Caractérisation thermophysique et mécanique de briques de terre stabilisées en vue de l'isolation thermique de bâtiment

Résumé Cet article présente une étude expérimentale portant sur la caractérisation de matériaux locaux utilisés dans l'isolation thermique des batiments. Ces matériaux sont des briques de terre compressées et stabilisées au ciment. La conductivité thermique et la chaleur spécifique de matériaux à base de latérite incorporant de la pouzzolane et de la sciure de bois a été déterminée. Les résultats montrent que les blocs en latérite + pouzzolane ou en latérite + sciure de bois sont meilleurs isolants thermiques que les blocs en latérite simple. Cependant, ces matériaux composites utilisés pour l'enveloppe du batiment doivent avoir des résistances mécaniques suffisantes pour être utilisés dans la construction. La mesure des propriétés mécaniques telles que la résistance à la compression et la résistance à la traction a montré un faible écart entre les résistances des trois types de matériaux étudiés. Les résistances en traction de la latérite et de la latérite + pouzzolane sont voisines et environ deux fois plus élevées que celle de la latérite + sciure de bois. Ces résultats permettent de préciser les conditions d'utilisation optimale de ces matériaux pour l'enveloppe du batiment.

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Thermophysical and mechanical characterization of stabilized clay bricks for building thermal insulation

An experimental study was carried out in order to determine the properties of local materials used for building thermal insulation. Cement stabilised compressed clay bricks were tested. The thermal conductivity and specific heat of laterite based materials incorporating pouzzolane or sawdust was determined. The experimental results showed that pouzzolane + laterite bricks or sawdust + laterite bricks give better thermal insulation than simple laterite bricks. However, these composite materials used for building shielding must present sufficient mechanical strength to the suitable for constructions. The measurement of mechanical properties such as compressive strength and tensile strength showed little difference between the strength of the three studied materials. The tensile strengths of the laterite and the laterite + pouzzolane bricks were similar. They were about two times that of the laterite + sawdust bricks. The obtained thermal and mechanical results permit to specify the optimal use conditions of the tested materials.

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Effects of calcium nitrate and triisopropanolamine on the setting and strength evolution of Portland cement pastes

The purpose of this work is to study the effect of using some admixtures such as calcium nitrate and triisopropanolamine on the setting and hardening process of cement pastes at 20°C temperature. Tests were performed on specimens cast from various mixtures prepared with two types of cements. The results indicate that calcium nitrate alone acts as a setting accelerator, but has relatively little beneficial effect on the long term period development of mechanical resistances. Regardless of the cement type used, triisopropanolamine used alone performed well as a hardening accelerator at all ages. The combined addition of calcium nitrate and triisopropanolamine produced at very early age significant and promising results with respect to both setting and hardening acceleration. Continuous compressive strength increase was observed with time.     

Vibrational coherence of self-organized silver nanocrystals in f.c.c. supra-crystals

Fabrication of devices from inorganic nanocrystals normally requires that they are self-organized into ordered structures. It has now been demonstrated that nanocrystals are able to self-organize in a 'supra'-crystal with a face-centred cubic (f.c.c.) structure. The physical properties of nanocrystals self-organized into compact arrays are quite different from those of both isolated nanocrystals and the bulk phase. The collective optical and magnetic properties of these nanocrystal assemblies are governed mainly by dipolar interactions. Here, we show that nanocrystals vibrate coherently when they are self-organized in f.c.c. supra-crystals. Hence, a phase relation exists between the vibrations of all of the nanocrystals in a supra-crystal. This vibrational coherence can be observed by a substantial change of the quadrupolar low-frequency Raman scattering peak. Although a change in electronic transport properties has previously been observed on self-organization of silver nanocrystals, vibrational coherence represents the first intrinsic property of f.c.c. supra-crystals.     

Calculation of the long-range order parameter from atom probe data

In addition to the known diffraction techniques, field-ion microscopy with atom probe is well established for determining the long-range order parameter. The site occupation probabilities of the chemical species on the different sublattices may be estimated from experimental profiles. However, this evaluation method demands that the superstructure planes be identified unequivocally from the data. This condition is not fulfilled in all cases. We propose a new analytical method for which this condition need not be met.     

Hydration heat kinetics of concrete with silica fume

The objective of this study was to evaluate the influence of silica fume on the hydration heat of concrete. Portland cement was replaced by silica fume in amounts from 10 % to 30 % by mass in concrete with w/(c+sf) ratios varying between 0.25 and 0.45. A superplasticizer was used to maintain a fluid consistency. The heat of hydration was monitored continuously by a semi-adiabatic calorimetric method for 10 days at 20 °C. The calorimetric study indicated that the hydration was modified by the presence of silica fume. In the early stages, the silica fume showed a high activity and accelerated the hydration rate as compared to that of the reference concrete. The fine silica fume particled provided nucleation sites for hydrates growth. Then the pozzolanic activity took over and increased both strength and the hydration heat. A substitution of Portland cement by 10% with silica fume produced greater strength and cumulative heat of hydration as compared to that of the reference concrete.     

Trends in cigarette smoking: the Minnesota Heart Survey, 1980-1982 through 2000-2002.

This paper reports population-based secular trends in smoking prevalence and tobacco exposure among smokers. The Minnesota Heart Survey (MHS) assessed smoking in probability samples in the seven-county Minneapolis-St. Paul metropolitan area. Five surveys were conducted in 1980-1982, 1985-1987, 1990-1992, 1995-1997, and 2000-2002 using similar sampling strategies and consistent protocols. Participants were metropolitan area residents of Minnesota, aged 25-74 years, with the addition of 75-84-year-olds in the last three surveys. In men, age-adjusted self-reported prevalence of current smoking steadily declined from 32.9% in 1980-1982, to 23.0% in 1995-1997, and to 20.6% in 2000-2002. In women, self-reported smoking was 31.8% in 1980-1982, 18.5% in 1995-1997, and 19.5% in the latest survey. Age-adjusted self-reported quantity of cigarettes consumed among smokers declined over the same period. Changes from 1995-1997 to 2000-2002 were not significant. Compared with Whites, Black participants had higher levels of smoking and later onset of the decline in smoking prevalence. A decline in smoking prevalence seems to have leveled off or reversed between the two most recent survey periods (1995-1997 through 2000-2002). Focus on smoking cessation should continue, especially in the subpopulation that smokes more than the majority.     

Kovacic's algorithm and its application to some families of special functions

We apply the Kovacic algorithm to some families of special functions, mainly the hypergeometric one and that of Heun, in order to discuss the existence of closed-form solutions. We begin by giving a slightly modified version of the Kovacic algorithm and a sketch proof.