Influence of MWCNT/surfactant dispersions on the mechanical properties of Portland cement pastes

This work studies the reinforcing effect of Multi Walled Carbon Nanotubes (MWCNT) on cement pastes. A 0.35% solid concentration of MWCNT in powder was dispersed in deionized water with sodium dodecyl sulfate (cationic surfactant), cetylpyridinium chloride (anionic surfactant) and triton X-100 (amphoteric surfactant) using an ultrasonic tip processor. Three concentrations of each surfactant (1mM, 10mM and 100mM) were tested, and all samples were sonicated until an adequate dispersion degree was obtained. Cement pastes with additions of carbon nanotubes of 0.15% by mass of cement were produced in two steps; first the dispersions of MWCNT were combined with the mixing water using an ultrasonic tip processor to guarantee homogeneity, and then cement was added and mixed until a homogeneous paste was obtained. Direct tensile strength, apparent density and open porosity of the pastes were measured after 7 days of curing. It was found that the MWCNT/surfactants dispersions decrease the mechanical properties of the cement based matrix due to an increased porosity caused by the presence of surfactants. © Published under licence by IOP Publishing Ltd

Deflection hardening of sustainable fiber–cement composites

In the present study sisal fiber–cement composites reinforced with 4% and 6% of short fibers were developed and their physical–mechanical behavior was characterized. To ensure the composite sustainability and durability, the ordinary Portland cement matrix was modified by adding fly ash and metakaolin, and the natural aggregate was substituted by 10% and 20% of recycled concrete aggregate. Flat sheets were cast in a self-compacted cement matrix and bending tests were performed to determine the first crack, postpeak strength and toughness of the composites. Cyclic flexural tests were carried out to determine the stiffness variation of composite due to cracking formation and propagation. It can be seen that the reinforcement provided by short sisal fibers for recycled cement matrices guaranteed a composite with multiple cracking and an increase of strength after the first crack. Reduction of stiffness and increase the damping capacity of composite are verified with progressive cracking.CAPES (PVE Program: Project 047/2012) and CNPqBrazilian agency FAPESBBrazilian agency CAPES through the PVE Progra

Cement composites reinforced by short curaua fibers

ABSTRACT The development of an eco-friendly material that could reduce CO 2 emission and that could aggregate value to a natural fiber, setting man at the countryside and raising the income of populations from poor regions is a challenge. Lignocellulosic fibers are cheap and are a readily available reinforcement, requiring only a low degree of industrialization for their processing. The main drawback of using cement composites reinforced with lignocellulosic fibers is that the fibers can be mineralized inside the alkaline environment. In this work, Portland cement was partially replaced by metakaolinite in order to produce a matrix free from calcium hydroxide, avoiding thus the problem of fiber mineralization. Cement composites reinforced with 2, 4 and 6% of short curaua fibers, were manufactured. The composites were submitted to four pointing bending tests in order to determine their mechanical behavior. The results obtained were compared with those found for cement composites reinforced with sisal fibers

Computer simulation for the evaluation of recombination strategies in intrapopulation recurrent selection in Eucalyptus.

Intrapopulation recurrent selection (IRS) has proven to be a promising breeding method in eucalyptus, mainly through being easier to carry out when compared to reciprocal recurrent selection (RRS). However, the recombination strategies in IRS that have not yet been compared. Thus, the purpose of this study was to verify the efficiency of different recombination methods in IRS. To do so, computer simulation was used considering different heritabilities (0.1, 0.5, 1.0), different initial allelic frequencies (0.2, 0.8) and allelic interactions without dominance and with complete dominance. The initial population consisted of 1000 individuals, which were selected at random for the beginning of cycle zero. These individuals were interbred two by two. Three selection strategies were carried out and, consequently, three recombination methods: recombine the best individuals selected within the best progenies; the best individuals phenotypically selected regardless of their genealogy; or selection in the mean value of the best progenies selected. It was observed that recombination of the best individuals regardless of their genealogy and of the best individuals within the best progenies provided for gains superior to recombination having only the mean of the progenies as reference. The average degree of dominance and the heritability of the trait should be considered at the time of choosing the method of selection followed by recombination

Dissipative collisions in 16^{16}O + 27^{27}Al at Elab_{lab}=116 MeV

The inclusive energy distributions of fragments (3$\leq$Z$\leq$7) emitted in the reaction $^{16}$O + $^{27}$Al at $E_{lab} =$116 MeV have been measured in the angular range $\theta_{lab}$= 15$^\circ$ - 115$^\circ$. A non-linear optimisation procedure using multiple Gaussian distribution functions has been proposed to extract the fusion-fission and deep inelastic components of the fragment emission from the experimental data. The angular distributions of the fragments, thus obtained, from the deep inelastic component are found to fall off faster than those from the fusion-fission component, indicating shorter life times of the emitting di-nuclear systems. The life times of the intermediate di-nuclear configurations have been estimated using a diffractive Regge-pole model. The life times thus extracted ($\sim 1 - 5\times 10^{-22}$ Sec.) are found to decrease with the increase in the fragment charge. Optimum Q-values are also found to increase with increasing charge transfer i.e. with the decrease in fragment charge.Comment: 9 pages, 4 figures, 1 tabl

Study of temperature effect on macro-synthetic fiber reinforced concretes by means of Barcelona tests: An approach focused on tunnels assessment

This paper presents an experimental investigation on the applicability of the Barcelona (BCN) test to evaluate the mechanical properties of a macro-synthetic fiber reinforced concrete (MSFRC) submitted to high temperature environments (up to 600 °C). BCN tests demonstrated that the MSFRC gradually loses tensile strength an energy consumption density with increasing temperature. Temperatures of 400 °C and 570 °C shown to be critical to the MSFRC mechanical performance. The residual mechanical behavior of the macro-synthetic fibers was not affected by the temperature up to 100 °C. For higher temperatures, the reinforcement showed that may lose part of its crystallinity compromising the MSFRC post-cracking performance. The constitutive model used to determine the stress-strain curves of the MSFRC was capable to reproduce the composite behavior after the event of a fire

Índice de vegetação por diferença normalizada (NDVI) de cultivares de soja sob três níveis de disponibilidade hídirca no solo.

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Effect of accelerated aging on the properties of microconcrete reinforced with sisal fibers

Neste trabalho dois microconcretos, com e sem metacaulinita, foram dosados para produção de compósitos laminados reforçados com fibras longas de sisal. Para garantir uma matriz livre de hidróxido de cálcio foi determinado o teor ideal de metacaolinita por análise termogravimétrica de pastas com teores de substituição de 20%, 30% e 40 nas idades de 4 h, 10 h, 24 h, 7 dias, 28 dias e 730 dias. Laminados com 30% de metacaolinita foram submetidos a ensaio de flexão após 28 dias de cura em água e após 12 e 25 ciclos de molhagem e secagem. A análise da microestrutura foi realizada a partir de microscopia eletrônica de varredura e espectroscopia de raios X por energia dispersiva (EDS). Os resultados demonstram que, no microconcreto contendo apenas cimento, o hidróxido de cálcio foi lixiviado da matriz e depositado na superfície da fibra, o que resultou em ruptura brusca devido ao enfraquecimento das fibras. Com a adição de metacaolinita, a matriz mostrou-se livre de hidróxido de cálcio, e o comportamento mecânico pós-fissuração não foi alterado pelo envelhecimento. No entanto, a variação de temperatura e umidade do ciclo molhagem-secagem induziu fissuração do microconcreto, o que negativamente afetou a resistência à formação da primeira fissura do compósito e seu modo de rupturaIn this study, two microconcrete samples, with and without metakaolin, were designed for the production of laminated composites reinforced with long sisal fibres. In addition to the reference microconcrete, another microconcrete was produced by replacing cement by metakaolin.The ideal metacaolin content that would guarantee a free calcium hydroxide matrix was determined by thermogravimetric analysis of blends with 20%, 30% and 40% substitution contents at the ages of 4 h, 10 h, 24 h, 7 days, 28 days and 730 days. Laminates produced with cement substitution by 30% metakaolin were submitted to flexural test after 28 days of curing in water, and after 12 and 25 wetting and drying cycles. The analysis of the microstructure was performed by scanning electron microscopy and dispersive energy X-ray spectroscopy (EDS). The results demonstrate that in the microconcrete containing only cement, the calcium hydroxide was leached from the matrix and deposited on the surface of the fibre. Thus, the aged composites showed a sudden rupture due to the weakening of the fibres. With the incorporation of metakaolin, the matrix showed to be free of calcium hydroxide, and the mechanical behaviour after cracking was not altered by aging. However, the temperature and humidity variation due the wet-drying cycle has induced cracking of the microconcrete, which detrimentally affected the strength at crack initiation of these composites and their rupture mod