Experimental and Theoretical Study of Earth-Moist Concrete

No abstract

Water layer thickness of silica fines and their effect on the workability of cement pastes

Concrete is used in infrastructure and in buildings. It is composed of granular materials of different sizes and the grading of the composed solid mix covers a wide range. The overall grading of the mix, containing particles from 300 nm to 32 mm, determines the mix properties of the concrete. The properties in fresh state (flow properties and workability) are for instance governed by the particle size distribution (PSD) and the resulting particle packing (PP). One way to further improve the packing is to increase the solid size range, e.g. by including particles with sizes below 300 nm. Possible materials, which are currently available, are limestone and silica fines like silica fume (mS) and nano-silica (nS). This paper addresses the characterization of six different silica fines with respect to their application in cement paste. Given that the fines provide by far the highest percentage of specific surface area in a mix, their packing behavior and water demand is of vital interest for the design of concrete. In the present work, different mixes are compared and analyzed using the mini spread-flow test method. In this way, a deformation coefficient derived by the spread-flow test is confirmed to correlate with the product of computed specific surface area (SSA) based on measured PSD and intrinsic density of the individual silica fines. Similarly, correlations with equal accuracy are found with a computed SSA using the BET method. With the flow experiments of different mixes it is possible to derive an individual deformation coefficient of the silica particles. It is demonstrated that the computed and the BET surface area values have a constant ratio (0.76 to 0.70). Finally, the value of a constant water layer thickness around the powder particles (24.8 nm) is computed for all silica fines at the onset of flowing. This implies the possibility to predict the flow behavior of paste only based on the knowledge of their SSA, either determined by computation or by BET measurements

Photocatalysis applied to concrete products - part 2 : influencing factors and product performance

The second part of this three-part article series addresses the influence of physicochemical parameters on the degradation performance of concrete products containing photocatalytic active TiO2. The influence of process conditions like irradiance, relative humidity, pollutant concentration and flow rate on the degradation mechanism is investigated. Furthermore, a short overview on photo catalytic powders (especially TiO2) and their influence on the degradation of NO are presented. In addition the application of TiO2 coatings and their microstructural analysis is explained

Chloride intrusion and freeze-thaw resistance of self-compacting concrete with two different nano-SiO2

In this study two different types of nano-SiO2 were applied in self-compacting concrete (SCC), both having similar particle size distributions (PSD) but produced in two different processes (pyrogenic and colloidal precipitation). The influence of nano-SiO2 on transport phenomena in SCC was investigated using the accelerated rapid chloride migration test at different ages (28 and 91 days) as well as the long-term diffusion test. The freeze-thaw resistance, expressed by the scaling factor (Sn), was also studied. Additionally, the microstructural characteristics of the hardened concretes were investigated by FEG-SEM and MIP analyses. The obtained results demonstrate that the addition of 3.8% bwoc of nano-SiO2 improves the SCC durability due to the refinement of the microstructure and the reduction in the connectivity of the pores. Additionally, a small difference in the reactivity of both types of applied nano-SiO2 additives was demonstrated

Chloride intrusion and freeze-thaw resistance of self-compacting concrete with two different nano-SiO2

In this study two different types of nano-SiO2 were applied in self-compacting concrete (SCC), both having similar particle size distributions (PSD) but produced in two different processes (pyrogenic and colloidal precipitation). The influence of nano-SiO2 on transport phenomena in SCC was investigated using the accelerated rapid chloride migration test at different ages (28 and 91 days) as well as the long-term diffusion test. The freeze-thaw resistance, expressed by the scaling factor (Sn), was also studied. Additionally, the microstructural characteristics of the hardened concretes were investigated by FEG-SEM and MIP analyses. The obtained results demonstrate that the addition of 3.8% bwoc of nano-SiO2 improves the SCC durability due to the refinement of the microstructure and the reduction in the connectivity of the pores. Additionally, a small difference in the reactivity of both types of applied nano-SiO2 additives was demonstrated

APPLICATION OF STEEL FIBRES IN ALKALI-ACTIVATED MORTARS

Alkali-activated materials are ideal for the repair of concrete structures in harsh environmental conditions due to their high durability in chemically aggressive environments. However, slag-based mortars, in particular, are prone to shrinkage and associated cracks. In this respect, the application of steel fibres is one solution to reduce the formation of shrinkage induced cracks and to improve post cracking behaviour of these mortars. This study investigated the influence of two different types of steel fibres on the tensile properties of two alkali-activated mortars. Direct tensile tests and single fibre pull-outs were performed to analyse the determining failure modes both on macro and micro scale. Mechanical testing was accompanied by non-destructive testing methods such as digital image correlation and acoustic emission for a detailed analysis of the fracture process

Conceptual Modeling Enables Systems Thinking in Sustainable Chemistry and Chemical Engineering

This study aims to equip students with conceptual modeling skills to address compelling 21st-century challenges in chemistry and chemical engineering education. System-based concept mapping is a critical competence for analyzing global, often complex, problems. We examined how conceptual modeling could scaffold practical experimental design, transitioning from problem identification to testable hypotheses. We set up a project in which first-year undergraduates in chemical engineering work in groups of 5–6 students. Their task was to develop concrete hypotheses for assignments that center on finding sustainable solutions for polluted environments. A set of educational roles (i.e., lecturers, tutors, learning assistants, educational specialist, and project coordinator) were implemented to ensure that students could accomplish their main learning outcome; that is, to become familiar with the academic way of thinking and apply critical thinking skills as a team. Interviews were conducted after the project was finished and revealed that, while conceptual modeling helped students to structure their ideas (i.e., to learn how to design research questions, incorporate interventions, and test models), developing hypotheses remains a challenging task. Our findings brought us to the recommendations for teaching conceptual modeling in the curriculum rather than at the project level, allowing students to progressively transition from understanding and applying concept mapping in their first year into creating solutions within the context of solving complex real-world problems in the final year of their bachelor’s degree. The collaborative learning environment and project format employed in this work could spark new ways to teach science that facilitates systems thinking in chemistr