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Machbarkeitsstudie zur Zustandserfassung von Spannbetonschwellen mit dem Ultraschallverfahren
(2024)
Das niederfrequente Ultraschallverfahren zur zerstörungsfreien Zustandserfassung von Stahlbetonkomponenten findet im Bauwesen umfangreiche Anwendung. Multistatische Arrays sind am Markt verfügbar und ausgelegt für die Prüfung großflächiger Betonstrukturen. In einer Machbarkeitsstudie wird überprüft, ob eine Zustandserfassung an schlanken Spannbetonschwellen möglich ist. B70-Schwellen und B90-Weichenschwellen wurden hierfür im Labor, im Schienennetz und während der Wiederaufbereitung mit dem Ultraschallverfahren untersucht. Fokus ist die Detektion von Sickenrisse, Kopfrissen und Treibrissen im Bereich des Schwellenkopfes.
Die rekonstruierten Ultraschalldaten zeigen für intakt-klassifizierte Schwellen ein reproduzierbares Rückwandecho und weitere Reflexionen an den Schwellenaußenkanten. Die Ausprägung der Amplitude variiert je nach Degradation durch Belastung und Verwitterung. Defekt-klassifizierte Schwellen mit z. B. Kopfrissen oder unterseitigen Abplatzungen unterscheiden sich im Signalbild deutlich durch oberflächennahe Reflexionen bzw. verschobene Rückwandechos. Basierend auf den untersuchten Schwellen zeigt die Machbarkeitsstudie, dass eine zerstörungsfreie Zustandserfassung mit Hilfe des Ultraschallverfahrens grundsätzlich möglich ist.
Moisture testing and mapping
(2023)
Almost all building materials in civil engineering have an open porosity and interact with or are affected by the environmental conditions. Structures might suffer from effects such as moisture adsorption, carbonation, corrosion, penetration of salt ions and chemical substances, etc. In the hygroscopic range, these processes are mostly driven by diffusion. Due to the confinement of small pores ( 1 m), the Knudsen effect reduces the molecular diffusion. This reduction can become more significant in case of temporal changing pore systems because of physisorption of water vapor, carbonation, or chemisorption.
In this study, unstabilised earth blocks and earth masonry are investigated. In a first step, the pore size distribution of the blocks is measured and sorption isotherms are recorded in experiments. Besides the ordinary physisorption, the involved clay minerals undergo swelling or shrinking due to chemisorption. The following two effects must be considered: first, the reduction of the available pore space by the adsorbed water layer. For this, the Hillerborg sorption theory is used, which is a combination of the well-known Brunauer-Emmett-Teller sorption theory and the Kelvin equation. This allows the computation of adsorbed water layers even in curved pore geometries. Second, the variation of the initial pore size distribution due to chemisorption needs to be modelled. Based on these two models, the effective diffusion coefficient can be predicted. For validation, arrays of relative humidity sensors were embedded into a free-standing earth masonry wall. This monitoring was carried out over more than a year to have a broad variety of environmental conditions and was located in Berlin, Germany.
The prediction of the effective diffusion coefficient can also be transferred to other processes and allows the investigation of materials having temporarily changing pore systems. Examples are the carbonation of cementitious materials, alkali silica reaction, calcium leaching of long-lasting structures, etc. This effect becomes most prominent in the meso-pore range and might alter the effective diffusion coefficient by more than 100 %.
Introduction of a monitoring system for Bingham fluids in additive manufacturing with concrete
(2022)
Freeform additive manufacturing of concrete structures is a rising technology in civil engineering with several fascinating advantages. Nonetheless, to ensure reliability and structural integrity, standards and quality control are required in the future to bring this technology into the market. As the concrete is manufactured continuously, continuous quality control of the printing process is also required, i.e. comprehensive process monitoring. At BAM, a test rig will be installed, enabling the printing of concrete structures with a maximum size of 2 m x 1 m x 1 m (l x w x h). Here, process monitoring is the focus of the test rig. In this study, we show the results of the first pump tests, including the measurement of several parameters such as temperature and pressure along the supply system, i.e. from the concrete pump to the printer head.
In general, wind turbines transform the kinetic energy of the wind into electric power. Thereby, the wind turbine blades are facing unsteady loads which are transferred to the hub to generate a rotation of the turbine’s axis. This brief introduction focuses on the aerodynamics of the blades and the corresponding loads. Starting with the basic flow field and loads of an airfoil, terms like stagnation point, boundary layer, Reynolds number, transition, and separation are introduced. For different geometries, lift and drag coefficient curves are discussed. Then, full wings will be considered, including their three-dimensional flow field due to wing tip vortices and crossflows. As a main source of increased loads, unsteady effects are explained in more detail such as gusts, tower passing, earth boundary layer crossing, free stream turbulences, yaw misalignment, etc. At the end, extra loads due to an oscillating free stream are introduced.
The majority of ultrasonic devices used for non-
destructive testing in civil engineering require contact with the
surface of the concrete (specimen), which significantly increases
the time required for the measurement. This makes it impractical
for extensive investigation of large-scale structures such as bridge
decks, foundations, or tunnels.
In a pioneering approach, fluidic oscillators are used as contact
free ultrasonic sources to overcome the aforementioned limi-
tations. These robust and cost-effective actuators require only
pressurised air and are ideally suited for harsh environments. At
a constant supply pressure, they generate a continuous mono-
frequent actuation signal. Further, varying the supply pressure
via a fast pressure regulator was found to generate a frequency
modulated signal which enabled time-of-flight measurement with
an added advantage of increased signal to noise ratio. To
demonstrate the feasibility of this novel idea of non-contact
ultrasound, the results of the initial tests are presented.
Das Bauwesen erfordert sehr hohe Flächenleistungen, so dass der physische Kontakt zur Oberfläche möglichst vermieden werden sollte, um eine hohe Verfahrgeschwindigkeit der Sensoren zu gewährleisten. Andererseits weisen luftgekoppelte, membranbasierte Aktuatoren durch den Impedanzsprung sehr hohe Verluste auf. Daher stellt das schnelle berührungslose Ultraschallmessen immer noch eine ungelöste Herausforderung im Bauwesen dar.