Wissenschaftliche Artikel der BAM
To date, the destructive extraction and analysis of drilling cores is the main possibility to obtain depth information about damaging water ingress in building floors. The time- and costintensive procedure constitutes an additional burden for building insurances that already list piped water damage as their largest item. With its high sensitivity for water, a ground-penetrating radar (GPR) could provide important support to approach this problem in a non-destructive way. In this research, we study the influence of moisture damage on GPR signals at different floor constructions. For this purpose, a modular specimen with interchangeable layers is developed to vary the screed and insulation material, as well as the respective layer thickness. The obtained data set is then used to investigate suitable signal features to classify three scenarios: dry, damaged insulation, and damaged screed. It was found that analyzing statistical distributions of A-scan features inside one B-scan allows for accurate classification on unknown floor constructions. Combining the features with multivariate data analysis and machine learning was the key to achieve satisfying results. The developed method provides a basis for upcoming validations on real damage cases.
Muon Tomography of the Interior of a Reinforced Concrete Block: First Experimental Proof of Concept
(2021)
Quality assurance and condition assessment of concrete structures is an important topic world-wide due to the aging infrastructure and increasing traffic demands. Common topics include, but are not limited to, localisation of rebar or tendon ducts, geometrical irregularities, cracks, voids, honeycombing or other flaws. Non-destructive techniques such as ultrasound or radar have found regular, successful practical application but sometimes suffer from limited resolution and accuracy, imaging artefacts or restrictions in detecting certain features. Until the 1980s X-ray transmission was used in case of special demands and showed a much better resolution than other NDT techniques. However, due to safety concerns and cost issues, this method is almost never used anymore. Muon tomography has received much attention recently. Novel detectors for cosmic muons and tomographic imaging algorithms have opened up new fields of application, such as the investigation of freight containers. Muon imaging also has the potential to fill some of the gaps currently existing in concrete NDT. As a first step towards practical use and as a proof of concept we used an existing system to image the interior of a reference reinforced 600 kg concrete block. Even with a yet not optimized setup for this kind of investigation, the muon imaging results are at least of similar quality compared to ultrasonic and radar imaging, potentially even better. The data acquisition takes more time and signals contain more noise, but the images allowed to detect the same important features that are visible in conventional high energy X-ray tomography. In our experiment, we have shown that muon imaging has potential for concrete inspection. The next steps include the development of mobile detectors and optimising acquisition and imaging parameters.
Moisture and salt loads of concrete can significantly change its microstructure and consequently lead to chemical and mechanical degradation. However, the non-destructive investigation of moisture and salt present in concrete is still difficult. In order to address and compare the sensitivity of different methods concrete samples with different pore systems realized by varying the w/z ratios have been fabricated. The focus of this study was put on the analysis of ultrasonic long and trans waves measured on the surface and in transmission mode. The results show clear dependencies of all applied methods. With the radar and microwave methods predominantly changes in the concrete moisture could be detected, whereas the different pore systems were not observed to alter the signals. In contrast for the ultrasound method also the ongoing hydration as well as the nature of the pore system strongly influenced the signals. As a consequence in a subsequent drying experiment it was also tried to delineate the effects of moisture and hydration. All specimens have been re-saturated under pressure and the drying experiment was repeated using the same multi-sensor approach.
Measuring the moisture content of floor screeds is usually done with minor destructive testing methods like Darr drying or the Calcium Carbid (CM) method. These require small samples, deliver only punctual information and still have proven not to be very reliable. Hence, a study has been made using the standard destructive tests as well as a suite of non-destructive testing methods working out their use for moisture determination. In this study five partners from research institutes and industry worked together and intensively researched different technologies. The main focus was put on the varying sensitivity of the measuring techniques in different moisture ranges. Especially for low moisture contents ( or ‘critical’ moisture contents when the screed is dry enough to be covered with the final floor finish), several commercial devices including the most commonly used CM-method failed to determine the correct moisture content for cementitious samples. Hence the need for more accurate, if possible non-destructive methods is high, taking also into account that the chemistry (and physical properties) of screeds may vary strongly depending on their origin and purpose.
Non-destructive testing methods are mostly applied and established for the detection of embedded mounting parts or structural defects in building elements. The assessment of the concrete microstructure or microstructural changes like chemical alterations or the formation of microcracks, e.g. due to material aging, freeze-thaw cycles, alkali-silica reaction and ettringite, is not in the focus of ndt research though. Concrete moisture and enhanced salt contents, which usually trigger all chemical microstructural changes, are other material properties, lacking reliable ways of measuring. But, the assessment of such material properties, on the long term also in a depth resolved manner, is definitely important, when the sustainability of our concrete infrastructure buildings shall be evaluated.
New consideration like the potential use of ndt, in particular the combination of different methods and alternate ways of data analysis are subject of research currently undertaken at BAM. These approaches involve for example working towards (i) a deeper understanding of how to measure moisture distributions reliably and follow transport phenomena, (ii) the use of stray phenomena in radar and ultrasound to locate material inhomogeneities or (iii) the application of LIBS for the delineation of diffusion and migration processes but also (iv) the use of new tools for data analysis like data fusion. First results are presented and new ideas discussed.
At BAM a multi-sensor robot system BetoScan is used for the investigation of reinforced concrete floors affected by corrosion in parking garages.
Potential maps, as well as the distribution of concrete cover and moisture can be assessed simultaneously and data can be collected contactlessly. In order to evaluate the extent of degradation adequately and to divide the investigated structure into zones with defined damage classes, large data sets have to be collected and interpreted manually. Thus, to promote an efficient data evaluation framework, which could speed up and simplify the evaluation of large data sets, an unsupervised data fusion is of major interest. However, taking into account that collected data do not certainly coincide in space, a scattered data interpolation method should be applied prior data fusion.
In the paper, a case study involving a BetoScan data set acquired from a reinforced concrete floor of a parking garage in Germany is presented. The data set includes potential mapping, covermeter based on eddy current, as well as microwave moisture measurements. Among the examined methods for interpolation of scattered data, kriging shows to yield smooth interpolated data plots even in the case of very sparse data. In the post-processing step, the investigated structure is efficiently segmented into zones using clustering based data fusion methods, which prove to be robust enough also for handling noisy data. Based on the minimization of the XB validity index, an unsupervised selection of optimal segmentation into damage classes is derived.
Am Beispiel der Ertüchtigung einer Brücke in Bremen wird gezeigt, wie mit den zerstörungsfreien Prüfverfahren (ZfP) Radar- und Ultraschall die genau Lage vorgespannte Bewehrung gefunden bzw. bestätigt werden kann. Im Rahmen von Ertüchtigungsmaßnahmen an Spannbetonbrücken werden häufig Kernbohrungen durchgeführt, die die vorhandenen Spannbewehrungen nicht beschädigen dürfen. Es wird die Vorgehensweise beschrieben, wie mit ZfP ein wesentlicher Beitrag geleistet wird, das Risiko einer Beschädigung zu minimieren und somit eine bessere Planungssicherheit für die Ertüchtigungsmaßnahmen zu schaffen. Dabei werden auch die Grenzen der beiden Verfahren aufgezeigt.
In kerntechnischen Anlagen werden Revisionszeiträume genutzt, um unter anderem Anlagenteile nach dem Stand der Technik zu ertüchtigen und um Nachrüstungsmaßnahmen zur Erhöhung der Betriebssicherheit durchzuführen.
Solche Anlagenteile werden häufig mittels Dübelplatten befestigt. Die Dübel sind in Stahl-betonbauteile einzubringen, die kraftwerkstypisch mit dichter Bewehrung versehen sind. Zerstörungsfreie Untersuchungen zur genauen Ortung bewehrungsfreier Orte sind hierzu erforderlich. Bei dieser vermeintlich einfachen Prüfaufgabe wurde deutlich, dass in der Praxis Bedarf an verbesserter Anwendungssicherheit der zur Verfügung stehenden ZFP-Verfahren besteht, die eng mit der Ausbildung von Prüfern verbunden ist. Dieser Beitrag berichtet über die
bislang im Bereich ZfPBau erstmalig umgesetzte Strategie aus
Verfahrensanweisung und Schulung zur zuverlässigen Ortung von Bewehrung an Stahlbetonbauteilen. Sie kann als Muster für weitere ZfPBau-Schulungen zur zuverlässigen Anwendung der Verfahren dienen.
Bereits seit vielen Jahren stehen Anwendern der zerstörungsfreien Prüfung im Bauwesen (ZfPBau) zuverlässige Geräte für Radar-, Wirbelstrom- und Ultraschallmessungen an Betonbauteilen zur Verfügung. Doch erst die Kombination dieser drei Verfahren an einer Messfläche erlaubt den maximalen Informationsgewinn über die innere Konstruktion von Stahlbeton- und Spannbetonbauteilen. Mit dem OSSCAR-Bauwerkscanner wurden diese drei Verfahren erstmals zur kombinierten
und automatisierten Datenaufnahme an einem Scannerrahmen und unter einer Software vereint. Die Ergebnisse werden bildgebend in frei wählbaren Schnitten dargestellt, was eine Rekonstruktion von Bauteilen erlaubt, für die keine Planunterlagen vorliegen.