Size effects in the biaxial tensile-compressive behaviour of concrete: physical mechanisms and modelling

This paper analyses failure mechanisms and apparent size effects in the biaxial tensile-compressive behaviour of concrete. To this end, a probabilistic discrete crack model is used to numerically determine failure surfaces in the tensile-tensile and tensile-compressive loading range, on account of size effects being considered as volume effects. The results (failure mechanisms, crack patterns, volume effects, etc.) are discussed in some detail with respect to the applied loading state. Size effects on the failure surface are quantified in terms of stress-invariant ratios at peak load for 8 loading paths. It is found that size effects decrease with increasing hydrostatic pressure,i.e. when passing from the tensile loading range into the tensile-compressive range. This can be explained by the activation of friction at the crack lips in a stable crack propagation, which regularises mechanical volume effects, and thus apparent size effects.     

Bimodal distribution of angiographic measures of restenosis six months after coronary stent placement

BACKGROUND: Restenosis has been perceived as the tail end of a normal distribution of the response of the vessel to the intervention. However, recent studies have described a bimodal distribution for de novo lesions after percutaneous transluminal coronary angioplasty. This finding suggests that some lesions may be more susceptible for restenosis. Whether this holds true for a wider spectrum of lesions undergoing stent placement is not yet known. The present study analyzes the frequency distribution of angiographic indexes of restenosis 6 months after coronary stent implantation. METHODS AND RESULTS: Quantitative angiographic evaluation was performed in 1084 lesions of 1084 patients before, immediately after, and 6 months after successful Palmaz-Schatz stent placement; this represented 80.4% of patients eligible for follow-up angiography. Principal end points of the analysis were angiographic indexes of restenosis at 6 months. Twenty-two lesions that became totally occluded at follow-up were excluded from most parts of the analysis. Diameter stenosis, minimal luminal diameter (MLD), and lumen loss at 6 months did not follow a normal pattern; the bimodal pattern was demonstrated through deconvolution that yielded two separate normal components delineating two lesion populations, which developed distinctively different degrees of lumen renarrowing. The first and larger subgroup of lesions, which was less prone to restenosis, was centered around a mean value of 27% for diameter stenosis and 2.19 mm for MLD, whereas the second subgroup, with a greater tendency for restenosis, was situated around a mean value of 68% for diameter stenosis and 0.76 mm for MLD. The intersection point between the two theoretical normal distribution components was 53.5% for diameter stenosis and 1.09 mm for MLD at follow-up. CONCLUSIONS: Frequency-distribution curves of angiographic indexes of restenosis after coronary stent placement have a bimodal pattern, suggesting the existence of two distinct populations with different propensity to restenosis. These findings may encourage future efforts for the timely identification of the subset with a higher risk as the target of specific antirestenotic strategies.     

On the use of nanoindentation for cementitious materials

Recent progress in experimental and theoretical nanomechanics opens new venues in materials science for the nano-engineering of cement-based composites. In particular, as new experimental techniques such as nanoindentation provide unprecedented access to micromechanical properties of materials, it becomes possible to identify the mechanical effects of the elementary chemical components of cement-based materials at the scale where physical chemistry meets mechanics, including the properties of the four clinker phases, of portlandite, and of the C-S-H gel. In this paper, we review some recent results obtained by nanoindentation, which reveal that the C-S-H gel exists “mechanically” in two different forms, a lowdensity form and a high-density form, which have different mean stiffness and hardness values and different volume fractions. While the volume fractions of the two phases depend on mix proportions, the mean stiffness and hardness values do not change from one cement-based material to another; instead they are intrinsic properties of the C-S-H gel.

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Résumé Les récents progrès en “nanomécanique”, aussi bien sur le plan théorique qu’expérimental, ouvrent de nouvelles perspectives en science des matériaux pour la nano-ingénierie des composites à base de ciment. Grace à de nouvelles techniques expérimentales telles que la ‘nanoindentation’, qui permet d’avoir un accès sans précédent aux propriétés micromécaniques des matériaux, il devient notamment possible d’identifier les effets mécaniques des composants chimiques élémentaires à l’échelle où la chimie rejoint la mécanique; cela inclut les propriétés des quatre phases de clinkers, de la portlandite et du gel de C-S-H. Dans le présent article, nous analysons quelques résultats récents obtenus par nanoindentation; ces résultats révèlent que le gel de C-S-H existe “mécaniquement” sous deux formes différentes, l’une à faible densité et l’autre à forte densité. La valeur moyenne du module d’élasticité, de la dureté, ainsi que la fraction volumique de ces deux formes sont différentes. Alors que la fraction volumique des deux phases dépend de la formulation du mélange, les valeurs moyennes du module d’élasticité et de la dureté sont identiques d’un composite à l’autre; il s’agit de propriétés intrinsèques du gel de C-S-H.

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Editorial Note Prof. Franz-Josef Ulm is a RILEM Senior Member. He was awarded the 2002 Robert L’Hermite Medal. He is Associate Editor forConcrete Science and Engineering.     

Statistical Indentation Techniques for Hydrated Nanocomposites: Concrete, Bone, and Shale

Concrete, bone and shale have one thing in common: their load-bearing mineral phase is a hydrated nanocomposite. Yet the link between material genesis, microstructure, and mechanical performance for these materials is still an enigma that has deceived many decoding attempts. In this article, we advance statistical indentation analysis techniques that make it possible to assess, in situ , the nanomechanical properties, packing density distributions, and morphology of hydrated nanocomposites. These techniques are applied to identify intrinsic and structural sources of anisotropy of hydrated nanoparticles: calcium–silicate–hydrate (C–S–H), apatite, and clay. It is shown that C–S–H and apatite, the binding phase in, respectively, cement-based materials and bone, are intrinsically isotropic; this is most probably due to a random precipitation and growth process of particles in calcium oversaturated pore solutions, which can also explain the nonnegligible internanoparticle friction. In contrast, the load-bearing clay phase in shale, the sealing formation of most hydrocarbon reservoirs, is found to be intrinsically anisotropic and frictionless. This is indicative of a 'smooth' deposition and compaction history, which, in contrast to mineral growth in confined spaces, minimizes nanoparticle interlocking. In all cases, the nanomechanical behavior is governed by packing density distributions of elementary particles delimitating macroscopic diversity.     

The nano-mechanical signature of Ultra High Performance Concrete by statistical nanoindentation techniques

Advances in engineering the microstructure of cementitious composites have led to the development of fiber reinforced Ultra High Performance Concretes (UHPC). The scope of this paper is twofold, first to characterize the nano-mechanical properties of the phases governing the UHPC microstructure by means of a novel statistical nanoindentation technique; then to upscale those nanoscale properties, by means of continuum micromechanics, to the macroscopic scale of engineering applications. In particular, a combined investigation of nanoindentation, scanning electron microscope (SEM) and X-ray Diffraction (XRD) indicates that the fiber-matrix transition zone is relatively defect free. On this basis, a four-level multiscale model with defect free interfaces allows to accurately determine the composite stiffness from the measured nano-mechanical properties. Besides evidencing the dominant role of high density calcium silicate hydrates and the stiffening effect of residual clinker, the suggested model may become a useful tool for further optimizing cement-based engineered composites.     

Flexible Universal Networks—A New Approach to Telecommunication Services?

GPRS, WLAN integration, Mobile Number Portability and UMTS are some of the new technologies which are foreseen to carry many new service applications. In the near future, it is expected that mobile networks will go beyond connecting people and will connect an increasing number of machines, appliances, consumer devices, sensors and actuators. Also, users will connect through personal area networks and the importance of user profiles and service profiles will grow. Given the current strictness of network design, those services will further increase the complexity of today's networks. The reason is established design principles, which require new, dedicated network elements for new service applications. In addition, the applications contain their own subscriber data, usually stored locally in the application's database (distributed across the network), managed from their own management systems and with their own interfaces to Subscriber Care and Accounting Systems. In current designs, databases like home location register, AAA-Servers, MMS, Number Portability and others keep a significant amount of identical data that have to be kept consistent across all applications. For Network Operators, the effort to plan, to administrate and to introduce new services in those networks increases dramatically. This paper presents a concept to simplify current and future telecommunication networks. The concept contains a number of measures that complement each other but can be implemented independently one from another. In addition, a Telco project is presented (Evolute) where some of the ideas presented in the paper have been implemented.     

Deconvolution am Beispiel von Infrarotspektren verschiedener Milcheiwei?stoffe

Zusammenfassung Die Theorie der Deconvolution von IR-Spektren wird dargestellt and an Milcheiweißspektren veranschaulicht. An einem einfachen Beispiel aus der milchwirtschaftlichen Praxis (Kühlung von Milch) werden die Vorteile der Deconvolution demonstriert. Es wird jedoch ausdrücklich darauf hingewiesen, daß die Deconvolution sinnvoll dosiert eingesetzt werden muß, damit keine unphysikalischen Effekte verstärkt werden, welche zu Fehlinterpretationen führen können.

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Deconvolution of milk protein infra-red spectra

Summary The theory of deconvolution of infra-red spectra is presented and illustrated using milk protein spectra. The advantage of this method is impressively demonstrated and its relevance for dairy farming shown using a simple example, the cooling of milk.

     

Multiple equality sets and post machines

Equality sets of finite sets of homomorphisms are studied as part of formal language theory. Some particular equality sets, called Merge_k(k-COPY), are investigated. These languages are combinatorially difficult, and are full semiAFL generators of the recursively enumerable sets, and are semiAFL generators of the class MULTI-RESET, provided k ? 3. To accomplish this characterization, equality sets are related to multihead and multitape Post machines operating in real time. A Post machine has a one-way input tape and Post tapes as storage tapes, which in the multihead version are scanned from left to right by a write head and several read heads. By simulating Post machines by multiple reset machines, and vice versa, several new characterisations of the class MULTI-RESET are obtained, and it is shown that for multihead and multitape Post machines linear time is no more powerful than real time, and two Post tapes or, alternatively, three heads on one Post tape are as powerful as any finite number of heads or tapes. Finally, some complexity bounds for equality sets and Post machines are discussed.     

The scratch test for strength and fracture toughness determination of oil well cements cured at high temperature and pressure

Recent advances in scratch test analysis provide new ways to relate measured scratch test properties not only to strength properties but fracture properties of materials as well. Herein, we present an application of such tools to oil well cements cured at high temperatures and pressures. We find a concurrent increase of strength and toughness of different oil well cement baseline formulations which we relate to the water-to-binder ratio for a series of cementitious materials prepared with cement and silica flour. The scratch test thus emerges as a self-consistent technique for both cohesive–frictional strength and fracture properties that is highly reproducible, almost non-destructive, and not more sophisticated than classical compression tests, which makes this ‘old’ test highly attractive for performance-based field applications.