Autogenous shrinkage of concrete: a balance between autogenous swelling and self-desiccation

According to physical analyses, the driving force of autogenous shrinkage of concrete is the change in the capillary pressure induced by self-desiccation in its cement matrix. Self-desiccation is caused by the balance between the absolute volume reduction (chemical shrinkage) and the building up of the capillary network. The aim of this study was to quantify the influence of the cement characteristics on the chain of mechanisms leading from hydration to autogenous deformations. Four parameters were selected: (i) for clinker, the amount of C₃A and free lime and the SO₃/K₂O ratio; (ii) for cement, the fineness. To master the experimental area, 16 cements were prepared at the laboratory from pure raw materials. An important number of characterizing techniques were used in the experimental study. Their choice was based on the important parameters drawn from the physical analysis: setting time, suspension-solid transition, hydration kinetics through isothermal calorimetry and nonevaporable water, chemical shrinkage, evolution of relative humidity, capillary porosity and autogenous shrinkage. Using different techniques allowed to determine the precise mechanism of action of each parameter. Results showed that these mechanisms are generally different, even if their macroscopic consequences may be identical. This point will probably be useful for modeling and determining the industrial keys reducing the autogenous shrinkage. The physical mechanisms involved in autogenous deformations were further understood. In particular, this study shows that initial autogenous shrinkage should be considered as a balance between the self-desiccation and an initial swelling phase. The influence of the four parameters considered on this last phenomenon were also characterized.     

Verres formes dans les systemes Ga2S3La2X3MnS (X = S,O)

Preparation of glasses. Glass forming domains in the ternary systems [Ga₂S₃, (La₂S₃, La₂O₂S or La₂O₃), MnS]. Thermal behaviour. U.V. and visible spectroscopy. E.P.R. spectra. Manganese has an octahedral environment of sulfur.     

Correction of inner filter effect in mirror coating cells for trace level fluorescence measurements

The sensitivity of the spectrofluorometric technique can be improved by a factor of about 3.6 using a mirror coating cell. In the case of a large working range, the nonlinear relationship due to the absorbance of solutions between concentration of the analyte of interest and fluorescence intensity (called inner filter effect) must be corrected. This paper suggests a universal inner filter correction equation based on the physical absorbance phenomenon of a mirror coating cell that only depends on the solution absorbance spectra and the cell parameters. These parameters are determined with rhodamine b standard solutions and a simplex method-based mathematical fitting. The methodology has been successfully applied to the correction of classical and synchronous spectra in absorbent media. The partial least squares (PLS) quantification of a mixture of trace levels (approximately 1-10 microg L(-1)) of six polycyclic aromatic hydrocarbons (PAHs) by synchronous fluorescence is possible, even in an absorbent matrix. This simple method allows extension of the analytical field of fluorescence quantification to a large working range in absorbent solutions.     

Characterization of the granular packing and percolation threshold of reactive powder concrete

Reactive powder concrete (RPC) is a new cement-based material developed through microstructural engineering. RPC is composed of very fine powders: sand, crushed quartz and silica fume, all with particle sizes comprised between 300 and 0.02 μm, and a low water content, W/C<0.20. A very dense matrix is achieved by optimizing the granular packing of these powders. This compactness confers to RPC ultrahigh strength and durability.

The hydration kinetic of the cementitious matrix using electrical conductivity and isothermal calorimetry is presented. A linear relationship between the logarithm of conductivity ln(σ) and the degree of hydration , has been found:

ln(σ)=A_v−B_v

where A_v depends on the nature and the dosage of the different mineral and organic components, and B_v is an invariant related to the granular packing. From 19 different RPC compositions, an average value of 12.1 with a standard deviation of 1.7 were found for B_v. The capillary porosity percolation threshold was also deduced from this relationship. It has been found that when the degree of hydration is equal to 26%, the capillary pore space is discontinuous. This last result is in agreement with the prediction of the NIST microstructural model by Bentz and Garboczi [D., Bentz, E. Garboczi, Percolation of phases in a three-dimensional cement paste microstructural model, Cem Concr Res 21 (2) (1991) 325–344]. A continuous determination of the degree of hydration based on electrical conductivity is proposed.     

Processing and properties of tape-cast alumina/zirconia laminates composites

In the present work, laminar ceramic structures formed by layers of alumina and partially stabilized zirconia were fabricated by water-based tape casting. Rheological, physical and mechanical properties of slurries and laminates were evaluated. The laminates consisted of stacked alumina and zirconia green tapes produced by thermopressing. Pyrolysis was carried out at 450 °C and sintering at 1500 °C. The alumina/zirconia laminates were studied for a better understanding of the formation behavior and crack propagation at the laminate interface. The flexural strength values of laminates depend on the stress state on their surface. The laminates with the highest amount of zirconia layers presented low strength values (6.7 MPa), while the laminates with more alumina layers had a higher strength level (57.7 MPa). This is because these laminates have alumina layers on the surface which are in a state of residual compressive stress.     

A taxonomy of argumentation models used for knowledge representation

Understanding argumentation and its role in human reasoning has been a continuous subject of investigation for scholars from the ancient Greek philosophers to current researchers in philosophy, logic and artificial intelligence. In recent years, argumentation models have been used in different areas such as knowledge representation, explanation, proof elaboration, commonsense reasoning, logic programming, legal reasoning, decision making, and negotiation. However, these models address quite specific needs and there is need for a conceptual framework that would organize and compare existing argumentation-based models and methods. Such a framework would be very useful especially for researchers and practitioners who want to select appropriate argumentation models or techniques to be incorporated in new software systems with argumentation capabilities. In this paper, we propose such a conceptual framework, based on taxonomy of the most important argumentation models, approaches and systems found in the literature. This framework highlights the similarities and differences between these argumentation models. As an illustration of the practical use of this framework, we present a case study which shows how we used this framework to select and enrich an argumentation model in a knowledge acquisition project which aimed at representing argumentative knowledge contained in texts critiquing military courses of action.     

Une structure derivee de celle du spinelle la structure de Fe0.76 Yb2.16 S4

Single crystals of so-called “FeYb₂S₄ spinel” were prepared and their crystal structure was solved. The cubic cell, space group Fd3m, has a lattice constant $\text{a}\text{= 10.69}\text{A}\text{?}$. From X-rays determinations, its content is (Fe_0.76 Yb_2.16 S₄) × 8. Iron is divalent and ytterbium is trivalent. Ytterbium is in two series of octahedral sites, which are partially filled : 0.96 Yb and 0.04 ? on every 16d position, 0.12 Yb and 0.88 ? on every 16c position. Only 0.76 iron atom is in every 8a position of the spinel.     

Temperature‐induced surfactant‐ mediated pre‐concentration of uranium assisted by complexation

Cloud‐point extraction (CPE) was used with lipophilic chelating agent to extract uranium(VI) from aqueous solutions. The methodology used is based on the formation of metal complexes soluble in a micellar phase of a non‐ionic surfactant, Triton X‐114. The metal ions complexes are then extracted into the surfactant‐rich phase at a temperature above the cloud‐point temperature. The influence of surfactant concentration on extraction efficiency was studied and the advantage of adding 8‐hydroxyquinoline (8HQ) as lipophilic chelating agent was evidenced. High extraction efficiency was observed, indicating the feasibility of extracting U(VI) using CPE. This study describes a four‐step process—(1) extraction, (2) thermo‐induced phase splitting, (3) back‐extraction and (4) second phase splitting—for the recovery of uranium from water. In our conditions, the extraction yield is quantitative and the concentration factor obtained is superior to 100. After stripping with a diluted nitric acid solution (pH < 1), the system can be recycled through a new four‐step cycle. Copyright © 2006 Society of Chemical Industry