2,961 research outputs found
A dark energy multiverse
We present cosmic solutions corresponding to universes filled with dark and
phantom energy, all having a negative cosmological constant. All such solutions
contain infinite singularities, successively and equally distributed along
time, which can be either big bang/crunchs or big rips singularities.
Classicaly these solutions can be regarded as associated with multiverse
scenarios, being those corresponding to phantom energy that may describe the
current accelerating universe
Mechanical behavior of hot-mix asphalt made with recycled concrete aggregates from construction and demolition waste: a design of experiments approach
[Abstract:] The present work is a re-evaluation of previous research on the durability of hot-mix
asphalt made with recycled concrete aggregates from construction and demolition waste (CDW) with
a different approach. Response surface methodology (RSM) was used to conduct this study. The kind
of natural aggregates (schist and calcite-dolomite), the recycled concrete aggregates percentage (0%,
20%, 40% and 60%) and the water saturation (0% and 100%) were the pertinent factors for this
methodology. Indirect tensile stress (ITS) was determined in mixtures fabricated with 0%, 20%, 40%
and 60% recycled concrete aggregates. According to the results, the ITS of the bituminous mixtures
increases as the percentage of recycled concrete aggregate increases. This behavior is more significant
when calcite-dolomite is used as a natural aggregate. Water saturation has the same influence in
both natural aggregates. The indirect tensile strength ratio (ITSR) was calculated to evaluate the
stripping potential. According to the Spanish specifications, the results suggest that the percentage
of CDW that can be used for hot mixes is 17% when schist is used as natural aggregate and 14%
for calcite-dolomite
Texture Evolution of AZ31 Magnesium Alloy Sheet at High Strain Rates
In the current contribution the mechanical behaviour at high strain rates of AZ31 magnesium alloy sheet is studied. Uniaxial deformation properties were studied by means of tensile split Hopkinson pressure bar (SHPB) at different temperatures. The influence of the strain rate and temperature on the deformation mechanisms was investigated by means of electron backscatter diffraction (EBSD) and neutron diffraction. It is shown that twinning plays an important role on high strain rate deformation of this alloy, even at elevated temperatures. Significant evidence of prismatic slip as a deformation mechanism is observed, also at warm temperatures, leading to the alignment of directions with the tensile axis and to a spread of the intensities of the basal pole figure towards the in-plane direction perpendicular to the tensile axis. The rate of decrease of the CRSS of non-basal systems is observed to be slower than at quasi-static rates. Secondary twinning and pyramidal slip were also outlined for some conditions. At warm temperatures, in contrast to quasi-static range, a generalized dynamic recrystallization is not observed. Moreover, the activation of rotational recrystallization mechanisms is reporte
Evaluation of the Resilient Modulus of Hot-Mix Asphalt Made With Recycled Concrete Aggregates From Construction and Demolition Waste
[Abstract] This paper reports the influence of the percentage of recycled aggregate (RCA) from construction and demolition waste (C&DW) together with the percentage of binder (L), curing time (t) and temperature (T) of the samples on the stiffness of a hot asphalt mixture. The study was carried out using the response surface methodology (RSM). The resilient modulus was chosen to estimate the stiffness of the mixture. The percentages of RCA studied were 0% (control), 5%, 10%, 20% and 30%, whilst 3.5%, 4% and 4.5% were those chosen for the binder content. Before compacting the samples, they were left into the oven to cure. Curing time, or pretreatment time, were set at 0 (control), 2 and 4 h. The samples were subjected to temperatures of 0, 10 and 20 °C. The natural aggregate is of the hornfels type. All the specimens studied showed high stiffness at low temperatures. According to this research, temperature proved to be the most influential factor on the decrease in the resilient modulus and, conversely, the percentage of recycled aggregate is not a significant factor in the range of values studied.Portugal. Fundação para a CiĂȘncia e Tecnologia; UIDB/04730/202
Effect of RecA inactivation and detoxification systems on the evolution of ciprofloxacin resistance in Escherichia coli
Background
Suppression of SOS response and overproduction of reactive oxygen species (ROS) through detoxification system suppression enhance the activity of fluoroquinolones.
Objectives
To evaluate the role of both systems in the evolution of resistance to ciprofloxacin in an isogenic model of Escherichia coli.
Methods
Single-gene deletion mutants of E. coli BW25113 (wild-type) (ÎrecA, ÎkatG, ÎkatE, ÎsodA, ÎsodB), double-gene (ÎrecA-ÎkatG, ÎrecA-ÎkatE, ÎrecA-ÎsodA, ÎrecA-ÎsodB, ÎkatG-ÎkatE, ÎsodB-ÎsodA) and triple-gene (ÎrecA-ÎkatG-ÎkatE) mutants were included. The response to sudden high ciprofloxacin pressure was evaluated by mutant prevention concentration (MPC). The gradual antimicrobial pressure response was evaluated through experimental evolution and antibiotic resistance assays.
Results
For E. coli BW25113 strain, ÎkatE, ÎsodB and ÎsodB/ÎsodA mutants, MPC values were 0.25 mg/L. The ÎkatG, ÎsodA, ÎkatG/katE and ÎrecA mutants showed 2-fold reductions (0.125 mg/L). The ÎkatG/ÎrecA, ÎkatE/ÎrecA, ÎsodA/ÎrecA, ÎsodB/ÎrecA and ÎkatG/ÎkatE/ÎrecA strains showed 4â8-fold reductions (0.03â0.06 mg/L) relative to the wild-type. Gradual antimicrobial pressure increased growth capacity for ÎsodA and ÎsodB and ÎsodB/ÎsodA mutants (no growth in 4 mg/L) compared with the wild-type (no growth in the range of 0.5â2 mg/L). Accordingly, increased growth was observed with the mutants ÎrecA/ÎkatG (no growth in 2 mg/L), ÎrecA/ÎkatE (no growth in 2 mg/L), ÎrecA/ÎsodA (no growth in 0.06 mg/L), ÎrecA/ÎsodB (no growth in 0.25 mg/L) and ÎrecA/ÎkatG/ÎkatE (no growth in 0.5 mg/L) compared with ÎrecA (no growth in the range of 0.002â0.015 mg/L).
Conclusions
After RecA inactivation, gradual exposure to ciprofloxacin reduces the evolution of resistance. After suppression of RecA and detoxification systems, sudden high exposure to ciprofloxacin reduces the evolution of resistance in E. coli.Plan Nacional de I+D+i 2013-2016 and the Instituto de Salud Carlos III (projects and PI17/01501 and PI20-00239)SubdirecciĂłn General de Redes y Centros de InvestigaciĂłn Cooperativa, Ministerio de EconomĂa, Industria y Competitividad, Spanish Network for Research in Infectious Diseases (REIPI; RD16/0016/0001 and REIPI RD16/ 0016/0009
Twinning and grain subdivision during dynamic deformation of a Mg AZ31 sheet alloy at room temperature
The microstructural evolution of an AZ31 rolled sheet during dynamic deformation at strain rates of âŒ103 sâ1 has been investigated by electron backscatter diffraction, X-ray and neutron diffraction. The influence of orientation on the predominant deformation mechanisms and on the recovery processes taking place during deformation has been systematically examined. The results have been compared with those corresponding to the same alloy tested quasi-statically under equivalent conditions. It has been found that strain rate enhances the activation of extension twinning dramatically, while contraction and secondary twinning are not significantly influenced. The polarity of extension twinning is even reversed in some grains under selected testing conditions. Significant grain subdivision by the formation of geometrically necessary boundaries (GNBs) takes place during both quasi-static and dynamic deformation of this AZ31 alloy. It is remarkable that GNBs of high misorientations form even at the highest strain rates. The phenomenon of recovery has been found to be orientation dependen
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