This paper presents the results of an interlaboratory study of the rheological properties of cement paste and ultrasound gel as reference substance. The goal was to quantify the comparability and reproducibility of measurements of the Bingham parameters yield stress and plastic viscosity when measured on one specific paste composition and one particular ultrasound gel in different laboratories using
1 Problemstellung 1.1 Weiche und fließfähige Betone in der Praxis Seit geraumer Zeit ist ein Trend hin zu weichen bis fließfähigen Betonen zu verzeichnen [1]. Insbesondere unter schwierigen Betoniersituationen (enge Bewehrungslagen, filigrane Bauteilgeometrie, komplizierte Schalung oder große Anzahl an Einbauteilen) ist es vorteilhaft, die verbesserte Verarbeitbarkeit für eine effiziente Herstellung zu nutzen und damit Betonierfehler (Bild 1) zu vermeiden. Der beschriebene Trend wurde erst durch die Weiterentwicklung der Betonchemie, insbesondere die breite Verwendung hochwirksamer PCE-Fließmittel, ermöglicht. Weil mit zunehmender Konsistenz auch die Gefahr der Entmischung ansteigt, stellt die Mischungsstabilität für eine zielsichere Anwendung fließfähiger Betone eine maßgebliche Frischbetoneigenschaft dar, die zwingend sicherzustellen ist. Im Rahmen des Beitrags werden daher die Zusammenhänge von Fließfähigkeit und Mischungsstabilität beschrieben und Konzepte vorgestellt, wie im Entwurfsprozess diese beiden Eigenschaften in Einklang gebracht werden können. Weiterhin werden erste Ansätze zur Prüfung der Mischungsstabilität unter Rütteleinwirkung beschrieben, die am Institut für Baustoffe entwickelt wurden.
Stability of flowable concrete -conception and testingEspecially in difficult moulding situation it is necessary to apply flowable concrete. However, with an increase in flowability the risk of segregation increases. Thus stability gains in importance in case of compaction and has to be considered in design like flowability. In contrast to Self-Compacting Concrete (SCC) neither models and strategies exist to describe and ensure stability in design of concrete nor performance-tests are available to assess stability. The current article describes concepts how stability of concrete can be integrated in mixture design. There for, the different proportions of granular particles (aggregates), flowable component (paste), their properties and interaction under vibration are described. Furthermore refined test methods from SCC are introduced to assess sedimentation of concrete under vibration.
The impact of low-pressure treatment on the crystal structure, morphology, and chemical composition of ettringite, due to their major importance with respect to processability (i.a., drying conditions) and to the analysis of ettringite-containing samples, is examined utilizing X-ray diffraction, thermogravimetric analysis, Raman spectroscopy, and environmental scanning electron microscopy. Synthetic ettringite was treated for various durations (5 min up to 72 h) and at two different levels of low-pressure (4.0 mbar and 60 µbar). Evaluation showed a correlation between the procedural parameters (time and pressure), the chemical composition, and the morphology of ettringite. The experiments reveal that, when exposed to 4 mbar pressure, nearly no changes occur in the ettringite’s morphology, whereas the crystals undergo swelling and slight deformations at very low pressures (60 µbar and 35.3 nbar), which is attributed to the loss of bound water and the partial transformation from ettringite to quicklime, anhydrite, and calcium aluminate. Furthermore, the strongly dehydrated ettringite shows the same morphology.
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