Mechanical and structural assessment of laboratory- and field-compacted asphalt mixtures

Compaction forms an integral part in the formation of the aggregate orientation and structure of an asphalt mixture and therefore has a profound influence on its final volumetric and mechanical performance. This article describes the influence of various forms of laboratory (gyratory, vibratory and slab-roller) and field compaction on the internal structure of asphalt specimens and subsequently on their mechanical properties, particularly stiffness and permanent deformation. A 2D image capturing and image analysis system has been used together with alternative specimen sizes and orientations to quantify the internal aggregate structure (orientation and segregation) for a range of typically used continuously graded asphalt mixtures. The results show that in terms of aggregate orientation, slab-compacted specimens tend to mimic field compaction better than gyratory and vibratory compaction. The mechanical properties of slab-compacted specimens also tend to be closer to that of field cores. However, the results also show that through careful selection of specimen size, specimen orientation and compaction variables, even mould-based compaction methods can be utilised with particular asphalt mixtures to represent field-compacted asphalt mixtures.     

Time dependent viscoelastic rheological response of pure,modified and synthetic bituminous binders

Bitumen is a viscoelastic material that exhibits both elastic and viscous components of response and displays both a temperature and time dependent relationship between applied stresses and resultant strains. In addition, as bitumen is responsible for the viscoelastic behaviour of all bituminous materials, it plays a dominant role in defining many of the aspects of asphalt road performance, such as strength and stiffness, permanent deformation and cracking. Although conventional bituminous materials perform satisfactorily in most highway pavement applications, there are situations that require the modification of the binder to enhance the properties of existing asphalt material. The best known form of modification is by means of polymer modification, traditionally used to improve the temperature and time susceptibility of bitumen. Tyre rubber modification is another form using recycled crumb tyre rubber to alter the properties of conventional bitumen. In addition, alternative binders (synthetic polymeric binders as well as renewable, environmental-friendly bio-binders) have entered the bitumen market over the last few years due to concerns over the continued availability of bitumen from current crudes and refinery processes. This paper provides a detailed rheological assessment, under both temperature and time regimes, of a range of conventional, modified and alternative binders in terms of the materials dynamic (oscillatory) viscoelastic response. The rheological results show the improved viscoelastic properties of polymer- and rubber-modified binders in terms of increased complex shear modulus and elastic response, particularly at high temperatures and low frequencies. The synthetic binders were found to demonstrate complex rheological behaviour relative to that seen for conventional bituminous binders.     

Influence of dual-frequency plasma-enhanced chemical-vapor deposition Si<Subscript>3</Subscript>N<Subscript>4</Subscript> passivation on the electrical characteristics of AlGaN/GaN heterostructure field-effect transistors

The influence of dielectric stress on the direct current (DC) electrical characteristics of AlGaN/GaN heterostructure field-effect transistors (HFETs) has been investigated. Dual-frequency plasma deposition was used to vary the amount of stress induced by a passivating dielectric on the surface of the devices. Initial data suggested a strong influence from the induced dielectric stress, but the low-frequency, radio-frequency (RF) excitation of the plasma deposition process was found to induce a severe nonreversible damage to the exposed AlGaN surface through N ion bombardment. The consequence is a drastic reduction of the sheet carrier concentration and mobility of the two-dimensional electron gas (2DEG). Subsequently, an alternative damage-free technique using a helium precursor was used to obtain compressive films. Based on the results, uniform dielectric stress has a minimal impact on the polarization charges within the AlGaN barrier.     

A double torsion fracture mechanics and Auger electron spectroscopy approach to the study of stress corrosion cracking in low alloy steels

The applicability of the double torsion fracture mechanics test to the study of stress corrosion cracking (SCC) in steels was assessed by evaluating the behaviour of the low alloy steels AISI 4140 and En3OA, exhibiting both high (1400 MNm^–2) and low (765 MNm^–2) yield strengths respectively. An optical method for measuring crack growth rate and a load relaxation method for computing it were compared and found to give similar results. The test was shown to be eminently suitable for the study of SCC in high yield strength steels and those in a temper embrittled condition. The influence of trace impurities on the SCC susceptibility was examined using Auger electron spectroscopy to determine the type and amount of grain boundary segregants. The degree of segregation of trace impurities was shown to have a profound effect on the stress intensity-crack velocity (K-V) diagram by reducingK _IC and increasing the reaction rate as shown by the increased slope in stage III of theK-V diagram. An anomalously low threshold stress intensity was observed in as-quenched AISI 4140 and this was attributed to residual stresses produced by the phase transformations occurring during quenching.     

Low-Dimensional Waveguide Grating Fabrication in GaN with Use of SiCl<Subscript>4</Subscript>/Cl<Subscript>2</Subscript>/Ar-Based Inductively Coupled Plasma Dry Etching

Optimized fabrication of submicron-sized features in gallium nitride (GaN) with the use of inductively coupled plasma (ICP) dry etching, based on SiCl₄/Cl₂/Ar gas mixture, is presented. Dense periodic patterns, i.e., 400-nm-period gratings, were transferred into a gallium nitride waveguide under different etching conditions. ICP power, radiofrequency (RF) power, chamber pressure, and Ar/Cl₂ gas mixing ratio were altered during the experiment. Depths of fabricated grating couplers up to 670 nm were achieved. The most suitable etching conditions are discussed with the assessment based on etching selectivity, scanning electron microscopy (SEM) observation of grating tooth slope, hard-mask erosion process, and etched surface morphology.     

Spin injection in n-type resonant tunneling diodes

We have studied the polarized resolved photoluminescence of n-type GaAs/AlAs/GaAlAs resonant tunneling diodes under magnetic field parallel to the tunnel current. Under resonant tunneling conditions, we have observed two emission lines attributed to neutral (X) and negatively charged excitons (X⁻). We have observed a voltage-controlled circular polarization degree from the quantum well emission for both lines, with values up to −88% at 15 T at low voltages which are ascribed to an efficient spin injection from the 2D gases formed at the accumulation layers.     

Sydney Soil Model. II: Experimental Validation

This paper presents simulations of the mechanical behavior of reconstituted and natural soils using a new model presented in a companion paper and referred to as the “Sydney soil model.” It is demonstrated that the performance of the proposed model is essentially the same as that of modified Cam clay model when describing the behavior of clays in laboratory reconstituted states. The model has also been employed to simulate the drained and undrained behavior of structured clays and sands, including calcareous clay and sand. Five sets of conventional triaxial tests and one set of true triaxial tests have been considered. It is demonstrated that the new model provides satisfactory qualitative and quantitative modeling of many important features of the behavior of structured soils, particularly in capturing various patterns of the stress and strain behavior associated with soil type and structure. A general discussion of the model parameters is also included. It is concluded that the Sydney soil model is suitable for representing the behavior of many soils if their ultimate state during shearing can be defined by an intrinsic and constant stress ratio M* and a unique relationship between mean effective stress and voids ratio, i.e., a unique p′-e curve.     

Mix design considerations of foamed bitumen mixtures with reclaimed asphalt pavement material

In the present work, a mix design parametric study was carried out with the aim of proposing a practical and consistent mix design procedure for foamed bitumen mixtures (FBMs). The mix design parameters that were adopted in the study are mixing and compaction water content (MWC), compaction effort using a gyratory compactor and aggregate temperature. This parametric study was initially carried out on FBMs with virgin limestone aggregate without reclaimed asphalt pavement (RAP) material and a mix design procedure was proposed. This proposed methodology was also found to apply to FBMs with RAP. A detailed consideration was also given to characterising the RAP material so as to understand its contribution to the mechanical properties of FBMs. Optimum MWC was achieved by optimising mechanical properties such as indirect tensile stiffness modulus and indirect tensile strength (ITS-dry and ITS-wet). A rational range of 75–85% of optimum water content obtained by the modified Proctor test was found to be the optimum range of MWC that gives optimum mechanical properties for FBMs. It was also found that the presence of RAP influenced the design foamed bitumen content, which means that treating RAP as black rock in FBM mix design is not appropriate. To study the influence of bitumen and water during compaction, modified Proctor compaction and gyratory compaction were employed on mixes with varying amounts of water and bitumen. By this, the work also evaluated the validity of the total fluid (water + bitumen) concept that is widely used in bitumen–emulsion-treated mixes, and found it not to be applicable.