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Electrochemical half-cell potential measurements for the detection of reinforcement corrosion
(2014)
This specification describes the application of electrochemical half‐cell potential measurements (frequently also called potential mapping) for the detection of reinforcement corrosion in reinforced concrete structures. Areas of corroding reinforcement steel can be located in a nondestructive manner by means of this procedure. Half‐cell potential measurements are used in order to detect chloride‐induced corrosion. However, it is not recommended in order to assess the risk of carbonation‐induced corrosion. For this purpose the determination of the carbonation depth and the concrete cover appear to be more appropriate.
The content of this specification exclusively refers to the application of mobile, local variable reference electrodes, which are only placed on the concrete surface while measuring. The technique distinguishes itself thereby from the range of corrosion monitoring systems with stationary installed reference electrodes and sensor systems, respectively, whereby it is possible to continuously track measurements within the area of the installed electrodes. However, these methods are not dealt with in this specification.
Fibre-optic sensors need to be more established in the sensor market. Their advantages have unquestionably been verified by numerous demonstrations. However, there are some open questions leading now and then to restraints in the user's community. The paper dicusses examples where fibre-optic sensors provide outstanding knowledge about the structure's behaviour, but application is often challenging. Requirements are listed and open questions to be solved are discussed. Short outlook to standards useful for better design, characterization and application is given. Standards are the basis for establiching SHM systems, especially for safety-critical structural diagnostics.
The combination of different types of sensors to multi-sensor devices offers excellent potential for monitoring applications. This should be demonstrated by means of four different examples of actual developments carried out by Federal Institute for Materials Research and Testing (BAM): monitoring and indoor localization of relief forces, a micro-drone for gas measurement in hazardous scenarios, sensor-enabled radio-frequency identification (RFID) tags for safeguard of dangerous goods, and a multifunctional sensor for spatially resolved under-surface monitoring of gas storage areas. Objective of the presented projects is to increase the personal and technical safety in hazardous scenarios. These examples should point to application specific challenges for the applied components and infrastructure, and it should emphasize the potential of multi-sensor systems and sensor data fusion.
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.
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.
Utilization of coherent phase information in complex wave fields forms the basis of interferometric time series analysis. The concept is known since decades, but until about 15 years ago there have been no practical implementations. Meanwhile seismic interferometry is used in a wide range from investigations of the earth’s deep crust to engineering applications. Focused on monitoring and imaging the MIIC (Monitoring and Imaging based on Interferometric Concepts) project as part of the German GeoTechnologien program has contributed to this development. Special attention was given to the transfer of methodology to different length scales that range from centimeters, in laboratory applications, over geotechnical scales to even kilometers in seismological applications. General purpose methods and open source software was developed, which can be used on all scales. The core of the MIIC software is a Python library organized in different modules for various processing tasks. A graphical user interface facilitates the creation of processing routines by visualizing connections and dependencies of variables and by checking the consistency of data types. Example applications have included carbon sequestration, salt mine and railroad embankment monitoring as well as imaging changes in concrete constructions.
Coda Wave Interferometry (CWI), a method to evaluate subtle changes of elastic wave velocity in a medium, has been proven to be effective to detect small changes or ultrasonic velocity in concrete caused by load, temperature, moisture, damage or other means. While classical CWI is just able to determine velocity changes globally in relatively large areas between and around pairs of transmitters and receivers, several approaches have been proposed to identify the area affected by the changes more precisely. Most of them are based on the calculation of sensitivity kernels for de-correlation of signals measured at a specific state against a reference. Others follow simplified approaches. In a laboratory setup a concrete specimen of 1:5 x 1:5 x 0:5 m3 was compressed at a certain point. Maximum loads of 20 to 100 kN have been applied in 5 to 10 kN steps in various cycles. The specimen is equipped with 18 embedded ultrasonic broadband piezo transceivers (60 kHz central frequency). Ten of these receivers have been connected to a multiplexer and ultrasonic transmitting and receiving equipment in a way that allowed almost continuous two way measurements between all sensor pairs. Even simple ways to evaluate the data (e.g. crosscorrelation between signals at different load states) allowed pinpointing the load center at least approximately. A more detailed data evaluation either using CWI or even more one of the more sophisticated localization algorithms gave “sharper” results in terms of localization and a better correlation between load and velocity change/de-correlation. The results are used in upcoming monitoring systems for concrete structures.
The combination of radio-frequency identification (RFID) tags with different types of sensors offers excellent potential for applications with regard to identification, diagnosis, and monitoring. This should be demonstrated by means of two examples of actual developments carried out by the Federal Institute for Materials Research and Testing (BAM). The Identification and diagnosis of concrete components is a major task in the maintenance of critical infrastructure, for instance concrete bridges with heavy traffic volume. A feasibility study investigates the application of RFID sensor systems for this task. The second example reviews the transportation of dangerous goods. Using modern technologies enables promising possibilities to reduce accidents and to avoid non-conformity with transportation regulations. Project results demonstrate an innovative technical solution for monitoring of dangerous goods transports with RFID sensor systems.
In dem von der Forschungsinitiative Zukunft Bau geförderten Projekt
zur „3D Kartierung von Bauwerksoberflächen“ wurden Verfahren zur Ortung von Rissen und Rissverläufen an historischen Gebäuden und Skulpturen weiterentwickelt und optimiert. Diese neuen Entwicklungen zur effizienten 3D Erfassung, Darstellung und zum Monitoring von Rissen basieren auf der aktiven Thermografie, einem Trackingsystem und Ultraschall.
Systematische Untersuchungen zur Zuverlässigkeit und zu Messgenauigkeiten der Messverfahren im Labor wurden durch den kombinierten Einsatz der Verfahren an Skulpturen aus historischem Beton an der Giebichensteinbrücke in Halle ergänzt.