Enhanced Optical Dichroism of Graphene Nanoribbons

The optical conductivity of graphene nanoribbons is analytical and exactly derived. It is shown that the absence of translation invariance along the transverse direction allows considerable intra-band absorption in a narrow frequency window that varies with the ribbon width, and lies in the THz range domain for ribbons 10-100nm wide. In this spectral region the absorption anisotropy can be as high as two orders of magnitude, which renders the medium strongly dichroic, and allows for a very high degree of polarization (up to ~85) with just a single layer of graphene. The effect is resilient to level broadening of the ribbon spectrum potentially induced by disorder. Using a cavity for impedance enhancement, or a stack of few layer nanoribbons, these values can reach almost 100%. This opens a potential prospect of employing graphene ribbon structures as efficient polarizers in the far IR and THz frequencies.Comment: Revised version. 10 pages, 7 figure

Disorder Induced Localized States in Graphene

We consider the electronic structure near vacancies in the half-filled honeycomb lattice. It is shown that vacancies induce the formation of localized states. When particle-hole symmetry is broken, localized states become resonances close to the Fermi level. We also study the problem of a finite density of vacancies, obtaining the electronic density of states, and discussing the issue of electronic localization in these systems. Our results also have relevance for the problem of disorder in d-wave superconductors.Comment: Replaced with published version. 4 pages, 4 figures. Fig. 1 was revise

Comportamento à tracção directa de elementos laminares de betão armado reforçado com fibras de aço

Quando comparado com o betão simples, o betão reforçado com fibras de aço, BRFA, apresenta melhor comportamento à fendilhação e maior capacidade de absorção de energia. Em consequência, a utilização de BRFA, em determinadas aplicações, pode ser uma mais valia em termos técnicos e económicos. O presente trabalho tem como principal objectivo avaliar a influência das fibras de aço no comportamento à tracção de elementos laminares de betão armado. Os principais resultados experimentais e numéricos são apresentados e discutidos

Post-cracking behaviour of steel fibre reinforced concrete (RILEM Recommendations)

Recently, RILEM TC 162-TDF Committee has proposed a specimen configuration, test procedures and flexural tensile strength parameters to characterize the post-cracking behaviour of the steel fibre reinforced concrete (SFRC). The approach proposed by RILEM is still yet under discussion, mainly, the ability of the flexural tensile strength parameters to simulate the post-cracking behaviour of the SFRC on the verifications to the ultimate and serviceability limit states. Taking the recommendations of the RILEM, the influence of the fibre content, the concrete age, and the percentage of cement replaced by fly ash on the bending behaviour of the SFRC was analysed in the present work. The influence of the fibre distribution on the fracture surface of the specimens was also considered. The experimental campaign was carried out with a cost competitive SFRC developed for industrial pavements. Bending tests with concrete specimens reinforced with conventional bars and steel fibres were also performed for developing a reinforcement system adjusted for panels of industrial pavements heavily loaded. The results obtained are presented and analysed.Fundação para a Ciência e a Tecnologia (FCT) - POCTI/34793/99

Tensile stress–crack width law for steel fibre reinforced self-compacting concrete obtained from indirect (splitting) tensile tests

In this work, the fracture mode I parameters of steel fibre reinforced self-compacting concrete (SFRSCC) were derived from the numerical simulation of indirect splitting tensile tests. The combined experimental and numerical research allowed a comparison between the stress-crack width (σ - w) relationship acquired straightforwardly from direct tensile tests, and the σ - w response derived from inverse analysis of the splitting tensile tests results. For this purpose a comprehensive nonlinear 3D finite element (FE) modeling strategy was developed. A comparison between the experimental results obtained from splitting tensile tests and the corresponding FE simulations confirmed the good accuracy of the proposed strategy to derive the σ – w for these composites. It is concluded that the post-cracking tensile laws obtained from inverse analysis provided a close relationship with the ones obtained from the experimental uniaxial tensile tests.The studies reported in this paper are part of the research project LEGOUSE (QREN, project nº 5387). This project is co-supported by FEDER through COMPETE program (“Programa Operacional Factores de Competitividade”). The materials were supplied by Radmix and Maccaferri (fibres), SECIL (cement), SIKA and BASF (superplasticizers), Omya Comital (limestone filler), and Pegop (Fly ash).The studies reported in this paper are part of the research project LEGOUSE (QREN, project nº 5387). This project is co-supported by FEDER through COMPETE program (“Programa Operacional Factores de Competitividade”). The materials were supplied by Radmix and Maccaferri (fibres), SECIL (cement), SIKA and BASF (superplasticizers), Omya Comital (limestone filler), and Pegop (Fly ash)

Avaliação da adequabilidade das leis propostas pela Rilem para caracterizar o comportamento fendilhado do betão reforçado com fibras de aço

Nos últimos dois anos têm sido efectuados ensaios de flexão, segundo as recomendações do grupo de trabalho TC 162-TDF da RILEM, para caracterizar o comportamento à flexão do betão reforçado com fibras de aço (BRFA). Os resultados obtidos foram utilizados, em conjunto com um modelo numérico desenvolvido, por forma a avaliar o desempenho das leis constitutivas recomendadas pelo referido grupo. Foi dado especial ênfase à modelação do comportamento do BRFA fendilhado, quer por intermédio de lei tensão-extensão (σ-ε), que por meio de lei tensão abertura de fenda (σ-w). A investigação numérica efectuada é descrita e os principais resultados obtidos são apresentados e comentados.Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BSAB/291/2002-POCTI, POCTI/34793/99 "Cost competitive steel fibre reinforced concrete for industrial pavements