Use of electromagnetic two-layer wave-guided propagation in the GPR frequency range to characterize water transfer in concrete

Abstract

The objective of this paper is to adapt a recent innovative technique for extracting and exploiting the Electromagnetic (EM) waveguide dispersion of civil engineering materials by means of GPR, and allowing to monitor the water ingress front during the absorption process for various concrete mixes. This technique is based on an inversion procedure that applies the Electromagnetic Waveguide Model (WGM) to invert phase velocity dispersion curves in their modal form. A Parallel homogenization model, derived from the Lichtenecker-Rother equation, has been employed to extend the waveguide model from a one-layer to a two-layer medium. The WGM outputs are then used to estimate the geometric parameters of the propagation medium and offer a primary application to water transfer monitoring in concrete through capillarity effects. The initial WGM validation is carried out on FDTD-simulated propagation signals, while the second validation relies on GPR data acquired from homogeneous materials. Then, a broad-based experimental study is conducted for the purpose of correlating electromagnetic waveguide dispersion parameters with both the geometric and hydric characteristics of various concrete mixes. Results obtained using the two-layer WGM serve to monitor the water ingress front during an absorption process. These results are then compared to the moisture gradients generated on cores using gammadensimetry, which is set as the reference. This procedure yields a number of trends, which in turn provide key information on the conditioning state of the studied concretes.

Introduction

For decades, a considerable body of research has focused on studying the durability of civil engineering structures through the use of non-destructive testing (NDT) methods, whose features vary in degree of reliability. Among these methods, electromagnetic techniques are primarily based on an analysis of the temporal content of electromagnetic (EM) waves propagated in concrete [1], [2], [3], [4], [5], [6], [7]. Besides the efforts involved in interpreting EM wave dispersion relative to the dielectric nature of materials [8], certain applications derived from seismic techniques have revealed the existence of a second dispersion phenomenon due to the geometry and boundary conditions of the propagation medium [9], [10], [11], [12]. Within the GPR field of application, the notion of geometric dispersion has been introduced as a multi-modal propagation of EM waves through waveguides [13], [14], [15]: in passing through the waveguide layer, EM energy undergoes multiple reflections, leading to a series of constructive interferences among the various propagation modes. Under these conditions, the use of temporal methods to estimate permittivities and/or thicknesses through time peak picking, from WARR (Wide Angle Reflection and Refraction) or CMP (Common Mid-Point) configurations [16], [17], [18] becomes totally inappropriate, mainly for two-layer media with very thin layers or without enough dielectric contrasts at the interfaces.

In the presence of a strong dielectric contrast between the various layers composing the investigated medium, the EM energy is "trapped" in the upper layer of the structure, a situation that necessitates a modal approach to studying EM wave propagation [19], [20]. Arcone [21] is recognized as the precursor of this approach for the EM characterization of a thin ice layer bounded by air on its upper surface and liquid water below. Since the air/ice and ice/water dielectric contrasts are extremely high (i.e. strong reflectors), the ice layer therefore behaves like an EM waveguide. In assuming that the dominant frequencies of dispersive waves are sufficient to calculate the waveguide cutoff frequency, Arcone [21] was able to determine the dielectric permittivity of various ice layers.

More recently, van der Kruk et al. [22], [23] demonstrated the possibility of deriving the physical properties of each layer constituting the dielectric medium from the dispersion of GPR waves propagated in waveguides with both strong [23] and weak [22] permittivities. The results of these studies have shown that the propagation of Transverse Electric (TE) modes is less sensitive to EM leakage phenomena, which implies the use of "broadside" antenna orientations in order to generate such modes [23], [24]. As a follow-up to this work, a new inversion method has been implemented to estimate the dielectric permittivity and geometry of waveguides.

This paper will develop an adapted innovative procedure, from these last works [19], [20], [21], [22], [23], [24], for applying EM waveguide dispersion within the spectral domain in order to characterize two layers present in concrete structures, whose formation is due to water penetration into dry concrete and which decompose the investigated medium into two parts, i.e. dry and wet layers presenting a gradient between the two parts. An underlying question linked to this study is about the robustness of this proposed approach on a realistic two-layer medium with gradient.

This paper will thus begin with an introduction to the Parallel model and modal propagation theory, as adapted to the case of multilayer waveguide media delimited by both air (upper surface) and a strong reflector (lower surface). The next part will develop an analytical EM model, called the Waveguide Model (WGM), to be associated with an inversion procedure in a way that allows estimating the thicknesses of the studied medium by inverting the phase velocity dispersion curves. This model will be validated using both synthetic data generated from a FDTD (Finite Difference Time Domain) numerical model and experimental data stemming from the EM characterization of standard materials (simple cases with planar layers) using multi-offset GPR antennas. The experimental study conducted during the final step will associate the inversion procedure applied to GPR measurements with both the physical and hydric characteristics of two-tested concrete mix designs presenting realistic imbibition gradient fronts.