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811.
812.
The behavior of two series of concrete slabs exposed to sulfate-bearing soils was investigated by a numerical model called STADIUM. In addition to the diffusion of ions and moisture, the model also accounts for the effects of dissolution/precipitation reactions on the transport mechanisms. The simulations yielded by the model were compared to the actual degradation of the slabs after 8 years of exposure. The microstructural alterations of concrete resulting from the penetration of magnesium, chloride and sulfate ions were studied by backscatter mode scanning electron microscope observations and energy-dispersive X-ray analyses. The comparison of both series of data indicates that the model can reliably predict the various features of the microstructural alterations of concrete.
Résumé Le comportement de deux séries de dalles sur sol en béton exposées à des sols chimiquement agressifs a été étudié à l'aide d'un code de calcul numérique appelé STADIUM. Ce modèle permet de décrire le transport couplé de l'eau et des ions dans des matériaux poreux non-saturés en prenant en considération l'influence des réactions chimiques. Les résultats des simultations de la dégradation du béton après huit ans d'exposition à des ions chlore, sulfate et magnésium. Les observations ont été réalisées par microscopie électronique à balayage. Des analyses par dispersion des rayons X ont également été effectuées. Les données démontrent clairement que le modèle perment de prédire avec précision le comportement du béton soumis à différents types d'agression chimique.


Editorial Note Laval University (Canada) is a RILEM Titular Member. Prof. J. Marchand was awarded the 2000 Robert L'Hermite Medal. He is Editor in Chief for Concrete Science and Engineering and Associate Editor for Materials and Structures. He participates in RILEM TC 186-ISA ‘Internal Sulfate attack’.  相似文献   
813.
The failure of the conductor of the 460 kV overhead transmission line located along the crossing of the Paraná River was investigated. As a result of this failure, a blackout took place in the southern states of Brazil in January of 2002, reaching approximately 67 million inhabitants. Investigation of the external aluminium layer of the ACSR conductor near the fracture showed typical static deformation marks and dynamic fretting wear tangential marks, both associated with the presence of Al2O3 debris. Additionally, encrusted silicon particles were observed on the external surface of the strands. The internal Al layer showed elliptical deformation marks also associated with Al2O3 debris. Both Al2O3 and Si particles are efficient abrasive material, which associated with slight relative motion of metallic surfaces (clamp/strand and strand/strand) can promote fretting wear. Two types of fracture surface of the Al strands were identified: 45° and quasi-normal surfaces, the former being the predominant type. Delamination and particle detachment were the mechanisms of superficial degradation observed on the Al strands, indicating that the rupture of the strands occurred under a gross slip fretting regime induced by sub-conductor oscillation. Inspection of the internal surface of as-cast Al–10%Si spacer clamps revealed different stages of intense circumferential wear caused by the preferential cracking and particle detachment of the eutectic constituent of the as-cast microstructure. This intense wear reduces the clamping contact pressure, which allows higher displacement amplitude and leads to the critical fretting regime. Finally, a few suggestions are discussed to minimise the occurrence of future failures.  相似文献   
814.
Experimental data on the pulsed laser radiation confinement in compensated GaAs and ZnSe with deep impurity levels are reported for the laser wavelength λ=1.55 μm and a pulse repetition frequency of up to 100 kHz. It is demonstrated that an increase in the pulse repetition rate is accompanied by a decrease in the energy confinement threshold and by an increase in the radiation attenuation coefficient. These effects are explained by the accumulation of nonequilibrium charge carriers related to a dependence of the recombination time constant on the concentration of free impurity centers.  相似文献   
815.
Spatial distributions of the average and coherent intensities of the electromagnetic field in a stochastic, strongly fluctuating discrete nonlossy medium are studied by experimental and theoretical methods. It is established that, at a certain density of the medium, the average field intensity near the source immersed into this medium can significantly exceed the value in the free space.  相似文献   
816.
The initial steps of the development of prestressed concrete containment (PCC) for nuclear power plant (NPP) with pressurized water reactors (PWR) in the former USSR are analyzed. The constructive and technological decisions, accepted for primary PCC of Novovoronez NPP, such as the positioning of reinforcement elements and seaths in cylindrical wall and dome of the containment, the anchorage of reinforcement element ends, the technological aspects of concrete works, system and technology of a high level of biaxial pressing on a thin-wall structure at large wrapping angles of power reinforcing strands and etc. are observed. Experience won through the construction and operation of the primary PCC served as a basis for development of a new generation of improved unified PCC (IUPCC) for serial NPP, equipped with PWR of capacity of 1000 MW. The IUPCC is actually a cylinder 45 m in diameter and 54-m high covered with a gently sloping spherical dome. Thickness of cylinder wall is 1200 mm and that of dome wall is 1100 mm. The principle novelty of this PCC is the type and positioning of reinforcement strands. The paper describes strand arrangement and their anchorage in IUPCC. In the vertical part of PCC, strands are arranged on a helical-loop scheme and both strand ends are firmly anchored at the ring girder. Each strand is bended at the bottom of the containment. In the dome, strands are grouped on the orthogonal-loop scheme with the anchorage on one side and with bend of loop on the opposite side of the ring girder. To prevent the leakage of gases and to ensure tightness of the IUPCC an inner metal 8-mm liner with special anti-corrosion coating is designed. Monitoring and checking the stress and strain state of IUPCC is possible during the building, testing as well as operating periods. If any defects or decreased prestress of concrete are detected it is possible to tighten or replace the strands. It is noted that the more than 20 IUPCCs are in-service in Russia, Ukraine, and Bulgaria where NPP with PWR of capacity of 1000 MW were constructed.  相似文献   
817.
Coarse filters for shape matching   总被引:3,自引:0,他引:3  
This article describes the coarse shape filters that support the 3D, Internet-based search engine ShapeSifter, which aims to locate parts already in production that have a shape similar to a desired new part. The search engine analyzes the target model's shape characteristics and performs a similarity match on the database's contents. Our main challenge is identifying shape metrics that produce effective characterizations of 3D models for similarity comparison. We focus on using three novel convex hull-based indices to carry out a preliminary coarse filtering of candidates prior to more detailed analysis. We also describe the crucial role played by two databases of benchmark objects. Unlike other shape-matching systems, we chose ShapeSifter's architecture (multistep query), 3D representation (triangulated mesh), and implementation (Java3D) to allow deployment online once it matures  相似文献   
818.
819.
820.
Consideration was given to the decision making procedures based on the fuzzy messages of experts whose preferences on the set of collective decisions can also be fuzzy.  相似文献   
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