• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 63
  • 41
  • 18
  • 15
  • 13
  • 6
  • 2
  • 2
  • 1
  • 1
  • Tagged with
  • 197
  • 197
  • 78
  • 35
  • 35
  • 29
  • 26
  • 24
  • 17
  • 16
  • 16
  • 15
  • 14
  • 14
  • 13
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
11

Reconstruction of Paleoclimate Time-Series in the Peace-Athabasca Delta, Northern Alberta, from Stable Isotopes in Tree-Rings

Bailey, Joscelyn Nesto-Leigh 01 July 2008 (has links)
The isotopic labelling of carbon in tree-rings varies as a function of growth season temperature and relative humidity. The isotopic labelling of oxygen in tree-rings varies as a function of source-water isotopic composition and humidity-dependent evaporative enrichment of leaf water during the growth season. The season of carbon-isotope labelling was identified statistically as late-spring to early-fall (April to October) for temperatures and relative humidity with a three-year weighted (50-30-20) carry-over due mainly to stored photosynthates. The season of oxygen-isotope labelling was identified statistically as a combination of a winter (pDecember to March) source-water signal (temperature-dependent precipitation isotope composition) with a late-spring to early-fall (April to October) humidity signal (evaporative enrichment of leaf water). A two-year carry-over was attributed to the residence time of soil water, but no notable photosynthate carry-over was identified. Carbon- and oxygen- (mechanistic) isotope response surface models were then compared and contrasted to regression-based bivariate and univariate models. It was found that in most cases the isotope response surface models were the best means of predicting isotopic labelling when environmental data are known. The carbon-isotope response surface was used to reconstruct 50-years (AD 1900-1954) of relative humidity data by introducing measured carbon isotope values and instrumental growth season temperature. During the analysis of the oxygen-isotope response surface we found an isotope-temperature relation that appears to reflect circulation-dependent damping. To verify this we introduced scaled values of the North Pacific index as a proxy for this suppression. The coupling of the isotope response surfaces generated a humidity reconstruction that is also thought to be driven by atmospheric circulation. Our reconstruction shows that the fluctuations in temperature range have not exceeded the natural variability in the instrumental record of the 20th century; however, the atmospheric moisture (humidity) reconstruction predicts a directional drying trend in the Peace-Athabasca Delta that appears to reflect increasingly zonal circulation in western Canada over this period.
12

The frequency of tropopause-level thick and thin cirrus clouds as observed by CALIPSO and the relationship to relative humidity and outgoing longwave radiation

Cardona, Allison Leanne 10 October 2008 (has links)
Thin cirrus clouds play an important radiative role in the earth's atmosphere and climate system, yet are one of the least understood components of the climate system. With the use of data from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), thin cirrus and thick cloud distributions in the tropics are analyzed at 121, 100, and 82 hPa. Observations obtained between December 2006 and November 2007 show that thin cirrus between 30°N and 30°S occur in close proximity to regions of intense convection and are positively correlated with low values of outgoing longwave radiation (OLR). In conjunction with the CALIPSO data, water vapor data from the Earth Observing System (EOS) Microwave Limb Sounder (MLS), OLR data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at http://www.cdc.noaa.gov/, and linearly interpolated NCEP reanalysis temperature data were used. These data were used to examine how thick and thin cirrus cloud fractions at 121-hPa and 100-hPa are related to relative humidity with respect to ice (RHI), temperature, and OLR. Our observations show that both RHI and convection play important roles in the development and maintenance of thick and thin cirrus clouds at the pressure levels of interest. The highest fractions of clouds are almost always seen within OLR values representative of convection and at relatively high values of RHI. However, when peaks in cloud fraction are found above the convective threshold, higher RHI values are needed than are needed when convection is responsible for the formation and maintenance of these clouds.
13

Dynamic subdivided relative humidity model of a polymer electrolyte membrane fuel cell

Headley, Alexander John 19 November 2013 (has links)
The development of a control-oriented dynamic relative humidity model for a polymer electrolyte membrane (PEM) fuel cell stack is presented. This model is integrated with a first law based thermal model, which tracks energy flow within four defined control volumes in the fuel cell; the cathode channel, anode channel, coolant channel, and fuel cell stack body. Energy and mass conservation equations are developed for each control volume. On top of mass conservation, electro-drag and osmosis models were also implemented within the model to account for the major modes of vapor transfer through the membrane between the anode and cathode. Requisite alterations to the thermal model as well as mass flow rate calculations are also discussed. Initially, the model utilized a single lumped control volume for the calculation of all values each channel (anode and cathode). This lumped value method is computationally inexpensive, and makes the model optimal for control design. However, investigation of the mass-based Biot number showed the need for greater granularity along the length of the channels to properly capture the relative humidity dynamics. In order to improve the resolution of the model, while still minimizing the computation expense, the model was subdivided into a series of lumped value models. The cathode channel was the point of focus as it is the major concern from a controls perspective. This method captures the proper trends found in far more complex CFD models, while still maintaining a quick calculation time. Different levels are subdivision (3 and 6 submodels) are investigated, and the differences discussed. Particularly, temperature range, relative humidity range, the effect on the modeled voltage, and calculation time are compared. This control-oriented model is low order and based on lumped parameters, which makes the computational expense low. Formulation of this model enables the development of control algorithms to achieve optimal thermal and water management. / text
14

Apvaliosios ir smulkintosios medienos tūrio nustatymas svėrimo būdu / Measuring the bulk of logs and flaked wood by means of weighing it

Ažukas, Kastytis 15 June 2009 (has links)
Magistro darbe atliekama medienos tūrio nustatymo svėrimo būdu galimybių analizė. Darbo objektas – Apvalioji ir smulkintoji mediena naudojama medienos drožlių plokščių gamybai. Darbo tikslas - Ištirti apvaliosios ir smulkintos medienos, skirtos medžio drožlių plokštės gamybai, tankio kitimo dėsningumus ir jos svorio konvertavimo į tūrį ypatumus. Darbo metodai - Empirinio tyrimo metodas paremtas duomenų rinkimu medieną sveriant, atrenkant mėginius ir juos džiovinant. Kiekybinis palyginimas - statistinis duomenų apdorojimas paremtas dispersine ir regresine analize ir taikant koreliacinius ryšius. Visą empirinę medžiagą sudarė 180 medvežių imtis su spygliuočių, lapuočių ir smulkintoji mediena. Tiriama medienos tankio ir drėgnio koreliaciniai ryšiai, taip pat šių parametrų kintamumas nuo medžių rūšies, meteorologinių sąlygų: juos kertant ir sandėliuojant. Darbo rezultatai. Nustatyta, kad didžiausią įtaką medienos tankiui turi medienos drėgnis, o didžiausia drėgnos medienos masė yra žiemą. Pavasarį medienos masė mažėjo, o lengviausia buvo vasarą. Didėjant medienos santykiniam drėgniui, medienos tankis didėja nepriklausomai nuo medžių rūšies, tai rodo, kad medienos drėgnis ir jo kintamumas priklauso nuo metų laiko. Tiek lapuočių, tiek spygliuočių medienos masės priklausomybės nuo santykinio drėgnio kreivė žiemos metu yra aukščiausia, lyginat su pavasario ir vasaros kreivėmis. Tai dar kartą įrodo, kad mediena drėgniausia žiemos metu. Nustatytas drėgnio... [toliau žr. visą tekstą] / In this Master's paper an analysis is carried out to calculate the probability of estimating the volume of timber using the method of weighing. Work -study : Logs and flaked wood used for producing wood cutting panelling The aim of the work: Analysing the changes in thickness of logs and flaked wood used for producing wood cutting panelling and converting its weight into bulk. Methods of work: Empiric method based on weighing the timber, sorting out the samples and drying them. Quantitative analysis - processing the statistic data based on dispersive and regressive analysis and using correlation relations. The empiric analysis covered 180 trucks of softwood, deciduous wood and cutting.The correlation between the thickness and the humidity has been analysed. It has been also analysed how the thickness and humidity change in different species of trees, meteorological conditions, cutting down the trees and storing them. The results of the work: It has been decided that the greatest influence on the thickness of the wood has the density of the wood. The greater the relative density of the timber, the bigger the thickness of the timber. And it doesn't depend on the kind of the wood. It shows that the thickness of the wood and its changing depends on the season of the year. The curve of relative density in winter is the highest if ompared with the one in spring or in winter. The rate of the variation of density has been fixed from 11,5 to 32,2 %. As it is seen the range was great... [to full text]
15

Roxithromycin : a solubility and stability study / Elzet van Niekerk

Van Niekerk, Elzet January 2011 (has links)
Roxithromycin is a semi-synthetic, macrolide antibiotic, derived from erythromycin A. It acts as a bacteriostatic drug at low concentrations and a bactericidal drug at high concentrations. It binds to the 50S subunit of the 70S ribosome, which causes the reversible inhibition of RNA-dependent bacterial protein synthesis. It is well known that active pharmaceutical ingredients (APIs) may exist in numerous solid states. Differences in the solid state significantly influence the physical and chemical properties of an API. The in vivo performance of a dosage form will also be influenced by the solid state properties of a given pharmaceutical active. The amorphous characteristics of APIs have a significant impact on their performance and thus offer the potential for exciting new pharmaceuticals. Whilst amorphous forms of poorly soluble APIs are more soluble than their crystalline counterparts, they tend to be physically unstable, which makes their formulation into solid dosage forms quite challenging. Roxithromycin has only 50% oral bioavailability due to its poor aqueous solubility and for this reason, its potential for optimal therapeutic effect are limited. Poor solubility is thus an important obstacle in formulation development. During this study, amorphous forms of roxithromycin were prepared via quench cooling, and desolvation of chloroform- and ethyl acetate solvates. These amorphous forms were characterised by means of several techniques, whilst their solubilities and stabilities were also investigated. The outcomes of the solubility studies illustrated the complexity of this API and its amorphous forms with regards to their interactions with water. Solubility studies confirmed the superior solubility of the roxithromycin glass (prepared through quench cooling) and amorphous forms (desolvation of solvates) over the roxithromycin monohydrate in water. The solubility in water improved in the order of roxithromycin monohydrate < roxithromycin glass < roxithromycin glass powder < amorphous chloroform desolvate. The roxithromycin monohydrate, as well as the amorphous forms of roxithromycin demonstrated stability over a one-month period of exposure 40°C and relative humidity (RH) of 75%. The roxithromycin glass powder tended to revert to the more stable crystalline monohydrate after week 3 of stability testing. The roxithromycin glass at lower temperatures of 25°C and 30°C (both at 75% RH) tended to transform into the more crystalline form at week 4 of the study. These transformations were, however, not as significant as during the 40°C / 75% RH study. The conclusion could therefore be made that this transformation into the crystalline form was more temperature – than moisture dependant. At a higher temperature (at identical humidity conditions), the transformation into the crystalline form was much faster. Stability studies on the two roxithromycin desolvates were also performed in order to determine whether these amorphous forms, would differ, with regards to their stability, from the glass prepared through heating and cooling. It was determined that the desolvates were more stable than the roxithromycin glass. / Thesis (MSc (Pharmaceutics))--North-West University, Potchefstroom Campus, 2012
16

Roxithromycin : a solubility and stability study / Elzet van Niekerk

Van Niekerk, Elzet January 2011 (has links)
Roxithromycin is a semi-synthetic, macrolide antibiotic, derived from erythromycin A. It acts as a bacteriostatic drug at low concentrations and a bactericidal drug at high concentrations. It binds to the 50S subunit of the 70S ribosome, which causes the reversible inhibition of RNA-dependent bacterial protein synthesis. It is well known that active pharmaceutical ingredients (APIs) may exist in numerous solid states. Differences in the solid state significantly influence the physical and chemical properties of an API. The in vivo performance of a dosage form will also be influenced by the solid state properties of a given pharmaceutical active. The amorphous characteristics of APIs have a significant impact on their performance and thus offer the potential for exciting new pharmaceuticals. Whilst amorphous forms of poorly soluble APIs are more soluble than their crystalline counterparts, they tend to be physically unstable, which makes their formulation into solid dosage forms quite challenging. Roxithromycin has only 50% oral bioavailability due to its poor aqueous solubility and for this reason, its potential for optimal therapeutic effect are limited. Poor solubility is thus an important obstacle in formulation development. During this study, amorphous forms of roxithromycin were prepared via quench cooling, and desolvation of chloroform- and ethyl acetate solvates. These amorphous forms were characterised by means of several techniques, whilst their solubilities and stabilities were also investigated. The outcomes of the solubility studies illustrated the complexity of this API and its amorphous forms with regards to their interactions with water. Solubility studies confirmed the superior solubility of the roxithromycin glass (prepared through quench cooling) and amorphous forms (desolvation of solvates) over the roxithromycin monohydrate in water. The solubility in water improved in the order of roxithromycin monohydrate < roxithromycin glass < roxithromycin glass powder < amorphous chloroform desolvate. The roxithromycin monohydrate, as well as the amorphous forms of roxithromycin demonstrated stability over a one-month period of exposure 40°C and relative humidity (RH) of 75%. The roxithromycin glass powder tended to revert to the more stable crystalline monohydrate after week 3 of stability testing. The roxithromycin glass at lower temperatures of 25°C and 30°C (both at 75% RH) tended to transform into the more crystalline form at week 4 of the study. These transformations were, however, not as significant as during the 40°C / 75% RH study. The conclusion could therefore be made that this transformation into the crystalline form was more temperature – than moisture dependant. At a higher temperature (at identical humidity conditions), the transformation into the crystalline form was much faster. Stability studies on the two roxithromycin desolvates were also performed in order to determine whether these amorphous forms, would differ, with regards to their stability, from the glass prepared through heating and cooling. It was determined that the desolvates were more stable than the roxithromycin glass. / Thesis (MSc (Pharmaceutics))--North-West University, Potchefstroom Campus, 2012
17

The Survival and Recovery of ϕ6 Virus from Fomites

Bearden, Richard L, II 09 May 2015 (has links)
Viral transmission from the environment can occur via fomites, but there is uncertainty about which factors most affect viral persistence on fomites. Children are a population highly susceptible to viral infection, and sharing common fomites like toys may spread infection. The objective of this research was to assess the survival of enveloped viruses on the surfaces of children’s toys, using bacteriophage ϕ6 as a surrogate for enveloped human viruses. The survival of infectious ϕ6 virions was observed over a 24 hour period at 22°C and relative humidities of 40% & 60%. On the surface of children’s toys, ϕ6 was better able to persist at 60% RH (log10 reduction< 2 log10) over a 24 hour period than it was at 40% RH (log10 reduction> 6 log10). If ϕ6 virus persists on toy material for up to 24 hours, then viral transmission via shared fomites is certainly significant.
18

Padrões eletrostaticos em uma superficie de silica : um modelo para o acumulo de cargas eletricas em isolantes / Electrostatic patterning of a silica surface: a new model for charge build-up on a dielectric solid

Gouveia, Rubia Figueredo 26 July 2005 (has links)
Orientador: Fernando Galembeck / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-08-05T15:23:30Z (GMT). No. of bitstreams: 1 Gouveia_RubiaFigueredo_M.pdf: 1322724 bytes, checksum: 354ae5145ed6ce8d69bae986b50bda01 (MD5) Previous issue date: 2005 / Resumo: Atualmente muitas tecnologias importantes são baseadas nos princípios eletrostáticos, como por exemplo, a pintura eletrostática, a reciclagem de termoplásticos e a eletrofotografia. No entanto, mesmo considerando todo o desenvolvimento da eletrostática, ainda existem muitas questões importantes não respondidas, quando se considera a eletrização de materiais isolantes. Na maioria dos casos não se conhece quais são as espécies portadoras de cargas, nem como elas podem ser detectadas e identificadas. Trabalhos anteriores deste laboratório mostraram que os portadores de cargas em látexes são íons simples (K, RSO4, etc). Neste trabalho foram estudados os padrões eletrostáticos criados em uma superfície de sílica, recoberta por um conjunto de eletrodos interdigitados de ouro. As amostras foram preparadas utilizando técnicas microlitográficas e sua superfície foi investigada usando as microscopias de força atômica (AFM) e de varredura de potencial elétrico (SEPM). Foram obtidos padrões eletrostáticos reprodutíveis; a aplicação de uma diferença de potencial elétrico entre os eletrodos produz padrões de potenciais elétricos que desaparecem rapidamente quando os eletrodos são curto-circuitados e aterrados. O tempo de relaxação desses padrões foi de aproximadamente 10 minutos sob umidade de 70% e eles são fortemente influenciados pela umidade relativa da atmosfera, apresentando um tempo de decaimento superior a 1 hora, quando a superfície é exposta a uma umidade relativa de 10%. Os resultados são interpretados usando um modelo simples baseado na quimisorção de espécies presentes na atmosfera acompanhada de reações eletródicas, podendo ser aplicado a outros isolantes. / Abstract: Many current important technologies are based on electrostatic principles, as for instance electrostatic painting, polymer recycling and photocopying. However, even considering its long history, there are still many open questions in the study of electric phenomena in insulators. For instance, how can charge carriers in insulators be detected, identified and quantified? In this laboratory the examination of latexes showed that the charge carriers are K e RSO4 ions. This work describes reproducible electric potential patterns created on silica surfaces, covered with sets of interdigitated parallel gold electrodes by using microlithography techniques. The patterns are detected by concurrent atomic force microscopy (AFM) and scanning electric potential microscopy (SEPM). The images were acquired while electrodes were biased and also after they were all short-circuited and grounded, producing reproducible electrostatic patterns. Characteristic times for pattern formation and relaxation are in the order of 10 minutes at 70% relative humidity and they are strongly influenced by the relative humidity of the atmosphere, showing a relaxation time in the order of 1 hour, when the surface is under 10% RH. The results are interpreted using a model based on the chemisorption of atmospheric water and electrochemical activity at the silica surface. The sum up, this work describes a new approach to the study of the electrification of insulator surfaces that produces reproducible results and can be applied to many other cases. / Mestrado / Físico-Química / Mestre em Química
19

Impacto do ambiente do incubatório na produção de pintos de corte / Environmental impact of the hatchery production of broiler chicks

Poderoso, Fabiana Maria Gonçalves de Lima 20 August 2018 (has links)
Orientador: Irenilza de Alencar Nääs, Marta dos Santos Baracho / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Agrícola / Made available in DSpace on 2018-08-20T11:48:14Z (GMT). No. of bitstreams: 1 Poderoso_FabianaMariaGoncalvesdeLima_M.pdf: 1087757 bytes, checksum: 1db07cd336e1192d99ffb8537b7a5e02 (MD5) Previous issue date: 2011 / Resumo: No Brasil, a avicultura é uma atividade relativamente nova, apesar de ter seu início na década de 30, como atividade fornecedora de subproduto, o esterco para os cafezais, se tornando uma atividade econômica produtora de proteína de alto valor biológico, somente a partir da década de 60. Hoje ocupa um lugar privilegiado na economia mundial devido ao trabalho de muitos profissionais ligados à atividade, sendo os principais pontos que ocorreram grandes melhorias: avanço tecnológico nas áreas de produção, nutrição e sanidade, permitindo uma melhora significativa nos índices zootécnicos. O incubatório é o local onde se obtêm o produto final, ou seja, do pintainho de um dia, tem a responsabilidade de disponibilizar aos produtores, pintos de qualidade para maximizar o desempenho das aves a campo. O presente trabalho teve como objetivo geral avaliar a influência da temperatura e da umidade relativa do ar no ambiente do incubatório, sobre as respostas da taxa de eclosão durante o processo de incubação e nascimento. Os dados de temperatura e umidade relativa do ar foram registrados em uma das salas das incubadoras e em uma das salas de nascedouro do Incubatório São José no município de Amparo - SP e analisados com relação ao índice de eclosão dos ovos incubados que foi analisado no dia do nascimento dos pintos. Foram rastreadas 48 bandejas, tendo capacidade de 96 ovos cada bandeja, até o nascimento desses pintos, sendo estas divididas seis quadrantes na incubadora sendo oito bandejas em cada quadrante classificadas como lotes e após foram monitoradas na sala de nascedouro que foi dividida em quatro quadrantes. Os dados foram submetidos à análise estatística descritiva por lote e análise de comparação de médias, através do teste F na ANOVA e do teste não-paramétrico de Kruskal-Wallis. Os resultados obtidos foram: a temperatura do ambiente influenciou o descarte total de ovos no 14º dia de incubação (de 35.9ºC à 37.0°C) e no 21º de incubação (de 37,0 °C à 37,3 °C) e, comparados com a idade das matrizes (42 semanas e 56 semanas), identificou-se que a matriz mais velha obteve maior número de descartes nas variáveis analisadas como: ovos inférteis, mortalidade de 0 a 7 dias e ovos quebrados, a serem o volume maior do total de ovos descartados. E, somente houve influência da umidade relativa do ar aos 21 dias de incubação ( de 52,2% à 62%). Conclui-se que houve influência do ambiente térmico e da idade da matriz no descarte total de ovos no processo de incubação / Abstract: Poultry science is responsible for studying the birds. In Brazil poultry production is a relatively new activity, although had its beginning in the 30's decade as an activity that supplied waste which was used in coffee production. It became an economical activity that produces protein of high biological value just after the decade of 60. Today Brazil occupies a privileged place in the world's economy due to the work of several professionals connected to this activity. The areas were the greatest technological improvements occurred are: production, nutrition and health, allowing significant enhancement in animal production indexes. The hatchery is the place where the final product is obtained, the one day old pullet, and that has the responsibility of given the producers high quality chicks in order to maximize their performance in the field. The objective of this research is to evaluate the influence of ambient temperature and relative humidity of the air in the hatchery on the response during the process in both incubator and hatcher. Air temperature and relative humidity data will be registered in a incubator and hatcher at the Incubatório São José, county of Amparo - SP and they will be analyzed with relation to the hatcher index at the first day. Forty eight trays with 96 eggs each were screened trays until the birth of these chicks. The hatchery was divided in six quarters and eight incubator trays in each quadrant were classified as lots and they were monitored after the hatcher room, which was divided into four quadrants. The data were analyzed by descriptive statistics and analysis of batch mean comparison, using the F test in ANOVA and nonparametric Kruskal-Wallis. The obtained results were: the temperature of the environment influenced the discarding total egg on day 14 of incubation and 21 of incubation and compared with age of the array, we identified that it took the variables as: infertile eggs, mortality 0-7 days and broken eggs, to be the bulk of total eggs discarded. There was only influence of relative humidity at 21 days of incubation. It is concluded that there was influence of thermal environment and age of female breeder in total egg discarding in the process of incubation / Mestrado / Construções Rurais e Ambiencia / Mestre em Engenharia Agrícola
20

Creep properties of cementitious materials : effect of water and microstructure : An approach by microindentation / Rôle de la microstructure et effet de l'eau sur les propriétés de fluage des liants : une approche par micro-indentation

Zhang, Qing 13 February 2014 (has links)
Les matériaux cimentaires tels que le béton, le ciment et le plâtre sont largement utilisés dans la construction, les matières premières dont ils sont faits étant abondantes sur Terre. Cette tendance ne devrait pas changer dans les prochaines décennies. Mais ces matériaux subissent l'impact du fluage. Le fluage des matériaux cimentaires est une problématique complexe. D'une part, dans les matériaux cimentaires, le fluage est souvent couplé avec d'autres phénomènes tels que le séchage, l'hydratation et la fissuration, et peut être influencé par différents paramètres comme la température, le niveau de contrainte, la teneur en eau et la formulation. D'autre part, la mesure du fluage par un test macroscopique traditionnelle du fluage requiert du temps (il est recommandé de réaliser l'essai de fluage du béton sur plusieurs mois afin de donner une caractérisation fiable du fluage à long terme) et s'avère fastidieuse, puisque les paramètres expérimentaux doivent être bien contrôlés sur de longues périodes de temps. Cette thèse étudie la micro indentation à l'échelle de la pâte de ciment ou du plâtre pour évaluer les propriétés de fluage propre à long terme des matériaux cimentaires, en comparant les fonctions de fluage obtenues par des tests de micro indentation de quelques minutes avec celles obtenues par des expériences macroscopiques de fluage réalisées pendant de longues années. Pour la pâte de ciment, la comparaison a été faite à l'échelle du béton à l'aide d'une certaine homogénéisation. L'étude a validé le fait que un test de micro indentation de quelques minutes peut fournir une mesure des propriétés à long terme de matériaux cimentaires. Une fois validée la technique d'indentation, nous avons étudié l'effet de la microstructure (c'est-à-dire la distribution des phases) et celui de l'eau sur le fluage propre à long terme des matériaux cimentaires. L'effet de la microstructure a été étudiée sur des matériaux tels que des pâtes de C3S et de C2S ainsi que sur des compacts de C-S-H synthétique, de portlandite (CH) et leurs mélanges préparés par compression de poudres. Une attention particulière a été consacrée à créer des compacts avec de grandes fractions volumiques de phase cristalline. Pour tous les échantillons examinés, nous avons identifié le bon modèle micromécanique qui permette de prédire les résultats. Le choix du modèle micromécanique concorde avec les observations microstructurales. L'effet de l'humidité relative a été étudié par le conditionnement et l'indentation de certains de ces matériaux (par exemple la pâte de C3S, de compact de C-S-H et de compact de CH) dans différentes humidités relatives allant de 11% à 94%. L'humidité relative a eu un effet significatif sur le fluage : pour tous les matériaux testés, une plus grande humidité a conduit à un fluage plus important. Le compact de portlandite fut le plus sensible à l'humidité relative, sans doute parce que le fluage se produit au niveau des interfaces entre les cristaux de portlandite. Pour la pâte de C3S, une relation simple a été identifiée entre les propriétés de fluage à long terme et la teneur en eau. Enfin, nous avons proposé des modèles micromécaniques qui permettent la prédiction des propriétés de fluage à long terme de matériaux cimentaires avec une large gamme de fraction volumique de phase cristalline et sur une gamme d'humidités relatives étendue / Cementitious materials such as concrete, cement and gypsum are widely used in construction, as the raw materials of which they are made are abundant on Earth. Such trend is unlikely to change in the coming decades. But these materials suffer from creep. The creep of cementitious materials is a complex issue. On one hand, in cementitious materials creep is often coupled with other phenomena such as drying, hydration and cracking, and can be influenced by various parameters such as temperature, level of stress, water content and mix design. On the other hand, measuring creep by traditional macroscopic creep testing is time-consuming (creep test on concrete is recommended to be carried out over several months in order to provide a reliable characterization of long-term creep) and tedious, since experimental parameters need to be well controlled over extensive periods of time. This thesis studied microindentation at the scale of cement paste or gypsum plaster for the assessment of long-term basic creep properties of cementitious materials, by comparing creep functions obtained by minutes-long microindentation testing with those obtained with macroscopic creep experiments which lasted up to years. For cement paste, the comparison was made at the scale of concrete with the aid of upscaling tools. The study validated that minutes-long microindentation testing can provide a measurement of the long-term creep properties of cementitious materials. With the validated indentation technique, we studied the effect of microstructure (i.e., the distribution and the spatial organization of phases) and of water on long-term basic creep of cementitious materials. The effect of microstructure was studied on materials such as C3S pastes and C2S pastes as well as on compacts of synthetic C-S-H, portlandite (CH) and their mixtures prepared by compaction of powders. For all samples considered, we identified the right micromechanical model that allows predicting the results. The choice of micromechanical model was consistent with microstructural observations. The effect of relative humidity was studied by conditioning and testing some of those materials (i.e., C3S paste, compact of C-S-H, and compact of CH) in various relative humidities ranging from 11% to 94%. Relative humidity had a significant effect on creep: for all materials tested, a greater humidity led to a greater creep. The compact of portlandite was the most sensitive to relative humidity, probably because creep occurs at interfaces between portlandite crystals. For C3S paste, a linear relation was identified between long-term creep properties and water content at relative humidities ranging from 11% to 75%.Finally, we proposed micromechanical models that allow predicting long-term basic creep properties of cementitious materials with a wide range of volume fraction of crystalline phase and over a wide range of relative humidities

Page generated in 0.0755 seconds