RESUMO O concreto é um material com elevada frequência de utilização em construções brasileiras e em edificações de países ao redor do mundo. Fatores como os custos reduzidos dos materiais, elevado volume de pesquisa e resistência elevada, colaboram para a grande utilização do concreto como material de construção. Entretanto, o concreto tem baixa resistência à tração, fissura facilmente e apresenta pequena capacidade de deformação. O concreto reforçado com fibras (CRF) foi desenvolvido na década de 60 e é um material que apresenta uma boa resposta mecânica em termos de resistência residual após a fissuração da matriz e consequentemente um comportamento mais dúctil do que o concreto sem fibras. Entretanto, o efeito da adição de fibras no concreto restringe-se somente a fase pós-fissuração do processo de carregamento. As fibras não tem efeito no comportamento que antecede a carga de pico. Avanços recentes em nanotecnologia têm possibilitado a produção de nanopartículas, em especial, nanotubos de carbono. Tratam-se de nanopartículas que podem ser utilizadas como reforço da matriz cimentícia para atuar ao nível de sua nanoestrutura. A pesquisa proposta estuda o comportamento de concretos contendo a incorporação de nanotubos de carbono e também de concretos reforçados com fibras de aço (CRFA) juntamente com a adição de nanotubos de carbono. A pesquisa tem o objetivo de avaliar melhorias na resposta dos concretos e dos CRFA, a partir da incorporação das nanopartículas de carbono. Foram realizados ensaios de compressão e de flexão em três pontos em corpos de prova prismáticos entalhados. Foi constatado que os nanotubos de carbono permitem reduzir a absorção de água e melhorar a mobilidade das misturas (de concreto e de CRFA) no estado fresco. Também foram observados aumentos, na resistência à compressão axial e resistências flexionais dos materiais concreto e CRFA quando da adição de nanotubos de carbono.

ABSTRACT Concrete is a material with a high frequency of use in Brazilian constructions and in buildings in countries around the world. Factors such as reduced material costs, high research volume and high resistance, contributed to the wide use of concrete as a building material. However, concrete has low tensile strength, cracking easily and has a low deformation capacity. Fiber reinforced concrete (FRC) was developed in the 1960s and is a material that presents a good mechanical response in terms of residual strength after cracking of the matrix and consequently a more ductile behavior than concrete without fibers. The effect of adding fibers to the concrete is restricted only to the post-cracking phase of the loading process. The fibers have no effect on the behavior that precedes the peak load. Recent advances in nanotechnology have enable the production of nanoparticles, in particular, carbon nanotubes. The nanoparticles can be used to reinforce the cement matrix to act at the level of its nanostructure. The proposed research studies the behavior of concretes containing the incorporation of carbon nanotubes and also of steel fiber reinforced concrete (SFRC) with the addition of carbon nanotubes. The research aims to evaluate improvements in the response of concretes and SFRC, based on the incorporation of carbon nanoparticles. Compression and flexural tests were performed on notched prismatic specimens. It has been found that nanotubes reduce the absorption of water and improve the mobility of mixtures (concrete and SFRC) in the fresh state. Increases in axial compression strength and flexural strengths of concrete and SFRC materials were also observed when adding carbono nanotubes.

Abstract The thermal modification has proved to be an effective method to improve the durability of wood elements. Until now, most applications of thermally modified wood have been limited to non-structural applications since the wood’s heat treatment leads to a significant reduction in mechanical performances. However, there is no knowledge about these treatments applied on the wood species more used in Portuguese constructions. In order to evaluate the effect of heat treatment on the wood species of Maritime pine (Pinus pinaster), Eucalyptus (Eucalyptus globulus), Beech (Fagus sylvatica), Acacia (Acacia melanoxylon) and Oak (Quercus faginea), an experimental program consisted of mechanical bending tests and dimensional stability tests (shrinkage, swelling and equilibrium moisture content) was defined. For comparison purposes, all tests were made with test specimens of non-modified and thermally modified wood using four temperature values (200ºC, 240ºC, 260ºC and 300ºC) and three exposed periods (2, 4 and 8 hours). The obtained results show that the behaviour of the studied experimented species was similar to the one of other species already studied in the available literature. It was possible to point out that mechanical properties decrease with the severity of the thermal treatment, while the dimensional stability improved.

ABSTRACT The purpose of this study is to develop data mining models to predict restrained shrinkage crack widths of slag mortar cementitious composites. A database published by BILIR et al. [1] was used to develop these models. As a modelling tool R environment was used to apply these data mining (DM) techniques. Several algorithms were tested and analyzed using all the combinations of the input parameters. It was concluded that using one or three input parameters the artificial neural networks (ANN) models have the best performance. Nevertheless, the best forecasting capacity was obtained with the support vector machines (SVM) model using only two input parameters. Furthermore, this model has better predictive capacity than adaptative-network-based fuzzy inference system (ANFIS) model developed by BILIR et al. [1] that uses three input parameters.

Abstract Risk management is of paramount importance for the success of tunneling works and it is linked to the main excavation methods used: the Conventional Excavation Method (CEM) and the Tunnel Boring Machine Method (TBM). Considering the importance of the “Safety and Health” criterion for the choice of the excavation method, the fact that this criterion is usually mostly focused on the structural component, and taking in account that there is no research showing the advantages of one of the methods over the other, this research intends to conduct a comparative risk analysis between both methods, taking into consideration the different constraints that might appear. In order for this comparison to hold true, a risk evaluation is presented, analysing 12 risks and 4 risk factors in 3 phases, so that the impact of the different variables can really be appraised. This research is made in a scenario of the construction of a 3.5 km-long tunnel in a non-urban area, with an unproblematic rock mass. The final result will be a detailed analysis of the influence of “Safety and Health” criteria, useful for both the technical and the scientific community, something that has never been done before.

ABSTRACT Carbonation and chloride action are the two leading causes of degradation in reinforced concrete structures. Despite the combined action of these two mechanisms being a reality, there is little research on the effect of this combination in cementitious matrices. Furthermore, the incorporation of fly ash in cementitious matrices has been frequently used in order to make the matrix more resistant to the action of chlorides. On the other hand, it is known that Ca(OH)2 existing in the matrix is consumed by the pozzolanic reactions, which makes easier the carbonation front advance. Therefore, this paper presents a study of the behavior of cementitious matrices, with and without fly ash, subjected to the combined action of chlorides and carbonation. Two different kinds of mortars were produced: reference (only cement CEM I 42.5R) and fly ash mortar (40% replacement of cement). After curing, the specimens were subjected to three different wetting-drying cycles, considering NaCl solutions and CO2 atmospheres. Afterwards, chloride profiles and carbonation front were measured. Results show that carbonation has a direct influence on chloride penetration, decreasing it for cement mortars with 0% fly ash and increasing it for mortars with 40% cement replacement by fly ash. Moreover, the evolution of carbonation is also influenced by chloride presence, which decreases under the combined action.

Preparing a high fluidity paste is a major step in ultra-high performance concrete (UHPC) development with respect to its self-compacting ability as well as its ultra-high strength. In this regard, some experiments have been carried out in order to study various superplasticizer (SP) addition methods and times. Among these procedures, stepwise and delayed methods seem to be more efficient compared to direct addition of SP with or immediately after water addition. However, few studies regarding water addition time and method have been conducted since now. In this research work, the effects of water and SP addition methods on the fluidity of paste were investigated. The results demonstrated that stepwise and delayed water beside delayed SP addition remarkably reduce the flow time. This maximum fluidity was achieved after totally 15 minutes of mixing including 3 minutes after 70% of water addition to powder, as first-part water, 6 minutes after SP addition and finally an extra 6 minutes after second-part water which is 30% of the total water. Based on this procedure, the opportunity for developing self-compacting and durable UHPC could be accessible. Furthermore, using higher content of aggregates and supplementary cementitious materials would be possible due to higher fluidity of the paste which finally results in an eco-efficient UHPC.