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In this study, an Impulse Response Function analysis of pile response to sine-sweep excitation by a low cost, portable Shaker was used to identify defects in piles. In straightforward impact-echo methods, echoes from the pile toe and defects are visible in the time domain measurements. However, these echoes are not present in the time domain records of piles subjected to sine-sweep excitations, due to interactions between the input and output signals. For this reason, the impulse response function in the time domain has been calculated and is able to identify the echoes from pile impedance changes. The proposed methodology has been evaluated both numerically and experimentally. A one-dimensional pile-soil interaction system was developed, and a finite difference method used to calculate the pile response to sine-sweep excitation. The numerical simulations indicate that impulse response measurements with a synthesized logarithmic, sine-sweep excitation could be an effective tool for detecting defects in piles. The methodology was further tested with field trials on 6 cast in situ concrete test piles including 1 intact pile and 5 defective piles subjected to sine-sweep excitations by a shaker. In 5 of the 6 cases the echoes from the pile toe could be identified in the deconvoluted waveforms—the impulse Response functions. Damage detection is more difficult and dependent on the selection of the optimal regularization parameter. Further research and optimization of the deconvolution process is needed to evaluate the effectiveness compared to standard pile integrity testing methods.
To complement the information provided by deterministic seismic imaging at length scales above a certain resolution limit we present the first application of adjoint envelope tomography (AET) to experimental data. AET uses the full envelopes of seismic records including scattered coda waves to obtain information about the distribution of absorption and small-scale heterogeneity which provide complementary information about the investigated medium. Being below the resolution limit this small-scale structure cannot be resolved by conventional tomography but still affects wave propagation by attenuating ballistic waves and generating scattered waves. Using ultrasound data from embedded sensors in a meter-sized concrete specimen we image the distribution of absorption and heterogeneity expressed by the intrinsic quality factor Q−1 and the fluctuation strength ɛ that characterizes the strength of the heterogeneity. The forward problem is solved by modeling the 2-D multiple nonisotropic scattering in an acoustic medium with spatially variable heterogeneity and attenuation using the Monte-Carlo method. Gradients for the model updates are obtained by convolution with the back-propagated envelope misfit using the adjoint formalism in analogy to full waveform inversion.
We use a late coda time window to invert for absorption and an earlier time window to infer the distribution of heterogeneity. The results successfully locate an area of salt oncrete with increased scattering and concentric anomalies of intrinsic attenuation. The resolution test shows that the recovered anomalies constitute reasonable representations of internal structure of the specimen.
Die immer komplexere Konstruktionsweise von Neubauten einerseits und die alternde Infrastruktur andererseits erfordern in manchen Fällen eine dauerhafte Überwachung, um besondere Einwirkungen und gegebenenfalls Schädigungen rechtzeitig und genau zu erkennen. Hierfür ist eine Vielzahl von Methoden und Sensoren verfügbar. Das Portfolio weist aber insbesondere bei der Detektion langsamer, räumlich begrenzter Veränderungen Lücken auf. Hierfür sind bisher sehr aufwändige Untersuchungen oder Installationen notwendig.
In der vorliegenden Arbeit werden Sensoren und zugehörige Auswertemethoden für die Ultraschalltransmission vorgestellt, die sich für die zerstörungsfreie, dauerhafte Überwachung von Beton eignen. Direkt oder nachträglich in Betonkonstruktionen eingebaut, ermöglichen sie eine nicht nur lokale, sondern größere Raumbereiche umfassende Dauerüberwachung von Änderungen der Materialeigenschaften.
Das Prinzip der Ultraschalltransmission und die verschiedenen Einflussparameter werden vorgestellt. Zu letzteren gehören neben der Belastung und Schädigung auch Umweltparameter wie Temperatur und Feuchte. Verschiedene Methoden zur Datenanalyse, wie z. B. die Codawelleninterferometrie, ermöglichen eine Detektion kleinster Veränderungen. Die in den Beton einzubettenden Ultraschallsensoren werden vorgestellt und ihr Einbau und Betrieb beschrieben. Als Beispiele für Anwendungen werden Frost-Tauwechsel-Experimente im Labor, die Detektion von lokalen Lasten im Technikumsmaßstab und der Einsatz an realen Brücken diskutiert. Die Sensoren sind zum Teil bereits seit mehreren Jahren in Probeobjekte eingebettet und liefern zuverlässig wertvolle Daten.-------------------------------------------------------------------------------------------------------------------------------------------------------------
Monitoring of concrete constructions by embedded ultrasonic sensors:
Challenging new constructions and the ageing infrastructure are increasing the demand for permanent monitoring of loads and damages. Various methods and sensors are used for this purpose. But the technologies available today have difficulties in detecting slowly progressing locally confined damages. Extensive investigations or instrumentations are required so far for this purpose.
In this study we present new sensors and data processing methods for ultrasonic transmission, which can be used for non-destructive permanent monitoring of concrete. They can be mounted during construction or thereafter. Larger volumes can be monitored by a limited number of sensors for changes of material properties.
The principles of ultrasonic transmission and influencing factors are presented. This latter include load, damages as well as environmental parameters as temperature or moisture. Various methods for data processing, e. g. coda wave interferometry are introduced. They allow the detection of very small changes in the medium. The embedded sensors are shown including mounting and operation. Application examples so far include small scale laboratory freeze-thaw experiments, localizing loads in larger concrete models and monitoring load effects on real structures. Some sensors are operating already for several years.
Ein Ultraschallsystem mit großer Apertur, genannt LAUS – Large Aperture UltraSound, wird vorgestellt und die technischen Eigenschaften erläutert. Es besteht aus zwölf Prüfköpfen, die beliebig auf Betonoberflächen mit Unterdruck befestigt und jeweils als Sender oder Empfänger genutzt werden. Alle möglichen Kombinationen ergeben 132 Einzelmessungen, die zur Rekonstruktion des durchschallten Volumens genutzt werden. Das System wurde an zwei Bauwerken erfolgreich erprobt. Die Dicke einer sehr stark bewehrten Fundamentplatte eines Fallturms wurde bestimmt. Die Messungen erfolgten in einem Raster auf einer Fläche von 2,0 × 1,8 m2 und die Rückseite konnte eindeutig abgebildet werden. Weitere Anzeigen aus dem Inneren der Platte zeigen das Potenzial des Systems. An einem Brückenbauwerk wurden indirekt Verdichtungsmängel lokalisiert. Dazu wurden mit dem LAUS-System die im Brückenträger verlaufenden Spannkanäle durch fast 2 m Beton abgebildet.
Ultrasonic echo and transmission techniques are used in civil engineering on a regular basis. New sensors and data processing techniques have lead to many new applications in the structural investigation as well as quality control. But concrete structures in the nuclear sector have special features and parameters, which pose problems for the methods and instrumentation currently available, e.g. extreme thickness, dense reinforcement, steel liners or special materials. Several innovative ultrasonic techniques have been developed to deal with these issues at least partly in lab experiments and pilot studies. Modern imaging techniques as multi-offset SAFT have been used e. g. to map delaminations. Thick concrete walls have successfully been inspected, partly through a steel liner. Embedded ultrasonic sensors have been designed which will be used in monitoring networks of large concrete structures above and below ground. In addition, sensitive mathematical methods as coda wave interferometry have been successfully evaluated to detect subtle changes in material properties. Examples of measurements and data evaluation are presented.
The legend continued: The International Symposium NonDestructive Testing in Civil Engineering (NDT-CE) had returned to Berlin. The Bundesanstalt für Materialforschung und -prüfung (BAM) and the Technical University of Berlin (TUB) hosted this prestigious event in the industrial landmark building Peter-Behrens-Halle and the Test Site Technical Safety (TTS) in Horstwalde in September 2015. Almost 300 participants from more than 35 countries gathered to present recent research, exchange knowledge and to lest the newest equipment in NDT-CE. This symposium, held regularly in various locations all over the world, was organized for the third time in Berlin and is by far the largest of its kind.
This article presents a unique method of installing a special type of embedded ultrasonic transducers inside a 36-m-long section of an old bridge in Germany. A small-scale load test was carried out by a 16 ton truck to study the temperature and load influence on the bridge, as well as the performance of the embedded transducers. Ultrasonic coda wave interferometry technique, which has high sensitivity in detecting subtle changes in a heterogeneous medium, was used for the data evaluation and interpretation. The separation of two main influence factors (load effect and temperature variation) is studied, and future applications of wave velocity variation rate Φ for structural health condition estimation are discussed. As a preliminary research stage, the installation method and the performance of the ultrasonic transducer are recognized. Load- and temperature-induced weak wave velocity variations are successfully detected with a high resolution of 10−4%. The feasibility of the whole system for long-term structural health monitoring is considered, and further research is planned.
Reinforced concrete bridges are iconic parts of modern infrastructure. They are designed for a minimum service life of 100 years. However, environmental factors and/or inappropriate use might cause overload and accelerate the deterioration of bridges. In extreme cases, bridges could collapse when necessary maintenance lacks. Thus, the permanent monitoring for structure health assessment has been proposed, which is the aim of structural health monitoring (SHM). Studies in laboratories have shown that ultrasonic (US) coda wave interferometry (CWI) using diffuse waves has high sensitivity and reliability to detect subtle changes in concrete structures. The creation of micro-cracks might be recognized at an early stage. Moreover, large-volume structures can be monitored with a relatively small number of US transducers. However, it is still a challenge to implement the CWI method in real SHM practical applications in an outdoor environment because of the complex external factors, such as various noise sources that interfere with the recorded signals. In this paper, monitoring data from a 36-m long bridge girder in Gliwice, Poland, instrumented with embedded US transducers, thermistors, and vibrating wire strain gauges, is presented. Noise estimation and reductionmethods are discussed, and the influence of traffic, as well as temperature variation, are studied. As a result, the relative velocity variation of US waves following the temperature change with a very high precision of 10−4% is shown, and a good bridge health condition is inferred. The influence of lightweight real traffic is negligible. The study verified the feasibility of the implementation of the CWI method on real bridge structures.
Detection of Multiple Cracks in Four-Point Bending Tests Using the Coda Wave Interferometry Method
(2020)
The enlargement of the cracks outside the permitted dimension is one of the main causes for the reduction of service life of Reinforced Concrete (RC) structures. Cracks can develop due to many causes such as dynamic or static load. When tensile stress exceeds the tensile strength of RC, cracks appear. Traditional techniques have limitations in early stage damage detection and localisation, especially on large-scale structures. The ultrasonic Coda Wave Interferometry (CWI) method using diffuse waves is one of the most promising methods to detect subtle changes in heterogeneous materials, such as concrete. In this paper, the assessment of the CWI method applied for multiple cracks opening detection on two specimens based on four-point bending test is presented. Both beams were monitored using a limited number of embedded Ultrasonic (US) transducers as well as other transducers and techniques (e.g., Digital Image Correlation (DIC), LVDT sensors, strain gauges, and Fiber Optics Sensor (FOS)). Results show that strain change and crack formation are successfully and efficiently detected by CWI method even earlier than by the other techniques. The CWI technique using embedded US transducers is undoubtedly a feasible, efficient, and promising method for long-term monitoring on real infrastructure.