Shear-thickening behavior of high-performance cement grouts — Influencing mix-design parameters

Rheology of concrete is of great importance to its flow performance, placement and consolidation. A full understanding of fresh concrete flow behavior can be achieved through a good understanding of paste rheology. Cement pastes exhibit a complex rheological behavior affected by several physical and chemical factors, including water-to-cement ratio (w/c), high-range water-reducer (HRWR) type and dosage, and cement characteristics. An experimental investigation was carried out to evaluate the effect of w/c, HRWR–cement combinations, and supplementary cementitious materials (SCM) on the pseudoplastic behavior of high-performance cement grouts. Grout mixtures proportioned with w/c of 0.30, 0.33, 0.36, and 0.40, various cement–HRWR combinations, and cement substitutions by 8% silica fume were investigated. The incorporation of HRWR can lower the yield stress of mixtures, thus enhancing deformability, while silica fume improves mechanical and durability performances.High-performance structural grouts are shown to exhibit shear-thickening behavior at low w/c and shear-thinning behavior at relatively higher w/c. Mixtures made with polycarboxylate HRWR acting by steric effect exhibited greater shear-thickening behavior compared to those made with polynaphthalene sulfonate-based HRWR acting by electrostatic effect. The paper discusses the effect of mixture parameters on non–linear rheological behavior of various grout mixtures prepared with different w/c, HRWR–cement combinations, and silica fume.     

Comparison of measured and predicted performance of heat storage unit packed with spheres of a local material

The present paper presents a comparison between the measured and predicted performance of a sensible heat storage unit. The storage unit is packed with spheres manufactured from a powder of certain type of Egyptiain clay (trade name “tafla”) produced at Abu-Siebera, Aswan. A special experimental set up was designed to predict the performance of the storage unit. The temperature distribution of the solid spheres was measured at different mass flow rates and at different time intervals. A comparison was made between these measured values and the corresponding values obtained using a numerical model based on finite difference techniques. The results showed a reasonable agreement between the measured and predicted temperature distribution. Finally, the pressure drop through the storage unit was measured and compared with the corresponding pressure drop calculated using two different empirical formulae.     

Fuzzy approach for optimal energy-management of a domestic photovoltaic panel

This paper introduces a new switching means giving a real time optimum connection mode of domestic appliances on either the electrical grid or a photovoltaic panel (PVP) output. A fuzzy algorithm makes the decision which ensures the optimal energy-management based on PVP generation and appliances states with respect to energy-saving criteria. Validation is driven on a 1 kW peak (kWp) PVP and domestic appliances of different powers installed at the Energy and Thermal Research Centre (CRTEn) in the north of Tunisia. Results confirm that energy saved during daylight is 80–90% of the PVP generated energy.     

Mechanosynthesis,crystal structure and magnetic characterization of M-type SrFe12O19

M-type strontium hexaferrite was prepared by mechanosynthesis using high-energy ball milling. The influence of milling parameters, hematite excess and annealing temperature on magnetic properties of SrFe₁₂O₁₉ were investigated. Commercial iron and strontium oxides were used as starting materials. It was found that mechanical milling followed by an annealing treatment at low temperature (700 °C) promotes the complete structural transformation to Sr-hexaferrite phase. For samples annealed at temperatures from 700 to 1000 °C, saturation magnetization values (M_s) are more sensitive to annealing temperature than coercivity values (H_c). The maximum M_s of 60 emu/g and H_c of 5.2 kOe were obtained in mixtures of powders milled for 5 h and subsequently annealed at 700 °C. An increase in the annealing temperature produces negligible changes in magnetic saturation and coercivity. An excess of hematite as a second phase produces a slight decrease in the saturation magnetization but leads to a significant increase in coercive field, reaching 6.6 kOe.     

Effects of inorganic nitrate and nitrite consumption on cognitive function and cerebral blood flow: A systematic review and meta-analysis of randomized clinical trials

ABSTRACT

We conducted a systematic review and meta-analysis of randomized clinical trials examining the effect of inorganic nitrate or nitrite supplementation on cognitive function (CF) and cerebral blood flow (CBF). Two databases (PubMed, Embase) were searched for articles from inception until May 2017. Inclusion criteria were: randomized clinical trials; participants >18 years old; trials comparing a nitrate/nitrite intervention with a control. Thirteen and nine trials were included in the meta-analysis to assess CF and CBF, respectively. Random-effects models were used and the effect size described as standardized mean differences (SMDs). A total of 297 participants (median of 23 per trial) were included for CF; 163 participants (median of 16 per trial) were included for CBF. Nitrate/nitrite supplementation did not influence CF (SMD +0.06, 95% CI: -0.06, 0.18, P = 0.32) or CBF under resting (SMD +0.14, 95% CI: -0.13, 0.41, P = 0.31), or stimulated conditions (SMD + 0.23, 95% CI: -0.11, 0.56, P = 0.19). The meta-regression showed an inverse association between duration of the intervention and CBF (P = 0.02) but no influence of age, BMI or dose (P < 0.05). Nitrate and nitrite supplementation did not modify CBF or CF. Further trials employing larger samples sizes and interventions with longer duration are warranted.     

Steady state theoretical model of fired clay hollow bricks for enhanced external wall thermal insulation

This paper proposes a theoretical model to study the steady state thermal behavior of fired clay hollow bricks for enhanced external wall thermal insulation. The study aims at the development of new materials and structural components with good thermal material properties, with respect to energy saving and ecological design. Thermal insulation capacity of two external walls of different thicknesses, constructed of locally produced bricks, is studied. The basic brick units used for the investigation are small-size bricks with eight equal cavities or recesses and big-size bricks with twelve equal recesses. Their recesses configuration has been varied to perform the assessment. The insulation materials injected within brick recesses during the assessment are granulated cork and expanded polystyrene. The improvement in the thermal performance of the walls will be the result of optimization among the various factors such as brick cavity configurations, integration of insulation within brick recesses and the cavity surface emissivities. So emphasis is given to the study of the impact of these factors singly or in combination on the overall thermal resistance of walls in order to find out the best design solutions to maximize their thermal insulation capacity. Computer modeling and calculations performed, for steady state conditions, show that the increase in hollow brick cavity height contributes to the improvement of the overall thermal resistance of the order of 18–20%. The improvement could significantly increase to the range of 88.64% and 93.33%, if the bricks used are injected with the insulating material. If the cavity surface emissivities are lowered to 0.3, the improvement will be 72.73–78.33%. The results have also shown that replacing the cork by expanded polystyrene (EPS), having lower thermal conductivity, would not improve significantly the overall thermal resistance. This improvement is 9.08% for a wall of small-size bricks having configuration BS2CV and 8.34% for a wall of big-size bricks having configuration BB3CV.     

Synthesis and characterization of photoluminescent cerium-doped yttrium aluminum garnet

Powder phosphor yttrium aluminum garnet (YAG), doped with trivalent cerium (Ce³⁺) is synthesized by sol-gel method. The formation of YAG and YAG:Ce (cerium-doped yttrium aluminum garnet) was investigated by means of X-ray diffraction (XRD). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were also used. The purified crystalline phases of YAG and YAG:Ce were obtained at 1000 °C. The maximum average grain size is about 20-23 nm for undoped samples and 28-34 nm for doped samples. The crystalline YAG:Ce emission shows one peak in the range 480-535 nm with the maximum near 520 nm. Photoluminescence (PL) intensity of 5d → 4f transition of Ce³⁺ increased with increasing annealing temperature. With increasing the concentration of Ce³⁺, the photoluminescence peak shifts towards the red region.     

Chemical composition and in vitro digestibility of some Spanish browse plant species

Samples of leaves, flowers and fruits of four browse species were collected from uplands in the province of León (northwestern Spain) at different sampling times (from spring till autumn). The browse species were Erica australis (Spanish heath), Cistus laurifolius (laurel‐leaved rock rose), Quercus pyrenaica (hoary oak) and Rosa canina (wild dog rose). A large variability in chemical composition and in vitro digestibility and gas production kinetics was observed among species and among sampling times within each species. The ranking order of the browse plants according to their in vitro digestibility and gas production kinetics was R canina > Q pyrenaica > C laurifolius > E australis. In the leaves of R canina and Q pyrenaica, crude protein content, digestibility and parameters of gas production tended to decrease throughout the growing season (P < 0.05 in most cases), whereas cell wall contents followed the opposite trend. Seasonal variations were less pronounced in the other browse species. There were significant (P < 0.05) correlations between in vitro digestibility and chemical composition of the browse plants, positive with the crude protein content and negative with the fibre fractions. Copyright © 2004 Society of Chemical Industry     

Compositions of anthocyanins in blackberry juice and their thermal degradation in relation to antioxidant activity

This study investigated the compositions of anthocyanins in blackberry juice and the effect of heating (70, 80, and 90?°C) on individual anthocyanins, as well as its impact on antioxidant activity. The results showed that five anthocyanins were identified, including cyanidin (Cy)-3-O-glucoside (1), Cy-3-O-arabinoside (2), Cy-3-O-malonyl-glucoside (3), Cy-3-O-dioxalyl-glucoside (4), and the Cy derivative (5). The degradation rates of individual anthocyanins (1–4) followed the first-order reaction kinetics. Unexpectedly, the amount of the Cy derivative (5) was observed to slightly increase in temperature. At the same temperature, the half-lives (T _1/2) values of each anthocyanin (1–4) significantly differed (P?<?0.05). Maximum of the T _1/2 values were observed with Cy-3-O-dioxalyl-glucoside (T _1/2 values ranged 9.48–3.54?h) and exhibited relatively resistant stability. Moreover, the T _1/2 values of Cy-3-O-glucoside were higher than Cy-3-O-arabinoside and Cy-3-O-malonyl-glucoside. The antioxidant activity of blackberry juice reduced with the decreasing of total anthocyanins. These results demonstrated that the time dependency of anthocyanins degradation and antioxidant capacity were strongly correlated in blackberry juice.     

Improved performance and energy management strategy for proton exchange membrane fuel cell/backup battery in power electronic systems

The objective of this paper is to propose an approach to analyze the reliability of proton exchange membrane fuel cell and backup battery in the power electronic systems by improving the strategy of energy management. This approach is based on the research of critical causes generating the degradation of power sources in the power electronic application and their undesirable effect. The analysis of potential failure modes that affect the fuel cell and the auxiliary source is developed by using analysis tools such as the Failure Mode and Effects Analysis (FMEA). The undesirable factors have to be taken into account in designing the topology of power converter in order to improve the performance of a fuel cell power system. In this context the authors propose to integrate a multiphase converter to minimize the current ripple and ameliorate the dynamic response of exchange membrane fuel cell. The improvement of battery lifetime is also ensured by the incorporation of an appropriate charge cycle that promises the full state of charge and avoids the overcharge.