Flexural strengthening of reinforced concrete beams with carbon fibers reinforced polymer (CFRP) sheet bonded to a transition layer of high performance cement-based composite

Resistance to corrosion, high tensile strength, low weight, easiness and rapidity of application, are characteristics that have contributed to the spread of the strengthening technique characterized by bonding of carbon fibers reinforced polymer (CFRP). This research aimed to develop an innovate strengthening method for RC beams, based on a high performance cement-based composite of steel fibers (macro + microfibers) to be applied as a transition layer. The purpose of this transition layer is better control the cracking of concrete and detain or even avoid premature debonding of strengthening. A preliminary study in short beams molded with steel fibers and strengthened with CFRP sheet, was carried out where was verified that the conception of the transition layer is valid. Tests were developed to get a cement-based composite with adequate characteristics to constitute the layer transition. Results showed the possibility to develop a high performance material with a pseudo strain-hardening behavior, high strength and fracture toughness. The application of the strengthening on the transition layer surface had significantly to improve the performance levels of the strengthened beam. It summary, it was proven the efficiency of the new strengthening technique, and much information can be used as criteria of projects for repaired and strengthened structures.

Desenvolvimento de microssensores eletroquímicos e aplicações no estudo de processos dinâmicos interfaciais utilizando microscopia eletroquímica de varredura

O trabalho descrito nesta tese mostra de forma detalhada a fabricação e caracterização de diferentes microssensores eletroquímicos os quais têm sido recentemente utilizados como sondas em grupo de técnicas conhecida como Scanning Electrochemical Probe Microscopy (SEPM). Desta forma, a caracterização de superfícies pode ser feita explorando diferentes fenômenos interfaciais relevantes à Ciência. Neste sentido, as interfaces de materiais cristalinos como hidroxiapatita (materiais dentários) e calcita foram o foco de estudo neste trabalho. Assim, diferentes técnicas SEPM foram exploradas no sentido de se obter informações relevantes relacionadas aos processos dentários, como a erosão ácida e hipersensibilidade. Inicialmente, microeletrodos de platina foram desenvolvidos empregando uma metodologia convencional na qual são utilizados microfibras encapsuladas em capilares de vidro. Scanning Electrochemical Microscopy (SECM) no modo amperométrico foi utilizada para obtenção de informações com relação às alterações de topografia do esmalte dentário causadas pelo contato com substâncias ácidas. Adicionalmente, SECM foi empregada no estudo do transporte de espécies eletroativas em amostras de dentina e investigações relacionadas ao bloqueio dos túbulos empregando-se cremes dentais comerciais foram realizadas. A permeação de peróxido de hidrogênio pela dentina também foi estudada. Os resultados de SECM foram comparados com imagens SEM obtidas nas mesmas condições experimentais. Microeletrodos de membrana ionófora íon-seletiva (Ion Selective Microelectrodes-ISMEs) sensíveis a íons cálcio também foram desenvolvidos e caracterizados, com subsequente aplicação em SECM no modo potenciométrico. A dissolução ácida de esmalte bovino (erosão dentária) foi investigada em diferentes valores de pH (2,5; 4,5; 6,8). Além disso, o transporte de íons cálcio através de membranas porosas sintéticas foi avaliado a uma distância tip/substrato de 300µm. Alterações no fluxo de íons cálcio foram correlacionadas em experimentos realizados na presença e ausência de campos magnéticos gerados por nanopartículas de magnetita incorporadas à membrana porosa. Microcristais de calcita facilmente sintetizados pelo método de precipitação foram utilizados como superfície modelo para investigações interfaciais, cujos resultados podem ser correlacionados aos materiais dentários. Desta forma, nanopipetas de vidro preenchidas com eletrólito suporte foram fabricadas e utilizadas como sonda em Scanning Ion Conductance Microscopy (SICM). O mapeamento topográfico de alta resolução espacial da superfície de um microcristal de calcita foi obtido utilizando o modo de varredura hopping mode. Adicionalmente, sondas multifuncionais ISME-SICM também foram desenvolvidas e caracterizadas para investigações simultâneas com relação às alterações topográficas e quantificação de íons cálcio sobre a superfície de um microcristal de calcita. A adição de reagentes ácidos no canal SICM promoveu a dissolução da superfície do microcristal, sendo obtidos dados cinéticos de dissolução. Investigações em meio neutro também foram realizadas utilizando a sonda ISME-SICM. Os resultados experimentais obtidos também foram comparados com aqueles oriundos de simulação computacional.

Nanophotonics of optical fibers

This review is concerned with nanoscale effects in highly transparent dielectric photonic structures fabricated from optical fibers. In contrast to those in plasmonics, these structures do not contain metal particles, wires, or films with nanoscale dimensions. Nevertheless, a nanoscale perturbation of the fiber radius can significantly alter their performance. This paper consists of three parts. The first part considers propagation of light in thin optical fibers (microfibers) having the radius of the order of 100 nanometers to 1 micron. The fundamental mode propagating along a microfiber has an evanescent field which may be strongly expanded into the external area. Then, the cross-sectional dimensions of the mode and transmission losses are very sensitive to small variations of the microfiber radius. Under certain conditions, a change of just a few nanometers in the microfiber radius can significantly affect its transmission characteristics and, in particular, lead to the transition from the waveguiding to non-waveguiding regime. The second part of the review considers slow propagation of whispering gallery modes in fibers having the radius of the order of 10–100 microns. The propagation of these modes along the fiber axis is so slow that they can be governed by extremely small nanoscale changes of the optical fiber radius. This phenomenon is exploited in SNAP (surface nanoscale axial photonics), a new platform for fabrication of miniature super-low-loss photonic integrated circuits with unprecedented sub-angstrom precision. The SNAP theory and applications are overviewed. The third part of this review describes methods of characterization of the radius variation of microfibers and regular optical fibers with sub-nanometer precision.

Structure dependencies for multiscale polymer composites using extrusion processes

Multiscale reinforcement, using carbon microfibers and multi-walled carbon nanotubes, of polymer matrix composites manufactured by twin-screw extrusion is investigated for enhanced mechanical and thermal properties with an emphasis on the use of a diverging flow in the die for fluid mechanical fiber manipulation. Using fillers at different length scales (microscale and nanoscale), synergistic combinations have been identified to produce distinct mechanical and thermal behavior. Fiber manipulation has been demonstrated experimentally and computationally, and has been shown to enhance thermal conductivity significantly. Finally, a new physics driven predictive model for thermal conductivity has been developed based on fiber orientation during flow, which is shown to successfully capture composite thermal conductivity.

Efeito da adição de microfibras de lã de vidro na tenacidade à ruptura de pastas de cimento para poços de petróleo

Although there are a wide variety of additives that act in fresh state, to adjust the properties of cement, there is also a search by additions that improve the tenacity of the cement in the hardened state. This, in turn, can often be increased by inserting fibers, which act on the deflection of microcracks. This study aimed to use a microfiber glass wool (silica-based) as an additive reinforcing the cement matrix, improving the rupture tenacity, in order to prevent the propagation of microcracks in the cement sheath commonly found in oil wells submitted to high temperatures. The fibers were added at different concentrations, 2 to 5% (BWOC) and varied average sizes, grinding for 90 s, 180 s, 300 s, 600 s. The cement slurries were made with a density of 1,90 g/ cm3 (15,6 lb/gal), using Portland cement CPP- Special Class as the hydraulic binder and 40% silica flour. The characterization of the fiber was made by scanning electron microscopy (SEM), particle size by sieving, X-ray fluorescence (XRF), X-ray diffraction (XRD) and thermogravimetry (TG / DTG). Were performed technological tests set by the API (American Petroleum Institute) by rheology, stability, free water, compressive strength, as well as testing rupture energy, elastic modulus and permeability. The characterization results showed good thermal stability of the microfiber glass wool for application in oil wells submitted to steam injection and, also, that from the particle size data, it was possible to suggest that microfibers milled up to 300 s, are ideal to act as reinforcement to the cement slurries. The rheological parameters, there was committal of plastic viscosity when larger lengths were inserted of microfiber (F90). The values obtained by free water and stability were presented according to API. The mechanical properties, the incorporation of microfiber to the cement slurries gave better rupture tenacity, as compared to reference cement slurries. The values of compressive strength, elastic modulus and permeability have been maintained with respect to the reference cement slurries. Thus, cement slurries reinforced with microfiber glass wool can ensure good application for cementing oil wells submitted to steam injection, which requires control of microcracks, due to the thermal gradients