defect detection in concrete using capacitive measurement techniques depends on factors such as sensor sensitivity, electrode configuration, operating frequency, and concrete's material properties. Advanced systems can detect defects down to millimetres and moisture content changes less than 0.2%, primarily at or near the concrete's surface.
However, the capability to detect deeper defects decreases with depth due to signal dispersion and attenuation. Practical resolution achieved is influenced by balancing sensitivity against penetration depth.
Typically, capacitive measurement is used alongside other non-destructive testing methods for comprehensive defect assessment in concrete, offering high-resolution detection for surface-level issues with limitations in depth.
What are the limits of Capacitive measure testing?
Capacitive measurement testing is effective for surface and near-surface evaluations in concrete, identifying moisture presence and surface-level defects, however, penetration depth is limited, making it less suited for deep assessments within thick concrete elements.
Accuracy can be compromised by the presence of reinforcement and environmental conditions such as temperature and humidity, which may skew results without proper calibration. Furthermore, the heterogeneous nature of concrete can affect the reliability of this method across different structures.
The method is not recommended for analysing deep internal defects or areas with dense metal reinforcement, where electromagnetic interference could lead to inaccuracies. It should also be supplemented with other testing methods for critical structural evaluations to ensure safety and integrity.
Capacitive measurement primarily offers qualitative insights into moisture and defect presence at or near the concrete's surface, signalling potential issues rather than providing detailed quantitative data on defect dimensions or exact moisture levels throughout the structure. For comprehensive assessments, integrating capacitive measurement with other diagnostic techniques is advisable for a more complete understanding of concrete conditions.
Ancillary information
Maturity of test: > 10 years
Qualification & interpretation : Inspector
Service disruption: No
Preliminary works: Yes
Time consumption Medium (one day)
Cost Low
Access to element 1 face
References and further information
Lu, L; Checkroun, M; Abraham, O; Maupin, V, Villain, G. Non-destructive evaluation of concrete using a capacitive imaging technique: Preliminary modelling and experiments, Cement and Concrete Research, Volume 40, Issue 12, 2010, pp. 1734-1743, ISSN 0008-8846, https://doi.org/10.1016/j.cemconres.2010.08.015.