284 research outputs found

    Nanotechnology and Cementitious Materials

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    General vie

    Numerical Simulation of Porosity in Cements

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    The pores in cementitious materials, their sizes and connectivity have an important influence on the durability of concrete. Several microstructural models have been developed to simulate the three-dimensional pore network in cement pastes. In this article, microstructures with the ic model are compared with experimental results. It is seen that despite having a resolution for the capillary pores very close to reality, the experimentally observed breakthrough diameter from mercury intrusion is much lower than the values obtained by applying an algorithm of mercury intrusion to the simulated microstructure. The effect of some of the most important input parameters on the pore sizes in the simulated microstructure explored. The phenomenon which seems best able to explain this discrepancy is that C-S-H is not in fact a phase with a smooth surface as represented in microstructural models, but a phase which grows as needles into the pore space, leading to very small water-filled capillary pores from quite young ages. The results demonstrate it will be extremely challenging to represent the porosity of real microstructures in microstructural models on the scale of hundreds of microns necessary to study macroscopic transport

    Equivalency points: Predicting concrete compressive strength evolution in three days

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    Knowledge of the compressive strength evolution of concrete is critical for activities such as stripping formwork, construction scheduling and pre-stressing operations. Although there are several procedures for predicting concrete compressive strength, reliable methodologies involve either extensive testing or voluminous databases. This paper presents a simple and efficient procedure to predict concrete strength evolution. The procedure uses an experimentally-determined parameter called the Equivalency Point as an indicator of equivalent degree of reaction. Equivalency Points are based on early age concrete deformation and temperature variations. Test results from specimens made from seven concrete types validate the approach. © 2008 Elsevier Ltd. All rights reserved

    Revealing design complexity: Lessons from the Open University

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    Design is an inherently complex activity. Design thinking is cognitively complex and design practice is contextually complex. This has implications for university-level design education which has traditionally displayed clear distinctions between the full-time and part-time undergraduate sectors, particularly in their teaching and learning strategies. However, a number of pressures and trends are evident which suggest that these two sectors are moving closer together. One of the drivers in this phenomenon is the need for students to be exposed to realistic levels of design complexity. This paper examines complexity in design and draws some significant parallels between modern design practice in general and the production of a new undergraduate course at the Open University. Both are used to illuminate design complexity. The paper suggests that some of the tools, techniques and approaches of part-time, undergraduate, distance design education might usefully be exploited in more traditional, full-time course models

    Impacto del uso de un cemento de bajo carbono en la mejora de la sostenibilidad de la producción de cemento

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    A preliminary assessment of conditions for the industrial manufacture of a new cementitious system based on clinker-calcined clay and limestone, developed by the authors, referred as “low carbon cement” is presented. The new cement enables the substitution of more than 50% of the mass of clinker without compromising performance. The paper presents the follow-up of an industrial trial carried out in Cuba to produce 130 tonnes of the new cement at a cement plant. The new material proved to fulfill national standards in applications such as the manufacture of hollow concrete blocks and precast concrete. No major differences either in the rheological or mechanical properties were found when compared with Portland cement. Environmental assessment of the ternary cement was made, which included comparison with other blended cements produced industrially in Cuba. The new cement has proven to contribute to the reduction of above 30% of carbon emissions on cement manufacture.Se presenta la evaluación preliminar de las condiciones de fabricación industrial de un nuevo sistema cementicio a partir del empleo de clínquer; arcillas calcinadas y piedra caliza; desarrollado por los autores; denominado “cemento de bajo carbono”. El nuevo cemento posibilita la reducción de más de un 50% de la masa de clínquer; sin comprometer el comportamiento del material. El presente trabajo presenta el monitoreo de la producción industrial en una planta en Cuba; de 130 t del nuevo cemento. El cemento obtenido cumple con las regulaciones nacionales de calidad y su empleo tiene similar rendimiento que el cemento Pórtland para la producción de bloques y hormigón de 25 MPa. Se realiza el análisis de impacto ambiental del cemento ternario mediante la comparación con otros cementos producidos industrialmente. El nuevo cemento puede contribuir a la reducción de más del 30% de las emisiones de CO2 asociadas a la manufactura de cemento

    Equivalency points: predicting concrete compressive strength evolution

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    Knowledge of the compressive strength evolution of concrete is critical for activities such as stripping formwork, construction scheduling and pre-stressing operations. Although there are several procedures for predicting concrete compressive strength, reliable methodologies involve either extensive testing or voluminous databases. This paper presents a simple and efficient procedure to predict concrete strength evolution. The procedure uses an experimentally-determined parameter called the Equivalency Point as an indicator of equivalent degree of reaction. Equivalency Points are based on early age concrete deformation and temperature variations. Test results from specimens made from seven concrete types validate the approach

    The ateneo as an effective model of continuing professional development:Findings from southern Argentina

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    Despite a sustainable research interest in different forms of teachers’ professional development, scant international attention has been paid to forms of professional development which are implemented in South America. Based on a qualitative research design, this study explores the impact of the ateneo as an innovative model of continuing professional development. An ateneo is a model which supports teacher reflection and change in teaching practices by concentrating on context-responsive practical issues such as lesson planning and delivery. The study was carried out with 22 teachers of English as a foreign language in southern Argentina. Data were gathered through the teachers’ lesson plans, whole group discussions, and the teachers’ final assignments to receive credits for completion of the ateneo. Drawing on thematic analysis, the participants envisaged the ateneo as a practice-oriented, dynamic, interaction-based, and personal as well as collective space for developing teaching skills and professional knowledge. In particular, findings show that the participants exhibited an improvement in lesson contextualization, sequencing and transitioning, maximization of resources, class time management, and reflective teaching. The study argues that the ateneo became successful given the shared teacher identity among the participants and the course tutors and the explicit focus on the teachers’ daily practices

    Deflection control for reinforced recycled aggregate concrete beams: Experimental database and extension of the fib Model Code 2010 model

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    Recycled aggregate concrete (RAC) has emerged as a viable solution for solving some of the environmental problems of concrete production. However, design guidelines for deflection control of reinforced RAC members have not yet been proposed. This study presents a comprehensive analysis of the applicability of the fib Model Code 2010 (MC2010) deflection control model to reinforced RAC beams. Three databases of long-term studies on natural aggregate concrete (NAC) and RAC beams were compiled and meta-analyses of deflection predictions by MC2010 were performed. First, the MC2010 deflection control model was tested against a large database of long-term tests on NAC beams. Second, a database of RAC and companion NAC beams was compiled and initial and long-term deflections were calculated using the MC2010 model. It was shown that deflections of RAC beams are significantly underestimated relative to NAC beams. Previously proposed modifications for MC2010 equations for shrinkage strain and creep coefficient were used, and new modifications for the modulus of elasticity and empirical coefficient β were proposed. The improved MC2010 deflection control model on RAC beams was shown to have equal performance to that on companion NAC beams. The proposals presented in this paper can help engineers to more reliably perform deflection control of reinforced RAC members.This is the peer-reviewed version of the article: N. Tošić, S. Marinković, and J. de Brito, ‘Deflection control for reinforced recycled aggregate concrete beams: Experimental database and extension of the fib Model Code 2010 model’, Structural Concrete, vol. 20, no. 6, pp. 2015–2029, 2019 [https://doi.org/10.1002/suco.201900035

    Characteristics of Copper-based Oxygen Carriers Supported on Calcium Aluminates for Chemical-Looping Combustion with Oxygen Uncoupling (CLOU)

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    Eight different oxygen carriers (OC) containing CuO (60 wt %) and different mass ratios of CaO to Al2O3 as the support were synthesized by wet-mixing followed by calcination at 1000 °C. The method of synthesis used involved the formation of calcium aluminum hydrate phases and ensured homogeneous mixing of the Ca2+ and Al3+ ions in the support at the molecular level. The performance of the OCs for up to 100 cycles of reduction and oxidation was evaluated in both a thermogravimetric analyzer (TGA) and a fluidized bed reactor, covering a temperature range of 800 to 950 °C. In these cycling experiments, complete conversion of the OC, from CuO to Cu and vice versa, was always achieved for all OCs. The reactivity of the materials was so high that no deactivation could be observed in the TGA, owing to mass transfer limitations. It was found that OCs prepared with a mass ratio of CaO to Al2O3 in the support >0.55 agglomerated in the fluidized bed, resulting in an apparent deactivation over 25 cycles for all temperatures investigated. High ratios of mass of CaO to Al2O3 in the support resulted in CuO interacting with CaO, forming mixed oxides that have low melting temperatures, and this explains the tendency of these materials to agglomerate. This behavior was not observed when the mass ratio of CaO to Al2O3 in the support was ≤0.55 and such materials showed excellent cyclic stability operating under redox conditions at temperatures as high as 950 °C.The authors thank Mohammad Ismail and Matthew Dunstan for helping with the XRD analysis and Alex Casabuena-Rodriguez and for helping with the SEM. This work was supported by the Engineering and Physical Sciences Research Council (EPSRC grant EP/I010912/1).This is the final version of the article. It first appeared from ACS via http://dx.doi.org/10.1021/acs.iecr.5b0117
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