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- Radar (21)
- Zerstörungsfreie Prüfung im Bauwesen (16)
- Non-destructive testing (12)
- Beton (10)
- Concrete (10)
- Masonry (10)
- Active thermography (7)
- Aktive Thermografie (7)
- Non-destructive testing in civil engineering (6)
- Impuls-Thermografie (5)
Monitoring of cracks in historic concrete structures using optical, thermal and acoustical methods
(2015)
Cracks are a major issue in the field of cultural heritage. In order to evaluate the significance of a crack, a long term monitoring of the damaged region is required. However, there is a lack of easy to operate tools for such monitoring measures. Therefore, new or existing methods for other applications have to be optimised for cultural heritage investigation. The paper describes the application of such crack observation methods on a historic concrete sculpture. Beside conventional methods, like mapping by hand and ultrasonic depth profiling, a novel tracking system is presented. Furthermore, the suitability of active thermography for the investigation of cracks was investigated. The results show promising prospects for these non-destructive techniques.
Active thermography is sensitive to inhomogeneities at and below the surface of objects investigated. Thus, it should be useful for detecting plaster delaminations on concrete. In this paper, the results of field and laboratory investigations into plaster-covered concrete were compared. For evaluating the bonding state of the plaster it is not sufficient to study only the thermal contrasts at the surface of the investigated objects. The experimental results suggest that the overall thermal behaviour has to be considered.
As shown recently, the quantification of damage in historic masonry structures is possible by using active thermography. In this paper, a case study is presented concerning systematic studies of the determination of damage size and prognosis of damage increase inside a sandstone column by using different approaches of active thermography. Various heating sources as well as impulse and periodic heating have been compared. Reproducible investigations in regular time intervals for structural monitoring are possible.
Pulse thermography is a non-destructive testing method based on infrared imaging of transient thermal patterns. Heating the surface of the structure under test for a short period of time generates a non-stationary temperature distribution and thus a thermal contrast between the defect and the sound material. In modern NDT, a quantitative characterization of hidden imperfections in materials is desired. In particular, defect depth and shape are of interest. The reconstruction of the defect from thermography data is a nonlinear inverse problem, and ill-posed. We propose an algorithm for the identification of subsurface defects based on the travel time of the reflected thermal pulse. Our work extends results by Lugin and Netzelmann, taking lateral thermal flows directly into account while retrieving the defect depth. This requires significantly less computational work. Quantitative information about the defect shape and depth is obtained. Application of our method to both thermography data generated by a finite element simulation and experimental heating of PVC test specimens with different defects yields good reconstruction of the actual defects.
In this article, we present a measurement procedure to gain information about depth and angle of open surface cracks. The method is based on a local excitation with, e.g., a laser. The resulting surface temperature is recorded with an infrared camera. Based on this data, crack-caused anisotropies in the lateral heat flow can be detected and exploited to characterise the cracks.
The experimental set-up is based on a Nd:YAG laser. The beam is focused on the test sample by using an optical scanner to generate the required lateral heat flow. The time resolved temperature distribution is recorded with a high-speed infrared camera (InSb FPA, 3 to 5 µm) providing a frame rate of up to 500 Hz.
Up to now, only qualitative information was gained from measurements of this type. Whereas the local transient behaviour of temperature distribution provides also quantitative information of the crack parameters. The general concept of the method presented herein has already been published [1], but the mentioned publication is focused on the crack depth only.
In this paper, we can show that it is possible to simultaneously resolve the angle and depth and, in particular, the depth of non-perpendicular cracks.
The delamination of a plaster layer, which covers a concrete structure, can be detected by active thermography. In this paper, a systematic series of measurements is described, where the gap between plaster and concrete was varied. These investigations were compared to FEM-simulations. The experimental and simulated data were in good agreement. It was found that the used experimental setup yields only marginal differences between fixed and lose plaster in the thermal behavior. However, significant characteristics within the temperature decays could be resolved, which are clearly related to the air gap.
Laser induced active thermography for the visualization of transport processes in building materials
(2009)
Non-destructive and minor destructive testing methods enable a classification of historic structures, building processes, applied building technologies and materials. Therefore, these methods should be part of the global investigation of historic buildings which is usually required before the planning of any building intervention. The case study presented herein encompasses experimental work carried out at the St. Servatius Church in Quedlinburg, Germany. Within a comprehensive restoration campaign, non-destructive on-site investigations were carried out with radar and ultrasonics. The results were combined with plans, views and cores recorded in advance.
Influence of concret properties on the calibration of radar, ultrasonics and active thermography
(2008)
Nach einer Einführung in die Puls-Phasen-Thermografie (PPT) wird ein neuer Ansatz für die Defekttiefenbestimmung im Frequenzraum mittels der sogenannten charakteristischen Frequenz des minimalen Phasen- und Amplitudenkontrastes zwischen Fehlstelle und ungestörtem Bereich entwickelt und anhand von systematischen Untersuchungen an Probekörpern der Bundesanstalt für Materialforschung und -prüfung und ergänzenden Simulationsrechnungen überprüft.
Die mit Rechteckimpuls-Spektral-Thermografie (RST) bezeichnete Methode, die auch als eine Rechteckimpuls-Thermografie (RIT) im Frequenzbereich oder eine durch Amplitudenauswertung ergänzte PPT für Rechteckimpulserwärmung verstanden werden kann, soll die existierenden Ansätze ergänzen und bietet eine Alternative für die Tiefenbestimmung von Defekten bei Messungen mit langen Erwärmungs- und Beobachtungszeiten, wie sie bei Anwendung von aktiver Thermografie im Bauwesen häufig vorkommen.
Radar und Datenfusion
(2007)
Es wird ein Überblick über den Stand der Technik der Bauzustandsanalyse von historischem Mauerwerk mit zerstörungsfreien Prüfverfahren gegeben. Einzelne Prüfverfahren und ausgewählte Praxiseinsätze werden vorgestellt, um einen Einblick in die Vielfältigkeit der Anwendungsmethoden, aber auch in die Komplexität der möglichen Verfahrenskombinationen und der Datenauswertung und -interpretation zu geben. Aus Erfahrungen, die bei vielfältigen Messeinsätzen an historischen Gebäuden gewonnen wurden, lässt sich die Vorbereitung und Durchführung eines Messeinsatzes mit zerstörungsfreien, zerstörungsarmen und zum geringen Anteil mit zerstörenden Prüfverfahren unter bestimmten Voraussetzungen standardisieren. Obwohl einige Arbeitsschritte bei der Durchführung der Messungen und der Auswertung, Interpretation und Darstellung der Messergebnisse schon automatisiert wurden, ist für die Durchführung der Untersuchungen mit ZfPBau-Verfahren der Einsatz von geschultem Personal erforderlich.
Radar - Case studies of the application of radar for non-destructive testing of concrete structures
(2007)
Struktur- und Feuchteuntersuchungen von Bauteil- und Bauwerksoberflächen mit der Impuls-Thermografie
(2005)
Das Impulsradarverfahren - ein Verfahren zur zerstörungsfreien Strukturaufklärung in Bauwerken
(2004)
Abstract
In the recent years the application of infrared thermography and especially its active approaches lock-in thermography, impulse themography and pulse phase thermography (PPT) has gained more importance for non-destructive testing (NDT). In this paper it is demonstrated that PPT is very well suited for NDT in civil engineering. The method is based on pulse heating of the investigated specimen and on the observation of the cooling down process on its surface with an infrared camera. The transient behaviour is afterwards analysed with Fast Fourier Transformation, enabling the creation of phase and amplitude images with enhanced visualisation of the defects. Results of European and National (i.e. DFG) funded projects are presented, for example the detection of voids and inhomogeneities close to the surface and the localisation of delaminations on plastered concrete and masonry specimen.
ZUSAMMENFASSUNG
Im Rahmen eines von der Deutschen Forschungsgemeinschaft geförderten Vorhabens zur zerstörungsfreien Prüfung im Bauwesen wird in der Bundesanstalt für Materialforschung und -prüfung (BAM) in Zusammenarbeit mit der Technischen Universität Berlin (TUB) die Puls-Phasen-Thermografie (PPT) weiterentwickelt, bewertet und eingesetzt. Die durch die Impuls-Thermografie (IT) aufgenommenen Daten werden mittels Fast Fourier Transformation (FFT) bearbeitet. Die durch die FFT gewonnenen Phasenbilder reduzieren die Störeinflüsse von Oberflächeninhomogenitäten und ungleichmäßiger Erwärmung.
Die dargestellten Ergebnisse zeigen deutlich, dass sich die PPT sehr gut zur zerstörungsfreien Prüfung im Bauwesen eignet. Insbesondere Inhomogenitäten im oberflächennahen Bereich, wie Ablösungen und Hohlstellen hinter Beschichtungen (Putz, CFK-Laminate etc.), aber auch Defekte in größeren Tiefen können zuverlässig geortet und bildgebend dargestellt werden.
Aufbau eines historischen Mauerwerkskörpers ("Obelix") zur Bewertung von zerstörungsfreien Verfahren
(2004)
Zusammenfassung
Auf dem Gelände der BAM entstand im Frühjahr 2003 ein Probekörper, der die Verbindung bildet zwischen Laborprüfkörpern und realen Bauwerken aus Mauerwerk. Er dient der Validierung und Weiterentwicklung von zerstörungsfreien Prüfverfahren zur Untersuchung von historischem Mauerwerk. Dabei wurde große Sorgfalt auf die Materialauswahl und den strukturellen Aufbau gelegt. Die Materialien sollten denen von realen, historischen Bauwerken entsprechen. Der strukturelle Aufbau ergibt sich aus praxisrelevanten Fragestellungen. Der Probekörper steht auch anderen Arbeitsgruppen für Verfahrensvalidierungen zur Verfügung.
The German Society for Non-destructive Testing (DGZfP) is a center of activity in research, development, application and dissemination concerning non-destructive testing (NDT) methods. The transfer of results from research to practical application is one of the main targets. Therefore the Technical Committee NDT in Civil Engineering has published ten guidelines providing information from scientifical background to practical application concerning NDT-methods like radar, ultrasonic and radiography. Considering radar for example the content and structure of these guidelines is described below. To contribute to harmonization of European regulations and standards these guidelines should be transferred into the English language area. They can serve as a basis for harmonized European standards.
Active infrared thermography in civil engineering - quantitative analysis by numerical simulation
(2003)
The cooling-down process of building structures after heating-up with an external radiation source was analysed to detect voids inside and below the surface. Quantitative results of concrete test specimen containing voids with different sizes at various depths will be presented here. The experimental results were compared to numerical simulations performed with a Finite Difference program developed at BAM.
ONSITEFORMASONRY is a research project funded by the European Commission under the 5. Framework Programme 1998-2002 in the Thematic Programme: Energy, Environment and Sustainable Development and the Key Action: The City of Tomorrow and Cultural Heritage.
The main objective of the project is the development and improvement of methodologies for the evaluation of the structure of historic masonry Cultural Heritages. For effective restoration and conservation of historic buildings, a detailed assessment of the structural safety and physical damages of the masonry structure is required. Therefore, typical masonry damages and the most frequent pathologies in each region have been identified and summarised in a catalogue of problems and damages. Selected non-destructive (NDT) and minor-destructive (MDT) techniques are performed by the partners including the development of software packages for fast and automated data analysis. The strategy for an effective and useful combination of different NDT and MDT methods will be worked out considering the results of case studies and taking into account the experiences of the consortium members. Recommendations and guidelines for the application of the integrated methodologies will be developed in close co-operation with end-users.
The structural safety, durability and performance of the infrastructure is of primary interest in every country. An efficient system for early and regular structural assessment as well as for quality assurance during and after the construction of new structures and of reconstruction processes is urgently required. At BAM, NDT methods to be applied in civil engineering are developed and their application improved. From the experience of on-site assessments, quality assurance systems and methodologies for regular inspections are elaborated. This work is mainly performed in the frame of externally funded research projects.
We concentrate on appropriate algorithmic processing of microwave Ground Penetrating Radar (GPR) data obtained from concrete specimens containing tendon ducts below steel reinforcement bars either via simulations or via experiments. The goal is to understand the physical phenomena of electromagnetic wave propagation and scattering in detail in order to optimize imaging algorithms like SAFT (Synthetic Aperture Focusing Technique as the NDT version of SAR) in its 3D diffraction tomographic FT-SAFT formulation (Fourier-Transform-SAFT). Therefore we start with the simulation of a "simple" specimen and switch to a more realistic one in order to check whether simulation and experiment can give comparable results. Basically, this could be confirmed, but, nevertheless, various parametric studies will still have to be performed in the future.
For the investigation of the complex inner structure of historic buildings, the application of radar has been increased during the last ten years. Although historic masonry in general is very inhomogeneous and although in some cases high absorption occurs due to enhanced moisture, radar has been successfully applied to get information on geometry, position of inclusions and about kind and size of existing damages. In this paper case studies will be presented concerning the localisation of metallic inclusions and the investigations of the structure of an exterior wall with a multiple leaf structure.
Ortung von Hohlstellen und Ablösungen in Betonkonstruktionen mit Radar und Impuls-Thermografie
(2003)
Non-destructive testing (NDT) of concrete structures is performed using pulse-echo methods. According to the type of the applied waves it can be classified as acoustic (Impact-Echo, Ultrasonics) and electromagnetic methods (Radar). The results are visualised through different imaging processes. This work is performed in the frame of a research project promoted by Deutsche Forschungsgemeinschaft (FOR384).The objective of data fusion is to use the complementary information of the different methods. Radar can detect metallic reflectors in concrete (metallic ducts and concrete reinforcement) very well. This method is not able to locate defects behind these reflectors (injecting defects, defects behind close concrete reinforcement), because the electromagnetic waves are completely reflected at metals. The acoustic methods are able to compensate this deficit i.e. acoustic waves can penetrate through metal. But acoustic waves in the ultrasonic range are completely reflected by air layers. Air layers have smaller influence on radar propagation, so that both methods complement each other.In order to be able to combine the NDT-data from several methods records at the same volume, the different data setsmodes of signals must be adapted. The ultrasonic and radar data have to be reconstructed with programs based on the Synthetic Aperture Focusing Technique (SAFT) before data fusion. Subsequently, a conversion of the data into a uniform format has to be carried out. This is a prerequisite in order to keep the data exchange between the project partners as simple as possible. After the data sets are imported and transferred into a common reference system, they can be processed with operations according to the purpose of the investigation. Results, which have been achieved in concrete test specimen with radar and ultrasonics, will be presented and will show the feasibility of the data fusion method.
Application of georadar for the determination of moisture content and distribution in masonry
(2003)
The application of impulse radar for structural investigation of concrete elements has been increased during the last years related to technical developments, i. e. of high frequency antennas. In this paper, case studies will be presented related to the location of reinforcing bars, tendon ducts and repaired concrete areas in concrete bridges, anchors and dowels in concrete highways and delaminations of layered structures as they are used for non-ballasted railway tracks. It is shown that impulse radar can be applied in case of regular inspection and for searching the cause of damages but also for quality assessment in civil engineering.
Anwendung der Impuls-Thermografie als quantitatives zerstörungsfreies Prüfverfahren im Bauwesen
(2002)
Zusammenfassung
Die Impuls-Thermografie ist eine aktive Methode zur Strukturuntersuchung von Bauteilen. Im Rahmen eines von der DFG geförderten Projektes wird in Kooperation mit der TU Berlin an der Bundesanstalt für Materialforschung und -prüfung zur Zeit die mögliche Anwendung der Impuls-Thermografie als zerstörungsfreies Prüfverfahren im Bauwesen untersucht.
Die Impuls-Thermografie soll zur Ortung von oberflächennahen Inhomogenitäten, bei denen es sich im Normalfall um Fehlstellen handelt, in typischen Bauteilen verwendet werden und nach Möglichkeit die Geometrie- sowie die Materialparameter der Inhomogenitäten quantitativ bestimmt werden.
Messungen werden derart durchgeführt, dass Oberflächen mit einem geeigneten Heizstrahler erwärmt, die anschließenden Abkühlungsprozesse mit einer Infrarot-Kamera betrachtet und thermische Bilddaten (Thermogramme) aufgezeichnet werden. Der Hauptansatzpunkt für die quantitative Auswertung der Messdaten ist die Betrachtung von Temperatur-Zeit-Kurven (Transienten) des Abkühlungsprozesses über fehlerfreien und fehlerhaften Bereichen des Probekörpers. Zusätzlich werden auf Basis der Finiten-Differenzen-Methode numerische Simulationen durchgeführt. Quantitative Ergebnisse werden sowohl durch den Vergleich der experimentellen Daten mit den Simulationsrechnungen als auch durch eine Anpassung analytischer Funktionen an die Transienten erzielt.