Conceptualizing the asphalt film thickness to investigate the Superpave VMA criteria

Asphalt binder film thickness (FT_b) is the key factor that is responsible for durability of asphalt mixtures. Mixtures with coarse aggregate gradations have difficulty meeting the Superpave minimum voids in mineral aggregate (VMA) criteria even though they tend to have thick asphalt films. In this study, the concept of asphalt binder film thickness (FT_b) was used to investigate the Superpave VMA criteria. Superpave aggregate gradations of three nominal maximum aggregate sizes (NMAS): 9.5, 12.5 and 19.0 mm, were used. Aggregate gradations passing above, below, crossover through, humped, and through restricted zone were all considered. Superpave Gyratory Compactor test data of 126 compacted asphalt mixtures were used in the study. The current Superpave VMA criteria relate the mixture durability with VMA and set the same VMA value for mixtures with the same NMAS regardless of other parameters. However, a poor relationship was found between VMA and FT_b (a durability measure) with an R ² ? 0.01, and yet a relatively good relationship (R ² ? 0.38) was found between voids filled with asphalt (VFA) and FT_b although the VFA volumetric phase is part of the VMA volumetric phase in the mixture. This result was justified by the poor relationship between VFA and VMA (R ² ? 0.13). Despite that the effective binder content (P _be ) as a volumetric phase represents the VFA in the mixture, the relationship between FT_b and P _be was found to be more significant (higher R ²) than the relationship between FT_b and VFA. In this study, asphalt mixtures that did fail the Superpave VMA criteria in some cases had adequate asphalt FT_b, and mixtures that passed the criteria did not necessarily have adequate FT_b. In conclusion, although the current Superpave VMA criteria are significant, findings of this study support the tendency to modify the current criteria.     

Laboratory characterization of directional dependence of permeability for porous asphalt mixtures

Water permeability is an important property for porous asphalt mixtures. Previous numerical modeling showed that the permeability of the porous asphalt mixtures varies in different directions and a single permeability cannot accurately evaluate the mixture’s directional permeability. To investigate the direction-dependent water permeability of the porous asphalt mixtures, a unidirectional permeameter was used to measure the permeability in twelve directions in the vertical plane (parallel to compaction direction) and twelve directions in the horizontal plane (perpendicular to the compaction direction) on two open graded friction course (OGFC) mixtures with different nominal maximum aggregate sizes, i.e., OGFC-13 and OGFC-10. Furthermore, a new multidirectional permeameter was designed which can control the rainfall intensity and adjust transverse slope to simulate the actual water flow process in pavement. The multidirectional permeability and void saturation of eight porous asphalt mixtures were determined by the multidirectional permeameter. Results show that the porous asphalt mixtures demonstrate direction-dependent permeability properties in both vertical and horizontal planes, whereas the dependence is less in the horizontal plane than that in the vertical plane. In the vertical plane, the minimum permeability occurs in the vertical direction and the maximum value occurs in the horizontal direction. In the horizontal plane, the permeability differs in different directions, but has no obvious relationship with directions. Increasing the air void content and the nominal maximum aggregate size of the mixtures can reduce the directional difference of the permeability. The void inside porous mixture cannot be entirely occupied by water when surface runoff occurs. Increasing the air void content and aggregate particle size can lead to an increase of the permeability and the void saturation in the porous asphalt mixtures.     

The effect of aggregate gradation on creep and moisture susceptibility performance of warm mix asphalt

It is clear that the purpose of mixture design is to select optimum asphalt content for a desired aggregate structure to meet the prescribed criteria. Aggregate makes up high proportion of volume and mass of mixtures; hence, it is considered as an important constituent of asphalt concrete. This study postulates that the gradation is an important characteristic of the aggregate in adoption of the optimum mixture. One aggregate source, three gradations and different percentages of Sasobit^® was used to manufacture hot mix asphalt and warm mix asphalt. The test results indicated that the aggregate gradation affects the rutting resistance and especially the moisture susceptibility of the introduced mixtures, differently. Rutting resistance was evaluated using the flow number parameter, and in order to determine the moisture sensitivity mechanism, a mechanical and visual inspection tests were carried out. At the end, it is concluded that the optimum aggregate gradation for these two types of mixtures is different.     

Performance evaluation of asphalt mixtures containing recycled concrete aggregates

Abstract

To evaluate the feasibility of using Recycled Concrete Aggregates (RCA) in asphalt mixtures, the coarse RCA and fine RCA were prepared as a partial replacement of the natural aggregates (NA). Different amounts of replacement of NA with RCA were investigated, and the mechanical properties and pavement performance of asphalt mixtures containing different proportions of RCA were analysed based on laboratory tests. The results indicated that with increasing the RCA percentage, the optimum asphalt content increased and the bulk density of mixtures decreased as well. Mixtures containing 40% coarse RCA or 20% fine RCA both showed satisfactory performance. Besides, the mixture containing 40% fine RCA had the highest asphalt content, but gave much better performance compared to the virgin mix except for its bad resistance to permanent deformation. Finally, the pavement performance of mixtures containing 60% coarse RCA and 50% coarse RCA were unacceptable.     

Dynamic softening mechanism of 30 % SiCp/2024Al composite during hot deformation process

Using Gleeb‐1500D simulator, the isothermal compression tests of 30 % SiCp/2024Al (volume fraction) are conducted at a temperature range of 623 K ‐ 773 K and a strain rate range of 0.01 s^‐1 ‐ 10 s^‐1. The softening mechanism of composites during hot deformation has been proposed based on the Zener‐Hollomon parameter Z, deformation temperature T and microstructure analysis. Cross slip of dislocation plays a dominant role under the conditions of lnZ≥59.634 and T≤673 K. While, deformation mechanisms such as cross slip, climb of dislocation and unzipping of the three dimensional dislocation network play a joint role when lnZ≤61.933 and T≥623 K. Particularly, dynamic recrystallization occurred when lnZ≤55.669 and T≥723 K. The cross slip, climb and unzipping of dislocation and dynamic recrystallization are the main softening mechanisms. The role of the dynamic recrystallization mechanisms become more significant and the critical strain of dynamic recrystallization decrease with the decrease of lnZ. Dynamic recrystallization nucleation mechanisms are mainly constituted of the subgrain combination and the bulging of the grain boundary.     

Experimental assessment of flue gas desulfurization residues and basic oxygen furnace slag on fatigue and moisture resistance of HMA

Basic oxygen furnace (BOF) slag and flue gas desulfurization (FGD) residues both are industrial wastes. Research on using BOF slag as a novel aggregate and FGD residues as a filler in road construction has benefits both in environment and economics. The main objective of this research was to evaluate the effect of FGD residues and BOF slag on the fatigue performance and moisture resistance of asphalt mixtures. The fatigue performance of asphalt mixture was conducted by means of indirect tensile fatigue test. Stress loading control mode, with four stress levels (300, 400, 500 and 600 kPa), was used in this research. Statistic t‐test was adopted, and it had approved the positive effect of BOF slag and FGD residues on the fatigue lives of asphalt mixture. Moisture resistance of asphalt mixture was investigated by retained Marshall stability test and tensile strength ratio test. Research results indicate that BOF slag and FGD residues can improve the fatigue and moisture resistance, when the BOF slag and FGD residues based asphalt mixture was designed properly.     

A long-term ultraviolet aging procedure on foamed WMA mixtures

Long-term thermal and ultraviolet (UV) aging procedures of asphalt mixtures are complicated, but can be simulated in the laboratory. The objective of this study was to investigate the influence of long-term thermal and UV aging on foamed warm-mix asphalt (WMA) mixtures. Rut resistance, indirect tensile strength (ITS), deformation, dissipated elastic energy, and fracture energy were measured for all mixtures. The experimental design included two aggregate sources; three aging states (unaged, thermal and UV aging); one water-bearing WMA additive and water foaming technology; two PG 64-22 binders, and three air void contents (2, 4, and 7 %). A total of 24 mixtures were evaluated and 144 specimens were made and tested in this study. The test results indicated that thermal and UV aging procedures had limited contribution in improving the rut resistance of a mixture as air void content was low. Unaged samples had the highest ITS values amongst three aging states while UV aged samples had the lowest. In addition, UV aged mixtures generally had greater dissipated energy than thermal aged mixtures regardless of foaming technology, aggregate source, and air void. Moreover, the foaming technology might reduce the stored elastic energy of the mixture due to additional water or released water from water-bearing additive. Furthermore, UV aging generally reduces the fracture resistance of an asphalt mixture than standard thermal aging. In addition, when using WMA foaming technology, aggregate source affects the fracture resistance of the asphalt mixture.     

Evaluation of asphalt–aggregate interaction based on the rheological properties

The silicon dioxide (SiO₂) and calcium oxide (CaO) analytical reagents are selected to prepare asphalt mastics and the effects of aggregate chemical composition on asphalt–aggregate interactions (AAI) are evaluated based on the complex modulus and phase angle. It is found that the oxide analytical reagents significantly affect the rheological properties such as complex shear modulus and phase angle, and the effects of CaO are greater than SiO₂ due to the stronger interaction between asphalt binder and CaO analytical reagents. Both the modulus stiffening ratio and the phase angle-based K. Ziegel-B coefficient could be used to evaluate the AAI, and the latter is the better index. Results show that the indexes increase with the test temperature, but decrease with the loading frequency, and tend to be constant. The higher adhesive strength between asphalt binder and limestone than basalt is likely attributed to the higher content of CaO in limestone aggregate and the stronger asphalt–CaO interaction.     

Reconsideration of the fatigue tests for asphalt mixtures and binders containing high percentage RAP

When applying reclaimed asphalt technology in a flexible pavement project, most performance concerns are related to low temperature and fatigue cracking since the stiffness of the HMA mixture could dramatically increase through adding a high percentage of reclaimed asphalt pavement (RAP) material. The purpose of this study is to evaluate asphalt mixtures with high RAP contents, prepared using two RAP addition methods, for their performance based on fatigue-cracking resistance rather than relying on volumetric properties. Asphalt mixture samples were prepared with three RAP binder content replacement percentages (30, 40 and 50%) using two preparation methods: the as-is RAP gradation (traditional method) and the splitting of the RAP gradation into coarse and fine fractions (fractionated method). Asphalt mixture beam fatigue and binder fatigue time-sweep tests were performed. Beam fatigue samples also underwent freeze–thaw cycling for freeze–thaw damage evaluation. Rather than basing the performance based solely on S–N_f curves to illustrate the fatigue performance, the beam fatigue test data was analysed through a dissipated energy approach. Faster fatigue degradation was observed for the 40% RAP binder and beam mixture when subjected to repeated loading. From a morphology aspect, this can be explained by the binder’s phase separation and physical hardening effects.     

Fatigue life and endurance limit prediction of asphalt mixtures using energy-based failure criterion

Fatigue cracking is one of the major types of distress in asphalt mixtures and is caused by the accumulation of damage in pavement sections under repeated load applications. The fatigue endurance limit (EL) concept assumes a specific strain level, below which the damage in hot mix asphalt (HMA) is not cumulative. In other words, if the asphalt layer depth is controlled in a way that keeps the critical HMA flexural strain level below the EL, the fatigue life of the mixture can be extended significantly. This paper uses two common failure criteria, the traditional beam fatigue criterion and the simplified viscoelastic continuum damage model energy-based failure criterion (the so-called G^R method), to evaluate the effect of different parameters, such as reclaimed asphalt pavement (RAP) content, binder content, binder modification and warm mix asphalt (WMA) additives, on the EL value. In addition, both failure criteria are employed to investigate the impacts of these parameters in terms of the fatigue life of the study mixtures. According to the findings, unlike an increase in RAP content, which has a negative effect on the mixtures’ fatigue resistance, a higher binder content and/or binder modification can significantly increase the EL value and extend the fatigue life as was proved before by other researchers, whereas WMA additives do not significantly affect the mixtures’ fatigue behaviour. A comparison of the model simulation results with the field observations indicates that the G^R method predicts the field performance more accurately than the traditional method.     

Effect of laboratory foamer on asphalt foaming characteristics and foamed mixture properties

In the United States, mechanical foaming is the most popular method for producing warm mix asphalt, which is the latest technology implemented to reduce the production temperature and/or enhance the compactability of asphalt mixtures. Three commonly used commercially available laboratory foamers to produce asphalt foams include the Wirtgen WLB 10S (Wirtgen foamer), the InstroTek Accufoamer (InstroTek foamer) and the Pavement Technology Inc. Foamer (PTI foamer). Though these foamers have been widely used in research studies and construction practice, it is still unknown whether they produce asphalt foams with the same quality and quantity. In this study, asphalt foaming characteristics produced by these three laboratory foamers were measured using a non-contact test set-up consisting of a laser device and a digital camera, and compared in terms of instantaneous volume expansion, foam stability and surface area evolution of foam bubbles. Additionally, the workability, coatability and mechanical performance of foamed mixtures prepared using these same laboratory foamers were compared against the conventional hot mix asphalt (HMA). Test results indicated that foamed asphalts produced by the Wirtgen foamer had the largest volume expansion and greatest foam stability, followed by those produced by the InstroTek foamer and the PTI foamer. The optimum foaming water content (W_opt) was determined for each laboratory foamer based on the workability and coatability results of the corresponding foamed mixtures. In addition, the performance evaluation of the foamed mixtures produced at W_opt values indicated equivalent mixture stiffness but greater moisture susceptibility as compared to the conventional HMA.     

Improved method of considering air void and asphalt content changes on long-term performance of asphalt concrete pavements

Mixture properties (aggregate gradation and volumetric quantities), rate of loading and environmental conditions are the most important factors that affect the |E*| values. The main objective of this study was to develop a rational approach to investigate and model the effect of air voids and asphalt content on the |E*| master curves and consequently predict pavement performance. In this study, |E*| tests were conducted on three asphalt concrete mixtures with the same aggregate gradation, but different binder grades. For each of these mixtures, the air void and asphalt contents were varied at three levels. It is found that the developed method provides a more accurate estimate of the effects of volumetric changes in hot mix asphalt. The application of the proposed approach would be most beneficial for quality control/quality assurance purposes, performance-related specifications and for estimating contractors' incentives and penalties, where |E*| is utilised to predict the pavement performance.     

Effects of long-term aging on moisture sensitivity of foamed WMA mixtures containing moist aggregates

Long-term aging of an asphalt mixture is complicated, but can be simulated in the laboratory. The objective of this study was to investigate the influence of long-term aging on moisture susceptibility of foamed warm mix asphalt (WMA) mixtures containing moist aggregate. Weight loss, indirect tensile strength (ITS) of dry and conditioned specimens, and deformation (flow) were measured for all mixtures. The experimental design included two aggregate moisture contents (0 and ~0.5% by weight of the dry mass of the aggregate); two lime contents (1 and 2% lime by weight of dry aggregate) and one liquid anti-stripping agent (ASA); one foaming WMA additive (Asphamin) and two foaming water contents (2 and 3%); and two aggregate sources. A common long-term aging procedure was used in this study. A total of 64 mixtures were evaluated and 256 specimens were made and tested in this study. The test results indicated that long-term aging improved the moisture resistance of WMA mixtures regardless of the ASA and moisture conditioning. In addition, aggregate source significantly affected the moisture resistance regardless of the foaming technology, ASA, and aggregate moisture content. The mixture with various hydrated lime contents exhibited similar moisture resistance under dry and wet conditions. The liquid ASA used in the WMA mixture showed a weaker resistance to the moisture damage in comparison with hydrated lime.     

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.     

Core electron level structure in C<Subscript>60</Subscript>F<Subscript>18</Subscript> and C<Subscript>60</Subscript>F<Subscript>36</Subscript> fluorinated fullerenes

The structure of C1s and F1s core electron levels in C₆₀F₁₈ and C₆₀F₃₆ fluorinated fullerenes has been studied by X-ray photoelectron spectroscopy using synchrotron radiation. It is established that C1s levels of carbon atoms not bound to fluorine in these compounds are shifted down by 1.0 and 1.6 eV relative to the C1s level in the usual C₆₀ fullerene, so that the binding energies of the core electron levels in C₆₀F₁₈ and C₆₀F₃₆ amount to E _b (C1s, C-C) = 285.7 and 286.3 eV, respectively. These values are characteristic and can be used for the identification of both homogeneous fluorinated fullerenes and combined materials comprising a mixture of various fluorinated fullerenes with each other and with different carbon-containing based materials.     

Longitudinal zero-sound dispersion,thermal conduction,and the LandauF 2 in3He

The Boltzmann equation for the distribution of Landau quasiparticles on the surface of the Fermi sphere is solved by an expansion in Legendre polynomials having symmetry appropriate to longitudinal sound. It is assumed that the collision operator can be replaced by three finite relaxation times and that the quasiparticle interaction includes a nonzero Landau parameterF _s ² . From the solution, the heat flux and temperature gradient in a zero-sound wave can be computed, and comparison with a corresponding phenomenological generalization of Fourier's law yields an expression relating thermal conductivity, ,F _s ² , and the parameter -c ₀ /V _F , wherec ₀ is the longitudinal zero-sound velocity andv _F the Fermi velocity. This expression should hold simultaneously with a second equation expressing the condition that zero sound should propagate undamped atO K, and thus we can solve for andF _s ² . We obtain the valueF _s ² =–2.99 (v _F =56.62 m/sec), which depends hardly at all on the experimental value of , specific heat, and sound absorption used. These estimates agree with earlier ones from transverse zero sound as to sign ofF _s ² , but it appears that complete quantitative consistency may necessitate invoking Landau parameters of third and higher order.     

Nonlinear dynamics of structures assembled by bolted joints

Summary The nonlinear transfer behaviour of an assembled structure such as a large lightweight space structure is caused by the nonlinear influence of structural connections. Bolted or riveted joints are the primary source of damping compared to material damping, if no special damping treatment is added to the structure. Simulation of this damping amount is very important in the design phase of a structure. Several well known lumped parameter joint models used in the past to describe the dynamic transfer behaviour of isolated joints by Coulomb friction elements are capable of describing global states of slip and stick only.The present paper investigates the influence of joints by a mixed experimental and numerical strategy. A detailed Finite Element model is established to provide understanding of different slip-stick mechanisms in the contact area. An advanced lumped parameter model is developed and identified by experimental investigations for an isolated bolted joint. This model is implemented in a Finite Element program for calculating the dynamic response of assembled structures incorporating the influence of micro- and macroslip of several bolted joints.List of symbols a acceleration - E ₀, E_t material moduli - F ₀ mass weighted excitation force - F _t tangential joint force - F generalized force - F _exc ^* excitation force - F _exc amplitude of excitation force - F _C0 spring element force - F _R0 friction element force - K _A, K_B normal stiffness - K _t tangential stiffness - L length of contact area - M _t transmitted joint torque - m _red reduced mass - p normal contact pressure - r effective radius - q generalized coordinate - z internal variable - x coordinate in the contact area - u relative displacement - u relative velocity - relative angle - friction coefficient - damping ratio - material parameter - ₀ equivalent slip limit - microslip parameter - excitation frequency Dedicated to Prof. Dr. Dr. h. c. Franz Ziegler on the occasion of this 60th birthday     

Evaluation of permanent deformation characteristics of unmodified and Polyethylene Terephthalate modified asphalt mixtures using dynamic creep test

One of the major types of plastics that can be found in Municipal Solid Waste (MSW) is Polyethylene Terephthalate (PET) which is a non-biodegradable semi-crystalline thermoplastic polymer, and is considered as polyester material. Generating large amount of waste PET, mainly as bottles, would cause environmental hazards by disposing in landfills. This paper aims to evaluate effects of utilizing waste PET flakes as modifier in asphalt mixture as an alternative solution to overcome the potential risks arise from producing large amount of waste PET as well as evaluating the deformation characteristics of unmodified and PET modified asphalt mixtures. To achieve this aim, different percentages of PET were designated for this investigation, namely: 0%, 0.2%, 0.4%, 0.6%, 0.8% and 1% by weight of aggregate particles, and dynamic creep test was performed at different stress levels (300 kPa and 400 kPa) and temperatures (10 °C, 25 °C and 40 °C). Consequently, Zhou three-stage model was developed. The results showed that permanent deformation characteristics of asphalt mixture were considerably improved by utilization of PET modification, when the permanent strain was remarkably decreased in PET modified mixture compared to the conventional mixture at all stress levels and temperatures. Besides, based on Zhou model, it was concluded that elastic and visco-elastic properties of asphalt mixture were improved by application of PET modification.     

Deformation behaviour of single crystals of InP in uniaxial compression

The deformation characteristics of indium phosphide (InP) single crystals under uniaxial compression have been examined as a function of strain rate, temperature and orientation. It has been shown that at temperatures below 0.55T _m (T _m=melting point; 1335 K) the material fractures in a brittle manner whereas at higher temperatures, within the range 0.55 to 0.71T _m, plastic deformation occurs by both slip and deformation twinning; above 0.71T _m, slip alone is the operative deformation mechanism. The observed operative slip systems are of the type {1 1 1} 0 1 1 which are characteristic of most Group IIIb-Group Vb compounds. Deformation twinning occurs predominantly on {1 1 1} planes but some activity is also observed on planes of the type {3 4 5}.     

Micromechanical finite element framework for predicting viscoelastic properties of asphalt mixtures

A micromechanical finite element (FE) framework was developed to predict the viscoelastic properties (complex modulus and creep stiffness) of the asphalt mixtures. The two-dimensional (2D) microstructure of an asphalt mixture was obtained from the scanned image. In the mixture microstructure, irregular aggregates and sand mastic were divided into different subdomains. The FE mesh was generated within each aggregate and mastic subdomain. The aggregate and mastic elements share nodes on the aggregate boundaries for deformation connectivity. Then the viscoelastic mastic with specified properties was incorporated with elastic aggregates to predict the viscoelastic properties of asphalt mixtures. The viscoelastic sand mastic and elastic aggregate properties were inputted into micromechanical FE models. The FE simulation was conducted on a computational sample to predict complex (dynamic) modulus and creep stiffness. The complex modulus predictions have good correlations with laboratory uniaxial compression test under a range of loading frequencies. The creep stiffness prediction over a period of reduced time yields favorable comparison with specimen test data. These comparison results indicate that this micromechanical model is capable of predicting the viscoelastic mixture behavior based on ingredient properties.