115 research outputs found
Use of waste oil/styrene-butadiene-rubber blends as rejuvenators for aged bitumen
No full textThis study elaborates the influence of blends formulated by waste oil (WO) and styrene-butadiene rubber (SBR) on thermo-mechanical and chemical compositional characteristics of rejuvenated bitumen. The results show that both WEO (waste engine oil) and WCO (waste cooking oil) can compensate viscous component for reclaimed bitumen, and the effect of WCO is more significant. However, WO-rejuvenated bitumen has considerable drawbacks in terms of temperature susceptibility and flow resistance, but dramatic improvement after being added with SBR has been noticed. Furthermore, the rejuvenation mechanism of WO/SBR blends is revealed by using FTIR tests, which shows that the physical blending mainly determines the rejuvenation mechanism when the WO/SBR blends are used in bitumen. The combination of WO and SBR is beneficial for improving the temperature susceptibility and rutting resistance of rejuvenated bitumen.Accepted Author ManuscriptPavement Engineerin
Effect of bio-oil on low-intermediate temperature properties of organosolv lignin-bitumen
No full textLignin, one of the most abundant natural polymers, has been extensively studied as liquid or solid additive in bituminous binders. Despite the fact the organosolv lignin in bitumen improves the overall resistance against oxidative aging, lignin could lead to binders of high thermal cracking sensitivity. In this study, a bio-based oil is implemented in a lignin modified bitumen to ameliorate characteristics, such as fatigue and thermal cracking resistance. Pressure aging vessel conditioning was applied to new binders formulated by different proportions of bio-oil to simulate the long-term aging. A series of rheological tests were performed. Based on the linear amplitude sweep test results, fatigue damage of lignin-bitumen could be reduced by increasing the oil content. According to relaxation test results, the addition of oil significantly decreased the ratio of residual stress and relaxation time. This study has shown preliminary conclusions on the use of bio-oil to improve the low-intermediate temperature performance of lignin-bitumen binders.Pavement EngineeringBio-based Structures & Material
Observational signatures of near-extremal rotating black holes
No full textWe are entering an exciting new era of imaging black holes with the help of the Event Horizon Telescope (EHT). This has stimulated many theoretical works predicting the signals that EHT may possibly observe and examining the type of properties of gravity that the signals can inform us. While these signals may in general depend on a complex nearby environment of a black hole, it is possible to expect some universal and striking signals for the case of near-extremal rotating black holes due to the existence of an enhanced conformal symmetry in the near-horizon region of such black holes. These particular signals may serve as a typical signature for identifying a near-extremal rotating black hole in the Universe. Moreover, the enhanced symmetry supplies powerful tools which enable one to do analytical computations for these interesting signals. From a practical perspective, astronomical observations have suggested that plenty of supermassive black holes are rotating very rapidly (i.e., they are in the near-extremal limit). Therefore, this thesis will focus on the optical observational signatures of high-spin black holes. In particular, we study the images of a point-like orbiting emitter (referred to as a “hot spot”) near the Innermost Stable Circular Orbit (ISCO) of a high-spin black hole. Images of such an emitter may reveal important features of the black hole event horizon since the emitter resides in the near-horizon region, thus the images can further inform us of the properties of the underlying gravity theory. We analytically compute the shadow of a near-extremal rotating black hole and the optical observables of a near-ISCO hot spot. A key feature of the optical appearance of such an orbiting hot spot is that there are many images of it moving on a vertical portion of the black hole shadow and having a rich structure. The computations rely on the geometric properties of the black hole spacetime and the motion of massive particles and photons in it. Many studies on black hole imaging are based on the assumption that the underlying gravity theory is general relativity (GR) and the motion of lights follows geodesic equations in the spacetime. Here we study alternative possibilities: a) we compute the influence of a plasma on the observational signature by taking into account its interactions with photons; b) we compute the observational signature based on gravity theories that go beyond GR, in particular the Scalar-Tensor-Vector (STVG) modified gravity (MOG) and the heterotic string theory. The obtained results may not only provide other possible templates for the EHT to test, but also propose a new way to distinguish different gravity theories
Balancing the sustainable component of ethylene-vinyl acetate for achieved better compatibility improvement of wax-based warm mix additives in bitumen
No full textThe ethylene-vinyl acetate (EVA) polymers are always doped into waxy bitumen to inhibit network of wax crystals in bitumen. However, the compatibility improvement behaviors between wax-based warm mix (WWM) additives and bitumen by EVA are not clear, and the sustainable components of EVA for corresponding WWM additives to achieve better compatibility improvement are also not determined. This paper investigated compatibility improvement behaviors between commonly used WWM additives and bitumen after the addition of EVA to obtain sustainable components of EVA through experimental method of activation energy of viscous flow (AEVF) and density function theory-molecular dynamic (DFT-MD) calculations. The results show that the repulsions between the end of main-chain with highest electronegativity in WWM additives and polar molecules of EVA can alleviate the aggregation behaviors of WWM additives and EVA displays the best and worst compatibility improvements for additives with shortest and longest carbon chains, respectively. The dispersed asphaltenes combined with EVA can form the composite wax inhibitors (WIs) systems to increase diffusion coefficient and reduce percentage increment values of cohesive energy density (CED) to further disrupt ordered degree of WWM additives. On this basis, the sustainable carbon numbers of main-chain for EVA that are slightly less than average carbon numbers of WWM additives will help to better improve the compatibility of WWM additives. This investigation can provide the inspiration on how to choose the sustainable components of EVA to achieve high-efficiency compatibility improvement for corresponding warm mix asphalts (WMAs) with different average carbon numbers.Pavement Engineerin
Characterization of nanoscale cracking at the interface between virgin and aged asphalt binders based on molecular dynamics simulations
No full textLow-temperature cracking is a major concern to improve the utilization of recycled asphalt mixture (RAM). A mechanism by which the crack propagates can provide a basis for advanced technological mitigation. Micro-crack formations in the interfacial proximity of the virgin and aged binders have been identified from electron microscopy tests. Atomic force microscopy (AFM) experiment showed the trilayer phases at the virgin-aged binder interface. In this study, molecular dynamics (MD) simulations were conducted to understand the nanoscopic crack propagation characteristics at the virgin-aged binder interface in the asphalt mixture with RAM. It was found that the blended binder of virgin and aged binders, and its interfaces with virgin and aged binders appeared to be the crack propagation zones. The relatively more significant matrix contraction of virgin binder and stiffer aged binder at a low temperature can cause more considerable tensile stress at the blended binder and its interfaces. Consequently, interfacial crack propagation became more profound and decreased the low-temperature cracking resistance
Laser shape variation influence on melt pool dynamics and solidification microstructure in laser powder bed fusion
No full textThe shape variation of the laser beam is evidently observed in the laser powder bed fusion (LPBF) process because of changes in laser incidence angle and misalignment between the build plate and the laser focus plane. This issue is particularly relevant in large-scale LPBF systems where the laser beam needs to scan a large build area. However, most LPBF modeling studies assume vertical laser radiation. The heat transfer, melt pool, and solidification evolution due to the laser shape variation have not been well addressed and quantified. In the present study, the temperature distribution, melt pool geometry and flow dynamics are captured via numerical modelling, and the grain morphology is characterized under various laser incidence angles. The results show that the melt pool depth becomes shallower, and the width is near the beam size as the laser beam becomes more elongated. The beam shape variation can affect the liquid flow pattern with increasing incidence angle, resulting in a larger vortex at the front of the melt pool and a smaller vortex at the rear of the melt pool. The thermal gradient increases and the solidification rate decreases as the laser incident angle becomes larger. The present study enhances the understanding of multi-physics in the LPBF process
Observability of zero-angular-momentum sources near Kerr black holes
No full textWe revisit monochromatic and isotropic photon emissions from the zero-angular-momentum sources (ZAMSs) near a Kerr black hole. We investigate the escape probability of the photons that can reach to infinity and study the energy shifts of these escaping photons, which could be expressed as the functions of the source radius and the black hole spin. We study the cases for generic source radius and black hole spin, but we pay special attention to the near-horizon (near-)extremal Kerr ((near-)NHEK) cases. We reproduce the relevant numerical results using a more efficient method and get new analytical results for (near-)extremal cases. The main non-trivial results are: in the NHEK region of a (near-)extremal Kerr black hole, the escape probability for a ZAMS tends to , independent of the NHEK radius; at the innermost of the photon shell (IPS) in the near-NHEK region, the escape probability for a ZAMS tends to
Observational signature of a near-extremal Kerr-Sen black hole in the heterotic string theory
No full textWe analytically study the optical appearance of an isotropically emitter orbiting near the horizon of a near-extremely rotating Kerr-Sen (KS) black hole which is an electrically charged black hole arising in heterotic string theory. We study the influence of the Sen charge on the observational quantities, including the image position, flux and redshift factor. Moreover, we compare the results with those for a near-extremal Kerr-Newman (KN) black hole, which is the charged rotating black hole in general relativity. We find quantitative corrections of the signatures of these charged black holes (both KS and KN) compare to that of a neutral Kerr black hole. This may serve as distinctive features of different black holes for future tests by the Event Horizon Telescope.</p
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