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The retroreflective corner echo is used, for example, in ultrasonic non-destructive testing of metals to find fatigue cracks in tubes or shafts. If the much weaker crack tip signal is additionally detected, the crack length can also be determined. A corner reflection occurs in cases of surface breaking cracks with predominantly perpendicular orientation to the surface. The intensity of the corner reflection depends on the angle of incidence and on the ultrasonic wave mode used. For the reliable detection of vertical surface breaking cracks in metals, transversal waves are commonly used, which propagate at an angle of 37° to 53° to the inspection surface. As shown in this contribution, the wide spread low frequency ultrasonic arrays with dry point contact sources available for ultrasonic testing of concrete also allow to receive corner echoes. These devices generate transversal waves in concrete structures with a large divergence of the sound field. A series of experiments was carried out with such dry point contact arrays on concrete specimens with artificial test defects and controlled induced cracks of different depths. The ultrasonic time-of-flight signals were recorded, exported and reconstructed utilising the SAFT (Synthetic Aperture Focusing Technique) algorithm. The SAFT reconstruction parameters were adjusted to visualize the corner echo indication. As will be shown, with this targeted processing, the reproducible detection of surface breaking cracks in concrete is possible. The retroreflective corner echo can thus be exploited in civil engineering for non-destructive inspection of concrete.
The corner echo is a well-known effect in ultrasonic testing, which allows detection of surface breaking cracks with predominantly perpendicular orientation to the surface as, for example, corrosion cracks in metal pipes or shafts. This echo is formed by two planes, the surface of the crack and the surface which the crack breaks. It can also be classified as a half-skip method, since a reflection of the pulse occurs on the backwall before the reflection at the defect takes place. In combination with the diffraction from the crack tip, the corner echo also allows crack sizing. As shown in this paper, the corner reflection can be used in civil engineering for nondestructive inspection of concrete. Commercially available low frequency ultrasonic arrays with dry point contact sources generate SH transversal waves with sufficient divergence of the sound field in order to detect corner reflections. Ultrasonic line-scans and area-scans were acquired with a linear array on flat concrete specimens, and the data were reconstructed by the Synthetic aperture focusing technique. If the angles and the area of reconstruction are chosen accordingly, the corner echo reflection can be distinguished from other ultrasonic information. The corner echo can thus be used as a method for deciding whether a crack is a partial-depth crack or a full-depth crack and thus for obtaining a statement about crack depth. This paper presents corresponding experimental results obtained on concrete specimens with artificial test defects and cracks induced under controlled conditions.
The method requires two parallel boreholes in the specimen in which two microwave antennae can be moved. The moisture content in the material can be calculated from the microwave intensity transmitted between the two boreholes. Moisture profiles along the boreholes can be obtained by moving the antennae in steps along the length of the boreholes and taking measurements at each step.
The microwave frequencies used in the laboratory measurements ranged from 8 to 16.5 GHz in steps of 0.5 GHz. The diameters of the antennae were between 7 and 9 mm, and of the boreholes between 8 and 12 mm. The microwave method produced measurement uncertainties between 0 and 2% by volume for all the materials studied in this report.
Ultrasound sensors should be embedded into concrete for monitoring concrete properties. These new longitudinal wave sensors with a center frequency of 60 kHz were examined regarding their suitability for ultrasonic measurements in concrete structures in terms of emission characteristics, sensitivity and frequency ränge.
For the measurement of the radiation patterns, the sensors were embedded vertically and horizontally in concrete cylinders. The directivity pattern was measured using a laser vibrometer.
The sensitivity of the sensor was determined in water using different sensors of the same type. It shows changes in the signal amplitude as well as variations in the frequency ränge for different transmitter-receiver combinations.
The attenuation of the concrete affects the achievable resolution of the measurements and thus, the maximum possible spacing of the sensors within a concrete element. Experimental tests helped optimizing the distances with respect to the required resolution and the effort of embedding the sensors. The signal attenuation in the concrete was measured in the frequency ränge of 60 kHz in response to various degrees of reinforcement and grain size. For this purpose, the sensors were cast at different distances in the specimens studied. The recorded Signals were evaluated for their amplitude and frequency spectrum.
The research project "Ultrasonic Net for Concrete Monitoring (UNeCOM)" aims at developing a methodology for an embedded ultrasonic network for the condition assessment of infrastructure constructions. Civil engineering structures made of concrete, which are located in tectonically active regions or undergo special loading conditions, may require continuous monitoring. It is important to assess the condition of the building and its stability to recognise and classify the effect of a seismic event or evolving damage at early stages before failure occurs. Embedded ultrasonic sensors offer the possibility to detect changes in the material and degradation mechanisms from inside the structure in areas which are difficult or impossible to inspect otherwise. In contrast to conventional ultrasonic testing methods, where the concrete surfaces are scanned with ultrasound probes, this new approach uses sensors, which are embedded into concrete, eliminating the effect of variable coupling conditions between sensors and concrete. This method allows an integral detection of changes in the concrete structure, for example due to seismic activities, to detect mechanical impacts, as well as degradation of the material due to overloading. Such methods have great relevance especially for the monitoring of constructions like power plants, bridges, offshore structures and other structures with high technical safety requirements. The sensor network can be controlled remotely through the internet which is also being used for data transfer. The embedded sensor network is designed to monitor structural damage and concrete degradation globally with high sensitivity.
The non-destructive assessment of cracks in concrete is a common task for which non-destructive evaluation solutions have been published. Primarily, these tests have been carried out on artificial cracks that have been created by using notches instead of natural cracks. This study evaluates a procedure designed to create reproducible and controlled cracks in concrete. The procedure is based on using expanding mortar in a series of blind holes. This is done in combination with carefully aligned reinforcement to guide the direction of the crack development. The depth of the crack is also controlled by reinforcement. Crack depth varies statistically in the range of the Maximum aggregate size (16 mm) used for concrete.
Abstract: Nondestructive evaluation (NDE) methods have received growing acceptance in many testing tasks in the assessment of concrete infrastructures. Substantial progress in NDE methods for concrete structures can be achieved by discussing the specifics of each testing scenario. Because of the large variety of testing scenarios, the testing tasks must be isolated into subtasks, which can then be solved by NDE methods. A classification scheme for NDE tasks is described and discussed in this paper. Four major groups have been identified: the construction process, the concrete structure, physical or chemical processes, and material properties. For each of these groups, a number of subtasks are described. Typical parameter ranges and Resolution requirements are illustrated and major influencing factors listed. Reference specimens may be designed to be used for performance evaluation, validation, and certification of NDE methods. The classifications can also be used to draft a research road map that benefits both the owners of infrastructure and the instrument developers.
Das Ziel des Forschungsvorhabens „Zerstörungsfreie Ortung von Gefügestörungen in Betonbodenplatten“ bestand in der Entwicklung eines Untersuchungskonzepts zur sicheren Ortung und Erfassung von Gefügestörungen in einseitig zugänglichen Betonplatten durch den kombinierten Einsatz verschiedener zerstörungsfreier Prüfverfahren. Hierfür wurde die Leistungsfähigkeit der folgenden Messverfahren hinsichtlich ihrer Aussageschärfe, Anwendungsgrenzen und Messgeschwindigkeit bei praxisnahen Randbedingungen untersucht:
- Impakt-Echo, manuelle Impaktauslösung
- Impakt-Echo, automatisierte Impaktauslösung
- Ultraschall-Echo mit multistatischem Array
- Impuls-Thermografie
- Radar
Als Versuchskörper dienten sechs liegend hergestellte Betonplatten der Abmessungen 200/200/30 cm aus zwei verschiedenen Betongüten und mit unterschiedlichem Bewehrungsgehalt (Bewehrungsgrade 0 %, 0 ,9 %, 2 ,6 %), von denen vier Platten auf trockenem und zwei Platten auf ständig nass gehaltenem Sand gelagert waren. Alle sechs Platten enthielten jeweils dieselben künstlich eingebrachten Fehlstellen: vier quadratische Kiesnester in unterschiedlicher Tiefe mit Kantenlängen zwischen 10 und 15 cm sowie drei oberflächennahe Hohlstellen (Ø 4/6/8 cm). Zur Erfassung eines möglichen Einflusses des Betonalters wurden die Messungen 4 Tage, 28 Tage sowie 180 Tage nach Herstellung der Platten durchgeführt.
Um aus den Untersuchungen einen breiten Nutzen für die Praxis zu erzielen, basierten die eingesetzten Messverfahren auf handelsüblichen Geräten, welche jedoch bezüglich ihres Messprinzips und insbesondere der Datenerfassung und -auswertung weiterentwickelt wurden. Die Fehlstellengröße wurde bewusst klein gewählt.
Mit Ausnahme des manuellen Impakt-Echo-Verfahrens, mit welchem unter Verwendung der Originalsoftware des Herstellers keine Fehlstellen nachweisbar waren, zeigten sich alle Messverfahren geeignet, unter verschiedenen Randbedingungen Gefügestörungen zu lokalisieren. Insgesamt wurde jeder Fehlstellentyp von zumindest einem Verfahren bei mindestens zwei Versuchskörpern detektiert. Es konnten jedoch an keiner Betonplatte mit einem einzelnen Messverfahren alle vorhandenen Fehlstellen an allen Messterminen geortet werden.
Am häufigsten wurden die beiden großen oberflächennahen Fehlstellen (Hohlstelle Ø 8 cm, Kiesnest 10 cm) erkannt. Am schwersten zu lokalisieren waren die kleine Hohlstelle mit 4 cm Durchmesser sowie die beiden auf der Unterseite gelegenen Kiesnester.
Die Untersuchungen zeigen, dass für eine erfolgreiche Anwendung zerstörungsfreier Messverfahren in der Praxis die Auswahl des am besten geeigneten Verfahrens bzw. einer Kombination verschiedener Verfahren jeweils auf die vorliegende Problemstellung abgestimmt sein muss.
Für Betonbodenplatten mit hohem Feuchtegehalt (junger Beton) oder starker Bewehrung ist entweder das Ultraschall- oder das Impakt-Echo-Verfahren geeignet. Das Ultraschall-Verfahren wird von hohen Bewehrungsgehalten zwar stärker beeinflusst als das Impakt-Echo-Verfahren, besitzt aber eine bessere Auflösung und ermöglicht auch im oberflächennahen Bereich eine quantitative Bestimmung der Fehlstellentiefe. Das Impakt-Echo-Verfahren kann wegen der niederfrequenten Impulsanregung Fehlstellen erst in größeren Tiefen und mit schlechterer Auflösung nachweisen, ist aber ideal geeignet zur Bestimmung der Plattendicke aufgrund seines signifikanten Rückwandechos. Sowohl das Radar-Verfahren als auch die Impuls-Thermografie können bei hohen Feuchte- und Bewehrungsgehalten nur sehr oberflächennah eingesetzt werden.
Bei trockenem Beton und geringen bis mittleren Bewehrungsgehalten liefert das Radar-Verfahren sehr gute Ergebnisse. In kurzer Zeit ermöglicht es eine flächendeckende Ortung der Tiefenlage und Größe von Inhomogenitäten bei guter Auflösungsgenauigkeit. Vorhandene Bewehrung wird deutlich abgebildet, kann aber darunterliegende Fehlstellen verschatten. Alternativ kann die Impuls-Thermografie für die Ortung oberflächennaher Gefügestörungen eingesetzt werden. Das Verfahren eignet sich insbesondere bei Oberflächen, die nicht direkt zugänglich sind, da zwischen Infrarotkamera und Messobjekt nur Sichtkontakt bestehen muss.
Die Betongüte hatte bei keinem eingesetzten Verfahren einen erkennbaren Einfluss auf die Messergebnisse. Streng genommen ist dies jedoch nur für den Einfluss des Wasserzementwertes, der Zementart und des Zementgehalts bei gleichbleibender Sieblinie und gleicher Kornart gültig. Bei Verwendung von Leichtzuschlag sind abweichende Ergebnisse denkbar.
Automated multi-sensor systems in civil engineering for condition assessment of concrete structures
(2012)
Validation of non-destructive testing methods is necessary to create a common basis where different systems can be compared and their applications and limitations be identified. This can be achieved through comparing the measurements taken by several systems used for a common diagnostic purpose under practical but controlled testing conditions. Well-designed small and large laboratory or field specimens promise such conditions.
The special concrete specimen (LCS) at BAM was constructed for validation purposes, in particular, to be used for evaluating the performance of echo methods. The thickness of the specimen is varying and it contains carefully designed built-in faults, such as voids, honeycombs and tendon ducts with various degrees of grouting defects. Since the geometry and condition of the defects are known, it can be used to compare the performance of radar, ultrasonic, impact-echo. The research was conducted within the Research group FOR384, sponsored by the German Research Society DFG.