ce/papers最新文献

Zementgebundene Baustoffe quellen bei permanenter Wasserlagerung, d. h., sie erfahren eine zeitabhängige Volumenzunahme. Wesentlich für das Quellverhalten zementgebundener Baustoffe ist der volumetrische Anteil an Zementstein. Für Bereiche, in denen Betonbauteile permanent einer Wasserumgebung ausgesetzt sind, wie zum Beispiel bei Gründungen im Grundwasser, kann das Quellverhalten ggf. die Tragwirkung verbessern, da sich Bauteile im Boden infolge des Quelldruckes verspannen. Dieser Beitrag untersucht das Quellverhalten von Zementstein aus CEM I und CEM III/B mit w/z-Werten von 0,3 und 0,5 bei 10 °C und 20 °C betrachtet. Zusätzlich wurden im Rahmen der Untersuchungen Zementsuspensionen verpresst, um Filtrationseffekte abzubilden. Neben dem Quellmaß zu verschiedenen Zeitpunkten (1, 3, 5, 7, 14, 28 und 56 Tage) wurde der Hydratationsgrad und Gehalt an Calciumhydroxid mit Hilfe der Thermogravimetrie und die Porosität mittels Quecksilberdruckporosimetrie als Materialkennwerte bestimmt. Die Untersuchungsmethoden erlauben einen Rückschluss von Porosität und Hydratphasenausbildung auf das Quellverhalten der untersuchten Zementsteine zu ziehen. Bei beiden Zementarten führen niedrigere Lagerungstemperaturen zu höheren Quellmaßen.

Alkali-silica reaction continues to be a challenging durability issue for portland cement-based concrete. While myriad of preventive options is known to reduce the risk of ASR, changes in availability and consistency of materials make either prescriptive or performance-based approaches difficult to develop and then quickly adapt. In general, the research community has supported industry with practical solutions based on empirically derived relationships, mostly from accelerated test methods and to a lesser extent realistic exposure/field structures. It is time to increase the level of science behind our approach. The research team represented in this talk is investigating a new methodology that combines the alkali availability needed to initiate ASR (aggregate specific) with the available alkali from the total cementitious blend. The relationship between reactivity of a supplementary cementitious material and the ASR expansion is also explored. This keynote lecture will: 1) Explore performance-based testing versus prescriptive approaches and why a hybrid approach should be considered ASR prevention; 2) Evaluate the relationship between accelerated laboratory tests, outdoor exposure blocks and field structures; 3) Examine the use of “non-traditional” supplementary cementitious materials and/or chemical admixtures to prevent alkali-silica reaction; 4) Propose future research needs and; 5) Make recommendations for how best to prevent alkali-silica reactivity following the proposed approach.

Shotcrete 3D Printing (SC3DP) applies concrete layer by layer using a wet-spray process. The resulting hardened concrete properties of the applied SC3DP layers (e.g. height, width or mechanical strength) are largely dependent on the selected material and process parameters. In this context, the nozzle geometry is an important influencing parameter. During printing, the velocity of the shotcrete jet is significantly influenced by the nozzle outlet diameter. Therefore, in the present study, the effect of the nozzle outlet diameter (15 - 30 mm) is investigated with regard to the resulting layer homogeneity, i.e. local density and aggregate distribution in the cross-section, and hardened concrete properties, i.e. flexural strength. By analysing the manufactured specimens, an uneven distribution of the aggregate is observed horizontally across the cross-section of the layers. An accumulation of aggregate is present in the core of the layer resulting in a cement paste-rich region in the edge areas. This leads to increased local densities in the core of the specimen. The application of the concrete with small nozzle outlet diameters results in the highest local densities and the highest flexural strength.

To study the effect of the main oxides and the minor components in slags on their reactivity as SCM, various glasses were synthesized to stepwise imitate a commercial slag of average chemical composition. First, a glass was produced from the main oxides CaO, Al₂O₃ and SiO₂. In a second step, the minor components MgO, Fe₂O₃, Na₂O and K₂O were added separately to the main oxide mix. A selection of two synthetic glasses was tested for their compressive strength contribution (up to 90 days) by substituting 20 wt.% of cement. After all testing times, the synthetic slags achieved a strength similar to that of the commercial product. The reactivities determined by heat flow calorimetry (R³ test) correlate with the calculation of NBO/T and the results of ²⁹Si MAS NMR showing that a decreased degree of polymerization enhances the reactivity. Apart from that, FTIR spectroscopy and ²⁷Al MAS NMR indicate a similar structure of the original and the synthetic slags.

Light metal die casting is usually performed using steel molds. However, these lead to a reduced quality of the casting due to the occurrence of metal corrosion on the surface and the incorporation of hydrogen into the casting as a result of required process chemistry. Ultra-high performance concrete based on alkali-activated slag can be used to produce mineral molds for aluminum casting. The use of reusable mineral molds not only enables the production of various thin-walled geometries. The risk of metal corrosion is eliminated and the concrete molds can withstand multiple cycles due to their thermal stability and high strength, making them potentially superior to the already common lost mineral molds.

Rice husk ash (RHA), an agricultural by-product, has been added as supplementary cementitious material (SCM) in concrete mixture to improve the compressive properties of recycled aggregate concrete (RAC) in recent years. This study aimed to optimise the mixture design of RAC considering two variables: the replacement ratio (wt.%) of recycled aggregate (RA) to natural aggregate (NA) with three levels (0%, 50% and 100%) and the replacement ratio (wt.%) of RHA to cement with three levels (0%, 10% and 20%). Compression test was implemented at concrete age of 28 days based on the full factorial experiment. By means of response surface methodology (RSM), the optimised RA replacement ratio and RHA replacement ratio can be calculated with respect to the compressive strength and E-modulus at 28 days, and vice versa the compressive strength and E-modulus at 28 days of RAC containing RHA can also be predicted. According to response surface modelling, the compressive strength reaches the maximum value when the RA replacement ratio is 0% and the RHA replacement ratio is 7%, and the E-modulus would reach the maximum when the RA replacement ratio is 17% and the RHA replacement ratio is 7%. The determination coefficient (R²) and adjusted coefficient (R²_adj) for the compressive strength model are 0.9632 and 0.9544 respectively, and for the E-modulus model are 0.9319 and 0.9157 respectively, showing that the models developed by RSM are relatively well correlated with the experimental results.

The use of textile reinforced concrete (TRC) is particularly suitable for producing sustainable, material-minimised components, as it allows for a significant reduction in the amount of concrete cover required compared to steel reinforced concrete (SRC). Instead of steel, fibres made of glass, aramid or carbon are used as reinforcement, which are processed from rovings into textiles with a polymer or mineral impregnation. Among these, carbon reinforcements have the highest tensile strength and alkali resistance, making them the most durable in concrete and requiring less maintenance over time. An innovative approach is the production of TRC structures by means of extrusion, in which the stiff, fresh concrete is continuously pressed through a shaping mouthpiece, giving the product its final shape. Within the scope of the CRC/Transregio 280, a new mouthpiece was developed that enables the horizontal introduction of stiff, impregnated textiles. The carbon TRC produced in this process showed a textile stress of up to 4,000 MPa. Additionally, solutions are presented that allow the characterization of stiff, fresh concrete and technical limits for shaping fresh, extruded TRC elements. The potential of this new production method is illustrated through the example of a compound component made of extruded TRC elements.