Effect of water and chloride contents and carbonation on the electromagnetic characterization of concretes on the GPR frequency band through designs of experiment

Abstract

Water content, chloride content and carbonation are three durability indicators that affect not only concrete properties and integrity, but also the ground penetrating radar (GPR) response. This article characterizes the effect of water and chloride contents as well as carbonation on the high-frequency electromagnetic (EM) properties of concretes using a coaxial-cylindrical cell operating within the large frequency band from 50 MHz to appprox. 1000 MHz, thus making possible to study dispersion effects on the complex relative permittivity of various concretes.During a second step, three specific designs of experiment (DoE), involving water and chloride contents, and then carbonation, are designed and studied in order to establish the relative sensitivity of these parameters compared to others in describing the mixes at three different frequencies.The complex dielectric properties are evaluated in the laboratory on many concrete specimens (numbering 24, 36 and 24 for the moisture, chloride and carbonation studies respectively). For these evaluations, the composition parameters consist of nature of aggregate, nature of cement, cement content and water-to-cement ratio (W/C) while conditions are described by degree of water saturation, chloride content or carbonation. In the case of the chloride and carbonation DoEs, the mixes are tested under both dry and saturated conditions.The models describing complex relative permittivity, in association with the designs of experiment, are multi-linear polynomials of the 4 or 5 identified engineering parameters, to be chosen depending on the study. The interpretation of datasets entails in independent calculation of model coefficients for the real and imaginary parts of relative permittivity, at the three chosen frequencies. This approach reveals the significant relationships existing between dielectric properties and composition parameters as well as their evolution versus frequency.The major results of this work can be summarized as follow:-The degree of water saturation amounts to about 80% on ε′r variability and 60% to 15% versus frequency on ε″r variability, for the first campaign.-The chloride content, while only visible in saturated mixes, represents approx. 40% and 50% with porosity (via the W/C ratio) accounting for some 30% and 40% on ε′r and ε″r variabilities, respectively.-Lastly, carbonation, while also only visible in saturated mixes, corresponds to a weight of about 14% in low frequencies and vanishes at high frequencies for ε′r variability; this amount rises to roughly 24% for ε″r variability.