|DGZfP-JAHRESTAGUNG 2001 |
|ZfP in Anwendung, Entwicklung und Forschung|
Berlin, 21.-23. Mai 2001 -Berichtsband 75-CD
German title: Fortschritte in der Untersuchung von Ingenieurbauwerken mit dem Impakt-Echo Verfahren
Das Impakt-Echo ist ein akustisches Verfahren, bei dem die Frequenz von Vielfachechos nach einer Impulsanregung gemessen werden. Es wird auf Betonbauteile angewendet, Messziele sind u. a. die Dickenbestimmung einseitig zugänglicher Bauteile, die Lokalisierung von Hüllrohren von Spannkanälen sowie generell die Ortung von Inhomogenitäten. Das Impakt-Echo Verfahren wird üblicherweise als Punkt-Messverfahren eingesetzt und konnte aus diesem Grund seit den ersten Entwicklungen vor 15 Jahren nur mit begrenztem Erfolg im Vergleich zu anderen Messmethoden verwendet werden. Schwierigkeit machten bisher sowohl die Messdatenerfassung als auch die Datenauswertung.
Die Entwicklung und Umsetzung eines automatisierten Systems zur Messdatenerfassung und -auswertung entlang von Messlinien und -flächen in der BAM stellt eine Fortentwicklung des Prüfverfahrens dar. Damit wurde es möglich, Bilder als zweidimensionale Querschnitte der untersuchten Struktur in Echtzeit direkt auf der Baustelle darzustellen. Die Vorteile dieses Verfahrens für den Nutzer hinsichtlich der Dateninterpretation und für den Eigentümer des untersuchten Objektes hinsichtlich anschaulicher Darstellungen werden in dieser Veröffentlichung vorgestellt. Es werden die Ergebnisse von Laboruntersuchungen mit dem Impakt-Echo System zur Analyse der Struktur von Betonbauteilen demonstriert.
Impact echo is an acoustical technique measuring the frequency of wave resonances in the structure produced by a mechanical impact. It is applied on concrete elements with 1-side access with the aim to determine thickness, locate cavities, ducts and inhomogeneities. Traditionally impact-echo is a punctual technique and because of this, only a limited success was registered in comparison to other methods. In particular data collection and interpretation are critical points in impact-echo. The development and application in BAM of an automated system for data collection and visualisation along measurement lines and on areas has signified a significant method progress. It was so possible to plot on-site in real time 2-dimensional data images representing sections of the investigated structure. The advantages of this method for the user in regard to data interpretation and for the owner of the structure in regard to understanding od the NDT results are then obvious. As demonstration example, the results of a laboratory investigation with the scanning impact-echo on a concrete slab with empty ducts are presented.
Applying a mechanical point impact with a small hammer on the object surface, a wave is generated and travels through the material, partly reflected by any internal reflector. The wave is almost completely reflected if the second material is air. A sensor beside the impactor picks up these reflections so that a time waveform is built up by the first and subsequent reflection arrivals. Data analysis is performed on the frequency spectrum of the waveform. The depth (d) of each reflector is calculated by dividing the wave velocity (v) by the measured frequency of the echo signal peak (f): d = v/2f.
An automated scanning impact-echo system has been developed at BAM to to solve some of the problems encountered by applying a punctual method as traditionally done since the development of this technique [1-3]. The BAM system collects 2- and 3-D sets of data (fig. 1) and it uses in-house software for real-time data display. It allows to view 2-D element sections by scanning the concrete surface in discrete regular steps along survey lines. The single spectra can so be plotted in series to obtain 2-dimensional data images. The commercial testing unit containing hammer and receiver (Olson IE II) allows reading frequencies up to circa 20 kHz..
|Fig 1: Principle of the scanning impact-echo method developed at BAM.|
In preparation of a measuring campaign on a real highway infrastructure, Michelsrombach Bridge (fig. 2), and aimed at site-testing a combination of NDT techniques), a specimen of the bridge deck was built in the laboratory with the first aim to locate the post-tensioning ducts (fig. 3 & 4). It has characteristics similar to the real bridge: the 25-cm thick concrete slab has 32-mm max. aggregate size and presents a reinforcement (Æ 12 mm) mat of 40 cm width with 3 cm concrete cover. The three metal ducts (Æ 4-cm and empty at the time of testing) have 6, 10 and 8 cm concrete cover.
Fig 2: View of Michelsrombach bridge.
Fig 4: Sketch of the specimen with position of the measurement lines in red (dimensions in mm).
Fig 3: The scanning system during data collection on the laboratory specimen.
The measurements lines run parallel and transversal to the ducts (fig. 4) and were made up of 1 cm spaced stations. At each point, the average of 3 single readings was recorded. Data presented here are plotted in frequency with depth calculation carried out using a compressional wave velocity of 4000 m/s.
The impactechogram from the measurement line A (fig. 5) is produced at a slab location undisturbed from the presence of the ducts This vertical section through the tested element presents on the horizontal axis the testing positions (where the unit with impactor and receiver is positioned on the concrete surface) and on the vertical axis the frequency or reflection depth. The image shows strong signal reflection (darker grey) at 8 kHz This horizontal black line corresponds to the resonant frequency of the acoustical wave between the top surface (where the wave is first generated) and the underside of the slab, where the wave is reflected back in the direction of the impact point. This wave reflection happens at the interface between concrete and air and allows to calculate the slab thickness (in this case equal to 25 cm) from the measured frequency. It is also known as the thickness frequency. Also visible in the image is a pattern of diagonal lines in two directions. As described in , these are the visible effect of the slab's lateral dimension and its vibration modes. In a 2-dimensional image of this kind, it is possible to visually distinguish these effects and separate them from the thickness information. It is also possible to notice at the thickness frequency that this horizontal line is disturbed by the slab's own vibrations and its intensity and frequency position change slightly in function of the merging of the two frequency peaks or of their subtraction from each other.
|Fig 5: Impactechogram at measurement line A, showing the thickness of the slab.|
The impactechogram collected along line B, across the ducts, (fig. 6) presents at 8 kHz the interrupted reflection from the slab underside. In comparison with fig. 5, this thickness reflection and the pattern of diagonal lines are much attenuated and more difficult to discern. This apparent signal attenuation is the effect of the focusing of the wave at the positions of the empty ducts, whose frequency peaks present now the maximum signal amplitude. The indirect reading of the 3 ducts appears at lower frequencies than the thickness frequency - between 7 and 8 kHz - approximately at horizontal position 15, 55 and 95 respectively. This means that the ducts were not located directly but as an apparent increase of element thickness (longer wave path around the duct). Note the small shift in relative frequency between the 3 empty ducts (7.7, 7.2 and 7.4 kHz).
|Fig 6: Impactechogram at measurement line B, showing the lateral position of the 3 emptx ducts.. These were only indirectly located by a shift of the thickness peak.|
On this concrete specimen, the scanning Impact Echo technique proved able to easily identify the slab thickness with good accuracy and the lateral position of the ducts. Between the avantages of this application, it has to be remembered that the concrete was new thus in likely good testing condition and that the ducts were empty, thus offering a large change of acoustical impedance . Disadvantage of this testing example was the fact that the ducts have small size, Æ 4 cm and present a concrete cover betwee 1.5 and 2.5 times the diameter of the duct. Nevertheless, the scanning Impact Echo, by plotting 2-dimensional element sections, greatly facilitates the interpretation of the IE data. Thanks to the higher density of data collection (station spacing: 1 cm) than for traditional punctual IE, small features, i.e. the ducts, can be detected and visualised..
Other favourable field of application of the scanning impact-echo method are for example the case of complex element geometry. The interpretation of the single frequency spectra would be in this case time consuming and difficult even foran expert user. Instead the BAM software allows to plot already on site, in real time, the impact-echo 2-dimensional images and permits an easier and faster data interpretation.
Furthermore, this type of data visualisation allows to detect the pattern of reflection peaks due to the vibrations of the element. This regular pattern is not visible and not recognisable in the single frequency spectra, where the single peaks are usually not interpreted or interpreted as noise or disturbances.