On the time dependent Schwarzschild - de Sitter spacetime

An imperfect cosmic fluid with energy flux is analyzed. Even though its energy density $\rho$ is positive, the pressure $p = -\rho$ due to the fact that the metric is asymptotically de Sitter. The kinematical quantities for a nongeodesic congruence are computed. The scalar expansion is time independent but divergent at the singularity $r = 2m$. Far from the central mass $m$ and for a cosmic time $\bar{t} << H^{-1}$, the heat flux $q$ does not depend on Newton's constant $G$.Comment: 8 pages, no figures, Sections 3 and 5 enlarged, one reference adde

Predicting mechanical properties of enhanced performance concrete using compressive strength

Mechanical properties of concrete are usually evaluated from compressive strength test results. Extensive literature, codes and regulations recommend the prediction of mechanical properties such as splitting and flexural tensile strength using the compressive strength of cylindrical specimens. These expressions are not related with the type of concrete and, generally, are only a function of the cylindrical compressive strength. It is the objective of this research work to investigate the validity of the existing relationships for enhanced performance concrete, obtained by replacing cement with fly ash for up to 60% in weight. The experimental program investigated the effect of replacing cement with fly ash on the mechanical properties, i.e. compressive strength, splitting-tensile strength and flexural-tensile behaviour. Results obtained show that, in most cases, a good linear correlation exists between the evaluated mechanical properties, i.e. splitting-tensile strength, flexural-tensile strength and elasticity modulus in flexure, and the square root of compressive strength. This further indicates that an increase in the compressive strength produces a less pronounced increase of the tensile strength

Gestão de resíduos de construção e demolição

Esta comunicação apresenta um estudo de gestão dos resíduos de construção e demolição na Zona Norte de Portugal. Para além da recolha de dados em documentos existentes foram feitos inquéritos e visitas às entidades que produzem e lidam com os RC&D, de forma a serem definidos aspectos como quantidades, tipos de materiais, e destinos finais dos resíduos

Viabilidade económica de uma central de tratamento de resíduos de construção e demolição

Comunicação apresentada no Encontro Novas Problemáticas para Gestão dos Resíduos, Beja, Dezembro de 2004.Nesta comunicação apresenta-se um estudo económico e de viabilidade duma central de reciclagem de resíduos de construção e demolição (RCD) a ser implantada na Zona Norte de Portugal. Começa-se por fazer a definição da central incluindo implantação e funcionamento interno para produção de agregados de reciclagem para utilização na construção civil. Depois, com base em vários cenários considerados, faz-se a avaliação económica e o estudo de viabilidade da central

L’emploi de métakaolin dans la production de béton écologiquement efficace

Les matériaux d'addition font actueÌlement pâÌtie des développements les plus récents dans la production du bétoÌr, caÌ Ì'üÍilisation des additions apporte uno amélioration des proprìáés mécâniques et de Ìa durabilité dü béton. D'autre part, leur utilisation a pour objectif de réduire la consommation de ciment. en conlribuant de maniere simple et économique à résoudre les problèmes liés à l'environnenent. Cet aÌlicle étudie les avantâges et Ìes limitations de la substin-rtion pafiielle du ciment par le nétakaolin. Les résultats obtgnus démontrent que cette substituüon apporte, pouÌ Ìe béton de même ordre dc résistanccà ìa compressionu, ne améliorationc onsidérablea u niveaud e Ìa durabilité.C e qui penÌet de conclwe quc lc métakaolin est un substitut efficace câpable de rédui.e le contenu de ciment pour l'obtention de bétons équivalents

Constitutive modelling of creep-ageing behaviour of peak-aged aluminium alloy 7050

The creep-ageing behaviour of a peak-aged aluminium alloy 7050 was investigated under different stress levels at 174 ∘C for up to 8 h. Interrupted creep tests and tensile tests were performed to investigate the influences of creep-ageing time and applied stress on yield strength. The mechanical testing results indicate that the material exhibits an over-ageing behaviour which increases with the applied stress level during creep-ageing. As creep-ageing time approaches 8 h, the material's yield strength under different stress levels gradually converge, which suggests that the difference in mechanical properties under different stress conditions can be minimised. This feature can be advantageous in creep-age forming to the formed components such that uniformed mechanical properties across part area can be achieved. A set of constitutive equations was calibrated using the mechanical test results and the alloy-specific material constants were obtained. A good agreement is observed between the experimental and calibrated results

Low cost high performance concrete using low quality fly ash

High performance concretes (HPC) usually have been produced using silica fume, high quality fly ash and carefully chosen aggregates. These constituents increase drastically the initial cost of HPCs. The objectives of this research work were twofold. On one hand, it intended to study the possibility of producing low cost HPC, with 28 day strengths in the range of 65 MPa, using low quality fly ash and locally available crushed aggregates. On the other hand, to verify the impact of carbon content of fly ash on the strength and durability of HPC. For this purpose it was decided to enhance the ‘as received’ fly ash by eliminating particles coarser than 75m, thus reducing the carbon content by 50%. Compressive strength and diffusion coefficient of concretes replacing 0, 20%, 40%, and 60% of Portland cement by ‘as received’ fly ash, and 20% and 40% replacement by ‘enhanced’ fly ash, were determined. Comparing the results obtained, it was found that HPC with up to 65 MPa can be made by replacing up to 40% of cement by ‘as received’ and ‘enhanced’ fly ash and using the crushed granite aggregates. It was also observed that the carbon content did have little impact on the strength and durability of concrete. Furthermore, it was observed that the durability of concrete, as measured by diffusion coefficient, increased drastically when fly ash replaced partially Portland cement

Durability of high-performance concrete with fly ash

High-performance concrete (HPC) is usually produced using high quality materials. These high quality constituents drastically increase the initial cost of HPC, hence hindering its more widespread use. Therefore, the main goal of this research project was to produce a low cost enhanced performance concrete or even a low cost HPC, using low quality fly ash and locally available crushed aggregates. In this way, a significant reduction in the use of Portland cement, as well as that of scarce natural resources, would be obtained. The effect of high percentages of fly ash and crushed aggregates on concrete performance was studied by comparing the mechanical and durability performances of such concretes with nominally similar types of concrete with no fly ash incorporated. The results obtained indicate that it is possible to produce concrete in the low range of HPC with up to 65 MPa at 56 days by replacing up to 40% of cement with as received fly ash using crushed granite aggregates. Furthermore, it was observed that workability and durability both increased significantly when fly ash partially replaced Portland cement