Abstract
The study presents the development of a new two-dimensional FEM numerical model describing the operation of two large open-ended coaxial probes designed to investigate the permittivity of concrete, and its constituents. This numerical simulation, combined with a capacitive approach describing the behaviour of the probes, enabled to prove the suitability of such device to determine the permittivity of dispersive dielectrics. Finding back the permittivity of a specified material by calculation of the S parameters, change of the reference plane and use of the capacitive model is the key to the proof. Measurements performed onto different materials show good similarities with the numerical simulations. Special considerations are mentioned concerning the size of the probe and its ability to measure the permittivity of heterogeneous materials made of large inclusions. Combination of such numerical tool and measuring device can be used as a non-destructive testing technique to assess the near surface permittivity of concrete structures or as a calibration technique for GPR measurements.
Highlights
The use of electromagnetic (EM) waves for the characterization of concrete moisture, either by Ground Penetrating Radar (GPR) [1,2,3] or by capacitive methods has been investigated by many researchers
The wave velocity is not easy to measure with usual GPR systems, especially those devoted to concrete structure investigations and so, a calibration is necessary
A probe which could be used on site for the assessment of dielectric constant could be useful for a fast calibration of GPR measurements and limiting drilling or cores
Summary
The use of electromagnetic (EM) waves for the characterization of concrete moisture, either by GPR [1,2,3] or by capacitive methods has been investigated by many researchers. Several coaxial transmission lines have been developed, and their ability to characterize both real and imaginary parts of concrete permittivity on a large frequency range was demonstrated [14,15,16] Their use is mainly limited to laboratory measurements because the insertion of the sample into a coaxial line requires a specific machining of the sample not. The software calculates the reflection coefficient at this exact point for different signal frequencies This reflection coefficient S11 depends on the electromagnetic properties attributed to the defined sample (dielectric permittivity, electrical conductivity, magnetic permeability) and is the basis to the comparison with experimental data. Computation time required to calculated S11 parameters over such frequency range is less than ten seconds
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