From Switching Arcs to Ball Lightning to Curing Cancer!

Previous modelling of switching has been through calculation of reductions in temperature of the arc at "current zero". "Enthalpy density" as a function of temperature is found to be an important property. New calculations now include an account of non-equilibrium electron density as a function of time through current zero and it is found that electron attachment rates, which are very large for SF6, could be a dominant property. Modelling discharges is having other successes, for example in explaining "ball lightning" observations inside of houses and aircraft, which suddenly appear, usually at glass windows. Discharge modelling suggests these observations might be explained by the production of "singlet delta" metastable molecules of oxygen in electrical discharges in air. If metastable densities are sufficient, electrons can be produced from the detachment of negative ions to produce radiation and explain ball lightning. An exciting new development is that plasmas from electric corona in air have been found to reduce the size of cancer tumours. These excited oxygen molecules have also been proposed as having a role in this remarkable interchange between classical electrical engineering and medical science

The Effect of Accelerator Dosage on Fresh Concrete Properties and on Interlayer Strength in Shotcrete 3D Printing

Recently, the progress in 3D concrete printing has developed enormously. However, for the techniques available, there is still a severe lack of knowledge of the functional interaction of processing technology, concrete rheology and admixture usage. For shotcrete 3D printing technology, we present the eect of accelerator dosages (0%, 2%, 4% and 6%) on fresh concrete properties and on interlayer strength. Therefore, early yield stress development up to 90 min is measured with penetration resistance measurements. Deformation of layers under loading is investigated with digital image correlation and a mechanical testing machine. One point in time (10 min after deposition) is examined to quantify vertical buildability of elements depending on the accelerator dosage. Four dierent interlayer times (0, 2, 5 and 30 min), which occur for the production of small and large elements as well as due to delay during production, are investigated mechanically as well as quantitatively with computed tomography regarding the formation of cold joints. With increased accelerator dosage, an instantaneous increase in early age yield stress and yield stress evolution was observed. An increase in interlayer time leads to a reduced strength. This is mainly attributed to the observed reduced mechanical interlocking eect of the strands. Finally, a model to describe interlayer quality is presented. In the end, advantages as well as limitations of the findings are discussed

Schalentragwerke mit funktionaler Gradierung

Betone für schlanke Schalentragwerke weisen zur Sicherstellung ausreichender Zugfestigkeiten oft einen hohen Stahlfasergehalt auf. Dies ist mit hohen ökologischen und monetären Kosten verbunden. Das Ziel war es daher, die Voraussetzungen für die Herstellung effizienter Schalentragwerke aus funktional fasergradierten Betonfertigteilen zu schaffen.Concrete for slender load-bearing shell structures often has a high steel fibre content to ensure sufficient tensile strength. This is associated with high ecological and financial costs. Thus, the aim of this project was to create the prerequisites for the production of efficient shell structures made of functional fibre-graded precast concrete elements

Additive Fertigung frei geformter Betonbauteile durch selektives Binden mit calciumsilikatbasierten Zementen

Die additive Fertigung erlaubt ein bisher nicht gekanntes Maß an geometrischer Freiheit bei der Gestaltung von Bauteilen. In der Medizin- und Dentaltechnik wird die additive Fertigung beispielsweise eingesetzt, um an die individuelle Anatomie des Patienten angepasste Prothesen und Implantate herzustellen. Im Maschinenwesen ermöglicht sie den Bau von multifunktionalen und formoptimierten Bauteilen und damit eine Senkung des Materialeinsatzes bei gleichzeitiger Steigerung der Leistungsfähigkeit. In der Luft- und Raumfahrttechnik wird sie genutzt, um mittels Topologieoptimierung oder durch das Zusammenfassen von ganzen Baugruppen zu einem einzelnen Bauteil Gewicht zu sparen [1]. (Aus: Motivation]Additive manufacturing allows a previously unknown degree of geometric freedom in the design of components. In medical and dental technology for example, additive manufacturing is used to produce prostheses and implants adapted to the individual anatomy of the patient. In mechanical engineering, it enables the construction of multifunctional and shape-optimized components thus reducing the amount of material used while simultaneously increasing performance. In aerospace engineering, it is used to save weight by means of topology optimization or by combining entire assemblies into a single component [1]. [Off: Motivation

Effect of Different Shear Rates on Particle Microstructure of Cementitious Materials in a Wide Gap Vane-in-cup Rheometer

Rheological properties of cementitious suspensions are affected not only by their mixture composition but also by process-related factors such as shear history. To enable a model-based description, investigations were carried out on the effect of shear history (shear rate variation over time) on the cement paste agglomeration state. Therefore, a Focused Beam Reflectance Measurement (FBRM) system and a wide gap rheometer were coupled to study the relation between shear history and in-situ chord length distribution simultaneously, indicating particle agglomeration. Hence, the effect of average shear rates (resulting from the applied shear profile), as well as shear rate distribution within the gap (local shear rates) on the particle agglomeration state have been investigated. The rheological properties of cement paste were evaluated with the Reiner-Riwlin approach. Furthermore, the agglomeration state of the particles was compared for different average shear rates and local shear rates at various positions of the FBRM probe. The results show that the median chord length increases in all positions when the average shear rate is decreased, indicating increasing particle agglomeration. Moreover, due to variable local shear rates at different FBRM probe positions, different agglomeration states are observed, resulting from two factors, shear rate dependent particle agglomeration and shear-induced particle migration

Effect of Pre-Shear on Agglomeration and Rheological Parameters of Cement Paste

Cementitious pastes are multiphase suspensions that are rheologically characterized by viscosity and yield stress. They tend to flocculate during rest due to attractive interparticle forces, and desagglomerate when shear is induced. The shear history, e.g., mixing energy and time, determines the apparent state of flocculation and accordingly the particle size distribution of the cement in the suspension, which itself affects suspension's plastic viscosity and yield stress. Thus, it is crucial to understand the effect of the mixing procedure of cementitious suspensions before starting rheological measurements. However, the measurement of the in-situ particle agglomeration status is difficult, due to rapidly changing particle network structuration. The focused beam reflectance measurement (FBRM) technique offers an opportunity for the in-situ investigation of the chord length distribution. This enables to detect the state of flocculation of the particles during shear. Cementitious pastes differing in their solid fraction and superplasticizer content were analyzed after various pre-shear histories, i.e., mixing times. Yield stress and viscosity were measured in a parallel-plate-rheometer and related to in-situ measurements of the chord length distribution with the FBRM-probe to characterize the agglomeration status. With increasing mixing time agglomerates were increasingly broken up in dependence of pre-shear: After 300 s of pre-shear the agglomerate sizes decreased by 10 µm to 15 µm compared to a 30 s pre-shear. At the same time dynamic yield stress and viscosity decreased up to 30% until a state of equilibrium was almost reached. The investigations show a correlation between mean chord length and the corresponding rheological parameters affected by the duration of pre-shear