Ségrégation externe des bétons frais: Modèle expérimental et influence des principaus facteurs

Résumé Un modéle expérimental de ségrégation externe a sété mis au point; il consiste à faire suivre à un seul granulat noyé dans une pate de ciment un trajet rectiligne à vitesse constante. On mesure simultanément la force nécessaire pour produire ce mouvement, en particulier au moment où le granulat s’extrait de la pate. Pour des pates à concentration en ciment usuelles (γ>0,530) l’extraction du granulat s’effectue par création d’une colonne de pate adhérente à ce dernier, la rupture intervenant toujours par traction dans la colonne elle-même. La force nécessaire pour extraire un granulat de la pate (force d’extraction ΔF_ex) s’exprime simplement par la relation: ΔF_ex = mSσ_t, avec: S: aire de la section diamétrale du granulat: σ_t: résistance à la traction de la pate de ciment fraiche: m: coefficient de conraction indépendant de la concentration en solide de la pate et de la dimension du granulat.

---

Summary An experimental model of external segregation has been devised. It consists of making a single aggregate grain embedded in cement paste follow a rectilinear path at constant rate. Simultaneous measurement is taken of the force necessary to produce this movement, particularly at the moment when the grain energy emerges from the paste. For paste with normal cement content (ψ>0.530) the extraction of the grain is activated by the formation of a column of paste bonded to the grain, rupture always being caused by tension in the column itself. The force necessary to extract a grain from the paste (extraction force ΔF_ex) is simply expressed by the equation: ΔF_ex = mSσ_t, with: S: area of the diametral section of the grain; σ_t: tensile strength of fresh cement paste; m: contraction coefficient independent of the solid concentration of the paste and of the grain size.

     

Isolated plantar venous thrombosis. Report of a case

We report the case of a patient with isolated plantar thrombophlebitis as a post operative complication of saphenectomy. Risk factors such as prolonged bed rest, perioperative inflammation and surgery of the greater saphenous vein itself should be considered. Moreover multiple episodes of superficial venous thrombosis had already occurred as complication of the superficial venous insufficiency. Literature on this unusual outcome is lacking. Ultrasound imaging revealed this superficial thrombosis. Usual echographic signs (non compressible vein, hypoechogenicity of the vessel lumen) can be found. The main problem is to differentiate veins from adjacent tendinous structures. Slow mobilisation of the toes and comparative analysis on contralateral foot are helpful. Thrombosis of the plantar veins must be considered as a possible diagnosis of unexplained plantar unilateral pains. Development of ultrasonic investigations and knowledge of its occurrence could further improve its diagnosis.     

Influence of the solvent on the conformations of poly(ethylene oxide) chains grafted on silica

Electron spin resonance (e.s.r.) of labelled grafted poly (ethylene oxide) has been used to estimate (as a function of temperature) the ratio between the populations of free-end segments in solution to those adsorbed onto the silica surface. The influence of the solvents such as C₆H₆, CHCl₃, C₄H₈O₂, C₆F₆ and CCl₄ has also been investigated. Better solvents produce greater swelling of the molecules, with more of the grafted layer extending into the solution. But solvent-surface and polymer-surface interactions must be taken into account to explain the results.     

Three-dimensional modelling of selective dissolution and passivation of iron-chromium alloys

A three-dimensional model has been developed for modelling the selective dissolution and passivation of alloys. The model has been used to simulate the passivation of iron-chromium alloys. The real structure of the alloy is taken into account (bcc in the present case), as well as the structure of the initial surface. The passivation is modelled in considering the formation of “oxide” nuclei, resulting from the presence of local chromium-rich clusters. During the dynamic evolution of the model, based on the Monte Carlo method, surface diffusion and dissolution of atoms occur according to probabilities dependent on the nature of the atom (Cr or Fe) and on its chemical environment. The conditions of simulation can be changed through a set of parameters defining the rules for surface diffusion, selective dissolution and number of Cr atoms in the Cr clusters required to initiate locally the passivation. The effects of these parameters on the simulation have been tested for an alloy containing 22 at.% Cr and compared with experimental data. The results show that the diffusion of Fe has little influence on the course of passivation while the diffusion of Cr has a marked effect. When the number of surface chromium atoms required to form a nucleus of passive film increases, the passivation becomes less rapid, with a marked effect on the composition of the passivated layer. The extent of the chromium enrichment in the passivated surface obtained in the model for the initial stages of passivation is not as high as the one measured experimentally in the stationary state of passivity.Other simulations have then been performed with various chromium contents in the alloy. The results show the existence of a transition, which is not sharp but progressive, between alloys that cannot be passivated to alloys that are passivated.     

Decentralising a service-oriented architecture

Service-oriented computing is becoming an increasingly popular paradigm for modelling and building distributed systems in open and heterogeneous environments. However, proposed service-oriented architectures are typically based on centralised components, such as service registries or service brokers, that introduce reliability, management, and performance issues. This paper describes an approach to fully decentralise a service-oriented architecture using a self-organising peer-to-peer network maintained by service providers and consumers. The design is based on a gradient peer-to-peer topology, which allows the system to replicate a service registry using a limited number of the most stable and best performing peers. The paper evaluates the proposed approach through extensive simulation experiments and shows that the decentralised registry and the underlying peer-to-peer infrastructure scale to a large number of peers and can successfully manage high peer churn rates.     

Influence of initial microstructuring of the cement paste in fresh mortar on early hydration-strength binding

This paper concerns the observation, interpretation and modelling of changes in mortar strength at the very beginning of its lifetime in relation to the advance of hydration, taking a special interest in the nature of the cement and that of the superplasticizer. A set of values was defined and determined experimentally in order to quantify the following:

Acute bacterial pneumonia in rats increases alveolar epithelial fluid clearance by a tumor necrosis factor-alpha-dependent mechanism

To study the rate and regulation of alveolar fluid clearance in acute pneumonia, we created a model of Pseudomonas aeruginosa pneumonia in rats. To measure alveolar liquid and protein clearance, we instilled into the airspaces a 5% bovine albumin solution with 1.5 microCi of 125I-human albumin, 24 h after intratracheal instillation of bacteria. The concentration of unlabeled and labeled protein in the distal airspaces over 1 h was used as an index of net alveolar fluid clearance. Since there was histologic evidence of alveolar epithelial injury, several methods were used to measure alveolar fluid clearance, including the use of experiments in rats with blood flow and the use of experiments in rats without blood flow, so that movement across the epithelial barrier would be minimized in the latter group. The results with each method were identical. We found that P. aeruginosa pneumonia increased alveolar liquid clearance over 1 h by 48% in studies with blood flow, and by 43% in rats without blood flow, compared with respective controls (P < 0.05). In both studies, this increase was inhibited with amiloride. However, propranolol had no inhibitory effect, thus ruling out a catecholamine-dependent mechanism to explain the increase in alveolar fluid clearance. An antitumor necrosis factor-alpha neutralizing antibody, instilled into the lung 5 min before bacteria, prevented the increase in alveolar liquid clearance in rats with pneumonia (P < 0.05). Also, TNFalpha (5 microg) instilled in normal rats increased alveolar liquid clearance by 43% over 1 h compared with control rats (P < 0.05). In normal rats instilled with TNFalpha, propranolol had no inhibitory effect. In conclusion, gram-negative pneumonia markedly upregulates net alveolar epithelial fluid clearance, in part by a TNFalpha-dependent mechanism. This finding provides a novel mechanism for the upregulation of alveolar epithelial sodium and fluid transport from the distal airspaces of the lung.     

Presence of UDP-N-acetylmuramyl-hexapeptides and -heptapeptides in enterococci and staphylococci after treatment with ramoplanin, tunicamycin, or vancomycin

Analyses of the peptidoglycan nucleotide precursor contents of enterococci and staphylococci treated with ramoplanin, tunicamycin, or vancomycin were carried out by high-pressure liquid chromatography coupled with mass spectrometry (MS). In all cases, a sharp increase in the UDP-N-actetylmuramoyl-pentapeptide or -pentadepsipeptide pool was observed. Concomitantly, new peptidoglycan nucleotide peptides of higher molecular masses with hexa- or heptapeptide moieties were identified: UDP-MurNAc-pentapeptide-Asp or pentadepsipeptide-Asp in enterococci and UDP-MurNAc-pentapeptide-Gly or -Ala and UDP-MurNAc-pentapeptide-Gly-Gly or -Ala-Gly in staphylococci. These new compounds are derivatives of normal UDP-MurNAc-pentapeptide or -pentadepsipeptide precursors with the extra amino acid(s) linked to the lysine epsilon-amino group as established by various analytical procedures (MS, MS-MS fragmentation, chemical analysis, and digestion with R39 D,D carboxypeptidase). Except for tunicamycin-treated cells, it was not possible to ascertain whether these unusual nucleotides were formed by direct addition of the amino acids to UDP-MurNAc-pentapeptide (or -pentadepsipeptide) or whether they arose by reverse reactions from lipid I intermediates to which the amino acids had been added.