Xavier Dérobert

On Variants of the Frequency Power Law for the Electromagnetic Characterization of Hydraulic Concrete

The objective of this paper is threefold: First, dielectric permittivity measurements on concrete specimens are compared among the different variants of Jonschers model. Each of these variants considers just a limited number of parameters (two or three), based on certain material-dependent and/or bandwidth-related simplifying assumptions. According to this setup, these variants affect the inherent “universality” of the model by imposing a tradeoff between representativeness and practicality. Such an effect becomes more pronounced for concrete mixtures with high moisture content. Next, we propose a computationally efficient two-step procedure for estimating the four model parameters; this procedure filters the linear parameters that contribute to model deviation from the frequency power law by applying the Kramers-Krönig relations. Only two parameters therefore need to be estimated numerically, whereas the filtered parameters can be estimated in closed form as the solution to a simple linear least-squares problem. All of these variants are applied to an array of concrete mixtures and then evaluated by their goodness of fit. The results obtained demonstrate that accounting for all model parameters via the proposed procedure yields the smallest fitting error, thus enhancing the data interpretation stage. Moreover, a parametric study has been carried out in order to correlate the dispersion parameter n present in each variant of Jonschers model with the physical and hydric characteristics of the concrete mixtures. This procedure has made it possible to derive trends providing information on the conditioning state of the studied media.

NON-DESTRUCTIVE TESTING OF FIRED TUNNEL WALLS: THE MONT-BLANC TUNNEL CASE STUDY

The investigation of fired tunnel walls typically relies on visual inspection and a comprehensive study of core samples. Visual inspection is limited to surface diagnosis, while core samples only provide a detailed image of the damaged zone at a single point. In order to gain an extensive view of the entire depth of the damaged zone as well as specific material properties, both the seismic refraction method and ground-penetrating radar investigation may be carried out. In the case of the Mont-Blanc Tunnel, these non-destructive methods have been tested in three zones: heavily damaged, moderately damaged, and sound. On the radargrams generated, several layers can be roughly identified; seismic refraction then confirms and characterises the existence of layers with contrasting mechanical properties.

Comparison of NDT techniques on a post-tensioned beam before its autopsy

Abstract Following the complete demolition of a prestressed concrete bridge in southern France, a suspected weak post-tensioned beam was retained for non-destructive testing (NDT). Ground penetrating radar, Ferroscan ® (covermeter), gamma-ray radiography and impact-echo methods have all been tested and their results then discussed after the autopsy of the beam by means of hydro-demolition. This paper describes the extent to which NDT surveys are able to respond to the needs of structural engineers, through the use of complementary NDT approaches. The introduction of a second technique should provide a more pertinent response while eliminating certain ambiguities either by improving measurement reliability or by focusing on questionable zones to obtain more precise local measurements. Afterwards, the problems still not adequately resolved by these techniques are pinpointed. This step concludes with the set of needs heretofore unmet by such techniques.

Time Delay and Permittivity Estimation by Ground-Penetrating Radar With Support Vector Regression

In the field of civil engineering, sounding the pavement layers is classically performed using standard ground-penetrating radar, whose vertical resolution is bandwidth dependent. The layer thicknesses are deduced from both the time delays of backscattered echoes and the permittivity of layers. In contrast with conventional spectral analysis approaches, this letter focuses on one of the machine learning algorithms, namely, the support vector machine, to perform time delay estimation and dielectric constant estimation of the medium from backscattered radar signals. This letter shows the super time resolution capability of such technique to resolve overlapping and fully correlated echoes within the context of thin pavement layer testing.

Applications of Ground Penetrating Radar in civil engineering — COST action TU1208

This paper focuses on the use of Ground Penetrating Radar (GPR) in civil engineering. Open issues in this field are identified and desirable advances in GPR technology, application procedures, data processing algorithms and analysis tools, are addressed. European associations, institutes and consortia interested in this topic are mentioned, together with the main relevant international events. The new COST (European COoperation in Science and Technology) Action TU1208 “Civil Engineering Applications of Ground Penetrating Radar” is presented, started in April 2013: this interdisciplinary project offers important research opportunities and will strengthen European excellence in all the fields concerning the success of GPR technique, with a main focus on its applications in civil engineering. Four Working Groups (WGs) carry out the research activities: WGI focuses on the design of innovative GPR equipment, on the building of prototypes, as well as on the testing and optimization of new systems; WG2 focuses on the GPR surveying of pavement, bridges, tunnels and buildings, as well as on the sensing of underground utilities and voids; WG3 deals with the development of electromagnetic forward and inverse scattering methods and of advanced data processing algorithms; WG4 explores the use of GPR in fields different from civil engineering and the integration of GPR with other nondestructive testing techniques. The COST Action TU1208 is still open to the participation of new parties: in this paper, information is provided for scientists and scientific institutions willing to join the Action and participate to its activities.

Complex Permittivity Frequency Variations From Multioffset GPR Data: Hydraulic Concrete Characterization

The objective of this paper is threefold. First, a parametric study is performed to evaluate the absorption-dispersion effect on ground penetrating radar (GPR) waveform spectra. Reference data for this evaluation correspond to synthetic GPR signals generated by one of the frequency power-law variants. The result of this paper justifies the need for processing within the spectral domain in order to calculate either propagation velocities or complex permittivities. Second, an innovative technique for extracting the electromagnetic (EM) dispersion of hydraulic concrete using GPR is presented. This technique is based on a coupling between modified forms of the 2-D Fourier transform and estimation methods of quality factors; moreover, it allows identifying dispersion phenomena as a variation in the complex permittivity as a function of frequency. For each method, an analytical validation is carried out on simulated propagation signals. For the third objective, an experimental study is conducted in order to correlate the EM dispersion indicator with both physical and hydrical characteristics of the various concrete mixtures. The complex permittivities obtained according to this “intermethod coupling” (applied to GPR measurements) are compared with those obtained from semidestructive measurements on cores using a cylindrical transmission line considered as the reference. This procedure makes it possible to derive trends that provide information on the conditioning state of the studied media.

Use of electromagnetic non-destructive techniques for monitoring water and chloride ingress into concrete

This paper deals with the use of three electromagnetic non-destructive in situ techniques to assess concrete conditions: electrical resistivity, capacimetry, and ground-penetrating radar. It shows the potential of these methods to monitor the ingress of water and chlorides into concrete. The electro- magnetic properties that are studied here are dielectric permittivity and electrical resistivity, both sensitive to volumetric water content and chloride content. Results are presented from an experi- mental study conducted on concrete slabs (and corresponding core cylinders) in a controlled labora- tory environment. Then, the discussion is focused on the ability of three electromagnetic techniques to assess the depth of the ingress front of different salt solutions and to discern between the 3 NaCl concentrations (0, 15 and 30 g/L).

Evaluation of concrete water content and other durability indicators by electromagnetic measurements

In order to use non destructive techniques (NDT) for the survey of reinforced concrete structures, it is important to show their ability to measure the cover concrete characteristics related to durability, in particular the concrete water and chloride contents. For this purpose, tests with two electromagnetic methods (GPR and capacitive probes) and impact echo method were carried out on 81 slabs of 9 different concrete mixes. Concrete porosity was ranging between 12.5 and 18%. Measurements were carried out at five different water contents. A real structure was also tested in situ. The NDT results are compared to concrete performance indicators such as porosity and water content, as well as chloride profiles. The comparisons show the complementarity of the methods to perform a pertinent diagnosis of concrete structures.

Support Vector Regression method applied to thin pavement thickness estimation by GPR

In the field of civil engineering, sounding the layers is classically performed using standard ground-penetrating radar (GPR), whose vertical resolution is bandwidth dependent. The layer thicknesses are deduced from both the time delays of backscattered echoes and the dielectric constants of the layers. In contrast with the conventional spectral analysis approaches, we propose in this paper to use one of the most powerful machine learning algorithm, namely the Support Vector Machine(SVM), to perform Time Delay Estimation (TDE) of backscattered radar signals. In particular, this paper demonstrates the super time resolution capability of such technique in the context of overlapping and totally correlated echoes when thin pavement layers survey is under scope.

A comparison of phase-shift and one-port coaxial cell permittivity measurements for GPR applications

The coarse and loose nature of unbound granular road materials presents a number of challenges for conventional permittivity characterisation approaches. An alternative that appears better suited to these materials involves measuring the phase-shift at discrete frequencies through a sample of known thickness. To validate this approach against more established methods, a comparison is required on materials that can be easily measured using either method. To this end phase-shift measurements were undertaken on a range of solid dielectric slabs including various types of stone, plastic and an artificial material. Permittivity predictions from this method were then compared to results from a one-port coaxial cell. As an additional comparison, and to better understand the results, the phase-shift test setup was also modelled using GPRMax software. To improve the predictions, reverberations within the test apparatus were minimized by isolating the direct wave using time-domain Blackman windowing. However, the narrow window necessary for this particular test setup also degraded the ability to detect frequency-dependent permittivity changes. Overall the phase-shift approach produced real relative permittivity predictions similar to that from the one-port coaxial cell. Despite limitations in the current approach, the results validate the phase-shift approach as a simple and rapid method of characterizing the permittivity of larger dielectric material samples of constant thickness.