1,720,963 research outputs found
Cool Cementitious Materials for a More Sustainable Urban Environment
No full textThe temperature increase in cities called the "urban heat island" (UHI) depends on the local microclimate (e.g., solar irradiation, population, buildings density, industrial activities, traffic, emissions, heat sources) and results not only in increased electricity consumption for indoor cooling but also in decreased outdoor comfort, especially in summer periods or in warm climate zones. As the world's urban population continues to grow, there is an urgency to make buildings more ecoefficient and reduce the impact of cities on climate change and global warming. In this framework, the use of cool concretes can mitigate the UHI and improve energy saving and outdoor comfort: cool concrete roofs and facade cement-based tiles can provide valid solutions to decrease the energy demand for air conditioning in building envelops; moreover, cool concrete paving blocks and pervious pavements can improve urban comfort for outdoor wellbeing. The present work shows the improved solar reflective performance of three cool cementitious solutions compared with traditional ones, with both white and colored surfaces, to fulfill both architectural demand and sustainability issues without any additional post-treatments or after placing steps, such as surface coating. Finally, the study is completed with the durability evaluation of the products' solar reflective performance
Accelerated Curing Of Cement Based Materials
No full textPortland cements are slow hardening materials and several techniques are used to expedite the attainment of a satisfactory strength, i.e., chemicals, reduction of particle size, heat. Increasing the temperature is by far the most effective. Dielectric heating is a well established technique in materials technology but the cement industry has so far expressed little interest in this technology, probably because the effects on the final properties of mortars and concrete are not well established yet and for the greater complexity of the process compared, for instance, to steam curing. Fear of radiation leakage from high power applicators not correctly designed and operated has placed a further constraint to the acceptance of this technique. Although early results of microwave heating were unsatisfactory, likely because of overheating effects, the availability of temperature controlled microwave applicators now makes it possible to choose temperature profiles that favourably influences the final properties of the materials. Here we report on the effects of microwave treatments on three types of modified portland cements: one contained microsilica and the others granulated slag or flyash from coal burning. In all cases an acrylic superfluidifier was used to keep water content low. The microwave treatment was given at 80°C for 0 to 16 h. The mechanical properties of the microwave treated samples were compared to those of samples treated in water at 80°C for 24 h. Mercury intrusion porosimetry, thermal analysis and SEM examination were also carried out to investigate the microstructural modifications brought about by the microwave treatment
Metodi elettrici nella tecnologia di malte e calcestruzzi
No full textThe application of electrical methods to the technology of cementitious materials opens up new possibilities for monitoring setting and hardening processes of fresh, admixtured pastes or for tracking the invasion of aggressive chemicals in hardened structures. The electrical conductivity can be related, by means of percolation models, to the porosity of the material and, therefore, to its strength. Innovative formulations with unusual properties can also be produced by controlling the electrical conductivity of hardened mortars. Further, energy efficient accelerated hardening techniques and contactless breaking techniques for decontamination and decommissioning processes in nuclear power plants are being developed
Electrical properties of fluidified portland cement mixes in the early stage of hydration
No full textThe electrical conductivity and the dielectric constant of cement mixes have been studied in the initial stages of setting to correlate the time evolution of the electrical parameters with the chemical and microstructural modifications. The formulations, based on a mix of Portland cement and microsilica, were fluidified with various amounts of an acrylic polyelectrolite and had a w/c ratio of 0.24.
Measurements were carried out at ambient temperature for times of up to 40 h, in the frequency interval 30 Hz–200 kHz. The conductivity showed a marked decrease correlated with the loss of connectivity of the macroscopic porosity (depercolation). The time to the drop of conductivity depended linearly on the concentration of superplasticizer whose retarding effect was clearly evidenced. A dielectric amplification phenomenon was observed which vanished after the porosity depercolated. The temperature profiles of mixtures with different concentrations of the superplasticizer were drawn and a simple model was developed to compute the specific thermal power as a function of time. A substantial coincidence was found, on the time axis, between depercolation and maximum rate of setting. Vicat tests were also carried out
Advancements In Modelling The Development Of Microstructure In Cement Pastes
No full textThe time evolution of some microstructural parameters (porosity, composition) and physical properties (electrical conductivity) of young cement pastes have been modelled by means of a numerical code (chemhyd3d) based on the percolation theory. The computational results have been compared with experimental data (for the electrical conductivity) and porosities reckoned by the Powers equation. Very good agreement has been found which indicated that the effects of the foremost parameters on the properties of mortars and concrete can now accurately predicted
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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