Permeability characteristics of carbonated concrete considering capillary pore structure

During carbonation process, the calcium phases present in cement are attacked by CO₂ and converted into CaCO₃ and the permeability of concrete is changing due to the change in porosity. The rate of carbonation depends upon porosity and moisture content of the concrete. Especially in underground reinforced concrete structures, the interior portion of concrete surface may be exposed to carbonation and the exterior portion of concrete surface exposed to wet soil or underground water. As carbonation proceeds from outer surface into internal portion of concrete, microstructure is also changed continuously from outer surface into internal portion of concrete. Even the deteriorations in the structures due to the carbonation have been reported more, research on permeability characteristics of concrete considering carbonation and micro-structural information is very scarce.In this study, the permeability coefficient in carbonated concrete is derived by applying a capillary pore structure formation model in carbonated cement mortar and assuming that aggregates do not affect carbonation process in early-aged concrete as a function of porosity. The permeability obtained from the micro-level modeling for carbonated concrete is verified with the results of accelerated carbonation test and water penetration test in cement mortar.     

Atmospheric Dependence on Dielectric Loss of 1/6Ba5Nb4O15· 5/6BaNb2O6 Ceramics

Samples of 1/6Ba₅Nb₄O₁₅·5/6BaNb₂O₆ along with the pure end members, Ba₅Nb₄O₁₅ and BaNb₂O₆, were sintered under low oxygen partial pressure. The degradation mechanisms of dielectric loss in this reducing atmosphere have been studied. We found that the degradation occurred primarily due to the formation of oxygen vacancies caused by the reduction of Nb⁵⁺. This was determined by measuring the electrical conductivity, and through X-ray photoelectron spectroscopy. More importantly, the dielectric loss of 1/6Ba₅Nb₄O₁₅·5/6BaNb₂O₆ samples with higher temperature stability was further decreased on sintering in a reducing atmosphere. This observation has been explained by considering the increased porosity and formation of a reduced second phase, Ba_0.65NbO₃.     

Dielectric Properties and Relaxation in (1−x)BiScO3–xBa(Mg1/3Nb2/3)O3 Solid Solutions

The dielectric properties and frequency dispersion associated with a dielectric relaxation were evaluated within the perovskite (1− x )BiScO₃– x Ba(Mg_1/3Nb_2/3)O₃ solid solution systems (0.7 ≤ x ≤ 1). With increasing BiScO₃, the room-temperature dielectric permittivity at low frequency (100 Hz) increased up to 115 at x = 0.7, and a dielectric relaxation phenomenon was evident. Relaxation parameters were analyzed using several Arrhenius-type equations, and the microwave dielectric property measurement using rectangular wave-guide method enabled confirmation of the extrapolated value of the Arrhenius plot. The result of the microwave dielectric property measurement was also checked with J -function fitting based on the frequency-dependent Gaussian distribution of the associated dielectric loss data at low frequency.     

Numerical prediction of residual stresses and birefringence in injection/compression molded center‐gated disk. Part I: Basic modeling and results for injection molding

The present study attempted to numerically predict both the flow‐induced and thermally‐induced residual stresses and birefringence in injection or injection/compression molded center‐gated disks. A numerical analysis system has been developed to simulate the entire process based on a physical modeling including a nonlinear viscoelastic fluid model, stress‐optical law, a linear viscoelastic solid model, free volume theory for density relaxation phenomena and a photoviscoelasticity and so on. Part I presents physical modeling and typical numerical analysis results of residual stresses and birefringence in the injection molded center‐gated disk. Typical distribution of thermal residual stresses indicates a tensile stress in the core and a compressive stress near the surface. However, depending on the processing condition and material properties, the residual stress sometimes becomes tensile on the surface, especially when fast cooling takes place near the mold surface, preventing the shrinkage from occurring. The birefringence distribution shows a double‐hump profile across the thickness with nonzero value at the center: the nonzero birefringence is found to be thermally induced, the outer peak due to the shear flow and subsequent stress relaxation during the filling stage and the inner peak due to the additional shear flow and stress relaxation during the packing stage. The combination of the flow‐induced and thermally‐induced birefringence makes the shape of predicted birefringence distribution quite similar to the experimental one.     

Numerical and experimental investigation of three-dimensional flow in extrusion dies

The univariant element, Q₁ P₀, and the multivariant elements, QP₀ and R P₀, are compared for the numerical simulation of the flow in extrusion dies. The pressure distribution obtained by using the Q₁ P₀ element was found to be afflicted with the checkerboard pressure mode. On the other hand, the multivariant elements, Q P₀ and R P₀, gave accurate and physically reasonable velocity and pressure distributions. The computed values of the pressure drop across extrusion dies matched well with the pressure drop determined experimentally.     

Properties of chitosan‐based biopolymer films with various degrees of deacetylation and molecular weights

An increase in the depolymerization of chitosan was found with an increased concentration of sodium perborate. Acetic anhydride was added to reacetylated chitosan in a molar ratio per gulcosamine unit, and the amide I band of IR spectra changed with the addition of acetic anhydride. Sixteen chitosans with various molecular weights (MWs) and degrees of deacetylation (DODs) were prepared. X‐ray diffraction patterns indicated their amorphous and partially crystalline states. Increases in the chitosan MW and DOD increased the tensile strength (TS). TS of the chitosan films ranged from 22 to 61 MPa. However, the elongation (E) of chitosan films did not show any difference with MW. TS of chitosan films decreased with the reacetylation process. However, E of chitosan films was not dependent on DOD. The water vapor permeabilities (WVPs) of the chitosan films without a plasticizer were between 0.155 and 0.214 ng m/m² s Pa. As the chitosan MW increased, the chitosan film WVP increased, but the values were not significantly different. Moreover, the WVP values were not different from low DOD to high DOD. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3476–3484, 2003     

Thermal and mechanical properties of the polymers synthesized by the sequential polymerization of propylene and 1‐hexadecene

The block copolymer of poly(1‐hexadecene) (PHD) and polypropylene (PP) was effectively synthesized by the sequential polymerization of propylene and 1‐hexadecene by using highly isospecific TiCl₃/Cp₂Ti(CH₃)₂ (Cp = cyclopentadienyl). The block copolymers had two separate melting temperatures of constituent blocks. The modulus of PHD–PP block copolymer was enhanced as the content of sequentially polymerized PP block was increased. The elongation at break showed positive deviation at the intermediate compositions from the simple additive values of constituent homopolymers. Shape memory effect which utilizes the crystalline PHD block as a reversible phase and the crystalline PP block as a fixed structure was examined. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1709–1715, 2002; DOI 10.1002/app.10551     

Influence of mixing cycle on the degree of mixing of calcite‐filled polyethylene upon stretching

The mixing cycle‐dependent degree of dispersion and degree of mixing of a calcite (calcium carbonate) agglomerate in high‐density polyethylene (HDPE), low‐density polyethylene (LDPE), and linear low‐density polyethylene (LLDPE) matrices upon stretching was investigated using three different techniques: mechanical property, morphological behavior, and image analyzer analyses. The mechanical properties analyzed in terms of the tensile strength and maximum elongation resulted in that the second mixing was the best for giving a better property for all systems except the LDPE system, which exhibited no significant difference between the second and third mixings. The morphological behavior of the three compounds were different, but no distinctive difference was observed to differentiate the degree of mixing from system to system. The number‐, weight‐, and z + 1‐average diameters of the air hole and the aspect ratio upon the stretching and mixing cycle were calculated to analyze the degree of mixing of the calcite‐filled composites. As a consequence, no difference in the average diameter of the air hole was obtained among the three systems, but the aspect ratios of the air hole varied significantly. Thus, the degree of dispersion and the degree of mixing may be influenced by the average calcite agglomerate size, the average diameter of the air hole, and the aspect ratio upon stretching and mixing cycles. Those factors would be formed by the difference in chemical characteristics upon various microstructures of polyethylene and its molecular weight and molecular weight distribution. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 311–321, 2003     

Encapsulation mechanism of N2 molecules into the central hollow of carbon nitride multiwalled nanofibers

Nitrogen molecules have been encapsulated into the central hollows of vertically aligned carbon nitride (CN) multiwalled nanofibers by dc plasma-enhanced chemical vapor deposition with C₂H₂, NH₃, and N₂ gases on a Ni/TiN/Si(1 0 0) substrate at 650 °C. X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectra showed the existence of nitrogen molecules in CN nanofibers. Elemental mapping images with electron energy loss spectroscopy of the CN nanofiber and catalyst metal, and optical emission spectroscopy spectra of the plasma showed the distribution of nitrogen atoms and molecules in the CN nanofiber, catalyst metal, and gaseous precursor, respectively. These studies showed that atomic nitrogen diffused into the catalytic metal particle because of the concentration gradient and then saturated at the bottom of the particle. Saturated nitrogen atom participated in the formation of the CN nanofiber wall but most of nitrogen was trapped in the central hollow of the nanofiber as molecules.     

Application of essential work of fracture concept to toughness characterization of high‐density polyethylene

Deformation and fracture toughness of high‐density polyethylene (HDPE) in plane‐stress tension was studied using the concept of essential work of fracture (EWF). Strain range for necking was determined from uniaxial tensile test, and was used to explain the deformation transition for 2‐staged crack growth in double‐edge‐notched tensile test. Through work‐partitioning, EWF values for HDPE were determined for each stage of the crack growth. Appropriateness of these EWF values to represent the material toughness is discussed. The study concludes that the EWF values for ductile polymers like HDPE may not be constant, but vary with the deformation behaviour involved in the crack growth process. POLYM. ENG. SCI., 47:1327–1337, 2007. © 2007 Society of Plastics Engineers