Polarization Charge Distribution in Gapped Graphene

We study the distribution of vacuum polarization charge induced by a Coulomb impurity in massive graphene. By analytically computing the polarization function, we show that the charge density is distributed in space in a non-trivial fashion, and on a characteristic length-scale set by the effective Compton wavelength. The density crosses over from a logarithmic behavior below this scale, to a power law variation above it. Our results in the continuum limit are confirmed by explicit diagonalization of the corresponding tight-binding model on a finite-size lattice. Electron-electron interaction effects are also discussed.Comment: 6 pages, 4 figures; expanded versio

The effect of loading direction on the compressive behaviour of a 3D printed cement-based material

Nowadays, additive manufacturing is being widely employed in several areas and is starting to be considered for the construction sector amongst the digital construction trend. The advantages that the additive manufacturing techniques can bring over the traditional construction methods are propelling multiple research projects within the field of 3D concrete printing. Technologies used for printing, material compositions and their rheological and mechanical properties are some of the research areas on 3D concrete printing. In this work, it is used a wet extrusion method for printing a cement-based mortar mixture. The compressive behaviour of printed specimens was evaluated based on the direction of loading. The results showed that with a proper printing process and rheological properties, in the case of the current mid-strength matrix, the effect of the layers interfacial behaviour on the compressive behaviour of printed specimens was reduced.(undefined

Development of cement-based mortars for 3D printing through wet extrusion

The construction sector is connoted as an extremely traditional business sector since long ago. However, due to the increase of the global competiveness, there is a demand on the development of new building materials and construction methods that can bring added value to the companies. The 3D concrete printing is a novel construction approach within digital construction that can offer a higher degree of optimization and flexibility for producing either structures or structural elements with complex geometries. One of the main challenges in the 3D concrete printing using wet extrusion is balancing properly the rheological and mechanical properties of the printable mixtures. In this study, several mixtures were developed and their capability for being used in 3D printing was assessed and discussed based on their rheological properties. The compressive strength of the matrices that could be properly printed are also presented.(undefined

Low-Complexity Equalisers for Offset Constellations in Massive MIMO Schemes

This work was supported in part by the European Regional Development Fund (FEDER), through the Competitiveness and Internationalization Operational Program of the Portugal 2020 Framework, in part by the Regional OP Centro under Grant POCI-01-0145-FEDER-030588, in part by the Regional OP Lisboa under Grant Lisboa-01-0145-FEDER-03058, in part by the FCT/MEC through national funds of MASSIVE5G Project under Grant SAICT-45-2017-02 and PES3N Project under Grant 2018-SAICT-45-2017-POCI-01-0145-FEDER-030629, in part by the UID/EEE/50008/2019 Project, and in part by the FCT Ph.D. under Grant SFRH/BD/108522/2015.Massive multi-input-multi-output (m-MIMO) schemes require low-complexity implementations at both the transmitter and the receiver side, especially for systems operation at millimeter wave (mmWave) bands. In this paper, we consider the use of offset constellations in m-MIMO systems operating at mmWave frequencies. These signals are designed to have either an almost constant envelope or be decomposed as the sum of constant-envelope signals, making them compatible with strongly nonlinear power amplifiers, which can have low-implementation complexity and high amplification efficient, making them particularly interesting for mmWave communications. We design and evaluate low-complexity frequency-domain receivers for offset signals. It is shown that the proposed receivers can have excellent performance/complexity trade-offs in m-MIMO scenarios, making them particularly interesting for future wireless systems operating at mmWave bands.publishersversionpublishe

Contribuição para a avaliação do comportamento térmico de edifícios tradicionais portugueses

Os edifícios tradicionais portugueses apresentam especificidades técnicas locais. O clima e os materiais de construção disponíveis localmente são fatores condicionantes dessas especificidades. Por exemplo, os edifícios tradicionais algarvios, as açoteias, têm a cobertura em terraço e são pintados de branco. Por sua vez, os edifícios de xisto e de tabique do Alto Douro têm a cobertura inclinada com beirais e vãos de pequenas dimensões. Os edifícios de granito e de tabique das regiões de Trás-os-Montes e do Minho também têm coberturas inclinadas com beirais. Contudo, com o fenómeno da globalização, tem havido uma tendência de uniformização dos padrões construtivos e as especificidades locais parecem ter vindo a ser negligenciadas. O aparecimento de problemas ambientais resultantes, em grande parte, da emissão de gases de efeito de estufa para a atmosfera provenientes do aumento do consumo energético nos edifícios implica a adoção de medidas conducentes à redução desse consumo. Pensa-se que o conhecimento das características técnicas dos edifícios tradicionais de cada região poderá contribuir para minimizar este consumo e tornar assim os edifícios novos ou os edifícios a reabilitar mais eficientes energeticamente e, implicitamente, mais sustentáveis. Neste sentido, foi desenvolvida uma metodologia experimental, para avaliar o coeficiente de transmissão térmica de elementos construtivos de edifícios, passível de ser aplicada “in situ” e em laboratório. Esta metodologia permite uma monitorização contínua desse desempenho térmico durante vários dias e em condições termo higrométricas reais. Nesta comunicação serão apresentados alguns dos resultados obtidos através da aplicação desta metodologia a elementos construtivos e materiais existentes nos edifícios tradicionais da região de Trás-os-Montes e Alto Douro

Numerical simulation of galvanized rebars pullout

The usage of rebars in construction is the most common method for reinforcing plain concrete and thus bridging the tensile stresses along the concrete crack surfaces. Usually design codes for modelling the bond behaviour of rebars and concrete suggest a local bond stress – slip relationship that comprises distinct reinforcement mechanisms, such as adhesion, friction and mechanical anchorage. In this work, numerical simulations of pullout tests were performed using the finite element method framework. The interaction between rebar and concrete was modelled using cohesive elements. Distinct local bond laws were used and compared with ones proposed by the Model Code 2010. Finally an attempt was made to model the geometry of the rebar ribs in conjunction with a material damaged plasticity model for concrete

Ductility and durability of strain hardening cementitious composites in the marine environment

Modern structures are being exposed to severe environments and the lack of durability is one of the most serious problems in concrete infrastructures. Structural concrete exposed to marine environment deserves special attention as the sea salts chemically react with the cement matrix and the steel reinforcement which results in loss of strength, cracking, spalling, etc. The challenges of Civil Engineering, especially within the structures in extreme environments, pose considerable expectations with regards to the development of fibre reinforced materials for the development of more resistant and durable solutions. In the present work, the behaviour of an Engineered Cementitious Composite (ECC) was studied. All the specimens prepared were cured in 4 types of environments: exposed to air (20ºC of temperature and 60% of humidity) immersed in tap water, immersed in salted water and immersed in seawater, all at an average temperature of 18ºC. A series of experiments, including compressive and direct tension tests were carried out to characterize the mechanical properties of the ECC materials while exposed to different environments. The most important characteristic of ECC, which include multiple-cracking behaviour at increasing tensile strains when subjected to increasing tensile loading, was confirmed in all types of curing environments. In all cases the cementitious composites performed well with regards to the strain hardening behaviour typically observed in these materials, although the cracking processes have shown different characteristics. Due to the ability of the material to control crack opening below extremely low values, typically under 100 µm, the durability of structures can be significantly improved when ECC materials are used in the in marine environments. It was shown also that the salted water does not represent well the effect of seawater while characterising ECC mechanical characteristics in the laboratory.The authors acknowledge the material suppliers Secil, Civitest, Sika, Saint-Gobain (Fibraflex) and Eurocálcio – Calcários e Inertes SA for providing the materials used in this study

Modeling disorder in graphene

We present a study of different models of local disorder in graphene. Our focus is on the main effects that vacancies -- random, compensated and uncompensated --, local impurities and substitutional impurities bring into the electronic structure of graphene. By exploring these types of disorder and their connections, we show that they introduce dramatic changes in the low energy spectrum of graphene, viz. localized zero modes, strong resonances, gap and pseudogap behavior, and non-dispersive midgap zero modes.Comment: 16 pages, lower resolution figure