Resumo em Inglês:

Abstract Self-compacting concretes (SCCs) are considered promising materials in the civil engineering field. Their main characteristic is the ability to compact only through gravitational force. Mineral additions such as rice husk ash (RHA) and fly ash (FA) are recommended to be used in SCCs during their mix designing, in order to increase fluidity and mechanical strength. These materials are also considered wastes from industry, without a certain destination, which contributes to environmental pollution. In this study, four mixtures of SCC were tested using RHA and FA with two different types of Portland cement, CEM CP IV and white CEM. For the fresh state tests, all of the SCCs mixtures showed satisfactory results. The SCCs with white CEM showed higher mechanical strength at 7 days than CEM CP IV. Analyzing the mineral additions, their use improved the mechanical strength of SCCs at 28 days, there is also observed a higher pozzolanic effect to RHA.

Resumo em Inglês:

Abstract This article presents a theoretical and numerical study of bolted sleeve connections with rectangular hollow sections (RHS) under axial tension and compression. The geometric form of a hollow section provides resistance for high axial loads, torsion and combines effects, spreading its utilization in truss systems. In this context, the sleeve connections proposed explore these characteristics of RHS and offer an attractive aesthetic appearance for the continuity of elements. The bolted sleeve connection with RHS is formed by two outer tubes connected by an inner tube and staggered bolts. Herein, a parametric study was developed for identification of the failure modes in the connection. Finite element models with different geometric parameters and number of bolts were created in commercial software. The width, depth and thickness of RHS tubes and diameter of bolts were variated. In the theoretical/numerical/parametric results, the yielding gross section failure, the fracture through the effective net area failure and the bearing failure were observed. These failure modes occurred in both outer and inner tubes. The load results were compared to determine the resistance capacity of sleeve connections. The theoretical formulations were evaluated for representation of the ultimate load of the failure modes.

Resumo em Inglês:

Abstract The objective of this article is to present an optimization problem formulation to reduce the total structural cost of prestressed doubly-symmetric and monosymmetric I-shaped steel simply supported beams with straight tendons. The optimization problem was implemented via MATLAB’s native Genetic Algorithm. The validation and evaluation processes adopted two examples from literature. The design method follows the Brazilian standard NBR 8800:2008 for the Ultimate and Serviceability Limit States. The best result was found for a monosymmetric case by up to 20.00% and 25.70%. Saving in material weight and installation of tendons, without exceeding the security limits, was also effective. Furthermore, the results presented an efficient alternative for structural engineering, providing a significant model for similar analyzes.

Resumo em Inglês:

Abstract Population growth in urban centers, together with the lack of physical space, has led to the construction of increasingly tall and slender buildings. Multiple-story structures present substantial challenges to civil engineering because they have specific requirements for their design, construction, and use. The increased number of foors leads to more lateral displacements resulting from horizontal actions. Under these conditions, to ensure system stability, structural bracing components are commonly adopted. In addition, along with the use of more resistant materials and new construction techniques, it is necessary to improve the methodologies adopted in the structural analysis to offer professionals in the area the conditions to undertake safer and more economical projects with better speed and efficiency. Thus, in this study, numerical analyses were applied to steel planar reticulated structures to evaluate their stability and strength when inserting bracing systems. The study compared the arrangement of the bars and analyzed the influence of the parameters of the bracing systems, such as the properties of the cross-section and the position of the inverted-V-braced system. The MASTAN2 program was used to perform nonlinear static assessments using reticulated finite elements that considered both geometrical and physical nonlinearities. It was observed that the inverted-V-braced system had a substantial impact on all of the structures that were analyzed, providing increased stiffiness and, as a result, significantly reducing the frame’s lateral displacement.

Resumo em Inglês:

Abstract Although delta ferrite is a very common phase in most austenitic stainless steels, studies about its formation and evolution during slab processing from industrial heats of tens of tons are scarce. The main objective of this research is to study the evolution of delta ferrite (quantity, chemical composition, morphology, and distribution) along the production route from the cast slab to the coil of an industrial heat of 80 tons of 304 stainless steel. Samples were extracted after the following processing steps: continuous casting, first and second hot-rolling pass, and solution-heat-treating, arriving at the final commercial condition. Sample analyses were carried out with several complementary microstructural characterization techniques: optical microscopy, scanning electron microscopy with energy dispersive spectroscopy (EDS), X-ray diffraction, and magnetic measurements of delta ferrite content (feritscope). Thermocalc® indicates that the present continuous cast slab solidifes according to the FA (ferrite-austenite) mode and the final microstructure should be completely austenitic in equilibrium conditions. Nevertheless, delta ferrite is detected along the processing steps, indicating that the steel is out of phase equilibrium. The ferrite content measured after solidification varies significantly across the as-cast slab thickness. Lower values are detected on the surfaces, followed by a gradual increase when moving into the slab, reaching a peak, and finally decreasing at the slab center. This pattern of delta ferrite content is named “M type” distribution. The average content of delta ferrite decreases after each subsequent processing step, namely the two hot-rolling passes and the solution heat-treating.

Resumo em Inglês:

Abstract The family of crystals known as Tutton's salts plays a significant role in physics and chemistry; because they are used in phase transition studies and to define models applied to materials. The importance of salts in material engineering is recent, as in applications in adiabatic degaussing refrigerators and solid-state anodes. Studies of the (NH4)2Ni(SO4)2∙6H2O and (NH4)2Co(SO4)2·6H2O are widely found in literature but do not occur for the mixture of both. In this research, we studied mixed crystals of the general chemical formula (NH4)2NixCo(1-x)(SO4)2·6H2O with x ranging from 0 to 1, utilizing x = 0.7. The objective is to study the modifications caused owing to the ion's weighted composition in the formation of the solid solution and compare it with the pure salts. For this, the growth of these crystals is discussed based on ICP-OES results and optical microscopy concerning the crystal growth theory. The discussion also relates the Raman spectra of the salts with molecular changes according to structured group theory, qualitatively characterizing its crystalline structure. Finally, a Single-crystal X-ray study solves and confirms the structure of pure salts and mixed salt, quantitatively characterizing their crystal structure.

Resumo em Inglês:

Abstract The simulation process was applied to the industrial circuit of the Juruti Bauxite Mine using mathematical models calibrated for the equipment and based on information from industrial sampling. The sampling campaign, mass balance routines and quantification of the process efficiency were discussed in Part I of this study and the model calibration and simulations process were the Part II objective. Simulation scenarios were defined considering the possible particle size curves for feeding the plant. Based on the simulated results, it was possible to observe which operations may be impacted by the increase in production, impacting the percentage of contaminants in the final product, indicating the necessity of equipment interventions to support increases in capacity. Another group of scenarios were established with the objective to evaluate the possibility of increasing the mass recovery and improving the quality of the fine product, by variations in the opening diameters of the apex and vortex finder of the five cyclone stages. Considering the simulated scenarios, the points of attention identified in the simulations were: (a) the washing efficiency could be reduced, in view of a consequent reduction in residence time connected to the change in characteristics of the ore fed to the plant; (b) increase capacity of the primary and secondary screens need to be done; (c) to keep the classification efficiencies, it is necessary install more cyclone units; and (d) changes in the apex diameter and vortex finder could generate results in terms of mass recovery and product quality.

Resumo em Inglês:

Abstract Continuous assessment of slope stability is important to the open pit design and operation. This article aims to present a tool for evaluating the stability conditions of rock slopes in mining, based on a global geotechnical database, using machine learning techniques. Different models are evaluated in this research: the general model, which uses all variables; the mathematical model, which uses only variables selected by the random forest (out-of-bag); and two expert-based models: the Q-Slope model and the Santos model. The validation of the model was done through the test sample, using partition confusion matrices aiming at reproducibility of the results. A study of the types of errors was carried out using Principal Component Analysis (PCA). The study of errors allowed the identification of samples that were inconsistent with the others. Afterwards, the models were redone and compared with the previous ones. The best performers are presented and discussed. The proposed methodology does not replace the classic analysis of slope stability. On the contrary, it contributes to engineers and geologists with a tool for monitoring the stability conditions of slopes in a mining operation. Slope stability analysis must be carried out throughout the mine's lifetime and, therefore, it is believed that the tool proposed here can optimize the selection of slopes most susceptible to instability.

Resumo em Inglês:

Abstract Flocculation is a complex process, usually dependant on various conditions to perform adequately. Nevertheless, the relevance and interaction of the process variables are not easily found in literature. In this study, nine variables of ultrafine quartz flocculation systems, defined in Part 1 of this study, were analysed. The effects of each variable and their interactions in the system were investigated based on flocculation and sedimentation tests. The results were statistically evaluated using the supernatant turbidity as experimental response. The evaluations resulting from the statistical approach indicated that suspension pH (5.49%) was the most significant variable, followed by flocculant concentration (4.07%) with the second greatest effect on the turbidity of the supernatant. Solid concentration, agitation intensity, and surfactant conditioning time (4.05%, 3.51% and 3.51%, respectively), also have significant effects on ultrafine quartz flocculation. The flocculant concentration and the surfactant conditioning time were the variables with the most significant interactions with the main variables. The Camp Number values showed a negative exponential relationship with the turbidity results, proving to be an important tool to evaluate flocculation. Backscattered electron scanning microscopy images of flocs formed in the presence of 30 g/t PAM showed compact flocs in the size range between 150 and 365 µm, with noticeable sphericity.

Resumo em Inglês:

Abstract An economic feasibility study must consider the uncertainties inherent to a mining project, whose risks must be quantified properly to enable accurate decision-making. Studies previously carried out through the Discounted Cash Flow (DCF) methodology in the project evaluated here - a quarry whose operations are currently interrupted, located in Pernambuco, Brazil, formerly taxed under the presumed profit regime - indicated a positive Net Present Value (NPV) in the deterministic scenario, therefore projecting a profitable project. However, a probabilistic analysis using Monte Carlo simulations indicated only a 49.98% occurrence probability for this NPV. An assessment focused on the company's taxation was never carried out, which is a gap that the present study intends to fill, in addition to evaluating the feasibility of immediate investment in this project. Furthermore, this is a gap in Brazilian literature in general, which does not take into account the taxation system in their economic assessments. In this context, considering scenarios whose taxation was based on real and presumed profit regimes, we reassessed the cash flows of this quarry and performed deterministic and probabilistic economic analyses, and compared the results of both scenarios. The sensitivity analysis indicated that the production rate would be the most impactful variable in the project's NPV, considering the six variables assessed. Hence, it was verified in both deterministic and probabilistic analyses that taxation under real profit, results in a higher economic return with a 56.08% probability of the NPV being positive and with the Internal Rate of Return (IRR) higher than infation (SELIC rate) at 4.81%; the taxation under the presumed profit, on the other hand, obtained respective probabilities of 46.54% and 3.23%. However, with the chances of obtaining some profit (NPV greater than zero) at the order of 50% and a minimal chance of the IRR being greater than the SELIC rate adopted at the time of this study, we would advise against investing in this venture. Moreover, even if the current moment is not the most suitable for investment in this sector, regardless of the production rate assessed in the probabilistic analysis, taxation on the real profit regime presented a greater economic return than taxation on the presumed profit regime., indicating that, for the parameters considered in this study, the first would be the most appropriate choice of tax system for this type of enterprise in Brazil.

Resumo em Inglês:

Abstract Mining in Brazil is one of the pillars that support its GDP (gross domestic product). Its execution requires technique and expertise. In a mining venture, the value chain drives the business to achieve its goals. Mine planning has a great influence on its performance, as it is through it that the organization is based to offer its products to the market and, consequently, the low compliance with the established premises will bring undesirable inconveniences. Within the planning, the short term is dedicated to operationalize the plans made possible by the long and medium term teams, in addition to attending to market variations. The indicators that control this process are of great importance, as an incorrect identification inserted in the various criteria that compose it can guide managers towards inefficient decision making. The objective of this study was, through lean methodology, to identify the main performance indicators that impact on the non-exception of the mining plan measured by the adherence and compliance indicators. In this method, the main causes of non-adherence and compliance with the mining plan were identified, enabling short and medium-term actions that leveraged the indicators. This study leveraged the average adherence and compliance by 17% and 54.9%, respectively, exceeding the target set for 6.5% and 49.2% respectively.