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Das Impulsradarverfahren hat sich in der zerstörungsfreien Prüfung im Bauwesen in den letzten Jahren etabliert. Mögliche Nutzer des Verfahrens sind oft daran interessiert, in einer ersten Abschätzung herauszufinden, ob Impulsradar bei ihrer Prüfaufgabe sinnvoll eingesetzt werden kann. Sofern die physikalischen Voraussetzungen für die Anwendbarkeit gegeben sind, ist danach die Machbarkeit von Interesse. Dazu ist die erzielbare Detektionstiefe von Bewehrungsstäben in Beton mit Radar von Bedeutung. In diesem Beitrag werden systematische Untersuchungen beschrieben, bei denen die erzielbare Detektionstiefe von Bewehrungsstäben in Betonbauteilen zunächst auf einfache Weise visuell quantifiziert wurde. Diese Untersuchungen wurden mit verschiedenen Antennenmittenfrequenzen durchgeführt. Dazu wurden Betonbauteile konzipiert, die bezüglich der Größen Einbautiefe, Bewehrung, Betonalter (Aushärtung des Betons) und Betonrezeptur variiert wurden. Am Ende stehen Kurven, aus denen die realistisch erzielbaren Detektionstiefen von Bewehrungsstäben in Beton in Abhängigkeit von Betonalter, Antennenmittenfrequenz und Betonsorte abgelesen werden können. Um den Einfluss der oberflächennahen Bewehrung und des Größtkorns der Gesteinskörnung auf die Detektionstiefen zu quantifizieren, sind tiefergehende Auswertungen mithilfe der POD(a)-Analyse erforderlich. Die Ergebnisse dieser beiden Einflussgrößen sind als Teil einer umfangreichen Dissertation zum Thema POD hier dargestellt.-----------------------------------------------------------------------------------------------------------------------------------------------------------
Quantification of depth measurement for the detection of metal rebars using Radar:
Among non-destructive testing methods in civil-engineering (NDT-CE) Radar or GPR (Ground Penetrating Radar) has been well established. As a first step the engineer has to make sure the chosen NDT-method is applicable. When the physical requirements for an application are fulfilled the engineer wants to know up to what depth he might detect reinforcement bars. This article summarizes the results of systematic testing with radar at well-defined test specimens with different concrete mixtures, rebars in various depths using different antenna frequencies for their detection. As a result simple detection curves allow the engineer to estimate the attainable detection depth of reinforcement bars depending on the concrete age, concrete mixture and antenna frequency. To quantify the influence of the reinforcement near the surface or the maximum grain size of the aggregate advanced data assessment using the theory of POD (Probability of Detection) is presented as part of a comprehensive PhD thesis.
Anhand eines erfolgreichen Messeinsatzes an einer Spannbetonbrücke, deren Querspannglieder zuverlässig lokalisiert und nachfolgend instandgesetzt werden sollen, wird die Vorgehensweise der Radarmessung mit zwei verschiedenen Polarisationsrichtungen der Antenne zur Unterscheidung von unterschiedlichen Arten der Bewehrungen ausführlich erklärt. Das Radarverfahren eignet sich sehr gut für die zerstörungsfreie Ortung von Bewehrung in Stahlbetonbauwerken. Insbesondere lässt sich mit dem Verfahren vorgespannte Bewehrung bis in einer Tiefe von ca. 30 cm und bei einem normalen Bewehrungsgrad (Maschenweite >10 cm) der schlaffen Bewehrung zuverlässig auffinden. Seit einigen Jahren wird das Radarverfahren erfolgreich im Vorfeld einer schadensfreien Kernbohrung im Rahmen von Ertüchtigungsmaßnahmen von Stahlbetonbrücken mit externen Spanngliedern angewendet. Aufgrund der Lage und dem Verlauf der vorgespannten Bewehrung in Hohlkastenseitenwänden und Plattenbalken kann eine vorgespannte Bewehrung von schlaffer Bewehrung in diesen Bauwerken sehr gut unterschieden werden. Diese Unterscheidungsmerkmale lassen sich aber bei Querspanngliedern in einer Fahrbahnplatte nur selten anwenden, da häufig die Überdeckung und der Abstand der schlaffen Bewehrung und der Querspannglieder ähnlich sind. In diesem Beitrag wird beschrieben, wie der gezielte Einsatz verschiedener Polarisationsrichtungen der Antenne zu einer zuverlässigen Unterscheidung von schlaffer Bewehrung und Spanngliedern beiträgt und damit zu einer verbesserten bildgebenden Darstellung der inneren Konstruktion der Fahrbahnplatte führt.
This work was sponsored by the Federal Highway Administration in cooperation with the American Association of State Highway and Transportation Officials. It was conducted in the second Strategic Highway Research Program (SHRP 2), which is administered by the Transportation Research Board of the National Academies. The project was managed by Monica Starnes, Senior Program Officer for SHRP 2 Renewal.
The research reported herein was performed by the Center for Advanced Infrastructure and Transportation (CAIT) at Rutgers University (RU); the Center for Transportation Infrastructure Systems (CTIS) at The University of Texas at El Paso (UTEP); the Federal Institute for Materials Research and Testing (BAM), Germany; and Radar Systems International, Inc. (RSI). Rutgers University was the coordinator and contractor for this project. Dr. Nenad Gucunski, professor and chair of Civil and Environmental Engineering and director of CAIT’s Infrastructure Condition Monitoring Program at RU, was the principal investigator. The other authors of this report are Dr. Soheil Nazarian, professor of Civil Engineering and director of CTIS at UTEP; Dr. Deren Yuan, research associate at CTIS at UTEP; Dr. Herbert Wiggenhauser, head of Non-Destructive Testing (NDT) in Civil Engineering at BAM; Dr. Alexander Taffe, leader of Combination and Automation of NDT of Buildings at BAM; Dr. Parisa Shokouhi, Alexander von Humboldt Research Fellow, hosted by BAM; and Doria Kutrubes, president of RSI. Arezoo Imani and Touraj Tayebi, graduate research assistants at RU, helped conduct the validation testing, data analysis, and web manual content preparation. Hoda Azari, a graduate research assistant, and Dr. Manuel Celaya, a research engineer at UTEP, assisted in the validation study as well. Hooman Parvardeh, research assistant at RU, helped build the reference database and develop the framework for the web manual, while Erica Erlanger, a research staff member at RU, edited the manuscript. Their contributions are gratefully acknowledged.
The research team also gratefully acknowledges contributions of the participants from industry and academia in the validation testing. The participants include NDT Corporation; Germann Instruments; Olson Engineering; Dr. Ralf Arndt, National Research Council associate at FHWA Turner–Fairbank Highway Research Center; Ingegneria Dei Sistemi S.p.A. (IDS), Italy; 3D-RADAR, Norway; Dr. John Popovics, University of Illinois at Urbana-Champaign; Dr. Jinying Zhu, The University of Texas at Austin; Rutgers University—Center for Advanced Infrastructure and Transportation; and The University of Texas at El Paso—Center for Transportation Infrastructure Systems. The contributions of these participants were critical for the evaluation and grading of the performance of NDT technologies.
Nuclear Power Plants have been in operation for ca. 50 years. Based on this experience, non-destructive testing tasks specific to thick and highly reinforced nuclear containment structures have materialized. The performance based Service life extension of existing NPPs also needs a measurement based decision to support continuing the service of the concrete part of the installations.
By nature, concrete is a very durable material and any natural Deterioration processes may take a long time to become critical to the structure. The experience of more than 50 years of service limits the testing tasks to a few ones which are not yet solved.
Research in NDT of concrete structures is performed by many research institutions all over the world with different technical and systematic approaches. Results are mainly obtained on laboratory specimens, sometimes additional field studies are reported. This research takes place independently without coordination and as a result, the outcomes mostly lack full comparability.
Software for data analysis has become indispensable and very powerful. This part of testing needs more attention when it comes to evaluate test results. Validation of NDE solutions is becoming a critical part in concrete structure testing.
Validation is by definition the proof that a customer´s requirements in the test are being met by the testing solution. This includes equipment, personnel qualification and data analysis. In the following tables, the research recommendation Validation is used in the sense, that proof of the performance of existing solutions needs to be adressed. In general, a validation methodology for NDE solutions for concrete testing in itself needs to be researched and established.
Comparability of research also needs an accepted and easily accessible reference. From experience, it is almost impossible to manufacture exact copies of test specimens at different locations. Round Robin tests are therefore needed to evaluate the performance of a test.
Data evaluation is generally done using dedicated software, sometimes Hardware dependant and not interchangeable between systems. Software is ever more increasingly becoming more powerful and sophisticated. An in-depth evaluation needs to address the comparability and validation of Software used for data analysis and evaluation. The vision of a unified software pool for NDE investigations would undoubtedly support research tremendously.
Quantitative NDE is mostly recommended to assess the condition of a structure. However, qualitative data can be very useful, especially for processes which change the material properties or deteriorate the structure (e.g. corrosion of reinforcement). The need for reliable baseline data is a key factor for such monitoring tasks.