Numerical Study of Scale Effects on the Stiffness Modulus of Rock Masses

The purpose of this paper is to contribute to the study of scale effects on the stiffness modulus of discontinuous rock masses from a mechanical point of view. The study has been conducted on the basis of the observation that the length and density of the natural discontinuities (cracks) in rock masses increase with the volume of the mass. Compressive tests on specimens containing open and closed flat cracks have been simulated through a numerical procedure that is based on the displacement discontinuity method and the fictitious stress method. Cracks have been generated, through a random process, with dips in the 0°–180° range. Both open and closed cracks have been considered. The numerical results obtained for specimens containing open cracks are found to approach the values obtained through an analytical solution. Then, the numerical method has been applied to the study of scale effects on the stiffness modulus of specimens with closed cracks. The numerical results show a decrease in the stiffness modulus with an increase in the size of the rock volume.     

Analysis of volatile components by direct injection of real-life samples by using a programmed-temperature vaporizer

A Chromatographic method is presented which allows the analysis of the volatile components of real-life samples without requiring the previous isolation and concentration of the compounds of interest. A programmed-temperature vaporizer (PTV) is used to enable the direct injection of the sample. A comparison of the proposed method with one involving a liquid-liquid extraction of the sample is also included.     

Kinetics of the curing reaction of unsaturated polyester resins catalyzed with new initiators and a promoter

In this article, the curing of unsaturated polyester resins catalyzed with a promoter [cobalt(II) octoate] and free‐radical initiators is presented. The new initiators were formed by the oxidation process of ethyl methyl ketone or cyclohexanone with hydrogen peroxide and the mixture of solvents containing hydroxyl groups. As a reference, a typical curing system containing ethyl methyl ketone hydroperoxide (Luperox) and the promoter was used. The differential scanning calorimetry runs were performed at different heating rates. The experimental data were fitted with the empirical kinetic model. First, the kinetic parameters (activation energy, frequency factor, and reaction order) were obtained with a single reactive process and with the nth‐order reaction f(α), the nth‐order reaction f(α) with autocatalysis, and the first‐order reaction f(α) with autocatalysis. Second, two or three different reactive processes with the nth‐order reaction f(α) for each step were used. The obtained values of the activation energies for the curing of the unsaturated polyester resins with the free radical initiator–cobalt(II) salt catalytic system were in the range 40–60 kJ/mol for the polymerization initiated by the redox decomposition of the initiators and 80–90 kJ/mol for the polymerization initiated by the thermal decomposition of the initiators at high temperatures. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1870–1876, 2006     

Solar light induced removal of arsenic from contaminated groundwater: the interplay of solar energy and chemical variables

The removal of arsenic by solar oxidation in individual units (SORAS) is currently being explored as a possible economic and simple technology to treat groundwater in Bangladesh and India. Hydroarsenicism affects also large regions of America, especially Argentina, Chile, Mexico and Peru. In this paper, the efficiency of arsenic removal by solar oxidation coupled with precipitation of iron (hydr)oxide, was assessed under various experimental conditions, both on samples of synthetic water and of groundwater of the province of Tucumán (Argentina). The results demonstrate that the underlying chemistry is very complex, and the efficiency is affected often in unpredictable ways by changes in the chemical matrix, or by changes in the operative conditions. Oxides generated from ferrous salts are more efficient than solids formed by hydrolysis of Fe(III); alkalinity contents (bicarbonate) is also important to permit the adequate precipitation. Addition of small amounts of citric acid (lemon juice) is beneficial, but at larger concentrations the effect is negative, probably because of interference in the formation of the solid. The effect of solar irradiation is variable, depending on the other experimental conditions. Although it is possible to remove As partially without solar irradiation under certain special conditions, a procedure versatile enough to cope with waters of different compositions must be based in the use of solar energy. Light plays the role of accelerating the oxidation of As(III) to As(V), and also affects the nature of the solid and, hence, its sorptive properties. The rationale of the effect of light is therefore appreciably more complex than in the case of heterogeneous photocatalysis with TiO₂.     

Induced expression of bacterial β-glucosidase activity in saccharomyces

The bglA gene which encodes a β-glucosidase from Bacillus polymyxa, has been expressed in Saccharomyces cerevisiae under control of the yeast CYC-GAL promoter. Strains have been constructed which carry the gene in different locations: in a multicopy plasmid, a single integration at the URA3 locus, or multiple integrations at the RDN1 locus. Integrative transformation at RDN1 yielded genetically stable clones with a high level of β-glucosidase activity. Coordinated overexpression of the GAL4 inducer protein further increased the level of enzyme activity, although eventually caused the lysis of the cultures. Diploid, triploid and tetraploid strains derived from the transformants with multiple integrations were constructed and expression of β-glucosidase activity in different conditions of growth was assayed. While per-cell activity increased with ploidy, specific activity was about the same in strains of equivalent genotype regardless of ploidy. Genetically stable and regulated expression in Saccharomyces of β-glucosidase activity is interesting for the development of strains able to ferment β-glycosidic sugars (i.e. cellobiose and lactose). From another point of view, the bglA product proved to be a convenient reporter enzyme to monitor heterologous gene expression.     

Combined effects of modified atmosphere packaging and lauric acid on the stability of pre-cooked fish products during refrigerated storage

The effects of lauric acid on pre-cooked products derived from minced cod or shrimp tails stored in the refrigerated state were studied. Lauric acid was employed either with or without modified atmosphere packaging (MAP:50% CO₂:50% N₂). Synergism between lauric acid and MAP on bacterial growth kinetics was found in pre-cooked minced fish products but not in pre-cooked shrimp tail products. Sensory properties of both products received the highest scores when treated with lauric acid and packed under a modified atmosphere. Only samples subjected to combined treatment were commercially acceptable after 1 month of storage.     

Microcutting culture and morpho-physiological changes during acclimation in two Lycium chilense cytotypes.

Lycium chilense, a deciduous perennial shrub, is one of the endangered native species of Patagonia due to sheep overgrazing. Chances of recolonization by seeds are scarce due to the limited density of propagules in the soil and very specific requirements for germination. The objective was to develop an in vitro propagation protocol that would help to perform reestablishment of this species in degraded areas of the Patagonian steppe. Seeds came from two provenances with different somatic number due to differences in ploidy level. Defoliated microcuttings were planted in test tubes with different growing media and taken to a growth chamber. Rooting percentage did not differ between origins, but higher values were encountered for medium without hormones. Subcultures increased significantly rooting percentage and reduced time to rooting. The leaves from micropropagated plants were thinner, did not exhibit hairs, and had poorly developed palisade parenchyma and less epicuticular waxes. In vitro leaves had lower stomatal density and their stomata were less functional when compared to acclimated leaves. A repopulation program of Lycium chilense based on microcutting culture, specially using subcultures, is feasible.     

Hydrosilylation crosslinking of silicone rubber catalyzed by bis (1,5-cyclooctadiene) di-μ,μ′-chlorodirhodium

Hydrosilylation vulcanization of addition silicone rubber catalyzed by bis (1,5-cyclooctadiene) di-μ,μ′-chlorodirhodium at 150° was studied. The course of vulcanization, rubber network density, and mechanical properties of the vulcanizate are discussed with respect to the structure and molecular weight of the reactive groups of telechelic and internal vinylmethylpolysiloxanes and crosslinking methyl (hydrogen) dimethyl siloxane copolymers. Molecular weights of the polysiloxane components as well as the ration of unsaturated and Si—H groups were optimized. The results are in agreement with the theory of rubber elasticity of polymer networks and show wide application possibilities of the catalyst used.     

Evaluating the effect of year of seroconversion on HIV progression in cohort studies. Italian Seroconversion Study

OBJECTIVES: To show how a spurious association between the calendar year of seroconversion and HIV progression arises as a result of censoring the follow-up of individuals at their last visit, when the individuals' visits are intermittent. DESIGN: A notional cohort of 1140 seroconverters and a cohort study of 1270 HIV-infected individuals seroconverted between 1985 and 1994, and followed up to December 1995 (the Italian Seroconversion Study cohort). METHODS: Failure times and rate of the patients attending the clinic over the study period were simulated for the notional cohort. Three separate scenarios with different probabilities of making a visit were considered. Standard survival analysis techniques were used to assess the effect of the year of seroconversion on HIV progression. The progression to a CD4 cell count of 200 x 10(6)/l according to the calendar year of seroconversion in the Italian Seroconversion Study was assessed using different censoring strategies. RESULTS: A spurious effect of the year of seroconversion consistently appeared in 100 repeated simulations. When ignoring the visits occurring after the first year of follow-up in the Italian Seroconversion Study cohort, results supported the hypothesis of no effect of the year from seroconversion. CONCLUSIONS: The choice of the censoring strategy is crucial when assessing the effect of year of seroconversion using survival analysis in cohort studies with intermittent visit structure. Different censoring strategies should be considered before firmly concluding that more virulent strains or the use of treatment are modifying the natural history of HIV disease from cohort studies of this nature.     

Complex structure and catalytic hydrogen ion reduction in Ni (II)-glutathione system

Polarographic studies on the reduction of hydrogen ions in the presence of Ni(II)-glutathione complexes revealed for alkaline solutions two catalytic waves. The catalytic reduction of hydrogen occurs only in the presence of square planar complexes which seem to be responsible for the catalysis. Besides the planar structure the metal-sulphur bond is necessary for the latter process.