Lab non destructive test to analyze the effect of corrosion on ground penetrating radar scans

Corrosion is a significant damage in many reinforced concrete structures, mainly in coastal areas. The oxidation of embedded iron or steel elements degrades rebar, producing a porous layer not adhered to the metallic surface. This process could completely destroy rebar. In addition, the concrete around the metallic targets is also damaged, and a dense grid of fissures appears around the oxidized elements. The evaluation of corrosion is difficult in early stages, because damage is usually hidden. Non-destructive testing measurements, based on non-destructive testing (NDT) electric and magnetic surveys, could detect damage as consequence of corrosion. The work presented in this paper is based in several laboratory tests, which are centered in defining the effect of different corrosion stage on ground penetrating radar (GPR) signals. The analysis focuses on the evaluation of the reflected wave amplitude and its behavior. The results indicated that an accurate analysis of amplitude decay and intensity could most likely reveal an approach to the state of degradation of the embedded metallic targets because GPR images exhibit characteristics that depend on the effects of the oxidized rebar and the damaged concrete. These characteristics could be detected and measured in some cases. One important feature is referred to as the reflected wave amplitude. In the case of corroded targets, this amplitude is lower than in the case of reflection on non-oxidized surfaces. Additionally, in some cases, a blurred image appears related to high corrosion. The results of the tests highlight the higher amplitude decay of the cases of specimens with corroded elements.Peer ReviewedPostprint (published version

Investigating Key Techniques to Leverage the Functionality of Ground/Wall Penetrating Radar

Ground penetrating radar (GPR) has been extensively utilized as a highly efficient and non-destructive testing method for infrastructure evaluation, such as highway rebar detection, bridge decks inspection, asphalt pavement monitoring, underground pipe leakage detection, railroad ballast assessment, etc. The focus of this dissertation is to investigate the key techniques to tackle with GPR signal processing from three perspectives: (1) Removing or suppressing the radar clutter signal; (2) Detecting the underground target or the region of interest (RoI) in the GPR image; (3) Imaging the underground target to eliminate or alleviate the feature distortion and reconstructing the shape of the target with good fidelity. In the first part of this dissertation, a low-rank and sparse representation based approach is designed to remove the clutter produced by rough ground surface reflection for impulse radar. In the second part, Hilbert Transform and 2-D Renyi entropy based statistical analysis is explored to improve RoI detection efficiency and to reduce the computational cost for more sophisticated data post-processing. In the third part, a back-projection imaging algorithm is designed for both ground-coupled and air-coupled multistatic GPR configurations. Since the refraction phenomenon at the air-ground interface is considered and the spatial offsets between the transceiver antennas are compensated in this algorithm, the data points collected by receiver antennas in time domain can be accurately mapped back to the spatial domain and the targets can be imaged in the scene space under testing. Experimental results validate that the proposed three-stage cascade signal processing methodologies can improve the performance of GPR system

Development of GPR data analysis algorithms for predicting thin asphalt concrete overlay thickness and density

Thin asphalt concrete (AC) overlay is a commonly used asphalt pavement maintenance strategy. The thickness and density of thin AC overlay are important to achieving proper pavement performance, which can be evaluated using ground-penetrating radar (GPR). The traditional methods for predicting pavement thickness and density relies on the accurate determination of electromagnetic (EM) signal reflection amplitude and time delay. Due to the limitation of GPR antenna bandwidth, the range resolution of the GPR signal is insufficient for thin pavement layer evaluation. To this end, the objective of this study is to develop signal processing techniques to increase the resolution of GPR signals, such that they can be applied to thin AC overlay evaluation. First, the generic GPR forward 2-D imaging scheme is discussed. Then two linear inversion techniques are proposed, including migration and sparse reconstruction. Both algorithms were validated on GPR signals reflected from buried pipes using finite difference time domain (FDTD) simulation. Second, as a special case of the 2-D GPR imaging and linear inversion reconstruction, regularized deconvolution was applied to GPR signals reflected from thin AC overlays. Four types of regularization methods, including Tikhonov regularization and total variation regularization, were compared in terms of accuracy in estimating thin pavement layer thickness. The L-curve method was used to identify the appropriate regularization parameter. A subspace method—a multiple signal classification (MUSIC) algorithm—was then utilized to increase the resolution of 3-D GPR signals. An extended common midpoint (XCMP) method was used to find the dielectric constant and the thickness of the thin AC overlay at a full-scale test section. The results show that the MUSIC algorithm is an effective approach for increasing the 3-D GPR signal range resolution when the XCMP method is applied on thin AC overlay. Furthermore, a non-linear inversion technique is proposed based on gradient descent. The proposed non-linear optimization algorithm was applied on real GPR data reflected from thin AC overlay and the thickness and density prediction results are accurate. Finally, a “modified reference scan” approach was developed to eliminate the effect of AC pavement surface moisture on GPR signals, such that the density of thin AC overlay can be monitored in real time during compaction

Inspección no destructiva de estructuras mediante georradar : análisis de daños por corrosión y por otras patologías

This thesis aims to studythe capacities and limitations of Ground Penetrating Radar (GPR) to support inspection, assessment, maintenance and rehabilitation tasks, helping to earlydetection and evaluation ofpathologies in unreinforced masonryconstructions, as well as in reinforcing concrete structures. One ofthe most common pathologies in existing reinforced concrete structures is corrosion damage. In fact, this is a pathologythat, in more advanced stages, may comprom ise the integrity of sorne structural elements, or e\en of the whole structural system. Corros ion dam age reduces the effecti\e cross section of steel reinforcing bars , dim inishing the bond between reinforcement and concrete, and affecting, dueto physical and chemical processes, the concrete that inwl\es the corroded steel reinforcing bars. In reinforced concrete structures, the corrosion ofsteel reinforcing bars is, in general, hidden. Accordingly , remotely or at distance assessment techniques are desirable , which aims to assess the integrity of steel elements, generally inaccessible through direct contact. The implementation of non-destructi\e tests (NDT) is a com mon practice when analysing pathologies in historical buildings, where inter'A:lntions should be as minimal invasi\e as possible . Sorne ofthe mostcommon methods to assess corrosion, such as the potential of corrosion, the petrographic analysis and the resistivimeter, present sorne disadvantages in comparison to the GPR. In fact, the uses of such methods require a considerable time, they are slightty invasi\e, and its application is localized, which means thattheir application only co\ers a reduced area of analysi s. In orderto assess the GPR capacity for detecting corros ion dam age, different laboratory tests were respecti\ely de\eloped, being the obtained results extremely satisfactory. Accordingly, it is possible to identify the following achie\ements: (i) a fast data acquisition planning methodologywas well established; (ii) the key elements and parameters for corrosion detection, within a qualitati\e analysis, were respecti\elyidentified; (iii) the contoureffects, related to a change on geometry, were identified and duly controlled; and (iv) a numerical approximation method, based on energy attenuation of GPR, for assessing the presence and degree ofcorrosion in damaged samples, was proposed with success. Obtained results are themsel\es a strong contribution of de\eloped research studies for the state of art. Four in-field tests, implemented on existing structures were respecti\ely developed, aim ing to apply and validate the methodologies, results and conclusions, obtained from laboratory tests. Toree case studies are related to historical constructions, belonging to the cultural heritage of the city of Barcelona, in Spain. The obtained results from GPR, combined, in sorne cases, with boreholes and seism ic tomography, allowed to identify pathologies dueto hum idity, corrosion and differential setttements. A fourth case study, de\eloped on a contemporary structure, in reinforcing concrete, and only using GPR, allowed to identifyand localize pathologies dueto corrosion damage. The laboratorytests and the case studies, using GPR, combined or not with other techniques, showed a strong capacityto detect and assess early stage pathologies, especially dueto corrosion damage, through fast, non-invasi\e and non­ destructi\e prospections. This strategy has econom ical and social advan tages . lndeed, a fast detection of pathologies , allow the rehabilitation of existing structures with a minimum amount of budget, when comparing with the needed resources to face damage in a more se\ere stage. The assessment ofpathologies in earlystages with GPR, especiallywhen combined with other techniques, allow then to ensure more resilient structures, by increasing their reliability, safety, and availability, and by decreasing their operational costs.En este trabajo de tesis se propone estudiar las capacidades y limitaciones de la tecnología de georradar para dar soporte a tareas de inspección, evaluación, mantenimiento y reparación mediante la detección temprana yla evaluación de patologías en construcciones de mampostería no reforzada yen estructuras de hormigón armado. Una de las patologías más comunes en estructuras es la que ocasiona la oxidación en elementos metálicos. Se trata de una patología que, en fases avanzadas, puede comprometer la integridad de los elementos o del conjunto estructural. La corrosión reduce la sección efectiva de los elementos metálicos o del armado, disminuye la adherencia entre acero y hormigón y afecta, por interacción química y física, el hormigón que rodea las armaduras corroídas. En estructuras de hormigón armado, la corrosión en las armaduras queda, en general, oculta. En este sentido, son deseables técnicas de prospección remotas o a distancia, con el propósito de evaluar elementos metálicos inaccesibles por contacto. El empleo de ensayos no destructi1.0s (NDT) y no invasi1.0s, es habitual en el análisis de patologías en edificios históricos, donde las inteMnciones deben ser mínimamente invasivas. Métodos comunes para evaluar la corrosión como el potencial de corrosión, análisis petrográfico y resistivímetro, tienen deswntajas frente al georradar. La utilización de estos métodos requiere un tiempo considerable, son lewmente invasi1.0s ysu aplicación es puntual de modo que las prospecciones suelen cubrir áreas relativamente reducidas constituidas por unos pocos elementos (armaduras). Para determinar la capacidad de la tecnología de georradar para detectar el fenómeno de la corrosión, se han diseñado diferentes experimentos en laboratorio. Los resultados obtenidos son razonablemente satisfactorios. Se ha definido una metodología para la planificación de prospecciones rápidas; Se han identificado los elementos y parámetros claw para interpretar, en análisis cualitati1.0s, la presencia de corrosión; Se han identificado y controlado los efectos de contorno relacionados con cambios en la geometría; Se ha propuesto una aproximación numérica, basada en la atenuación de la energía en la señal de georradar, para evaluar la presencia yel grado de corrosión en muestras afectadas . Este resultado constituye una contribución original y significativa del trabajo de tesis. Se han realizado cuatro campañas de prospección sobre construcciones y estructuras reales con objeto de aplicar las metodologías, resultados y conclusiones obtenidos en los ensayos de laboratorio. Tres casos de estudio se corresponden con construcciones históricas pertenecientes al patrimonio arquitectónico y cultural de Barcelona. Los resultados, obtenidos con prospecciones georradar combinadas, en algún caso, con sondeos y tomografía sísmica, han permitido identificar patologías causadas por la humedad, la corrosión y asentamientos diferenciales del terreno. Un cuarto caso de estudio, realizado sobre una estructura contemporánea en hormigón armado y utilizando únicamente tecnología de georradar, ha permitido identificar y localizar patologías ocasionadas por la corrosión. En los ensayos de laboratorio yen los casos de estudio descritos, la tecnología de georradar, combinada con otras técnicas, ha demostrado una buena capacidad para detectar y evaluar patologías en fases tempranas, especialmente por corrosión, mediante prospecciones rápidas, no invasivas y no destructivas. Esta estrategia tiene wntajas económicas y sociales. Una pronta detección de patologías, permite recuperar las construcciones con inwrsiones mínimas si se comparan con los importantes recursos que deben utilizarse frente a una elñdencia del daño en etapas sewras. La evaluación de patologías en etapas tempranas y con tecnología de georradar combinada con otras técnicas, nos orienta hacia construcciones más seguras y resilientes, y disminuyendo sus costos operativos.Postprint (published version