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A novel approach for the practical realization of the feeding lines in connected array of slots is presented. The proposed feed concept is based on a parallel-plate waveguide (PPW) structure and allows simplifying the unit cell design. When compared to the typical integrated coaxial feed, the PPW-based feeding structure can reduce the cost and complexity of the array, by resulting in a lower number of metal layers and by allowing for large diameter vias. Two examples of the unit cell design, based on this new feed concept, are presented; one consists in a single-polarized array of connected slot, covering the band from 70 to 140 GHz; another example is referring to a single- and dual-polarized connected slot array covering the 2-8 GHz range.

This paper presents an analytical and numerical investigation of the radiation features of an interesting class of electromagnetic bandgap (EBG) structures excited by an electric line source. The radiation from two-dimensional (2-D) lattices made with lossless dielectric cylinders, typically optimized by using the Bloch analysis of the corresponding 2-D photonic crystals, are originally investigated in terms of leaky waves. A thorough modal analysis of open waveguides, composed by a finite number of periodic chains of circular dielectric rods, is presented that shows a multiplicity of bound and leaky modes. Two radiation windows are identified and the relevant directive features are described in terms of properly excited dominant leaky modes. The possible excitation of higher order leaky modes and guided modes has been carefully considered, by also capturing the relevant residue contributions in a nonspectral representation of the excited fields. The final results on radiated fields by realistic truncated structures, obtained by ad-hoc software as well as full-wave EM simulators, are in excellent agreement with those predicted by the proposed leaky-wave approach.

The dynamics of the Alps and surrounding regions is still not completely understood, partly because of a non‐unique interpretation of its upper‐mantle architecture. In particular, it is unclear if interpreted slabs are consistent with the observed surface deformation and topography. We derive three end‐member scenarios of lithospheric thickness and slab geometries by clustering available shear‐wave tomography models into a statistical ensemble. We use these scenarios as input for geodynamic simulations and compare modeled topography, surface velocities and mantle flow to observations. We found that a slab detached beneath the Alps, but attached beneath the Northern Apennines captures first‐order patterns in topography and vertical surface velocities and can provide a causative explanation for the observed seismicity., Plain Language Summary: Present‐day surface deformation, including earthquakes, plate motion, and mass (re)distribution, results from processes operating at the surface and in the interior of the Earth. Understanding these processes and their coupling is of utmost importance in light of the hazard they pose to society. The Alps provide an excellent natural laboratory to understand such coupling. Here, we use seismic tomography models to constrain its upper‐mantle architecture. We further use these models to quantify forces originating from the resolved architecture and their effects on the present‐day surface deformation. The models can reproduce first‐order patterns in the observed topography and vertical surface motions. We found a causative correlation between the presence of a shallow slab attached to the overlying lithosphere in the Northern Apennines and the seismicity in the region. Our results allow us to better understand the transfer of internal forces to the surface, thereby helping to quantify the present‐day mechanical setup of the area., Key Points: Statistical ensemble of S‐wave tomography models is used to infer the Lithosphere‐Asthenosphere Boundary configuration and slab geometries in the Alps. The 3‐D upper‐mantle architecture from the statistics reproduce first‐order patterns in observed topography and Global Navigation Satellite Systems vertical velocities. A shallow/attached slab in the Northern Apennines is consistent with the mantle depth seismicity observed in this region., Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659, https://doi.org/10.5880/GFZ.4.5.2019.004, https://gfzpublic.gfz-potsdam.de/pubman/item/item_238001, http://ds.iris.edu/ds/products/emc-earthmodels/, https://doi.org/10.5281/zenodo.7071571, https://doi.org/10.5281/zenodo.6538257

It is well recognized that the arid climate is an essential parameter that causes various problems, such as shrinkage cracking, on concrete in hot weather, especially at an early age. Adrar's region is characterized by its severe aridity and is considered one of the world's warmest areas. The present experimental study aims to study the influence of this arid climate on concrete cracking. In this context, the influence on hardened concretes, made with local materials exposed to local climatic conditions and using the same concrete formulation used in this region, was investigated. The study of concrete cracking was conducted in this work carried out by Standard ASTM C1579. The concrete types investigated here are concrete without adjuvant, concrete with adjuvant, and concrete treated with a curing product. Each concrete underwent two modes of preservation; the first inside the lab and the second outside, in the open air. Also, this work provides results of measurements of the consistency, temperature of fresh concrete, the onset and development of cracks; the findings then compared. It found that those cracks were always present in concrete, and their appearance and evolution were faster for concretes exposed directly to the arid climate. However, it noted that cracks decreased by about 50% in length and 66% in width with the adjuvant's introduction. A further decrease of approximately 40% and 50% in length and width, also reported after treatment with the curing product. It should be noted that all the samples stored inside the laboratory showed more minor cracks whose appearance and evolution slower in comparison with those of samples kept outside, in the open air.

The recent failure of buildings because of punching shear has alerted researchers to assess the reliability of the punching shear design models. However, most of the current research studies focus on model uncertainty compared to experimentally measured strength, while very limited studies consider the variability of the basic variables included in the model and the experimental measurements. This paper discusses the reliability of FRP-reinforced concrete slabs’ existing punching shear models. First, more than 180 specimens were gathered. Second, available design codes and simplified models were selected and used in the calculation. Third, several reliability methods were conducted; therefore, three methods were implemented, including the mean-value first-order second moment (MVFOSM) method, the first-order second moment (FOSM) method, and the second-order reliability method (SORM). A comparison between the three methods showed that the reliability index calculated using the FOSM is quite similar to that using SORM. However, FOSM is simpler than SORM. Finally, the reliability and sensitivity of the existing strength models were assessed. At the same design point, the reliability index varied significantly. For example, the most reliable was the JSCE, with a reliability index value of 4.78, while the Elgendy-a was the least reliable, with a reliability index of 1.03. The model accuracy is the most significant parameter compared to other parameters, where the sensitivity factor varied between 67% and 80%. On the other hand, the column dimension and flexure reinforcement are the least significant parameters compared to other parameters where the sensitivity factor was 0.4% and 0.3%, respectively.

In Ethiopia, sawmills have poor capacity utilization primarily due to the outdated equipment that resulted in a low recovery rate and the production of a high amount of wastage. The lumber recovery rate is the output (lumber) of a log in the sawing process. In Ethiopia, Cupressus lusitanica is significantly used for lumber, for furniture production, construction, poles and posts. Sampled logs were processed according to the normal production rate and standard lumber dimension of the sawmill for the purpose of estimating the lumber recovery rate. The present study aimed to investigate the lumber recovery rate of C. lusitanica and the factors affecting it. A total of 26.93 m3 of lumber was produced by the sawmilling operation, representing 72.86% of the overall lumber recovery rate. Furthermore, the sawdust and slabs were recorded as 2.92 m3 (7.90%) of sawdust and 7.11 m3 (19.24%) of slabs, respectively. There were a number of factors that decreased the magnitude of the lumber recovery rate. It was observed that cutting using a wider saw kerf caused a reduction in the rate of lumber recovery owing to the generation of an increased quantity of sawdust. The lumbers were air-seasoned in the sawmill yard. Maximizing the volume of the lumber recovered from the logs can increase the sawmill profitability, lessen the effects of climate change, ensure the sustainable use of natural resources, enhance the energy efficiency and manage wood waste (e.g., recycling and prevention) for green economic development and industrial transformation. This species has a great demand in the wood industry of Ethiopia; hence, the plantation and yield of C. lusitanica must be expanded in order to provide sustainable forestry, protect valuable forest resources and safeguard the biodiversity in the country.

In this paper the strength of steel deck-concrete composite slabs is studied. Conventional slabs with open and dovetail profiles are studied with different designs, galvanized and stainless steel. Partial connection method (PCM) in Eurocode-4 is permitted for slabs with ductile longitudinal shear behavior. Ductility, as explained in the clause 9.7.3(3), depends on the fact that the failure load exceeds the load corresponding to the slip (0.1 mm) in more than 10%; then the slab is classified as ductile. Apparently, the aim of this criterion is to ensure the slab can undergo further deflections until the whole yielding of the cross-section is achieved, as required hypothesis by the PCM. The aim of this paper is to study the stresses developed at the cross-section of composite slabs with open and dovetail profile, in order to evaluate the yielding degree of the cross-section. Based on the results of this study, it is recommended to exclude open profiles from partial connection method (PCM), even if the slab is classified as ductile through Eurocode definition. Although all studied specimens are classified as ductile according to Eurocode-4 clause 9.7.3(3), the observed stress distribution at cross-section proves that yielding occurs in the slab with re-entrant profile only, whereas slabs with open-rib profiles remain almost elastic until their brittle failure.

This study aims to investigate the flexural behavior of high-strength thin slabs externally strengthened with fiber-reinforced polymer (FRP) laminates through a numerical simulation. A three-dimensional (3D) finite element (FE) model is created to simulate the response of strengthened reinforced concrete (RC) slabs under a four-point bending test. The numerical model results in terms of load-deflection behavior, and ultimate loads are verified using previously published experimental data in the literature. The numerical results show a good agreement with the experimental results. The FE model is then employed in a parametric study to inspect the effect of concrete compressive strength on the performance of RC thin slabs strengthened with different FRP types, namely carbon fiber-reinforced polymers (CFRP), polyethylene terephthalate fiber-reinforced polymers (PET-FRP), basalt fiber-reinforced polymers (BFRP) and glass fiber-reinforced polymers (GFRP). The results showed that the highest strength enhancement was obtained by the slab that was strengthened by CFRP sheets. Slabs that were strengthened with other types of FRP sheets showed an almost similar flexural capacity. The effect of concrete compressive strength on the flexural behavior of the strengthened slabs was moderate, with the highest effect being a 15% increase in the ultimate load between two consecutive values of compressive strength, occurring in the CFRP-strengthened slabs. It can thus be concluded that the developed FE model could be used as a platform to predict the behavior of reinforced concrete slabs when strengthened with different types of FRP composites. It can also be concluded that the modulus of elasticity of the composite plays a major role in determining the flexural capacity of the strengthened slabs.

Orientador: David de Carvalho Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Civil, Arquitetura e Urbanismo Resumo: Abstract: Mestrado Estruturas e Geotécnica Mestre em Engenharia Civil

Orientador: Leandro Mouta Trautwein Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Civil, Arquitetura e Urbanismo Resumo: Nas últimas décadas, lajes lisas têm sido amplamente empregadas em edificações, principalmente devido a sua concepção, que permite aliar eficiência estrutural aos requisitos arquitetônicos. Entretanto, esse sistema estrutural está sujeito à punção, um modo de falha que ocorre por cisalhamento em elementos delgados submetidos à carga ou reação concentrada. O objetivo desta pesquisa foi aplicar os conceitos das teorias de probabilidade e de confiabilidade na avaliação dos modelos de dimensionamento de ligações laje-pilar submetidas à punção, verificando as probabilidades de falha e índices de confiabilidade para cada estado limite de punção. Nove lajes lisas quadradas com 1800 mm de lado e com 130 mm de espessura foram ensaiadas até a ruptura, a fim de se verificar o comportamento desse sistema estrutural com ou sem a presença de aberturas. Além disso, os ensaios experimentais possibilitaram a obtenção dos parâmetros estatísticos das variáveis de projeto consideradas nas análises de confiabilidade, como aquelas relacionadas à resistência dos materiais, algumas propriedades geométricas e a carga de ruptura da laje. Uma investigação de modelos analíticos de previsão da carga de ruptura desse sistema estrutural complementou o programa experimental. Foram analisadas quatro especificações de projeto: EUROCODE 2 (2004), CEB-FIP/MC (2010), ACI 318 (2014) e ABNT NBR 6118 (2014). O estudo envolveu 249 resultados experimentais de carga de ruptura de lajes lisas com diferentes características físicas e geométricas para que estatísticas da variável erro de modelo pudessem ser construídas. Os resultados obtidos foram comparados com aqueles determinados através da modelagem teórica de Marques (2018) implementada computacionalmente. Por fim, a determinação das probabilidades de falha e índices de confiabilidade foi realizado com o acoplamento de algoritmos de confiabilidade aos modelos mecânicos de previsão da carga de ruptura de lajes lisas de concreto armado descritos anteriormente. Os algoritmos utilizados foram o Método de Simulação de Monte Carlo (MCS) e o Método de Confiabilidade de Primeira Ordem (FORM) Abstract: In last decades, flat slabs have been widely used in buildings, mainly due to their design, which aligns structural efficiency to the architectural requirements. However, this structural system is subject to punching, a failure mode that occurs by shearing in thin elements subjected to the load or concentrated reaction. The objective of this research was to apply the concepts of probability and reliability theories in the evaluation of the slab-column design models submitted to punching and also verify the probabilities of failure and reliability indexes for each punching limit state. Nine square slabs with 1,800 mm of side and 130 mm of thickness were tested until the rupture, in order to verify the behavior of this structural system with or without openings. In addition, the experimental tests allowed to obtain the statistical parameters of the design variables considered in the reliability analyzes, such as those related to the strength of the materials, some geometric properties and the load of rupture of the slab. An investigation of analytical models to predict the rupture load of this structural system complemented the experimental program. Four project specifications were analyzed: EUROCODE 2 (2004), CEB-FIP/MC (2010), ACI 318 (2014) and ABNT NBR 6118 (2014). The study involved 249 experimental results of load of rupture of flat slabs with different physical and geometric characteristics so that statistics of the model error variable could be constructed. The results were compared with those determined through the theoretical modeling of Marques (2018) computationally implemented. Finally, the determination of the probabilities of failure and reliability indexes was performed by coupling reliability algorithms to the mechanical models for predicting the load of rupture of flat slabs of reinforced concrete previously described. The algorithms used were the Monte Carlo Simulation Method (MCS) and the First Order Reliability Method (FORM) Doutorado Estruturas e Geotécnica Doutor em Engenharia Civil

A new miniaturized figure-of-eight coil (µCoil) for TMS applications has been developed taking advantage of the Flex circuit technology. First experiments on volunteers demonstrated the ability of the µCoil to elicit sensorial action potentials of the peripheral nervous system. The necessity of reducing the size of standard TMS stimulator arises from the poor spatial resolution of the latter. This study aims to model the µCoil and study the electromagnetic fields induced inside the arm during peripheral nerve stimulation. Results confirmed that the µCoil is capable of inducing a focalized electric field inside the tissues with amplitudes up to 70V/m consistent with the observed peripheral nerve stimulation in healthy volunteers.

The vibration serviceability assessment of slender and/or lightweight pedestrian systems with high sensitivity to walk-induced effects is rather challenging. In the same way, laminated glass (LG) is used in buildings for structural applications but still represents a not well known and vulnerable material. For pedestrian LG systems, the characterization of dynamic and mechanical parameters may require specific procedures which do not adapt from other constructional typologies. Among others, the mass of pedestrians is generally high compared with LG structural components. Size and restraints in LG may also lead to more pronounced vibration effects. For existing LG systems, moreover, knowledge of residual capacity may be rather difficult. In this paper, an original uncoupled experimental investigation is proposed to numerically address the accuracy and potential of low-cost laboratory body measures for vibration analysis of LG slabs to support (or even replace) field tests or more complex calculation approaches. A total of 40 experimental records are taken into account, in the form of body center of mass (CoM) acceleration time histories for an adult volunteer walking on a rigid concrete slab and equipped with a single high-precision, Wi-Fi triaxial sensor based on micro electromechanical systems (MEMS) technology. Body CoM records are elaborated and used as input for finite element (FE) nonlinear dynamic analysis in the time domain (WL1) of two LG slab configurations (GS1 and GS2) with identical geometry but different boundaries. A third reinforced concrete slab of literature (CS3) is also investigated for further assessment. Numerical parametric results from a total of 120 WL1-based nonlinear dynamic analyses are compared with FE numerical results based on a conventional deterministic approach (WL2) to describe walk-induced effects, as well as towards past field experiments (GS2). The accuracy and potential of the proposed procedure are discussed.

Specific to blast loading, bond shear tests between carbon fiber reinforced polymer (CFRP) strips and concrete have not yet been reported in literature, to the best knowledge of the authors. Given the high potential of strengthening concrete with CFRP to increase blast resistance, it is necessary to better understand the dynamic interaction between concrete and CFRP under blast impulse. This article presents a new experimental setup developed in order to study blast driven bond interaction between CFRP and concrete. An evaluation of the bond at the interface between the CFRP strip and the concrete is conducted in order to identify the parameters that affect the bond strength under blast loading. Several retrofitted specimens with different bond strength are tested. An explosive driven shock tube (EDST) set-up is used to generate the blast loading. Piezo resistive strain gauges and digital image correlation (DIC) measurement are used to record the debonding process and the evolution of the strain along the bonded area. As such, the bond behaviour is studied for three different bond lengths and the effect of interaction of the blast wave within the concrete is highlighted. The experimental tests demonstrate that under blast loading, the debonding between the CFRP strip and the concrete occurs due to a combined effect: interface stresses caused by the induced force in the CFRP and additional stresses resulting from the propagation of the blast induced stress wave within the concrete.

This study presents the results of an investigation of the effect of macro synthetic fibers (MSF) reinforcement on the load carrying capacity of concrete pavement slabs. Six concrete slabs having dimensions of 800 × 800 × 50 mm3 were prepared and tested under static loads at three different positions: interior, edge and corner of the slab. Three of the slabs were Portland cement concrete (PCC) and prepared as references. The other three slabs were macro synthetic fiber reinforced concrete (MSFRC). Mechanical properties examined in this study included compressive strength, splitting tensile strength, flexural strength and modulus of elasticity and ductility of PCC and MSFRC. The findings showed that the addition of MSF to PCC improved the load carrying capacity of concrete pavement slabs. Test results obtained indicated that the ultimate load carrying capacity of MSFRC slabs was increased by 24%, 20%, and 23% for interior, edge and corner loading positions, respectively.

Punching shear is a possible type of failure that occurs in reinforced concrete flat slabs, which can develop with an ultimate load below flexural capacity. Several researchers have studied the punching resistance of flat slabs over recent years. Although they have made great advances, there are codes that show different approaches to a singular design. Some codes show that there exist contradictions, even in the simplest situations, such as concentric loads. Most codes prescribe empirical expressions based in a theoretical model to analyze punching strength, but for flat slabs with holes around the column and shear reinforcement there are divergences between codes, justifying research in this area. This paper presents an experimental analysis of nine square reinforced concrete flat slabs under concentric loading (width: 1800 mm; thickness: 130 mm). The main variables used in the tests were: a) two square openings (150 mm) adjacent to the smallest side of the column and b) the use of shear reinforcement containing 3 layers, with 6 or 8 elements in each layer and radially distributed around the column. The research concludes that openings adjacent to the column affect punching shear strength, while the correct use of the shear reinforcement can minimize and even compensate this loss.

Structural design with highly flowable Fibre Reinforced Concrete has to duly take into account the preferential alignment of fibers, which can be governed through the rheological properties of the fluid mixture and the casting process and by the geometry of the structure. The possibility of predicting the fiber alignment, by tailoring the casting process, and of non-destructively monitoring it, can foster more efficient structural applications and design approaches. Focusing on UHPFRC slabs with pre-arranged casting defects, the flow-induced alignment of the fibers has been predicted by means of a suitable CFD modelling approach and hence monitored via a non-destructive method based on magnetic inductance properties of the fiber reinforced composite. The comparison between the assessed data on the fiber orientation and the crack patterns as visualized by image analysis supports the effectiveness of casting flow modelling and non-destructive fiber dispersion monitoring in supporting the structural design of elements made with highly flowable fiber reinforced cementitious composites.

A compact cylindrical micro-lens is designed for efficient out-coupling and collimation of THz radiation from a photoconductive THz antenna placed on the backside of a semiconductor substrate. The full radiation pattern of the 3D lens-antenna system is calculated rigorously by using the Green’s Function Volume Integral Equation Method in a form that exploits cylindrical symmetry. The lens is designed for an optimum amount of radiation emitted in a useful range of directions relative to the total emission taking into account the Purcell effect and radiation trapped in the semiconductor slab.

The technique presented in this paper points to the recovery of three dimensional rigid casts from natural molds of fossil organisms. The study of organisms preserved as molds in the rock (usually in slabs) is difficult, even if the natural mold is highly detailed. Traditionally, casts were made from each side of the natural mold, resulting in positive, soft partial specimens embedded in the rock. These kind of casts did not solve some issues related to the preservational mode and exposure of the natural molds. The application of this technique turns a natural rigid mold into a soft silicone rubber mold to make a final three dimensional rigid cast of the whole specimen devoid of sediment. This technique provides casts that preserve the details of the natural mold. The obtained casts allow and facilitate scientific anatomical studies but also provide excellent specimens for museum exhibition or educational purposes. La técnica que presentamos en esta contribución apunta a la obtención de réplicas rígidas de fósiles que se preservaron en las rocas como moldes naturales. El estudio de organismos preservados de esta forma (comúnmente en lajas) es dificultoso, aun si el molde tiene una excelente preservación. Tradicionalmente, dicha dificultad se ha resuelto realizando copias de los moldes naturales, obteniendo positivos (réplicas) parciales y flexibles del espécimen. Este tipo de copias facilitan el estudio pero no resuelven todos los problemas relativos a la preservación en sí misma y a la exposición del ejemplar fósil. La técnica que aquí desarrollamos convierte, mediante una serie de pasos intermedios, un molde natural rígido en uno flexible del que a su vez se obtienen copias rígidas, tridimensionales y libres de sedimento del espécimen fósil, que conserva todos los detalles del molde natural. Las copias obtenidas sirven no solo para facilitar el estudio anatómico morfológico de los restos fósiles sino también como material didáctico y de exhibición de gran calidad. Fil: Reuil, Santiago. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; Argentina Fil: Muzzopappa, Paula. Universidad Maimónides. Centro de Ciencias Naturales, Ambientales y Antropológicas; Argentina. Fundación de Historia Natural Félix de Azara; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

This work presents the results of an experimental and numerical investigation on the flexural behaviour of reinforced concrete (RC) slabs strengthened by carbon fibre reinforced polymers (CFRP) laminates applied according to the near surface mounted (NSM) technique, using stiff and flexible adhesives. Two study variables were analysed: i) the adhesive type and ii) the existence or not of pre-cracking on the slabs. The results show a clear dependence of slab’s flexural performance on the adhesive type: the use of flexible adhesive yields to 80% of the maximum load achieved with stiff adhesives. The existence of pre-damage did not affect the structural behaviour of the slabs. CFRP rupture was observed with the use of stiff adhesives while CFRP debonding has occurred with the flexible one. Finally, a higher ductility was observed when using flexible adhesive. A numerical model was worked out and calibrated to simulate the flexural behaviour of the tested slabs. The numerical simulations showed a very good agreement with the experiments. Besides the very good predictive performance in terms of load-displacement behaviour, the numerical model also correctly reproduced the failure modes obtained in the experiments and the differences in the bonding mechanisms of slabs strengthened with the stiff and flexible adhesives. It was demonstrated that the proposed numerical model can be used in engineering practice for the analysis and design of NSM CFPR strengthening systems for existing RC structures., FEDER funds through the Operational Program for Competitiveness Factors – COMPETE and National Funds through FCT (Portuguese Foundation for Science and Technology) under the project FRPLongDur POCI-01-0145-FEDER-016900 (FCT PTDC/ECMEST/1282/2014) and partly financed by the project POCI-01-0145-FEDER-007633. The first and third authors acknowledge the grants SFRH/BD/131259/2017 and SFRH/BSAB/150266/2019 provided by FCT, respectively, financed by European Social Fund and by national funds through the FCT/MCTES

Fibre reinforced concrete shows enhanced performance in statistically redundant bi-dimensional structural elements that undergo biaxial bending. However, the lack of reinforcing rebars in fibre reinforced structural elements may affect the structural ductility which may further affect the overall load bearing capacity of these structures. To investigate the influence of fibres in such elements, six concrete plates of 2000 × 2000 × 150 mm reinforced with steel fibres and/or reinforcing rebars are tested under a central concentrated load. Two of the elements are reinforced with only 35 kg/m3 of steel fibres, two are reinforced with 2-way conventional reinforcing rebars (35 kg/m3, in each direction) and two are reinforced with both steel fibres and rebars. The specimens are simply supported at the middle of each side by means of a bilateral restraint; the deflection response and cracking behaviour of all the specimens are recorded and compared. Moreover, the methodology introduced in the fib Model Code 2010 for design of steel fibre reinforced concrete is implemented to predict the ultimate load bearing capacity of these elements and its reliability is determined in comparison with the experimental values. The comparison of the behaviour of the specimens reinforced only with steel fibres, with those reinforced with steel rebars, shows the higher efficiency of steel fibres in terms of load carrying capacity, but with a lower ductility. The combination of steel fibres and rebars allows for a better exploitation of the capacity of both reinforcement solutions. Finally, the reliability of the approach implemented for the ultimate load prediction is shown and the need of rebars in providing ductility in fibre reinforced concrete members is underlined.

Many factors like nonlinear material properties, cracking of concrete, creep and shrinkage of concrete are the parameters which influence the deflections of slab strips. The literature review points out that analytical methods are required to predict the entire load deflection behaviour of restrained slab strips under short term loading. A method is proposed to predict the same for Traditionally Vibrated Concrete (TVC) and Geopolymer Concrete (GPC) slabs. The results are compared with the prediction of Rankine et al and Eyre and Kemp. The results show that the proposed method is able to predict the load deflection behaviour satisfactorily of restrained TVC/GPC slab strips.

This work describes an innovative application of structures of photonic crystal slabs for gas sensors. Structures consisting in double ring have been studied by simulation. Their working mechanism is based in the effect of a given target gas in the pores of the slab. The effect pursued is to translate the variation of refraction index of the gas inside the pores into a shift in the position of peaks of transmission associated to resonances in the device

Precast hollow core slabs (HCS) are technically advanced products in the precast concrete industry, widely used in the last years due to their versatility, their multipurpose potential and their low cost. Using three dimensional FEM (Finite Element Method) elements, this study focuses on the stresses induced by the prestressing of steel. In particular the investigation of the spalling crack formation that takes place during prestressing is carried out, since it is important to assure the appropriate necessary margins concerning such stresses. In fact, spalling cracks may spread rapidly towards the web, leading to the detachment of the lower part of the slab. A parametric study takes place, capable of evaluating the influence of the tendon position and of the web width on the spalling stress. Consequently, after an extensive literature review on the topic of soft computing, an optimization of the HCS is performed by means of Genetic Algorithms coupled with 3-D FEM models. Keywords

A three-story apartment building on the University of Miami campus in Florida, built in 1947 and scheduled for demolition in the spring of 2011, was used as a research test bed. Two identical strips of the one-way reinforced concrete (RC) slab of the first floor of the building were saw cut and load tested. In parallel with the well-established measurements of load and deflection, a continuous acoustic emission (AE) monitoring was carried out throughout the load test in order to estimate the AE evaluation parameters that relate to RC damage. The analysis of the AE results provides experimental evidence for the validity of AE technology as an assessment tool in the practice of in situ load testing.

A finite difference analysis was performed to investigate the behavior of one-way reinforced concrete slabs exposed to fire. The objective of the study was to investigate the fire resistance and the fire risk after extinguishing the fire. Firstly, the fire resistance was obtained using the ISO834 standard fire without cooling phase. Secondly, the ISO834 parametric fire with cooling phase was applied to study the effect of cooling time. Accordingly, the critical time for cooling was identified and the corresponding failure time was calculated. Moreover, the maximum risk time which is the time between the fire extinguishing and the collapse of slab was obtained. Sixteen one-way reinforced concrete slabs were considered to study the effect of important parameters namely: the concrete cover thickness; the plaster; and the live load ratio. Equations for heat transfer through the slab thickness were used in the fire resistance calculations. Studying the cooling time revealed that the slabs are still prone to collapse although they were cooled before their fire resistance. Moreover, increasing the concrete cover thickness and the presence of plaster led to an increase in the maximum risk time. However, the variation in the live load ratio has almost no effect on such time.

Este trabalho trata do projeto e construção de uma plataforma para a realização de ensaios dinâmicos especialmente com pessoas pulando, caminhando, etc. Procurou-se inicialmente localizar projetos já executados de plataformas de ensaios dinâmicos e estudar os carregamentos produzidos pelo movimento de pessoas em lajes e a resposta das estruturas a esses carregamentos. Foram estudados também os limites estabelecidos por diferentes normas para essas respostas dinâmicas, tendo em vista tanto o estado limite último como a estrutura em serviço, já que o corpo humano é bastante sensível às vibrações estruturais. Realizaram-se estudos paramétricos considerando várias configurações de lajes (diferentes vãos, espessuras e condições de apoio), buscando uma configuração que possuísse frequência natural próxima das frequências dos carregamentos humanos. A laje deveria ter dimensões apropriadas ao espaço físico disponível, frequência fundamental inferior a 5 Hz e flecha máxima instantânea compatível com as indicações da norma brasileira NBR6118:2007. Com base em esses critérios foi escolhida uma laje retangular maciça de concreto armado engastada em duas bordas opostas em vigas metálicas com conectores de cisalhamento tipo U. As outras duas bordas são livres. As vigas metálicas de apoio da laje, por sua vez, apóiam-se sobre 8 perfis metálicos (dois em cada canto da laje) que se apóiam dois a dois sobre colunas curtas de aço de perfil H. Perfis U de aço são soldados às quatro colunas, formando um quadro horizontal. Foram realizadas análises numéricas da plataforma de ensaios dinâmicos em vibração livre e forçada para obter as frequências naturais e os modos de vibração correspondentes, considerando o peso próprio da estrutura e o carregamento que simula o peso das pessoas. Obtida uma configuração de estrutura que cumprisse com os requisitos estipulados, foi feito o dimensionamento da laje levando-se em conta as recomendações da norma brasileira NBR6118:2007. A plataforma foi construída e já foi feito um estudo preliminar experimental para obtenção das primeiras frequências naturais. This paper is about the design and construction of a platform for dynamic tests especially with people jumping, walking, etc. Initially it was tried to find out projects already implemented in platforms and dynamic tests and to study the loads produced by movement of people on slabs and the structural response to these loads. The limits established by different standards have been also studied for these dynamic responses, taking into account the ultimate limit state, as well as the structure in service, since the human body is very sensitive to structural vibrations. Parametric studies were performed considering various configurations of slabs (different spans, thicknesses and conditions of support) have been done, looking for a configuration that could have natural frequency close to the frequencies of the human loads. The slab should have dimensions compatible with the available physical space, fundamental frequency below 5 Hz and maximum immediate deflection compatible with the indications of the Brazilian standard NBR6118: 2007. Based on these criteria was chosen a rectangular structure consists of a solid reinforced concrete rectangular slab studded in two opposite edges of steel beams with shear connectors type U. The other two edges are free. The steel beams supporting the slab, in turn, are supported on eight metal profiles (two in each corner of the slab) that are supported on two to two short columns of steel profile H. Profiles U in steel are welded to four columns, forming a horizontal frame. Numerical analysis of the dynamic test platform have been performed for free and forced vibration, for obtaining the natural frequencies and corresponding vibration modes, considering the self-weight of the structure and the load that simulates people's weight. After obtaining a structural configuration that fulfilled the stipulated requirements, the design of the slab taking into account the recommendations of the Brazilian standard NBR6118: 2007. The platform was built and has been done a preliminary experimental study to obtain the first natural frequencies.

The effects of creep and shrinkage on the time-dependent behaviour of reinforced concrete and composite steel-concrete slabs are discussed and procedures for the prediction of the long-term deflection are presented. The time-dependent deformations caused by creep and shrinkage are modelled using tractable formulations developed using the age-adjusted effective modulus method of analysis. The procedure includes the time varying nature of tension stiffening and the effects of time-dependent shrinkage-induce cracking. Sample calculations are provided. The methods are validated against a range of test data and are shown to provide reliable estimates of in-service deformations.

The use of high-strength steel (yield stress larger than 100 ksi) in reinforced concrete structures can provide an effective alternative to using conventional steel (yield stress up to 80 ksi). The goal of this study is to investigate if slabs with reduced quantities of high-strength steel reinforcement meet the serviceability criteria. Instantaneous and long-term deflections in slab specimens with conventional steel and reduced amounts of high-strength steel (as compared with conventional steel) were studied. Two sets of two reinforced concrete slabs, each 14 ft. long, 30 in. wide, were built. The depth, quantity, and type of longitudinal reinforcement were varied between specimens in each set. The first set of slabs consisted of a 4 in. thick specimen with conventional steel (Gr. 60) and a 6 in. thick specimen with high-strength steel (Gr. 120). The longitudinal reinforcement ratios were 0.7% and 0.2% respectively. The second set consisted of a 5 in. thick specimen with Gr. 60 steel and an 8 in. thick specimen with Gr. 120 steel. The longitudinal reinforcement ratios were 0.5% and 0.1% respectively. The slabs were designed such that the theoretical deflections at service loads were similar for the specimens within each set. The slabs will be loaded until the working strains are reached in the reinforcement, and then will be tested under this constant service-load.

Orientador: Armando Lopes Moreno Júnior Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Civil, Arquitetura e Urbanismo Resumo: Esse trabalho tem o objetivo de apresentar e comparar metodologias para avaliação estrutural de lajes alveolares protendidas em situação de incêndio. Assim, questões importantes devem ser abordadas no que diz respeito à análise desse elemento quando submetido a altas temperaturas. São elas: o comportamento quanto à flexão; o comportamento quanto ao cisalhamento; a aderência da armadura protendida no concreto; o efeito do confinamento da laje por meio da capa estrutural de concreto armado; a influência das características geométricas da seção no gradiente de temperatura; as perdas de protensão e o efeito do lascamento explosivo. Para alcançar esse objetivo, foi realizado um estudo do estado da arte da segurança contra incêndios, visando o comportamento estrutural e as diretrizes de projeto atualmente utilizados no meio técnico e acadêmico. Em seguida, dois estudos sobre ensaios de lajes alveolares, publicados em artigos científicos, foram selecionados e na sequência foram apresentadas metodologias a fim de avaliar teoricamente, por meio de equações matemáticas, os resultados publicados. Como complemento, foi executada uma modelagem computacional por meio de elementos finitos, com a ajuda do software ABAQUS®, buscando simular o gradiente de temperatura das lajes alveolares ensaiadas nos artigos publicados. O desenvolvimento do trabalho apontou que o método simplificado do Eurocode para a análise a flexão é mais conservador, com valor 1,9 vezes superior que o encontrado no ensaio, seguido pelos métodos dos 500 oC e das Zonas (1,7 vezes superior). Já o método do PCI apresentou valores mais próximos que os encontrados no ensaio (1,4 vezes superior). Para a análise ao cisalhamento, o método simplificado do Eurocode mostrou-se mais conservador, com valores variando de 1,3 a 2,0 vezes superiores ao encontrado no ensaio. Este, seguido pelo método tabular da ABNT, com valores variando entre 1,2 a 2,3, e depois pelo método tabular do Eurocode, com valores variando de 1,1 a 2,4. Com relação a modelagem computacional, destacou-se que certos aspectos, próprios do ensaio em escala real, interferem na resposta final do software de elementos finitos com relação ao gradiente da temperatura na seção transversal Abstract: The aim of the current study is to present and compare methodologies for the structural assessment of prestressed hollow-core slabs subjected to fire. Thus, important issues regarding the analysis of this element must be addressed whenever it is subjected to high temperatures, namely: its bending and shear behaviors; the prestressed reinforcement adhesion to the concrete; the slab confinement effect through the reinforced-concrete cover; the influence of the geometrical features of the section on temperature gradient; the loss of prestressing; and the explosive spalling effect. A study of the fire safety state of art was conducted to assess the structural behavior and the guidelines of projects, which are currently accepted within the technical and academic environments. Subsequently, two studies about trials performed in hollow-core slab, which were published in scientific papers, were selected. Then, methodologies were presented in order to theoretically assess the published results through mathematical equations. In addition, the ABAQUS® software was used to perform a computer modeling by means of finite elements as a way to simulate temperature gradient in the hollow-core slabs tested in the published articles. The current study found that the simplified Eurocode method used in the bending analysis was more conservative, and its value was 1.9 times higher than that found in the aforementioned assays. It was followed by methods such as the 500°C and the Zones (1.7 times higher). However, the PCI method showed values closer to those found in the assay (1.4 times higher). The simplified Eurocode method was more conservative in the shear analysis, and its values were 1.3 to 2.0 times higher than that found in the assay. It was followed by the ABNT tabular method, which presented values ranging from 1.2 to 2.3, as well as by the Eurocode tabular method, which values ranged from 1.1 to 2.4. With respect to computer modeling, it was highlighted that certain aspects, due the full-scale trial, interfere with the final response of the finite element software with respect to the temperature gradient in the cross section Mestrado Estruturas e Geotécnica Mestre em Engenharia Civil

We have written a Monte Carlo code to simulate the experimental results of a previously reported study. We were able to analyse the energy distributions of photons that reached the detector system after suffering several successive Compton scatterings in the target. We have also investigated how the number of multiply backscattered events depends on the target thickness and the energy of the primary photons.

The objective of this paper is to investigate the main factors governing the fire behaviour of a typical underground garage roof consisting of a flat concrete slab supported by a number of relatively slender columns. On the basis of the available literature, and of reasonable assumptions, the fire scenario is worked out first, by properly modelling the temperature evolution in the ambient surrounding the structural members. This preliminary step is instrumental in solving the heat transfer problem from the ambient to the structural members. The calculated thermal field makes it possible to evaluate the decay of the mechanical properties as a function of the temperature. The stiffness decay, together with the thermal deformations, often brings in significant redistributions of the internal forces. The main factors are studied, and some general considerations are drawn on the overall safety of the structure under examination.

The 90° waveguide bend is an important component of optical circuit applications. We propose several models for such a bend, some of them assisted by a two-dimensional photonic crystal with a bandgap in the desired range of operating frequencies. We show that a photonic crystal assisted bend reduces bending loss by several orders of magnitude for transverse electric modes. © 2011 World Scientific Publishing Company., Scientific and Technological Research Council of Turkey (107T569)

Both the flexural and shear strengths of reinforced concrete (RC) beams can be substantially increased using externally bonded fibre-reinforced polymer (FRP) reinforcement in the forms of sheets/strips/plates (all referred to as plates for brevity). Failures of such FRP-plated RC beams often occur by debonding of the FRP plate from the RC beam in a number of distinct modes. This paper provides a summary of the current understanding of the mechanics of debonding failures in FRP-plated RC beams largely based on the research of the authors and their co-workers. A systematic classification of possible debonding failure modes is presented following a brief outline of the common strengthening methods. The interfacial stresses and bond behaviour between FRP and concrete are then discussed before the mechanisms and processes of debonding failures are examined. Furthermore, advanced strength models for the key debonding failure modes are presented. The paper concludes with a brief discussion of future research needs.

A multifunctional all-FRP joint has been developed for the transfer of bending moments and shear forces in thermal insulation sections of concrete slab structures used in building construction. Tensile forces from moments are transferred by horizontal GFRP bars, while a pultruded cellular GFRP element transfers the compression forces. The shear forces are transferred by inclined GFRP bars and the webs of the GFRP element. The new joint considerably increases energy savings for buildings due to the low thermal conductivity of GFRP materials. The quasi-static behavior of the joint at the fixed support of cantilever beams was investigated. Two parameters were studied: shear- or moment-dominated loading mode and concrete strength. Results show that the all-FRP joint does not play a critical role at the ultimate limit state. Ductile failure occurs through concrete crushing. The GFRP bars lead to a significant improvement in joint performance compared with similar joints comprising steel bars. Higher concrete strength does not, however, significantly improve the ultimate load. [All rights reserved Elsevier]

Congreso celebrado en la Escuela de Arquitectura de la Universidad de Sevilla desde el 24 hasta el 26 de junio de 2015. Pelli Tower is being built in the City of Sevilla. A new concept of building Architecture is born. Singularities are being described in the article: structural typology, materials, loads considered in the calculations (seism and wind tunnel Test), and construction methods used in the execution. A list of the main quantities used in the Pelli Tower is included to emphasize the importance of the execution.

Based on defined temperature-dependent material properties, thermo-chemical and thermo-mechanical models were developed to predict the temperature progression and endurance of full-scale slab components exposed to ISO 834 fire conditions on their underside. Important geometrical changes that occur over long fire exposures of FRP materials, such as the radiative shielding effect of delaminating fiber layers and loss of fiber layers, were considered. The predicted temperature progression in the liquid-cooled and non-cooled slab components from the 2-D thermo-chemical model corresponded well with measured values up to 120min of exposure. The validated model allows the prediction of the hot face temperatures, which are very difficult to experimentally measure. The 3-D thermo-mechanical model accurately predicts the measured mid-span deflections and axial strains under serviceability loads in the liquid-cooled case during 120min of fire exposure. [All rights reserved Elsevier]

The quality of a continuously cast product depends on the process condition, which, in turn, depends on process parameters. To be able to set the correct parameter values, it is important that behavior of the steel being cast under different casting conditions is well understood. This will need the knowledge of the mechanical behavior of steels at temperatures near the solidus. This paper presents, in detail, how these properties were modeled as a function of steel grade, temperature, and stresses that arise during casting, These models have been used to develop an integrated system that is capable of simulating various process conditions that may be encountered during continuous casting. The details of how these models were developed, verified, and used to develop the integrated system are presented in this paper.

International audience; A magnetically controlled switch based on ferrite-loaded substrate integrated waveguide (FLSIW) is proposed. FLSIW consists of rectangular ferrite slabs loaded on the sidewall slots of SIW where the magnetic fields are strongest. With the application of an external magnetic bias on the ferrite slabs, the cutoff frequency of the FLSIW is changed, thereby making the SIW switchable. An X-band reflective type single pole single through (SPST) switch is demonstrated. The switch has a 1.1 GHz switchable bandwidth centered at 10.1 GHz with an insertion loss of less than 1 dB in the on state and an isolation of 20 dB in the off state.

The repair and retrofit/rehabilitation of existing concrete and masonry structures have traditionally been accomplished with externally bonded fiber-reinforced polymer (FRP) systems, steel plates, reinforced concrete (RC) overlays, and posttensioning, just to name some of the many techniques presently available. Fabric-reinforced cementitious-matrix (FRCM) composites have recently emerged as an additional strengthening technology. FRCM is a composite material consisting of a sequence of one or more layers of cement-based matrix reinforced with dry-fiber fabric. This paper has three objectives: (1) to review existing guidelines for tensile testing and calculation of FRCM material properties to be used in analysis; (2) to report on some of the results of an experimental program intended to characterize the tensile behavior of FRCM coupons; and (3) to discuss the performance and analysis of concrete RC slab-type elements strengthened with FRCM. The laboratory results demonstrate the technical viability of this new composite material system for strengthening flexural RC members.

The yield-line method of analysis is a long established and extremely effective means of estimating the maximum load sustainable by a slab or plate. However, although numerous attempts to automate the process of directly identifying the critical pattern of yield-lines have been made over the past few decades, to date none has proved capable of reliably analysing slabs of arbitrary geometry. Here, it is demonstrated that the discontinuity layout optimization (DLO) procedure can successfully be applied to such problems. The procedure involves discretization of the problem using nodes inter-connected by potential yield-line discontinuities, with the critical layout of these then identified using linear programming. The procedure is applied to various benchmark problems, demonstrating that highly accurate solutions can be obtained, and showing that DLO provides a truly systematic means of directly and reliably automatically identifying yield-line patterns. Finally, since the critical yield-line patterns for many problems are found to be quite complex in form, a means of automatically simplifying these is presented.

Due to the nature of the material, reinforced concrete elements normally need a complex computation process to find efficient sections. Recommendations of some main codes face the problem by offering very simple non-transparent all purpose predimensioning criteria. This paper demonstrates how transparency and accuracy are not in contradiction with simple predimensioning methods. This opens the possibility to make it easier to reach optimization of reinforced concrete elements.

International audience; We investigate the origin of the directivity of a dipole antenna embedded in a dielectric slab Fabry-Perot cavity. It is shown that the focusing effect is due to the angular selection rules in the Bragg-type mirror and is directly correlated to the frequency selectivity of the cavity. For a 6 GHz resonant device, having a quality factor of 100, the aperture of the beam pattern (E-H plane) is close to 12/spl deg/ in agreement with the arguments based on frequency and angular selectivities. Hence, this kind of cavity could be used in free-space based communication links.

We report a study of photonic crystal nanocavities as a function of lattice disorder, which is implemented by deliberate radius variations of the holes forming the photonic crystal. Using cross-polarized laser light scattering, we demonstrate that lattice disorder affects the cavity mode symmetry, as is crucially evidenced by measuring scattering resonances for different sample orientations and explained by group-theoretical analysis together with calculations of the mode profile. We also quantify how the increase of lattice disorder leads to a reduction of the cavity Q factors and to a spread of both Q factors and resonance wavelengths. The trends as a function of disorder and for different radii distributions are well reproduced by theoretical calculations when both intentional (deliberate) and intrinsic (residual) disorder are taken into account. The results shed light on the effects of disorder on cavity resonances and on the interplay between intentional and intrinsic disorder, yielding reliable estimations of residual disorder (which ultimately affects technology limits) from optical measurements. Interestingly, the disorder values derived from the variation of the Q factor are lower than those derived from the wavelength spread: this suggests the occurrence of subtle interhole correlation effects that turn out to be beneficial for cavity Q factors. Publisher PDF

Selected papers. Held as part of SPIE Photonics West 2010. We study propagation losses in slow light photonic crystal waveguides and show that dispersion engineering can reduce the loss. We develop an improved understanding of why and how this occurs and develop an new approach to modeling these devices that provides new design insights. Postprint

We show that anisotropic negative effective dispersion relation can be achieved in pure dielectric rod-type metamaterials by turning from the symmetry of a square lattice to that of a rectangular one, i.e. by breaking the rotation symmetry of effective homogeneous medium. Theoretical predictions and conclusions are verified by both numerical calculations and computer based simulations. The proposed anisotropic metamaterial, is used to construct a refocusing slab-lens and a subdiffraction hyperlens. The all-dielectric origin makes it more straightforward to address loss and scaling, two major issues of metallic structures, thus facilitating future applications in both the terahertz and optical range., Comment: Accepted for APL

We report on the first experimental evidence of guided resonances (GRs) in photonic crystal slabs based on aperiodically-ordered supercells. Using the Ammann-Beenker (quasiperiodic, 8-fold symmetric) tiling geometry, we present our study on the fabrication, experimental characterization, and full-wave numerical simulation of two representative structures (with different filling parameters) operating at near-infrared wavelengths (1300-1600 nm). Our results show a fairly good agreement between measurements and numerical predictions, and pave the way for the development of new strategies (based, e.g., on the lattice symmetry breaking) for GR engineering., Comment: 10 pages, 5 figures (minor revisions)

ISBN : 879-1-4244-4107-5; International audience; Metamaterials can prove to be good candidates for shields in EMC applications where weight reduction is a challenge. Indeed metamaterial slabs can provide the same reflective properties as conventional metallic screens but with a lower density and reduced weight. Another advantage is that they can be tailored to exhibit required frequency-selective properties. However, their performance in terms of shielding performance has yet to be evaluated. In this paper, a method to evaluate the shielding performance of a metamaterial slab consisting of arbitrary-shaped metallic inclusions in a dielectric host is proposed. The specific case of periodic metamaterials will be considered for both resonant and non-resonant inclusions in the effective-homogeneity limit.

La muralla-palacio de La Alhambra, que es Patrimonio de la Humanidad, se ve amenazada por el avance del Tajo de San Pedro (Figura1), un diedro de 65,5 m de altura, que corta la colina de La Alhambra y que se ha situado a sólo 23,8 m de ella. Existe en la zona un régimen tectónico de extensión que afloja el terreno y activa la caída de lajas. Otra causa importante del deterioro del Tajo es la erosión. El coeficiente de seguridad al deslizamiento del Tajo sometido al terremoto de periodo de retorno de 1.000 años es de 0,73. La solución propuesta para su preservación es una malla de alambre postesada de alto límite elástico y revegetación autóctona. La presión que puede llegar a aplicar la malla en servicio eleva el coeficiente de seguridad con sismo a un valor próximo a uno. Mediante una simulación en ordenador se ha analizado el impacto ambiental., San Pedro cliff (Figure 1) is a dihedral 65.5 m high, which has progressed to place itself at 23.8 m from the Alhambra wall-palace that is a heritage of the Humankind. Active normal faults surrounding the cliff have created an extension tectonic regime that loosens the ground and activates the slab falls. One of the faces of the dihedral is a major fault. The cliff’s factor of safety subject to the 1000 years return period earthquake is 0.73. The preservation solution proposed is a post-tensioned high elastic limit wire mesh and autochthonous vegetation. With this reinforcement, the factor of safety under that earthquake raises to a value around one. A computer simulation has permitted to analyse the environmental impact., Este trabajo se ha realizado con ayuda del proyecto de investigación BIA 2004-01302 del Ministerio de Educación y Ciencia.

The position-dependent exact-exchange energy per particle εx(z) (defined as the interaction between a given electron at z and its exact-exchange hole) at metal surfaces is investigated, by using either jellium slabs or the semi-infinite (SI) jellium model. For jellium slabs, we prove analytically and numerically that in the vacuum region far away from the surface εSlabx(z→∞)→−e2/2z, independent of the bulk electron density, which is exactly half the corresponding exact-exchange potential Vx(z→∞)→−e2/z [Horowitz et al., Phys. Rev. Lett. 97, 026802 (2006)] of density-functional theory, as occurs in the case of finite systems. The fitting of εSlabx(z) to a physically motivated imagelike expression is feasible but the resulting location of the image plane shows strong finite-size oscillations every time a slab discrete energy level becomes occupied. For a semi-infinite jellium, the asymptotic behavior of εSIx(z) is somehow different. As in the case of jellium slabs εSIx(z→∞) has an imagelike behavior of the form ∝−e2/z but now with a density-dependent coefficient that, in general, differs from the slab-universal coefficient 1/2. Our numerical estimates for this coefficient agree with two previous analytical estimates for the same. For an arbitrary finite thickness of a jellium slab, we find that the asymptotic limits of εSlabx(z) and εSIx(z) only coincide in the low-density limit (rs→∞), where the density-dependent coefficient of the semi-infinite jellium approaches the slab-universal coefficient 1/2., C.M.H wishes to acknowledge the financial support received from CONICET of Argentina. C.R.P. was supported by the European Community through a Marie Curie IIF -Grant No. MIF1-CT-2006-040222 and CONICET of Argentina under Grant No. PIP 5254. J.M.P. acknowledges partial support by the University of the Basque Country, the Basque Unibertsitate eta Ikerketa Saila, and the Spanish Ministerio de Educación y Ciencia -Grants No. FIS2006- 01343 and No. CSD2006-53.

We present a molecular dynamics analysis of the static friction between two thick slabs. The upper block is formed by N 2 molecules and the lower block by Pb atoms. We study the effects of the temperature as well as the effects produced by the structure of the surface of the lower block on the static friction. To put in evidence the temperature effects .we will compare the results obtained with the lower block formed by still atoms with those obtained when the atoms are allowed to vibrate (e.g., with phonons). To investigate the importance of the geometry of the surface of the lower block we apply the external force in different directions, with respect to a chosen crystallographic direction of the substrate. We show that the interaction between the lattice dynamics of the two blocks is responsible for the strong dependence of the static friction on the temperature. The lattice dynamics interaction between the two blocks strongly reduces the static friction, with respect to the case of the rigid substrate. This is due to the large momentum transfer between atoms and the N 2 molecules which disorders the molecules of the interface layer. A further disorder is introduced by the temperature. We perform calculations at T=20 K which is a temperature below the melting, which for our slab is at 50 K. We found that because of the disorder the static friction becomes independent of the direction of the external applied force. The very low value of the static friction seems to indicate that we are in a regime of thermolubricity similar to that observed in dynamical friction.