Project 2.1 – GPR inspection of transport infrastructures   (from the most recent paper to the oldest)

[wg2-p1-j9] L. B. Ciampoli, F. Tosti, M. G. Brancadoro, F. D’Amico, A.M. Alani, A. Benedetto, “A spectral analysis of ground-penetrating radar data for the assessment of the railway ballast geometric properties,” Non-Destructive Testing and Evaluation International (Elsevier), vol. 90, pp. 39-47, September 2017, doi: 10.1016/j.ndteint.2017.05.005 (Italy, United Kingdom)

Abstract: This paper presents a methodology for the assessment of railway ballast using ground-penetrating radar (GPR – 2 GHz horn antenna). The primary approach in this endeavour was the finite-difference time-domain (FDTD) simulations of ballast (a multi-stage process in terms of ballast size). To this effect, a combination of random sequential adsorption (RSA) and FDTD algorithms were applied. The results of the numerical simulation then were used to compare with the experimental investigations results using a container (methacrylate material) of the 1.5 1.5 0.5 m dimensions. Finally, the modelling of the frequency spectrum peak and the equivalent diameter of the ballast aggregates was developed.

[wg2-p1-j8] F. Benedetto, F. Tosti, A. M. Alani, “An Entropy-Based Analysis of GPR Data for the Assessment of Railway Ballast Conditions,” IEEE Transactions on Geoscience and Remote Sensing, vol. 55(7), pp. 3900-3908, July 2017, doi: 10.1109/TGRS.2017.2683507 (Italy, United Kingdom)

Abstract: The effective monitoring of ballasted railway track beds is fundamental for maintaining safe operational conditions of railways and lowering maintenance costs. Railway ballast can be damaged over time by the breakdown of aggregates or by the upward migration of fine clay particles from the founda- tion, along with capillary water. This may cause critical track settlements. To that effect, early stage detection of fouling is of paramount importance. Within this context, ground penetrating radar (GPR) is a rapid nondestructive testing technique, which is being increasingly used for the assessment and health monitoring of railway track substructures. In this paper, we propose a novel and efficient signal processing approach based on entropy analysis, which was applied to GPR data for the assessment of the railway ballast conditions and the detection of fouling. In order to recreate a real-life scenario within the context of railway structures, four different ballast/pollutant mixes were introduced, ranging from clean to highly fouled ballast. GPR sys- tems equipped with two different antennas, ground-coupled (600 and 1600 MHz) and air-coupled (1000 and 2000 MHz), were used for testing purposes. The proposed methodology aims at rapidly identifying distinctive areas of interest related to fouling, thereby lowering significantly the amount of data to be processed and the time required for specialist data processing. Prominent information on the use of suitable frequencies of investigation from the investigated set, as well as the relevant probability values of detection and false alarm, is provided.

[wg2-p1-j7] A. Benedetto, F. Tosti, L. Bianchini Ciampoli, A. Calvi, M. G. Brancadoro, A. M. Alani, “Railway ballast condition assessment using ground-penetrating radar – An experimental, numerical simulation and modelling development,” Construction and Building Materials, vol. 140, pp. 508–520, June 2017, doi:10.1016/j.conbuildmat.2017.02.110 (Italy, United Kingdom; TU1208 CBM Special Issue)

Abstract: This paper reports on the ground-penetrating radar (GPR)-based assessment of railway ballast which was progressively “polluted” with a fine-grained silty soil material. It is known how the proper operation of a ballast track bed may be undermined by the presence of fine-grained material which can fill progressively the voids between the ballast aggregates and affect the original strength mechanisms. This occurrence is typically defined as “fouling”. To this effect, a square-based methacrylate tank was filled with ballast aggregates in the laboratory environment and then silty soil (pollutant) was added in different quantities. In order to simulate a real-life scenario within the context of railway structures, a total of four different ballast/pollutant mixes were introduced from 100% ballast (clean) to highly-fouled (24%). GPR systems equipped with different air-coupled antennas and central frequencies of 1000 MHz and 2000 MHz were used for testing purposes. Several processing methods were applied in order to obtain the dielectric permittivity of the ballast system under investigation. The results were validated using the “volumetric mixing approach” (available within the literature) as well as by performing a numerical simulation on the physical models used in the laboratory. It is important to emphasize the significance of the random-sequential absorption (RSA) paradigm coupled with the finite-difference time-domain (FDTD) technique used during the data processing. This was proved to be crucial and effective for the simulation of the GPR signal as well as in generating synthetic GPR responses close to the experimental data.

[wg2-p1-j6]   C. Plati, P. Georgiou, A. Loizos, "A Comprehensive Approach for the Assessment of in-situ Pavement Density Using GPR technique," Near Surface Geophysics (EAGE), EAGE Near Surface Geophysics (NSG), vol. 14(2), pp. 117-126, April 2016, doi: 10.3997/1873-0604.2015043 (Greece; TU1208 NSG Special Issue)

Abstract: With the focus on quality assurance practices during pavement construction, the present research aims at investigating the compactability of hot mix asphalt using the ground-penetrating radar technique. Thus, density as an indicator of the compactability of hot mix asphalt is predicted using three different electromagnetic-mixing-theory-based density models (namely, the complex refractive index model, Rayleigh model, and Al-Qadi, Lahouar and Leng model), and the prediction performance is also investigated. The investigations are based on experimental data acquired, both in the laboratory and field, from new full-scale asphalt pavement sections with varying asphalt mixture compositions. The laboratory experiment, which involved the compaction of asphalt mixtures using the steel-segmented roller compactor, indicated that compaction mode affects the compactability of hot mix asphalt, whereas the analysis of field ground-penetrating radar experimental data revealed that the estimated electric permittivity εHMA during the compaction process could be considered a measure of the asphalt mix field compactability. The prediction performance of the three density models was evaluated using different methodological approaches with respect to the backcalculation of εs of the mix aggregates. The results indicated that, by utilizing the ground-penetrating radar field measured εHMA for the assessment of εs, the predicted Gmb values from the implementation of the above density models closely approach the ground-truth field-core bulk densities. Comparative evaluation of the three density models showed that the Al-Qadi, Lahouar, and Leng model exhibits the best prediction performance, which is comparable to nuclear/non-nuclear methods. In light of this, it could be argued that the ground-penetrating radar methodology coupled with novel algorithms can be an effective and efficient tool to improve the asphalt mix compaction process and assessment of in situ density.

[wg2-p1-j5] S. Fontul, E. Fortunato, F. De Chiara, R. Burrinha, M. Balderais, "Railways Track Characterization Using Ground Penetrating Radar," Procedia Engineering | Advances in Transportation Geotechnics III, vol. 143, pp. 1193-1200, February 2016, doi: doi:10.1016/j.proeng.2016.06.120 (Italy, Portugal; OPEN ACCESS)

Abstract: A proper quality control of the railway track condition and its monitoring since the construction phase are key factors for a long life cycle and for an efficient maintenance policy. For this purpose, suitable techniques, such as non-destructive tests, represent an efficient monitoring solution as they allow evaluating infrastructure characteristics continuously, saving time and costs, with minimal interferences on track use. Ground Penetrating Radar (GPR) is a fast and effective electromagnetic survey technique that enables the measuring of layers thickness, detection of changes on structure or on materials properties along the line. It can also detect different types of defects such as ballast pockets, fouled ballast, poor drainage, subgrade settlement and transitions problems, depending on their extension. These defects are generally the causes of vertical deviations in track geometry and they cannot be detected by the common monitoring procedures, namely the measurements of track geometry. GPR application to railways infrastructures at network level is relatively recent. In Portugal, rail inspection is performed with Plasser & Theurer EM120 equipment and recently 400 MHz IDS antennas were installed on it. GPR tests were performed on the Portuguese rail network and, as case study, a section of an in service tracks is addressed in this paper. A combined monitoring approach is presented, based on interpretation of the geometric parameter measurement, currently utilized for maintenance planning, together with GPR results, in order to detect the causes of the track deterioration and to plan more appropriate maintenance interventions.

[wg2-p1-j4] J. De Pue, M. Van Meirvenne, W. M. Cornelis,  "Accounting for Surface Refraction in Velocity Semblance Analysis With Air-Coupled GPR," IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing (JSTARS), vol. 9(1), pp. 60-73, January 2016; doi: 10.1109/JSTARS.2015.2439333 (Belgium; GPR 2014 Special Issue)

Abstract: The aim of this study is to evaluate the velocity semblance analysis technique for air-coupled common midpoint (CMP) measurements with a small antenna offset. The technique was originally developed for seismic surveys, assuming the small spread approximation. Owing to the strong refraction at the sur- face and shallow investigation depth, this assumption is not valid in the case of air-coupled ground-penetrating radar (GPR). To over- come this assumption, a modification to the method is proposed accounting for the refraction at the surface. Synthetic experiments were executed to demonstrate that the traditional method resulted in a persistent overestimation, whereas the modified method improved the results significantly. Two field experiments have been conducted to test the method under different field conditions. In a first experiment on a road test site, the modified method improved the estimation of depth and propagation velocity significantly. However, the technique failed to estimate the propagation veloc- ity and depth or objects in a second field test, due to rough terrain conditions and noise in the data. Therefore, an additional modi- fication was proposed, by incorporating in-line data as well. This improved the depth and velocity estimations significantly. Overall, this study demonstrates that the traditional velocity semblance analysis (TRAD) is not valid for air-coupled GPR. By accounting for the refraction at the surface and incorporating inline data, it is possible to successfully estimate depth and propagation velocity with small offset air-coupled GPR configurations.

[wg2-p1-j3]  J. Pedret Rodés, V. Pérez-Gracia, A. Martínez-Reguero,  "Evaluation of the GPR frequency spectra in asphalt pavement assessment," Construction and Building Materials (Elsevier), vol. 96, pp. 181–188, October 2015; doi: 10.1016/j.conbuildmat.2015.08.017 (Spain)

Abstract: Ground penetrating radar (GPR) is frequently used in pavement assessments, mainly using the evaluation of wave travel times. However, GPR data provide further information that could be used in order to determine the inner conditions and characteristics about materials. In this paper, the possible analysis of the frequency spectrum of GPR signals is analysed and discussed. Several tests were carried out in a portion of a highway in two different stages of its service life. Results highlight the relationship between the shape of the spectrum and the frequency signature with the structure and conditions of the pavement.

[wg2-p1-j2] X. Núñez-Nieto, M. Solla, A. Novo, H. Lorenzo, “Three-dimensional ground-penetrating radar methodologies for the characterization and volumetric reconstruction of underground tunneling,”  Construction and Building Materials (Elsevier), vol. 71, pp. 551–560, November 2014; doi: doi:10.1016/j.conbuildmat.2014.08.083 (Spain, Canada; INDUSTRY INVOLVEMENT; COOPERATION WITH IPC)

Abstract: This work presents the documentation and characterization of an ancient underground concrete tunnel using the ground-penetrating radar (GPR) method. Three-dimensional imaging methodologies were applied to create an accurate volumetric reconstruction of the underground tunneling space and the whole framework of galleries composing the main structure, which enabled for the dimensioning of the structure. Problems of moisture were also detected in a particular sector of the tunnel. In addition, FDTD modeling was used to improve the understanding of the GPR signal propagation and to assist the interpretation of the field GPR data. Both field and synthetic data have shown the capabilities of the method for the evaluation and characterization of this ancient construction.

[wg2-p1-j1] F. De Chiara, S. Fontul, and E. Fortunato, “GPR Laboratory Tests For Railways Materials Dielectric Properties Assessment,” Remote Sensing, vol. 6(10), pp. 9712-9728, October 2014; doi: 10.3390/rs6109712  (Italy, Portugal; OPEN ACCESS)

Abstract:  In railways Ground Penetrating Radar (GPR) studies, the evaluation of materials dielectric properties is critical as they are sensitive to water content, to petrographic type of aggregates and to fouling condition of the ballast. Under the load traffic, maintenance actions and climatic effects, ballast condition change due to aggregate breakdown and to subgrade soils pumping, mainly on existing lines with no sub ballast layer. The main purpose of this study was to validate, under controlled conditions, the dielectric values of materials used in Portuguese railways, in order to improve the GPR interpretation using commercial software and consequently the management maintenance planning. Different materials were tested and a broad range of in situ conditions were simulated in laboratory, in physical models. GPR tests were performed with five antennas with frequencies between 400 and 1800 MHz. The variation of the dielectric properties was measured, and the range of values that can be obtained for different material condition was defined. Additionally, in situ GPR measurements and test pits were performed for validation of the dielectric constant of clean ballast. The results obtained are analyzed and the main conclusions are presented herein.

Project 2.2 – GPR inspection of buildings

No journal papers on the GPR inspection of modern buildings, so far! Please see “WG4 – Archaeology and cultural heritage” for TU1208 papers on the GPR inspection of historical buildings.

Project 2.3 – GPR detection and localisation of buried utilities and voids, with a main focus to urban areas (from the most recent paper to the oldest)

[wg2-p3-j1] F. Sagnard, C. Norgeot, X. Dérobert, V. Baltazart, E. Merliot, F. Derkx, B. Lebental, “Utility detection and positioning on the urban site Sense-City using Ground-Penetrating Radar systems,” Measurement, vol. 88, pp. 318-330, June 2016; doi: 10.1016/j.measurement.2016.03.044 (France; INDUSTRY INVOLVEMENT; TU1208 Test site)

Abstract: This paper presents the design of a novel Ground-Penetrating Radar (GPR) test site that has been integrated into the mini-city demonstrator Sense-City located at University Paris-Est (France). This test site provides several sources of measurement interest expressed by the presence of a multilayered soil with significant dielectric contrasts, and various dielectric pipes and blades buried at various depths in trenches filled with a backfill soil different from the natural soil. This paper presents experimental Bscans associated with the pipe zone acquired by three different GPR systems at frequencies ranging from 300 MHz to 1.5 GHz. The interpretation and comparison of the raw Bscans have allowed to characterize the dielectric properties of the soil layers, and to detect the hyperbola signatures of the buried pipes. The results of this study will help to guide future developments on polarization, operating frequency and signal processing to extract parameters (orientation, dielectric characteristics, position and size) associated with pipes.

Project 2.4 – GPR inspection of construction materials (from the most recent paper to the oldest)

[wg2-p4-j5] X. Dérobert, J. F. Lataste, J.-P. Balayssac, S. Laurens, “Evaluation of chloride contamination in concrete using electromagnetic non-destructive testing methods,” NDT & E International, vol. 89, pp. 19-29, July 2017, doi: 10.1016/j.ndteint.2017.03.006 (France)

Abstract: We present the results of the sensitivity of some electromagnetic non-destructive testing (NDT) methods to chloride contamination. The NDT methods are resistivity, using a quadripole probe, capacitive technique, with few sets of electrodes, and radar technique, using different bistatic configurations. A laboratory study was carried out involving three different concretes with different water to cement ratios. The concretes were conditioned with different degrees of NaCl saturation by means of three solutions containing 0 g/L, 30 g/l or 120 g/l. The solution was homogenized in the concrete by using a specific procedure. Results show that the EM techniques are very sensitive to the chloride content and saturation rate and, on a second level, to the porosity. Multi-linear regression processing was performed to estimate the level of sensitivity of the NDT measurements to the three indicators. Values of ten ND observables are presented and discussed. At last, the uncertainties of the regression models are studied on a real structure in a tidal zone.

[wg2-p4-j4] S. Araujo, L. Delbreilh, L. Laguerre, H. Dumont, E. Dargent, C. Fauchard, “Rock permittivity characterization and application of electromagnetic mixing models for density/compactness assessment of HMA by means of step-frequency radar,” EAGE Near Surface Geophysics (NSG), vol. 14(6), pp. 551–562, December 2016, doi: 10.3997/1873-0604.2016031 (France)

Abstract: This work aims to determine the compactness/density of hot mix asphalt by measuring its permittivity by means of step-frequency radar. As hot mix asphalt is mainly made of rocks; their dielectric properties are measured in the frequency range of 0.5 - 4 Ghz with step-frequency radar, using cylindrical cavities. The results show that the rocks can be considered as low-loss dielectric. As electromagnetic mixing models are required to translate measured permittivity to the compactness, power law models and unified mixing rules are needed for laboratory experimental data. The slab permittivity of various compactness is determined with the help of the step-frequency radar system. This study shows that: (i) the selection of the electromagnetic mixing model has a critical impact on the accuracy of the calculated compactness; (ii) the choice of the host matrix for a family of unified mixing rules has huge consequences; and (iii) the best assessment of compactness/density is given by the complex refractive index model and Rayleigh and Böttcher models with an aggregate matrix.

[wg2-p4-j3] I. Rodríguez-Abad, G. Klysz, R. Martínez-Sala, J. P. Balayssac, J. Mené-Aparicio, “Application of ground-penetrating radar technique to evaluate the waterfront location in hardened concrete,” Geoscientific Instrumentation, Methods and Data Systems, vol. 5, pp. 567–574, December 2016; doi: 10.5194/gi-2016-21 (France, Spain; STSM Outcome; OPEN ACCESS)

Abstract: The long term performance of concrete structures is directly tied to two factors: concrete durability and strength. When assessing the durability of concrete structures, the study of the water penetration is paramount, because almost all reactions like corrosion, alkali-silica, sulfate, etc., which produce their deterioration, require the presence of water. Ground-penetrating radar (GPR) has shown to be very sensitive to water variations. On this basis, the objective of this experimental study is, firstly, to analyze the correlation between the water penetration depth in concrete samples and the GPR wave parameters. To do this, the samples were immersed into water for different time intervals and the wave parameters were obtained from signals registered when the antenna was placed on the immersed surface of the samples. Secondly, a procedure has been developed to be able to determine, from those signals, the reliability in the detection and location of waterfront depths. The results have revealed that GPR may have an enormous potential in this field, because excellent agreements were found between the correlated variables. In addition, when comparing the waterfront depths calculated from GPR measurements and those visually registered after breaking the samples, we observed that they totally agreed when the waterfront was more than 4 cm depth.

[wg2-p4-j2] H. Reci, T. Chinh Maï, Z. Mehdi Sbartaï, L. Pajewski, E. Kiri, “Non-destructive evaluation of moisture content in wood by using Ground Penetrating Radar,” vol. 5, pp. 575-581, December 2016, doi: 10.5194/gi-5-575-2016 (Albania, France, Italy; COOPERATION WITH NNC; STSM Outcome; OPEN ACCESS)

Abstract: This paper presents the results of a series of laboratory measurements carried out to study how the Ground Penetrating Radar (GPR) signal is affected by moisture variation in wood material. The effects of the wood fiber direction, with respect to the polarisation of the electromagnetic field, are investigated. The relative permittivity of wood and the amplitude of the electric field received by the radar are measured for different humidity levels, by using the direct-wave method in Wide Angle Radar Reflection configuration, where one GPR antenna is moved while the other is kept in a fixed position. The received signal is recorded for different separations between transmitting and receiving antennas. Direct waves are compared to reflected waves: it is observed that they show a different behaviour when the moisture content varies, due to their different propagation paths.

[wg2-p4-j1] X. Xiao, A. Ihamouten, G. Villain, X. Dérobert, “Use of Electromagnetic Two-layer Wave-Guided Propagation in the GPR Frequency Range to Characterize Water Transfer in Concrete,” NDT &  E International, 2016, vol. 86, pp. 164-174, March 2016; doi: 10.1016/j.ndteint.2016.08.001 (France)

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.

Project 2.5 – GPR estimation of water content in structures and soil (from the most recent paper to the oldest)

[wg2-p5-j2] F. Tosti, A. Benedetto, L. Bianchini Ciampoli, S. Lambot, C. Patriarca, E.C. Slob, “GPR analysis of clayey soil behaviour in unsaturated conditions for pavement engineering and geoscience applications,” EAGE Near Surface Geophysics (NSG), vol. 14(2), pp. 127-144, April 2016, doi: 10.3997/1873-0604.2016011 (Belgium, Italy, Netherlands, United Kingdom; TU1208 NSG Special Issue)

Abstract: Clay content is one of the primary causes of pavement damages, such as subgrade failures, cracks, and pavement rutting, thereby playing a crucial role in road safety issues as an indirect cause of accidents. In this paper, several ground-penetrating radar methods and analysis techniques were used to nondestructively investigate the electromagnetic behaviour of sub-asphalt compacted clayey layers and subgrade soils in unsaturated conditions. Typical road materials employed for load-bearing layers construction, classified as A1, A2, and A3 by the American Association of State Highway and Transportation Officials soil classification system, were used for the laboratory tests. Clay-free and clay-rich soil samples were manufactured and adequately compacted in electrically and hydraulically isolated formworks. The samples were tested at different moisture conditions from dry to saturated. Measurements were carried out for each water content using a vector network analyser spanning the 1 GHz–3 GHz frequency range, and a pulsed radar system with ground-coupled antennas, with 500-MHz centre frequency. Different theoretically based methods were used for data processing. Promising insights are shown to single out the influence of clay in load-bearing layers and subgrade soils, and its impact on their electromagnetic response at variable moisture conditions.

[wg2-p5-j1] A. Benedetto, F. Tosti, B. Ortuani, M. Giudici, M. Mele, "Mapping the spatial variation of soil moisture at the large scale using GPR for pavement applications," Near Surface Geophysics (EAGE), vol. 13, pp. 269-278, 2015; doi:  10.3997/1873-0604.2015006 (Italy; TU1208 NSG Special Issue)

Abstract: The characterization of shallow soil moisture spatial variability at the large scale is a crucial issue in many research studies and fields of application ranging from agriculture and geology to civil and environmental engineering. In this framework, this work contributes to the research in the area of pavement engineering for preventing damages and planning effective management. High spatial variations of subsurface water content can lead to unexpected damage of the load-bearing layers; accordingly, both safety and operability of roads become lower, thereby affecting an increase in expected accidents. A pulsed ground-penetrating radar system with ground-coupled antennas, i.e., 600-MHz and 1600-MHz center frequencies of investigation, was used to collect data in a 16 m × 16 m study site in the Po Valley area in northern Italy. Two ground-penetrating radar techniques were employed to non- destructively retrieve the subsurface moisture spatial profile. The first technique is based on the evalu- ation of the dielectric permittivity from the attenuation of signal amplitudes. Therefore, dielectrics were converted into moisture values using soil-specific coefficients from Topp’s relationship. Ground- penetrating-radar-derived values of soil moisture were then compared with measurements from eight capacitance probes. The second technique is based on the Rayleigh scattering of the signal from the Fresnel theory, wherein the shifts of the peaks of frequency spectra are assumed comprehensive indi- cators for characterizing the spatial variability of moisture. Both ground-penetrating radar methods have shown great promise for mapping the spatial variability of soil moisture at the large scale.