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This study evaluates the sensitivity of embedded ultrasonic sensors to detect propagating cracks within concrete. Because of the sensors constant coupling to the medium, highly sensitive data analysis techniques, such as the correlation between signals and their attenuation, are applied to detect changes in the signal due to propagating cracks.
The accuracy of the detected onset of the crack is evaluated with the nondestructive testing methods acoustic emission and Digital Image Correlation. It shows that with embedded ultrasonic sensors crack propagation can be detected before it is visible on the surface of the concrete.
In den letzten Jahren werden vermehrt trocken ankoppelbare Ultraschall-Punktkontaktprüfköpfe erfolgreich für Prüfaufgaben im Bauwesen angewendet. Oft werden mehrere Punktkontaktprüfköpfe in einem Gehäuse zu einer Strahlergruppe kombiniert, um die Schallbündelung zu verbessern oder um einzelne Prüfköpfe nacheinander als Sender und Empfänger zu betreiben und die Messsignale nach der Messung einer SAFT-Auswertung zuzuführen. Alternativ können die einzelnen Prüfköpfe einer solchen Gruppe laufzeitgesteuert angeregt und dadurch schon während der Messung das Prüfkopfschallfeld beeinflusst werden. Für diese Technik sind die Bezeichnungen laufzeitgesteuerte Ultraschall-Gruppenstrahler oder 'Phased Array' gebräuchlich. Mit den präsentierten Untersuchungen sollen die Anwendungsmöglichkeiten der Gruppenstrahlertechnik im Niederfrequenzbereich durch den Einsatz von Punktkontaktprüfköpfen verbessert werden. Dazu werden verschiedene Wellenarten eingesetzt und der Einfluss der Anordnung der Strahlerelemente im Array untersucht.
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.
Monitoring of cracks in historic concrete structures using optical, thermal and acoustical methods
(2015)
Cracks are a major issue in the field of cultural heritage. In order to evaluate the significance of a crack, a long term monitoring of the damaged region is required. However, there is a lack of easy to operate tools for such monitoring measures. Therefore, new or existing methods for other applications have to be optimised for cultural heritage investigation. The paper describes the application of such crack observation methods on a historic concrete sculpture. Beside conventional methods, like mapping by hand and ultrasonic depth profiling, a novel tracking system is presented. Furthermore, the suitability of active thermography for the investigation of cracks was investigated. The results show promising prospects for these non-destructive techniques.
This paper describes an innovation of a phased array using dry contact probes to test concrete components. Based on preceding work on pressure waves, a low-frequency ultrasonic phased array consisting of shear wave probes is used in the experiments. This configuration allows the variation and control of the sound field directivity during the measurement. The measurements are carried out on a concrete test block using a scanning laservibrometer and a scanner system in transmission mode. The laboratory measurements are compared to theoretical calculations to investigate the influence of the probes shape and dimension on the resulting sound field patterns. The comparison of the modelling and the measurements demonstrates the suitability of the point source synthesis as a tool for the optimization of probe array designs. Methods of coded are used to improve the signal to noise ratio.
Welcome to the first symposium on Knowledge Exchange for Young Scientists (KEYS) in Dar es Salaam, Tanzania, 9th – 11th June 2015. This symposium is part of a three phase series organised by BAM, and funded by VolkswagenStiftung. Each symposium will focus on relevant themes in the construction sector and will include adequate training as well as offer an opportunity to participate in informal discussions between key players and young scientists, which will facilitate the fundamentals of knowledge transfer.
Our world of cement and concrete faces tremendous challenges for the future. Without doubt cement-based materials will remain the most widely used construction materials in the world. Concrete can be made available everywhere in the world, and it is more sustainable than other materials. Nevertheless, in order to become even more sustainable, we need to find solutions to reduce CO2 generation for the cement production, to quickly solve problems of housing and infrastructures, and to cope with ever increasing customer needs in terms of performances.
There is no global optimum solution for concrete. Best concrete practices always has to be found locally and regionally based on raw materials, supply chains, and construction needs, but it can be inspired by other regions. In order to tackle the future challenges and build more innovative with concrete, we need to enhance interdisciplinary engineering skills and create networks between future decision makers.
Therefore, the KEYS symposia series offers a platform whereby young African scientists can get together with peers from Germany as well as with experienced academic engineers, industry specialists and scientists from across the globe for better implementation of research analysis in their respective fields of study. This primarily provides the opportunity to obtain further knowledge in the demand for the home-base innovation and growth of the education sector. It will also provide an opportunity to network with their peers and gain relevant and objective knowledge.
The theme for this first symposium is sub-Saharan African standards for cement and concrete research – raw materials, quality control and maintenance of cementitious products. The symposium is divided into 8 sections and includes presentations from 9 international keynote speakers and 22 young scientists, who were selected out of a high number of applicants based on the excellence of their scientific writing.
We would like to thank all participants for their valuable contribution to the symposium. We are blessed to receive tremendous input from the distinguished keynote speakers and we thank them for their attendance. Last but not least, we would like to acknowledge the funding body, VolkwagenStiftung, for their support. We hope you enjoy the symposium and find your engagement valuable with the entire team in sustaining your professional development in the global world of cement and concrete.
Carbon-based conductive coatings are complex composites, consisting of an organic or inorganic binder and conductive carbon components, for application as anodes in impressed current cathodic protection systems of reinforced concrete structures. The electrochemical properties of three coatings at different humidity and in saturated calcium hydroxide solution were studied by electrochemical methods, such as electrochemical impedance measurement, measuring of open circuit potential over time and galvanostatic polarization. The dissolved organic and inorganic carbons in electrolyte solution were quantified by using a photometric method. The structures of the coatings were investigated before and after the electrochemical tests by microscopy and scanning electron microscope/EDX analysis. The results of the electrochemical impedance measurements show that the tested coatings all have a relatively low resistance, which is between 100 and 200 Ω. The binder and the surface porosity influence the degradation behavior of those coatings. Especially the organic binder reacts with the strong alkaline medium under dissolving of organic carbon.
In civil engineering, the corrosion of steel reinforcements in structural elements of concrete bares a risk of stability-reduction, mainly caused by the exposure to chlorides. 3D computed tomography (CT) reveals the inner structure of concrete and allows one to investigate the corrosion with non-destructive testing methods. To carry out such investigations, specimens with a large artificial crack and an embedded steel rebar have been manufactured. 3D CT images of those specimens were acquired in the original state. Subsequently three cycles of electrochemical pre-damaging together with CT imaging were applied. These time series have been evaluated by means of image processing algorithms to segment and quantify the corrosion products. Visualization of the results supports the understanding of how corrosion propagates into cracks and pores. Furthermore, pitting of structural elements can be seen without dismantling. In this work, several image processing and visualization techniques are presented that have turned out to be particularly effective for the visualization and segmentation of corrosion products. Their combination to a workflow for corrosion analysis is the main contribution of this work.
Concrete is the most applied building material in modern times. It is present in all kinds of structures and no other material secms to rcplace concrete as the most selected building material in the near future. The prime advantages are the high performance, the easy production and a facile processability. The sustainability and durability are important requirements to concrete. Even if concrete is significantly less susceptible against factors promoting a damage, compared to other materials, there are still mechanisms affecting it. In particular, high-ways and hydraulic structures made of concrete worldwide show cases of alkali-silica reaction (ASR) damages. This article refers to experience at BAM Federal InstituteforMaterials Research and Testing for several decades to ASR research and darnage assessment. ASR is a severe darnage mechanism that can occur in concrete, when certain conditions related to composition and cxposition arise in a critical combination. The chemical reaction is based on the alkalis and the silica in the concrete and was first identified by Stanton in highway structures in California (Stanton, 1940). Actually three components are needed to initiate ASR: alkali-sensitive siliceous aggregates, alkalis coming from internal (cement or other components) or external sources (de-icing salt or sea water) and water. If all of these components are sufficiently present an alkali-silica gel could be formed (Hobbs, 1988). This gel itself needs additional space when it is formed. Furthermore it is able to swell in the presence of humidity, both processes generating an expansion pressure inside the concrete. As a consequence of intensive expansion, cracks can occur, which significantly lowers thc strength of the material. Parameters like amount and chemical composition of the gel as weil as the pattern and width of cracks can help to classify the ASR darnage (Swamy, 1992). The objective of this article is to demoostrate the microscopic work on ASR affected concrete samples, carried out to assess the damaging process or to evaluate various alkali-sensitive aggregates. It will explain the approach of ASR investigation from the macroscale to microscale, starting briefly with the condition assessment, over treatment and investigations of samples in the laboratory, to the use of different microscopical and analytical techniques. In the main part the characteristic features for identifying ASR in concrete are presented by using microscopic images. Especially the emphasis lies on the role of the aggregate.