Sensitivity of rheological properties of cement paste with SCC consistency

Robustness, which is defined as the capacity of a mixture to tolerate small variations in constituent elements, mixing parameters or temperature that are inevitable with production at any significant scale, is a key property to expand the practical implementation of self-consolidating concrete (SCC). At the first step, the effect of different amount of water, amount of superplastizer, mixing time, mixing speed and the addition time of superplastizer on the rheological properties of cement pastes with SCC consistency, has been evaluated. Two different SCC mix design concepts, the powder-type and VMA-type were selected for this research. The results show that the selected powder-type mix design is more robust than the VMA-type to a change in addition time of SP. Then, an experimental study was conducted to evaluate different mix design parameters by varying the binder combination, the type of PCE-SP and the addition of viscosity-modifying agent (VMA) to determine which particular parameters cause the largest difference in rheological behavior due to a variation in adding time of SP. In third step, the robustness of the loss of workability of flowable cement pastes, which is mainly affected by variations in the water content and the adding time of the superplasticizer, was evaluated. A comparison study was performed to develop a more accurate vision on robustness of cement paste, and also concrete, by taking into account the time-dependency of the rheological properties. Furthermore, the change in rheological properties of well-dispersed cement pastes subjected to different maximum shear rates has been conducted. It was shown that the choice of maximum shear rate during the rheological measurements is critical for the values of the rheological properties --Abstract, page iv

Determination of ultimate load and possible failure path for solid continuous media using adaptive refinement process

AbstractIn this study, an effective and practical, h-version, enrichment mesh generation, and finite element adaptive procedure for the non-linear solution of problems in continuous media is presented. Moreover, based on the gradient recovery rule, a general recovery technique is developed to measure error and refine mesh in general finite element solutions. The recovery technique is simple and cost effective to implement. The technique has been formulated for two dimensional problems by employing triangular elements. The formulation is consistent with non-linear formulations which iteratively equilibrate the continuous media problems.In the present study, in addition to correlating various norms (such as energy norm, L2 norm for stress and L2 norm for strain), a new norm, namely, deviating stresses norm (called J norm in this study), is also correlated by the authors to estimate the error rate in the finite element method. Based on the results of this study, the J norm can be used as a tool to estimate the error rate in the finite element method, and to determine the ultimate load and the possible failure path in continuous domains. For several numerical examples, the developed algorithms are demonstrated and the resulting meshes are presented

Synthesis and characterization of thermally stable poly(amide-imide)-montmorillonite nanocomposites based on bis(4-carboxyphenyl)-N,N'-pyromellitimide acid

Two new poly(amide-imide)-montmorillonite reinforced nanocomposites containing bis(4-carboxyphenyl)-N,N'-pyromellitimide acid moiety in the main chain were synthesized by a convenient solution intercalation technique. Poly(amide-imide) (PAI) as a source of polymer matrix was synthesized by the direct polycondensation reaction of  bis(4-carboxyphenyl)-N,N'-pyromellitimide acid  with 4,4'-diamino diphenyl sulfone in the presence of triphenyl phosphite (TPP), CaCl2, pyridine and N-methyl-2-pyrrolidone (NMP). Morphology and structure of the resulting PAI-nanocomposite films with 10 and 20% silicate particles were characterized by FT-IR spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effect of clay dispersion and the interaction between clay and polymeric chains on the properties of nanocomposites films were investigated by using UV-Vis spectroscopy, thermal gravimetry analysis (TGA) and water uptake measurements. KEYWORDS: Bis(4-carboxyphenyl)-N,N'-pyromellitimide acid moiety, Poly(amide-imide)-montmorillonite nanocomposite, Thermal properties Bull. Chem. Soc. Ethiop. 2013, 27(1), 95-104.DOI: http://dx.doi.org/10.4314/bcse.v27i1.1

Seasonal Variation in Exposure Level of Types A and B Ultraviolet Radiation: An Environmental Skin Carcinogen

Background: The main source of ultraviolet radiation (UVR) is the sun, affecting organs such as the skin, eyes, and immune system. According to American Conference of Governmental Industrial Hygienist (ACGIH) reports, the amount of UVR reaching the Earth’s surface is increasing yearly and is responsible for an increase in solar radiation‑related diseases. Aims: To investigate the amount of UVR reaching the Earth’s surface and understand the risk of UVR on disease among outdoor laborers in one of the central provinces of Iran.Materials and Methods: Arak city was divided into two geographic areas, and the weekly measurement of UVR was done in three locations) asphalt, grass and rooftop). To measure UVR, Hanger UV spectrometer, standard deviation (SD8‑A), and SD8‑B detectors were used. Amounts of UVR for a consecutive year and varying weather conditions were measured. Finally, values obtained were compared to ACGIH standards. Results: The minimum and maximum levels of UV type A radiation occurred in April 1.27 (0.724) W/m2 and September 7.147 (4.128) W/m2, these figures for UV type B were in March–April 0.005 (0.003) and September 0.083 (0.077). The maximum UVR is received between 11 and 15 o’clock.Conclusions: In the central cities of Iran, the minimum and maximum UV type A and B is received in March–April and in September, respectively. Based on the results, the angular position of the sun in the sky, cloud cover, and height from ground level affected the amount of UVR received, but the geographic locations studied did not.  Keywords: Outdoor Job, Skin Cancer, Carcinogen, Ultraviolet radiation, Ultraviolet A, Ultraviolet B, Ira

Toxicity of various silver nanoparticles compared to silver ions in the Ponto-Caspian amphipod Pontogammarus maeoticus (Sowinsky, 1894)

According to the increased probability of the presence of nanomaterials in the aquatic ecosystems, the present study examined the toxicity of three engineered silver nanoparticles (AgNPs) as well as silver ions in the Pontogammarus maeoticus, a brackish water benthic organism living in the littoral zone of the Caspian Sea. The animals were acutely exposed to different concentrations of two commercially prepared colloidal forms and a freshly prepared suspension of silver nanoparticles, plus AgNO3 during 48 hr. The number of mortalities was assessed and lethal concentration values were calculated using the EPA Probit Analysis Program. According to median lethal concentrations (LC50), the order of sensitivity of this amphipod to tested silver compounds was as: previously prepared AgNPs colloids > freshly prepared AgNPs suspension > AgNO3. Also the signs of nanoparticle accumulation were evident between the pereopods and pleopods of this gammarid; this accumulation could be one of the reasons for the higher toxicity of silver nanoparticles in comparison with silver ions in P. maeoticus. More acute and chronic studies are needed to understand the various aspects of nano-silver toxicity on amphipods in different salinities