Why is fresh self-compacting concrete shear thickening?

The rheological properties of fresh concrete are mostly described by means of the Bingham model. For self-compacting concrete, the Bingham model is applicable in a lot of cases, but some authors report that the rheological behaviour is non-linear. The apparent viscosity increases with increasing shear rate and the SCC shows shear thickening behaviour. Shear thickening becomes important in operations occurring at high shear rates, like mixing and pumping. In these cases, shear thickening should not be forgotten in order to avoid breaking of the mixer, pump or pipes.This paper will describe two possible theories for shear thickening behaviour of SCC, based on results published in the rheology literature. The first theory consists of the formation of so-called (hydro-)clusters, which are temporary assemblies of small particles. These clusters start being formed from a certain shear stress on: the critical shear stress. They cause the viscosity to increase with increasing shear rate. A second theory is based on grain inertia, where a part of the shearing force is transmitted through direct momentum transfer between solid particles. Results on cement pastes prove that the grain inertia theory is not the main cause of shear thickening in self-compacting concrete. The influence of several parameters on the shear thickening behaviour of SCC can be well explained by means of the cluster theory.     

Compaction and shear failure of refractory mortars – effects of porosity and binder hardening

With the aim to correlate the global properties of refractory mortars with the micro-mechanical processes, a series of uni-axial compression and shear tests was conducted. The test program was developed with the view that the shear grain slip and cracks are frequent failure mechanism under compressive loads. The micro-structural changes during compression were monitored by X-ray micro focus computed tomography. Discrete element modelling was used to highlight the effects of individual factors of influence. Mortars with a water glass binder of different maturity were tested. In compression the mortars demonstrated cracking and pore closure. Shear tests showed that the failure process consists of multiple local failure events. The combined effects of the porosity and immature binder promote increased tendency for crack branching and arrest. This results in low shear strength and high compressibility. Cohesion and interlocking between the grains prevents crack branching and increases the stiffness and the strength.     

High shear granulation of dolomite – I: Effect of shear regime on process kinetics

Wet granulation of previously unreported formulation system is presented. Dolomite powder is granulated under different shear regimes by using three-component binder formulation, water-molasses-polyvinylpyrrolidone. 1-D discretized population balance equation (PBE) and Equi-Partition of Kinetic Energy (EKE) coalescence kernel are applied to modelling granulation in a high shear mixer. Process modelling is focused to simulation of changes of the property of a group of entities, granule size distribution (GSD). The GSD predictions indicate the presence of coalescence growth as a dominant mechanism in the dolomite granulation process. Minor deviations between simulated and real GSDs signify the probability of other granulation mechanism(s) existence. A posteriori approach by integral method was used for coalescence rate constant estimation. This research highlights discrepancy in the coalescence rate of dolomite granulation process, between its early and later stages. Moreover, kinetic analysis of the high shear granulation process provides quantification of the macroscopic variable (impeller speed) influence on regarded property of a group of granules in terms of values of growth rate parameter.     

Evaluation of the shear characteristics of steel–asphalt interface by a direct shear test method

Shear characteristics of steel–asphalt interface under the influences of temperature, normal stress level and tack coat material were investigated. The direct shear tests were conducted on composite specimens with epoxy asphalt (EA) and polymer modified asphalt (PMA) tack coat materials at temperatures of 25 and 60 °C and normal stress levels of 0, 0.2, 0.4, and 0.7 MPa for each temperature. Results show that the failure modes include adhesive failure at the primer-tack coat interface and material failure of asphalt concrete. Steel–asphalt interface shows strain softening behavior until it reaches the sliding state. The shear strength and the shear reaction modulus increase with decreasing temperature and increasing normal stress levels. The specimens with EA tack coat provides much higher interface shear strengths than those with PMA tack coat at 25 and 60 °C. In addition, the failure envelopes of the shear strength and residual shear strength were obtained for combinations of tack coat materials and temperature conditions based on the Coulomb failure law.     

Breakage–reflocculation implemented by two-stage shear for enhancing waste-activated sludge dewaterability: Effects of shear condition and extracellular polymeric substances

The conditioning of waste-activated sludge (WAS) before dewatering is crucial for enhancing sludge dewaterability. The breakage–reflocculation that was implemented by two-stage shear (drastic first-stage shear for breakage and moderate second-stage shear for reflocculation utilizing the bioflocculation function) which was proposed as a novel WAS conditioning method with several advantages (simple operation, lower cost, and none added reagent) compared to traditional methods. Effects of the shear condition and extracellular polymeric substances (EPS) on breakage–reflocculation were orderly investigated. Two equations were developed by response surface methodology for predicting breakage–reflocculation conditioning performance. Analysis of variance (ANOVA) indicated that individual effects of first-stage shear rate (G₁), second-stage shear rate (G₂), second-stage shear time (t₂), and interactive effect of G₁G₂ were significant. More compact WAS flocs with better dewaterability and larger floc size formed through breakage–reflocculation. This was reflected in that the capillary suction time decreased by 16.9% and mean floc size increased by 24% under the optimum shear condition. In addition, the loosely bound EPS was revealed to be closely negatively correlated with breakage–reflocculation conditioning performance, indicating its adverse role in breakage–reflocculation. The breakage–reflocculation could be used as an independent conditioning method with low cost or a part of combined method.     

Comment on “Synthesis and electrorheological characteristics of titanate nanotube suspensions under oscillatory shear”

We comment on the recently published dielectric properties of titanate nanotubes (TNTs)- and titania nanoparticles (P25)-based electrorheological (ER) fluids in this note aiming to make a better understanding of their ER performance. Based on our Cole–Cole model plot, it is found that we can fit the dielectric spectra (dielectric constant/dielectric loss vs. frequency) of two ER fluids well with a better explanation of their polarizability by differentiating their ER performance.     

Nucleation and growth behavior of β-nucleated iPP during shear induced crystallization investigated by in-situ synchrotron WAXS and SAXS

Polymer crystallization under flow was investigated because final properties of polymers strongly depend on the crystalline structure and morphology formed during processing. In-situ synchrotron WAXS and SAXS were used to investigate the structure formation and morphological developments during quiescent and shear-induced crystallization of iPP with 3 different concentrations (0.01, 0.03 and 0.1 wt%) of β-nucleating agent. Under quiescent conditions, a high β-content was obtained at all those concentrations indicating the high β-nucleating efficiency of the nucleating agent. After application of shear, the β-nucleating ability at small concentration was retained, while at larger concentrations it was strongly retarded. Furthermore, the addition of β-nucleating agent was beneficial for the molecular alignment during processing, the anisotropic particles of β-nucleating agent were much more effective to align the molecular chains in the flow direction compared to isotropic ones. A crystallization scheme for the iPP β-nucleating agent under shear was proposed.     

Do hemostatic agents affect shear bond strength and clinical bond failure rate of orthodontic brackets?

To evaluate the effects of different hemostatic agents on the shear bond strength (SBS) in vitro and clinical bond failure rate of orthodontic metal brackets in vivo. A total of 100 human premolar teeth were randomly divided into five groups: control, blood, Viscostat, hydrogen peroxide (H₂O₂), and epinephrine. Teeth were bonded with same light-cured adhesive and composite. After storage in distilled water for 24 h, thermal cycling was used as an aging procedure on all samples. The brackets were subjected to an SBS test at a speed of 0.5 mm/min until bracket debonding. SBS values and the adhesive remnant index were evaluated. Ninety-nine patients (52 female, 47 males) undergoing routine orthodontic treatment were recruited for this controlled clinical study at bonding stages. All patients with bleeding on the buccal surface of any premolar tooth or teeth at bonding were included in this study. Over 6 months, the bond failure rate was calculated. Data were analyzed using one-way analysis of variance (ANOVA) and Tukey’s post-hoc test (p < .05). The McNemar test was used to compare bracket-bond failure. ANOVA showed a significant difference (p < .001) between the groups. No significant differences were found between the hemostatic agent groups (p > 0.05) in the in vitro part. The lowest failure rate was obtained in the control group rather than the hemostatic agent groups during clinical follow-up (p < 0.05). Each of the hemostatic agents (Viscostat, H₂O₂, and epinephrine) can be used for bleeding management during the orthodontic bonding process. Epinephrine application showed a high bond-failure rate at clinical follow-up.     

Growth,doping and characterization of epitaxial thin films and patterned structures of AlN,GaN, and AlxGa1−xN

Advancements in the doping of GaN and Al_xGa_1−xN thin films, and the growth of GaN and Al_xGa_1−xN structures on patterned heterostructure substrates via metalorganic vapor phase epitaxy are reported. The acceptor-type behavior of Mg-doped GaN films grown in N₂ diluents is presented. Net ionized impurity concentrations up to 8×10¹⁸ cm⁻³ and Hall mobilities up to ≈14 cm² V⁻¹ s⁻¹ were measured for Mg-doped films grown in N₂ in the as-grown condition. Donor and acceptor doping of Al_xGa_1−xN alloys was performed. Acceptor doping of Al_xGa_1−xN for x≤0.13 and donor doping for x≤0.58 were achieved for films deposited at 1100 °C. Lateral epitaxial overgrowth of GaN and Al_xGa_1−xN layers was investigated. The growth and coalescence of GaN and Al_xGa_1−xN stripes patterned in SiO₂ and/or SiN_x masks deposited on GaN, including aligned second lateral epitaxial overgrowth on initial laterally overgrown GaN layers, are discussed.     

Experimental Measurement and Correlation of the Solubility of Methyl-acetyl-naphthalene in n-Heptane,n-Octane,and n-Dodecane

1 INTRODUCTION 2-Methyl-6-acetylnaphthalene (2,6-MAN) is a type of white or pale yellow, powdery crystal, with a melting point of 332.15K. It is an important interme- diate[1] used for producing 2,6-naphthalene dicarbox- ylic acid (2,6-NDA), which has very extensive appli- cations in not only the light, electronic, and defense industries, but in many other areas. In particular, 2,6-NDA is an important monomer of liquid crystal polyester material (LCP) and polyethylene naphtha- lene-2…     

Comparison of supercritical fluid and solvent extraction methods in extracting γ-oryzanol from rice bran

Organic solvents were compared with supercritical CO₂ relative to efficiency for extracting lipid and γ-oryzanol from rice bran. A solvent mixture with 50% hexane and 50% isopropanol (vol/vol) at a temperature of 60°C for 45–60 min produced the highest yield (1.68 mg/g of rice bran) of γ-oryzanol among organic solvents tested. The yield of γ-oryzanol without saponification was approximately two times higher (P<0.05) than that with saponification during solvent extraction. However, the yield (5.39 mg/g of rice bran) of γ-oryzanol in supercritical fluid extraction under a temperature of 50°C, pressure of 68,901 kPa (680 atm), and time of 25 min was approximately four times higher than the highest yield of solvent extraction. Also, a high concentration of γ-oryzanol in extract (50–80%) was obtained by collecting the extract after 15–20 min of extraction under optimized conditions.     

First-principles calculations of electronic structures and optical,phononic, and thermodynamic properties of monoclinic α-spodumene

To further the applications of spodumene in ceramic materials, we performed first-principles density functional theory calculations using a quasi-harmonic approximation to investigate the electronic structures and optical, phononic, and thermodynamic properties of monoclinic types of α-spodumene with C2 and P2₁ structures. The electronic structures of C2- and P2₁-spodumene including band structure and density of states, were observed to be similar. These spodumene can be treated as wide-direct-band-gap insulators with band-gap widths of 5.537 eV for C2 -spodumene and 6.335 eV for P2₁-spodumene. The optical properties of C2- and P2₁-spodumene indicated that both C2- and P2₁-spodumene were found to be optically anisotropic and they can be used for UV shielding in optical devices. Additionally, we investigated the phononic properties of these spodumene to explore their thermodynamic characteristics, and found that both types are dynamically stable.     

Modelling the nonlinear shear stress–strain behavior of a carbon fabric reinforced polyphenylene sulphide from rail shear and [(45°,−45°)]4s tensile test

In this article, the nonlinear shear stress–strain relationship of a carbon fabric-reinforced polyphenylene sulphide is investigated by performing and comparing both the [+45°/−45°]_ns tensile test and the three-rail shear test. First, quasi-static and hysteresis tests are performed to obtain the data necessary for the material model. Then, the material constants are optimized by comparing finite element simulations with the data derived from the experiments. The conducted experiments are simulated and the results are compared with the experiments, with excellent correspondence. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Experimental electrohydrodynamics of poly(γ-Benzyl-L-Glutamate) in M-Cresol solution subjected to shear flow and electric fields

Electrohydrodynamics of a dilute solution of rigid macromolecules was experimentally studied in a continuation of previous theoretical work. We used poly(γ-benzyl-L-glutamate), having 4 different molecular weights ranging from 15,000 to 236,000, dissolved in m-cresol. Poly(γ-benzyl-L-glutamate) solutions were subjected to combinations of simple shear flow field and uniform electric field perpendicular to the shear direction. Transient birefringence and extinction angle were simultaneously measured using the phase-modulated birefringence method. Steady state results were compared with the theoretical prediction from previous works and rotational difiusivity and permanent dipole strength of PBLG were obtained from multiple parameter fitting. Consequently, the optical state of PBLG solution could be explained to a certain extent by the dimensionless field parameters established in the previous theory.     

Methanation of carbon dioxide and fluidization quality in a fluid bed reactor—the influence of a decrease in gas volume

A large decrease of fluidization quality was observed when methanation of carbon dioxide was carried out in a fluid-bed reactor even if the catalyst particles had the optimal properties for good fluidization. The cause of this phenomenon was explored by measuring pressure fluctuations, bubble frequency and extent of CO₂ conversions. The results indicated that the decrease of the fluidity was caused by a reduction in volume of reactant gases due to the reaction. The voidage in the emulsion phase is considered to be an important factor affecting the fluidity. The fluidization quality and contacting efficiency could be improved by such devices as baffle internals or two-stage spargers.     

Formation of β-cylindrites under supercooled extrusion of isotactic polypropylene at low shear stress

Formation of β-cylindrites of isotactic polypropylene under various wall shear stress (σ_w), supercooled temperature of melt (T_e) and crystallization temperature (T_c) has been investigated by polarized light microscopy (PLM), wide angle X-ray diffraction (WAXD), and differential scanning calorimeter (DSC). To have better control over the thermomechanical history, instead of a reciprocating screw, the samples were prepared by extruding supercooled melt through capillary die. β-cylindrites can be observed by PLM in the extruded specimen even at a lower σ_w (0.020 MPa), and the number of β-cylindrites nuclei increases rapidly with the lowering of T_e. The nucleation density of β-cylindrites increases with the raising of wall shear stress under a given T_e of 160 °C. Furthermore, at lower supercooled temperature of melt (145 °C), the radius of β-cylindrites decreases with the increasing of σ_w, and the number of β-cylindrites nuclei almost remain invariant. At relatively higher σ_w (0.090 MPa), a saturation of β-cylindrites nuclei is observed with decreasing T_c. A modified model based on above results has been proposed to explain the effect of the original structure of quiescent supercooled melt on the formation of β-cylindrites under low shear stress.