José Marcio Calixto

Using Converter Dust to Produce Low Cost Cementitious Composites by in situ Carbon Nanotube and Nanofiber Synthesis

Carbon nanotubes (CNTs) and nanofibers (CNFs) were synthesized on clinker and silica fume particles in order to create a low cost cementitious nanostructured material. The synthesis was carried out by an in situ chemical vapor deposition (CVD) process using converter dust, an industrial byproduct, as iron precursor. The use of these materials reduces the cost, with the objective of application in large-scale nanostructured cement production. The resulting products were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) and were found to be polydisperse in size and to have defective microstructure. Some enhancement in the mechanical behavior of cement mortars was observed due to the addition of these nano-size materials. The contribution of these CNTs/CNFs to the mechanical strength of mortar specimens is similar to that of high quality CNTs incorporated in mortars by physical mixture.

Intraocular pressure and ocular rigidity after LASIK

PURPOSE To assess the intraocular pressure (IOP) and ocular rigidity (E) at 1, 3, 6 and 24 postoperative months in eyes that underwent LASIK. METHODS The IOP measured by Goldmann applanation tonometer (GAT) and by the 5.5 and 10 gm. weights of a standardized Schioetz tonometer (ST) and the E measured by differential tonometry (the 5.5 and 10 gm. of ST) were assessed before and after LASIK (at 1, 3, 6 and 24 month) in 23 eyes. The volume of the corneal indentation (Vc) and IOP with ST resting on the eye (tonometric pressure - Pt) were also evaluated. RESULTS The average IOP (GAT) at 6 and 24 months after LASIK was lower than at 1 and 3 months and before LASIK. A statistically significant difference of E was found at 1, 3, 6 and 24 months. The reduction of the central corneal thickness (CCT) by LASIK was the cause of increasing values of Vc and decreased values of Pt. In turn, the changes in Vc and Pt were responsible for the lower IOP measurements by GAT and E changes. CONCLUSIONS LASIK reduces IOP readings by GAT but not the ones by ST (differential tonometry). LASIK also diminishes E. IOP (GAT) values are lower at 6 and 24 months. IOP evaluation by ST is more accurate than that by GAT. At 6 months, Vc, Pt and E parameters are stabilized (no changes relatively to 24 months measurements.

Microcracking of High Performance Concrete Subjected to Biaxial Tension - Compression Stresses

The results of an experimental investigation on the microcracking of high-performance concrete subjected to biaxial tension-compression stresses are presented. Short-term static tests and microcracking mapping were performed on 12.5 cm square by 1.25 cm thick plates. Strain controlled tests were executed in a biaxial testing machine constructed at the University of Texas. The primary variables studied were the deformations and the ultimate stress level at each stress ratio as well as the microcracking patterns and total crack lengths. For the microcracking study, the plates, after straining, were impregnated by an epoxy and then examined under a microscope. Microcracks were classified into simple and combined cracks, since this distinction allows for a much better representation of the microcracking process. A simple crack is either a bond or mortar crack where a combined crack contains both of these. For all stress ratios tested, the stress-strain behavior was directly related to the internal microcracking pattern. In all cases, the failure was directly related to the formation and propagation of the combined cracks.

Estudo comparativo dos critérios de dimensionamento ao cisalhamento longitudinal em lajes mistas de aço e concreto

resumo ABNT NBR 8800 (2008) prescribes two longitudinal shear design criteria for composite slabs with ribbed decking: m-k method and partial interaction method. The m-k method is considered the worldwide standard method. In this method, the longitudinal resistant shear strength is obtained from linear regression of test results. The partial composite method is an alternative to the m-k method and shall be used only for composite slabs with a ductile behavior. In this scenario, the objective of this paper is to compare these two methods with respect to test results of full size composite slabs employing steel-deck MF 50 manufactured by METFORM. The analysis of the comparative study shows good correlation between the two methods with respect to the prediction of the ultimate strength of the tested composite slabs.

Evaluation of pyrite and pyrrhotite in concretes

It is well known that aggregate characteristics can intensively interfere in concrete behavior especially when sulfides are presented in the aggregates. The lack of consensus to content limit value of these deleterious sulfur compounds in concrete structures for dams has motivated several investigations worldwide. Within this scenario, this work presents a methodology to evaluate the presence of pyrite and pyrrhotite in concretes produced with aggregates containing sulfides. For the study, rock samples from the Irape hydroelectric power plant area in Minas Gerais (Brazil) were used. This plant was built in a geological site where the rock presented sulfide levels of at least 3%. These rock samples were first ground and then used as aggregates in mortars, which were, during almost one year, subjected to three different exposed conditions: temperature of 23° ± 2°C and relative humidity of 95 to 100%; calcium hydroxide solution diluted in water kept at two different temperatures: room temperature and 50° C. The presence and amount of pyrrhotite were obtained from a leaching process of the material (aggregate or mortar) in a solution of hydrochloric acid. This procedure allowed also the evaluation of the pyrite content. The results showed that the amount of pyrite has remained virtually constant over time in the three exposure situations. This finding indicates that sulfur limits in aggregates should be set according to the type of iron sulfide presented and not solely by the total amount of sulfur.

Análise Numérica de Lajes Alveolares Protendidas Submetidas a Carregamentos de Longa Duração

Este artigo apresenta os resultados de uma analise numerica de lajes alveolares protendidas extrudadas submetidas a carregamentos de longa duracao. O modelo numerico considera os efeitos do tempo nas propriedades mecânicas dos materiais bem como nas propriedades reologicas, de modo a prever flechas, deformacoes e tensoes. O modelo leva em conta tambem variacoes no carregamento. Para a analise, cada laje e subdividida em um numero finito de elementos de barra cuja secao transversal e discretizada em elementos de concreto paralelos ao eixo de flexao, e elementos de armadura passiva. Para avaliacao dos resultados e considerada a area liquida de concreto. Os resultados do modelo numerico foram comparados com valores medidos em ensaios de duas series distintas de lajes alveolares protendidas. Cada serie de lajes possuia diferentes geometrias, taxa e distribuicao de cordoalhas. Deslocamentos no meio do vao de cada laje foram avaliados ate a idade de 127 dias apos o carregamento inicial. Boa correlacao foi obtida com o modelo numerico para cargas menores ou iguais as de servico.

Projeto de pilares retangulares esbeltos de concreto armado submetidos à flexão composta reta por métodos aproximados

Normas para projeto de estruturas de concreto armado estabelecem que a analise dos efeitos de 2a ordem em pilares esbeltos deve ser feita de modo a assegurar que para as combinacoes mais desfavoraveis das acoes de calculo, nao ocorra perda de estabilidade, nem esgotamento da capacidade resistente de calculo. Para isso e obrigatorio considerar a nao-linearidade fisica dos materiais, juntamente com a nao-linearidade geometrica. Os calculos dos efeitos de 2a ordem podem ser feitos por metodos aproximados ou pelo metodo geral. Neste trabalho o foco sera no calculo desses efeitos de 2a ordem com base nos metodos aproximados prescritos na NBR 6118: metodo do pilar-padrao com curvatura aproximada e metodo do pilar-padrao com rigidez aproximada. Dada a importância dos pilares e de sua estabilidade e importante que os metodos aproximados sejam capazes de apresentar solucoes seguras para o dimensionamento destes. Neste cenario, o objetivo desse trabalho e avaliar estes metodos aproximados em termos de seguranca, precisao e economia com relacao a resultados de ensaios de pilares encontrados na literatura. A analise deste estudo comparativo revela que o emprego do metodo da rigidez aproximada apresenta resultados mais proximos dos encontrados nos ensaios de pilares.

Avaliação “in loco” das perdas de protensão de cordoalhas engraxadas em lajes planas

Resumo This paper presents the results of “in situ” measurements of stress losses in unbonded tendons of post tensioned flat slabs. These flats slabs have approximately 350 m 2 in area and are 17 centimeter thick. They are part of a 16 story high hotel building. Load cells were employed to measure the forces in the tendons; they were placed in the vicinity of the active and the passive anchorages. The forces in the tendons were monitored from the time of their straining up to 170 days afterwards. The measurements reveal a frictional coefficient μ equal to 0.0528, a very close value to the one prescribed by NBR 6118 of 0.05. The stress loss due to the anchorage of the tendons was of 5.17% of the initial stress. The results indicate also that the remaining force near the passive anchorage is larger in relation to the value measured in the vicinity of the active one. The total stress loss measured so far (170 days) corresponds to 14 % of the initial post-tensioning stress.