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The aim of this study is to develop a constitutive model capable of describing the effects of confinement pressure on the mechanical behaviour of asphalt mixtures. The model is developed within the context of a Gibbs-potential based thermodynamic framework. Functional forms are assumed for the Gibbs potential and the rate of dissipation, which is maximised to obtain the constitutive model. The Gibbs potential and the rate of dissipation are formulated to have separate components corresponding to the spherical and deviatoric parts of the stress tensor. Additionally, one of the model viscosity parameters is formulated to be pressure-dependent. The efficacy of the model is demonstrated with the aid of experimental measurements conducted at four different confinement pressures ($0\comma\; 70\comma\; 140\comma\; 380\, {\rm kPa}$ 0 , 70 , 140 , 380 kPa ) and two different temperatures (40 and $55^{\circ }{\rm C}$ 55 ∘ C ) (Rahmani et al. 2013, Effect of confinement pressure on the nonlinear-viscoelastic response of asphalt concrete at high temperatures, Construction and Building Materials, 47, 779–788; 2017, Constitutive modeling of coupled aging-viscoelastic response of asphalt concrete, Construction and Building Materials, 131, 1–15; Bazzaz et al. 2018, A straightforward procedure to characterize nonlinear viscoelastic response of asphalt concrete at high temperatures, Transportation Research Record, 2672 (28), 481–492). Some of the test results are used to calibrate the model, while the remaining data are used for model validation. It is found that the model reasonably predicts the mixture responses in the axial and radial directions at different confining pressures and temperatures. [ABSTRACT FROM AUTHOR]

The use of high amounts of reclaimed asphalt pavement (RAP) in asphalt pavements has many economic and environmental benefits; however, there are concerns about brittleness and potential cracking of asphalt mixtures. One of the solutions to address this concern is through the inclusion of recycling agents (rejuvenators). The purpose of this study was to explore the effect of different types of recycling agents (biooils, vegetable oils, tall oil, aromatic extract, and paraffinic oil) on the rheological, microstructural, nanomechanical, and chemical properties of asphalt binder blends with high RAP content. Rheological properties were assessed using a dynamic shear rheometer. Atomic force microscopy was used to determine the microstructural characteristics and nanomechanical properties of the asphalt binder blends. A wavelet packet transform approach was proposed to quantify surface roughness characteristics. Fourier transform infrared spectroscopy was used to evaluate the chemical properties based on carbonyl and sulfoxide indices. Results indicated a correlation between the phases observed in the microstructure and rheological performance. Biooil recycling agents were the most effective in improving the microscopic distribution and rheological properties of binder blends, followed by vegetable oils. However, chemical analysis suggested that the addition of recycling agents did not reverse oxidative aging. Finally, the study recommended a rejuvenation index (RI) that quantified the effectiveness of recycling agents in improving blending and reducing stiffness and aging susceptibility. The RI signified that tall oil was the most susceptible to aging, followed by aromatic extract and paraffinic oil, whereas biooils and vegetable oils were the least susceptible to aging. [ABSTRACT FROM AUTHOR]

Warm-Mix Asphalt (WMA) is a widely used product, which proved a contribution to the reduction in asphalt mixing and compaction temperatures. This reduction leads to lower fuel consumption and smoke emission in asphalt plants. Most of the characterisation of binders used in WMA has focused in the past on measuring linear viscoelastic properties and associated Superpave parameters. Several studies have shown that the average stresses and strains of the asphalt mixture remain mostly within the linear viscoelastic response. However, localised strains in the binder phase of the mixture could reach values high enough to induce nonlinear viscoelastic and viscoplastic deformations. Therefore, this study focuses on an experimental and analytical evaluation of linear, nonlinear viscoelastic and viscoplastic responses of selected binders modified for use in WMA. The first part of the paper analyses the linear viscoelastic material properties and their ability to evaluate permanent deformation resistance. Then, the non-recoverable creep compliance parameter obtained from the Multiple Stress Creep Recovery (MSCR) test is analysed to assess the nonlinear response and permanent deformation of asphalt binders. The paper utilises a nonlinear plasto-viscoelastic (NPVE) approach to assess and quantify the nonlinear plasto-viscoelastic response of binders by separating the recoverable and irrecoverable strains measured in the MSCR test. Two WMA additives were included in this study by mixing them with polymer-modified and unmodified asphalt binders. Analysis of results showed that the NPVE approach captured a higher percentage of recovery than the NLVE approach. However, binder’s performance evaluation and ranking did not change by adopting the NPVE approach. The nonlinear viscoelastic parameters provided insight on the behaviour of asphalt binders mixed with WMA additives during loading cycles. Sasobit showed higher influence than Advera on binders in resisting permanent deformation by increasing the recoverable strain during the unloading phase. [ABSTRACT FROM AUTHOR]

This study utilizes close range photogrammetry (CRP) to measure the texture of asphalt pavement surfaces with a view to explaining the variation of friction measurements obtained using a GripTester. A handheld camera was employed to capture images at different locations on both lanes of pavement sections using a scale rule for identification of control points. Proprietary software were employed for creation and analysis of 3D models from the images to determine pavement surface texture parameters. Different scenarios were considered including pavement surface analysis before and after filtering for micro- and macro-texture separation. Thresholding with respect to height to analyze the top 1–2 mm of the surface was also considered. Texture parameters were then related to friction measured at the image capture locations. The lane with higher friction values generally showed higher individual texture parameters across different scenarios. However, meaningful texture-friction correlations along the lanes were only obtained with the top 2 mm of the surface. Stepwise regression indicated that the density of peaks (Spd) and the peak material volume (Vmp) best correlate (R 2  = 0.75–0.76) with friction, but the Spd is more influential. These parameters can be used as indicators of pavement surface friction during its service life. [ABSTRACT FROM AUTHOR]

• A framework was developed to characterize and model the damage and healing of asphalt. • Damage and healing were modeled by modifying the 'k' parameter in the 2S2P1D model. • The 'k' is expressed as a function whose parameters correlate to the damage resistance. • A graphical method is proposed to classify binders based on damage and healing behavior. • Results showed that adding modifiers to binders improves damage resistance and healing. This paper aims to develop a framework for characterizing damage and healing viscoelastic behaviour of asphalt binders. The framework is based on modifying the 2S2P1D (two springs, two parabolic elements, and one dashpot) viscoelastic model such that the evolution of the exponent 'k' of the parabolic creep function is associated with damage and healing behaviours. The model efficacy is corroborated through the analysis of viscoelastic response, damage, and healing of four asphalt binders (i.e. one unmodified asphalt and three other asphalts modified using low-density polyethylene and various additives.) These binders were subjected to strain sweep tests to determine their linear viscoelastic strain limit. In addition, frequency sweep tests at several temperatures were conducted to obtain binder linear viscoelastic parameters. Then, the binders were subjected to strain-controlled, cyclic progressive damage tests (continuous loading) as well as cyclic damage-healing tests (i.e. cyclic loading with rest periods). The test data were analysed to obtain the storage and loss moduli as functions of loading cycles, and they were modelled using the modified version of the 2S2P1D model. The exponent of the parabolic creep element 'k' was formulated using a hyperbolic cosine function that quantifies the deviation from the linear viscoelastic response during damage and healing. The results showed that the 'k' function was suitable to quantify the damage and healing responses of the asphalt binders at the test conditions. Furthermore, the parameters in the 'k' hyperbolic cosine function were used to create a plot for classifying binders based on their damage and healing characteristics. [ABSTRACT FROM AUTHOR]

This paper introduces an innovative methodology for estimating the ageing of asphalt concrete cores without extracting the binder. Asphalt concrete samples at different ageing stages (unaged, 3-month and 6-month aged) and with different percent air voids (4%, 7% and 10%) were analysed with low-field nuclear magnetic resonance (NMR). The transverse relaxation timeT2and relative hydrogen index (RHI) obtained from NMR measurements were related to the viscosity of the asphalt binder. The samples were analysed during cooling from 70°C to room temperature, showing increase in viscosity with decreasing temperature. There was a clear trend indicating higher viscosities in samples that were aged for a longer period and samples with higher percent air voids. The RHI andT2values obtained from low-field NMR measurements and the viscosity data calculated from measurements using a dynamic shear rheometer were correlated to develop a model that relates viscosity with RHI. [ABSTRACT FROM AUTHOR]

Fatigue cracking is one of the most serious distress modes affecting the serviceability of asphalt pavement structures. The inherent variability of asphaltic materials exhibited in fatigue test results, especially for specimens acquired from field pavements, makes the task of accurately predicting the material’s fatigue characteristics rather difficult. The problem is further exacerbated by the combined impact of a large number of factors, including loading conditions, material heterogeneity, ageing, construction quality and others. For these reasons, notable uncertainty is associated with the predicted fatigue life from laboratory tests based on the use of phenomenological models, which adopt deterministic input parameters despite the varying levels of uncertainty embedded in them. To investigate the effect of inherent uncertainty associated with asphalt mixtures on their fatigue life prediction, a probabilistic analysis approach is evidently needed. In this study, probabilistic analysis was applied to the fatigue life prediction model deduced from the viscoelastic continuum damage theory, based on testing various types of asphalt mixtures. The outcome of the analysis is a newly developed approach with the ability to predict the fatigue performance of asphalt mixtures at more consistent and reliable levels than current practice permits. [ABSTRACT FROM AUTHOR]

This paper presents findings on the evolution of the surface microstructure of two asphalt binders, one unmodified and one polymer modified, directly exposed to aging agents with increasing durations. The aging is performed using an accelerated weathering tester, where ultraviolet radiation, oxygen and an increased temperature are applied to the asphalt binder surface. Ultraviolet and dark cycles, which simulated the succession of day and night, alternated during the aging process, and also the temperature varied, which corresponded to typical summer day and night temperatures registered in the state of Qatar. Direct aging of an exposed binder surface is more effective in showing microstructural modifications than previously applied protocols, which involved the heat treatment of binders previously aged with standardized methods. With the new protocol, any molecular rearrangements in the binder surface after aging induced by the heat treatment is prevented. Optical photos show the rippling and degradation of the binder surface due to aging. Microstructure images obtained by means of atomic force microscopy show gradual alteration of the surface due to aging. The original relatively flat microstructure was substituted with a profoundly different microstructure, which significantly protrudes from the surface, and is characterized by various shapes, such as rods, round structures and finally 'flower' or 'leaf' structures. [ABSTRACT FROM AUTHOR]

A state-of-the-art review of key parameters that influence measurement and modeling of skid resistance of asphalt pavements is provided. Tire-pavement interaction/friction is discussed and the current harmonization method of friction measurements questioned. The latest developments on pavement surface texture measurement and characterization are highlighted. A critical review of aggregate properties affecting friction, the frictional properties of asphalt mixtures and the influence of environmental factors on skid resistance is presented. An overview of modeling efforts entailing different aspects of tire-pavement friction is also presented. The frictional performance of asphalt pavements largely depends on the type and quality of coarse aggregates used. The different hot mix asphalt (HMA) classifications generally have similar microtextures. Their frictional performance follows the same order as their macrotextures. There is need for experimentally-validated skid resistance prediction models, especially for warm surfaces. Such models should account for tire and pavement surface texture characteristics, and the influence of environmental factors. Some other research needs are also identified. [ABSTRACT FROM AUTHOR]

A constitutive model is developed and implemented in the finite element system three-dimensional computer-aided pavement analysis for the simulation of hot mix asphalt field compaction. The details of this model are presented in a companion paper (Masad et al., Finite element modelling of field compaction of hot mix asphalt. Part I: Theory,International Journal of Pavement Engineering, Accepted, 2014). This model is based on nonlinear viscoelasticity theory and can accommodate large deformations that occur during the compaction process. The model was used to study the influence of frequency and amplitude of vibratory compaction rollers on the level of compaction. In addition, it was used to analyse the influence of various methods for compacting longitudinal joints on the percent air voids near these joints. The model was used to simulate the compaction of asphalt pavements with different structures and compacted using various equipment and patterns. The finite element results of the level of compaction and percent air voids were in reasonable agreement with the measurements. The model offers the opportunity to simulate and predict the compaction of asphalt mixtures under various rolling patterns and for different pavement structures. [ABSTRACT FROM AUTHOR]

This paper presents the theoretical background for the development of a constitutive model that is used in the simulation of the compaction of asphalt mixtures. The constitutive model is developed to comply with the principles of thermodynamics, and is derived to represent the macroscopic behaviour of an asphalt mixture as a highly compressible viscoelastic material. The paper presents the details of the mathematical formulation and the computational implementation of the model in the finite element package computer-aided pavement analysis 3D. The capabilities of the compaction model and its sensitivity to changes in model's parameters are illustrated using simple numerical applications. In a companion publication, (Masad et al., Finite element modelling of field compaction of hot mix asphalt. Part II: Application,International Journal of Pavement Engineering, Accepted, 2014), the model is verified against field compaction measurements which demonstrate the ability of the model to capture the general trends of the compaction observed in the field. [ABSTRACT FROM AUTHOR]

The population and economy in the State of Qatar have been increasing in the past 5 years. Accordingly, traffic loading has also increased rapidly, which affected the performance of existing roads and highways. This high traffic loading merits consideration of the design and construction of long-lasting pavements that require minimal maintenance. The Transport Research Laboratory (TRL) in collaboration with the Public Works Authority (PWA) of Qatar constructed a field experiment that consisted of six different pavement sections in order to investigate the influence of using different materials and asphalt mixture designs on performance. This paper presents a comprehensive study for the field evaluation of the performance of these trial sections. The evaluation involved the use of the falling weight deflectometer (FWD) and a vehicle equipped with instruments for measuring permanent deformation and cracking. These field measurements were complemented with laboratory measurements on field cores: the dynamic modulus, flow number, and semicircular bending tests. The results revealed that the increase in temperature between winter and summer in Qatar reduced the stiffness of asphalt mixtures by about 80%. The sections in which polymer-modified bitumen was used had the lowest temperature susceptibility. Moreover, the results showed that the bitumen and aggregate type significantly affected the stiffness and the trial sections' resistance to rutting and fracture. [ABSTRACT FROM AUTHOR]

The State of Qatar is experiencing tremendous growth in infrastructure including road network and highways. The current methods used in design of asphalt pavements in the State of Qatar are empirical and might not be suitable for the design of long-lasting pavements. Given the significant increase in traffic, road authorities in the State of Qatar have been considering the use of mechanistic-empirical methods in the design and analysis of asphalt pavements. This study documents the results of a study in which the mechanistic-empirical pavement design guide (M-E PDG) software was used in the design of asphalt pavements with input parameters that were carefully selected to reresent local materials and climatic conditions. The selection of material properties was based primarily on specifications and design guides in the State of Qatar and on published literature about these materials. The mechanistic-empirical method was also used to assess the benefits of adopting the concepts of perpetual pavement design and also to compare the performance of pavement structures in which various bitumen grades, granular bases and chemically stabilised sub-base were used. A life-cycle cost analysis was carried out to determine the design with the highest net present value among the various options investigated. It is expected that the outcomes of this study would promote the use of mechanistic-empirical methods in the State of Qatar and the region. Inevitably, this will require significant efforts to calibrate material and damage prediction models used in the M-E PDG for more accurate representation of material properties and measured pavements performance. [ABSTRACT FROM AUTHOR]

This paper reports a laboratory investigation aimed at evaluating the potential of polymer binders to stabilize pavement subgrades in Qatar. The conclusions regarding the impact of the polymer stabilizers are based on comparisons with selected physical, chemical, mechanical, and microstructural properties of natural Qatari subgrade soil and soils stabilized with the traditional standard, portland cement (PC). The results demonstrate that the polymer binders modify the Qatari subgrade soils resulting in more favorable engineering properties: for example, the compressive strengths of the polymer-stabilized soils are superior to those of the unstabilized soils and those stabilized using PC. The mechanical properties of the stabilized and unstabilized soils were incorporated in the analysis of asphalt pavement sections using the state-of-the-art protocol for pavement design and analysis. Results of the analysis further demonstrated the benefits of using stabilized subgrade soils in improving pavement performance. Of particular practical importance is that polymer subgrade stabilization significantly reduces subgrade rutting. Consequently, polymer subgrade stabilization is a key consideration for perpetual pavements, especially considering the extremely high truck traffic in Qatar. Although the work reported in this paper was carried out with Qatari soil, the results are expected to be relevant to other types of soil and weather conditions experienced throughout the Middle East and in similar climates throughout the world. [ABSTRACT FROM AUTHOR]

This study analyzed the influence of asphalt binder nanostructures and inherent variability on their mechanical responses. This was achieved by reconstructing asphalt binder nanostructures using generative adversarial networks (GANs). GANs are able to detect hidden patterns in a given data set automatically, and they generate distribution-free data because they are not bound to specific probability functions. The mechanical responses of GANs-generated nanostructures were analyzed using finite-element (FE) analysis. The asphalt nanostructures were captured using atomic force microscopy (AFM). To overcome the limited number of AFM nanostructures, image augmentation techniques and stochastic random fields (RF) modeling were used to generate virtual images of asphalts' nanostructures, which were utilized as the expanded training data set for the GANs. This approach helped to enhance the GANs' generalization abilities and avoid training problems such as mode collapse. Modeling parameters were optimized to expedite the training process and reduce the computational time. The results demonstrated that GANs are capable of generating probable arrangements of nanostructures, thus expanding the body of knowledge regarding probabilistic analysis of asphalt mechanical behavior. The ability of GANs to reconstruct nanostructures indicates that the complex and variable nature of asphalt nanostructures can be identified and replicated by GANs, offering the opportunity to generate many replicates of nanostructures without the need to conduct many laboratory tests. The results successfully related the properties and distribution of the asphalt nanostructures to the variation in the mechanical response. This study proposes a methodology that combines aspects of AFM testing, GANs modeling, and FE modeling for the analysis of the mechanical response of asphalt binder nanostructures. The outcomes of this work link nanoscale properties to bulk properties. Consequently, practitioners and researchers can use the developed methodology to study uncertainty in mechanical response of asphalt binders and design blends with tailored properties and enhanced performance. Researchers also can benefit from this methodology to incorporate sustainable modifiers that enhance fundamental nanoscale properties of asphalt binders. Moreover, this work enables computational evaluation of various material designs that would be impractical to conduct in a laboratory environment, thus conserving resources. Finally, the methodology can be generalized to study nanostructures of other engineering materials to optimize their performance. [ABSTRACT FROM AUTHOR]

The skid resistance of asphalt pavement is a major characteristic of road safety. This study aimed at the development of a model that expresses skid resistance as a function of mixture gradation, aggregate texture and traffic level. The model was developed based on comprehensive measurements and analysis of asphalt mixture surface characteristics (friction, texture and skid resistance) in the laboratory and the field. In addition, aggregates were characterised using the aggregate imaging system after different polishing intervals in the Micro-Deval. The aggregate gradation was described using the two-parameter cumulative Weibull distribution function. The developed model provides an estimate of the skid number in the field. Consequently, this model can be used by engineers to select different combinations of aggregate types and mixture designs such that the desired level of skid resistance at a given traffic level is achieved. [ABSTRACT FROM PUBLISHER]

Many research studies and field experiences have shown a strong relationship between wet-weather accidents and pavement skid resistance. Therefore, measurement methods and models are needed to evaluate the safety level of driving on an asphalt pavement surface during its service life. The objective of this study was to develop a skid resistance prediction model based on measurable quantities such as aggregate shape characteristics, aggregate gradation, aggregate resistance to polishing, and traffic level. To achieve this objective, the skid number (SN) of asphalt pavement sections and traffic data were acquired and analyzed. In addition, statistical analysis was conducted to determine the relationship between different aggregate properties, pavement surface characteristics, and the measured SN values. The aggregate properties were measured using conventional test methods (acid insolubility, magnesium soundness, micro-Deval, and British polish value), and the Aggregate Imaging System (AIMS). The pavement surface characteristics were measured using the dynamic friction tester and circular texture meter. The statistical analysis led to the development of a new model for predicting the asphalt pavement SN as a function of traffic level, initial and terminal aggregate texture values measured using AIMS, and aggregate gradation described using the two-parameter Weibull distribution function. [ABSTRACT FROM AUTHOR]

Abstract: Moisture damage is a major factor in the deterioration of asphalt pavements. In order to combat this problem, it is essential to understand the effects of moisture on the adhesive and cohesive bonds in asphalt concrete mixtures. These effects can be quantified through the use of surface free energy, which is a thermodynamic material property that has been successfully used to select asphalt binders and aggregates that have the necessary compatibility to firm strong bonds and resist moisture damage. This study aimed at understanding the effects of material characteristics and additives on surface energy and the resulting bond between asphalt binders and aggregates. As such, the study involved measuring the surface free energy of 37 neat and polymer modified asphalt binders and 11 aggregates were measured. In addition, the surface free energy was measured for three asphalt binders after two anti-strip agents were added separately (six binder-anti-strip agent combinations) and for nine asphalt binders that were both short and long-term aged. The study also examined the effect of water pH on surface energy and water-aggregate adhesive bond. It was found that anti-strip agents, in general, reduced the cohesive bond energy of asphalt binder, allowing better wetting and adhesion to aggregates and increase in resistance to moisture damage. Aging of the asphalt either increased or decreased the cohesive bond depending upon the chemical composition of the unaged asphalt binder. Statistical analysis was conducted to rank the moisture resistance of asphalt binders and asphalt–aggregate combinations, respectively. The results showed that the pH of the water increased slightly due to contact with the aggregates, but did not significantly alter the total surface tension of the water or surface free energy components of the asphalt binder. [Copyright &y& Elsevier]

Abstract: This paper presents analysis of the effects of mechanical and physical material properties and loading conditions on the response of asphalt mixtures subjected to moisture diffusion processes. The analysis was conducted by using a finite element micromechanical model which couples the effects of moisture transport and mechanical loading. The maximum total force of resistance offered by the asphalt mixture microstructure model for a simulated displacement controlled load in the presence and absence of moisture was used as an indicator of its susceptibility to moisture damage. The results suggest that the diffusion coefficient of the asphalt matrix and aggregates, as well as the bond strength of the aggregate–matrix interface have the most influence on the moisture susceptibility of the materials. [Copyright &y& Elsevier]

The combined effect of moisture and mechanical loading on asphalt mixtures has been recognized as one of the main causes of premature deterioration of flexible pavements. This paper presents a micromechanical model of moisture-induced damage in asphalt mixtures. The model couples the effect of moisture diffusion and mechanical loading to quantify the level of damage within the mixtures. The mechanical properties of the materials are defined as a function of the amount of moisture content. The cohesive zone modeling technique is used to simulate adhesive damage at the aggregate-mastic interfaces. Damage is evaluated based on the location and time for crack initiation and propagation at the aggregate-mastic interfaces and on the level of strains and stresses within the bulk of the mastic. Results show that micromechanical models provide a better understanding of moisture damage mechanisms in asphalt mixtures and can guide the development of continuum damage models. [ABSTRACT FROM AUTHOR]

Increased loads on hot-mix asphalt (HMA) pavements have necessitated the development of new-generation asphalt mixture designs that rely on stone-on-stone contact among coarse aggregates in resisting and distributing applied loads. These stone-on-stone contacts, however, promote localized internal stresses that could cause aggregate fracture. Therefore, it is imperative that methods are developed to analyze the contribution of the coarse aggregate structure and properties to HMA performance during mixture design. This paper introduces an approach that combines the discrete element method (DEM) and image processing techniques to analyze the combined effects of aggregate gradation, shape, stiffness, and strength on HMA resistance to fracture. The DEM input parameters were determined based on measuring aggregate and HMA properties. Consequently, the model was used to quantify the internal forces in asphalt mixtures and determine their relationship to aggregate fracture which cannot be accomplished by conventional experimental methods. The model was successful to large extent in representing the influence of variability in aggregate properties and blending two sources of aggregates on mixture strength. The results showed that the required aggregate strength and optimum blending limits to achieve certain mixture strength are dependent on mixture design. [ABSTRACT FROM AUTHOR]

The presence of moisture in asphalt pavements detrimentally affects the bond between the aggregate and binder and the bond within the binder. The loss of these bonds leads to the deterioration of asphalt pavements. In regions with low rainfall, moisture diffusion is an important source of moisture transport in asphalt mixtures. The diffusion coefficient is a necessary input for models simulating moisture transport and, consequently, moisture damage in asphalt pavements. In this study, an experimental protocol was developed for determining the moisture diffusion coefficient of asphalt mixtures. The experimental set-up relies on measuring suction over time in test specimens using thermocouple psychrometers. The proposed protocol successfully determined a wide range of diffusion coefficients (i.e. from 5.67 × 10- 5 to 2.92 × 10- 6 cm2/s) of different asphalt mixtures. X-ray computed tomography was used to characterise the air void structure of test specimens. The results revealed good correlation between the diffusion coefficient value and the per cent of air voids and the average radius of air voids in the test specimens. The diffusion coefficient values determined in this study can be used as an input in models for determining the rate of moisture diffusion in asphalt mixtures with different per cents of air voids. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents analyses of the nonlinear viscoelastic behavior of unaged and aged asphalt binders tested using a dynamic shear rheometer (DSR) at several temperatures and frequencies. It was not possible to conduct all DSR tests at the same range of stresses, which is necessary for establishing the master curve for nonlinear viscoelastic materials. Therefore, the stress levels for each test, at a given temperature and frequency, were normalized by the ultimate stress level of that test. Consequently, all test results were transformed to a common range of normalized stresses that were used in establishing the master curve. A phenomenological model was used to obtain the creep response of the binders in the time domain from the normalized frequency domain measurements. Then, the Schapery single integral equation was used to model the binder nonlinear creep response. A master curve at a reference temperature of 30°C was formed using the time–temperature superposition principle (TTSP) at selected normalized stress levels. The Schapery’s nonlinear stress dependent parameters (g 1 g 2) were determined by vertical shifting the master curves at the different normalized stress levels. An aging shift factor was used to obtain the aged binder response from the properties of the unaged binder. The aging-time shift factor was found to be a function of temperature, but independent of stress level. The nonlinear viscoelastic model was implemented in the ABAQUS finite element (FE) software and used to back calculate the creep response of the unaged and aged binders. The FE results were in very good agreements with the experimental measurements. [Copyright &y& Elsevier]

Abstract: Compaction of asphalt mixes is a very important process that is known to influence the performance of these mixes. This study is aimed at developing a nonlinear model for the compaction process within a thermodynamic framework that exploits the notion of a body possessing multiple natural configurations. By specifying the forms for the stored energy, the rate of dissipation, the heat flux vector, etc., for the material, a specific model is developed to describe and study asphalt mix compaction. The phenomenon of compaction requires the development of a compressible viscoelastic material to describe the response of asphalt mixes. A numerical scheme was developed to solve the equations governing compaction and the results were compared with results from experiments that were carried out. The agreement with the experimental results are quite good indicating that this model may be used to predict the compactability of asphalt mixes and in the design of asphalt mixes that can be compacted in the field to achieve desirable performance characteristics. [Copyright &y& Elsevier]

This paper presents the development of new methods for measuring aggregate resistance to polishing, and degradation. These are important properties that influence pavement skid resistance and resistance to distresses under traffic loading. The aggregate imaging system is used to measure aggregate surface texture after different polishing time intervals in the micro-Deval test. Mathematical functions are then used to describe the initial texture, rate of polishing, and final texture. Aggregate abrasion is quantified by the percent difference in surface angularity before and after the micro-Deval test, while aggregate breakage is described by the weight loss in this test. The efficacy of the new methods is demonstrated through the analysis of aggregates from different sources that exhibit a wide range of properties. In addition, the new methods are verified by comparing the measured aggregate properties to the degradation of these aggregates in hot mix asphalt mixtures due to compaction forces. [ABSTRACT FROM AUTHOR]

Abstract: Asphalt concrete is the most widely used material for building the surface layer of pavements. It is a porous material that consists of a non-uniform arrangement of asphalt binder, aggregate particles and air voids. One of the primary factors controlling pavement performance is the fluid flow characteristics within the surface asphalt concrete layer. This paper focuses on the numerical simulation of fluid flow in the three-dimensional (3-D) microstructure of asphalt concrete, and the calculation of permeability from the flow field. The asphalt concrete microstructure was captured using the non-destructive X-ray computed tomography (CT) technique. X-ray CT images were processed in order to identify and retain interconnected air voids and eliminate isolated voids. This image processing enhanced the efficiency of the model as it does not have to solve for flow in isolated voids that do not contribute to fluid flow. The X-ray CT images were analyzed and the results were used to determine the relationship between air void distribution and permeability directional distribution or anisotropy. The computed permeability values were found to have good correlation with the experimental measurements. The major and minor principal directions of the permeability tensor were found to correspond to the horizontal and vertical directions, respectively. The results indicated that the non-uniform spatial distribution of air voids created more open flow paths in the horizontal directional than the vertical direction, and hence was the much higher permeability in the horizontal directions. [Copyright &y& Elsevier]

Asphalt mixes exhibit a viscoelastic-viscoplastic behavior, and the contribution of the different constitutive components depends on the temperature and rate of loading. This paper presents an anisotropic nonlinear viscoelastic-viscoplastic model for asphalt mixes. The influence of the anisotropic aggregate distribution is accounted for in both the viscoelastic and viscoplastic responses. The model’s parameters were determined based on the analysis of repeated creep triaxial tests on three asphalt mixes that included aggregates with different characteristics. The paper discusses the experimental factors that need to be considered in conducting the repeated creep tests in order to determine the model’s viscoelastic and viscoplastic parameters. It also discusses the relationship between these parameters and mix properties. The results show that the mix nonlinear viscoelastic response, hardening, dilation or contraction behavior, and damage depend on aggregate properties. [ABSTRACT FROM AUTHOR]

This paper evaluates the relationship between the air void distribution in asphalt mixes and their moisture susceptibility. Asphalt mix specimens were prepared to have different air void distributions through using two different gradations, using two different compaction angles in the Superpave gyratory compactor, and trimming the specimens at different locations. An x-ray computed tomography system was used to capture the internal structure of the specimens, and image analysis techniques were used to quantify the air void distribution and connectivity. Specimens in unconditioned (dry) and moisture-conditioned (wet) states were subjected to dynamic loading, and damage was evaluated using a crack growth index based on fracture theory. The parameters required for the crack growth index were obtained from mechanical and surface energy tests. The analysis results clearly demonstrate the influence of air void distribution on moisture susceptibility. Consequently, the analysis method presented in this paper can be used to evaluate alternative mix designs and internal structures, even when the same materials are used, in order to optimize the resistance to moisture damage. [ABSTRACT FROM AUTHOR]

Abstract: Form, texture, and angularity are among the properties of aggregates that have a significant effect on the performance of hot-mix asphalt, hydraulic cement concrete, and unbound base and subbase layers. Imaging techniques have provided a good means to quantify aggregate shape properties rapidly in spite of the fact that they might differ in the mathematical procedure and the instrumental setup they utilize. The validity of the mathematical procedure is essential for the results to be useful in quantifying aggregate shape. Some of the most widely used aggregate shape analysis techniques were evaluated in this paper. The analysis results revealed that some of the available analysis methods are influenced by both angularity and form changes and, consequently, are not suitable to distinguish between these two characteristics. Also, some of the analysis methods are quite adequate to measure both texture and angularity when changes are made to the image resolution and magnification level. The following analysis methods are recommended: wavelet analysis of gray images for texture; both the gradient method and tracing the change in slope of a particle outline method for angularity; aspect ratio for 2-dimensional form; and sphericity or the proportions of the three particle dimensions for 3-dimensional form. [Copyright &y& Elsevier]

Permanent deformation in hot mix asphalt is caused by a combination of densification (decrease in volume and hence increase in density) and shear deformation. The primary objective of this paper is to develop an elastoviscoplastic model that accounts for the influence of important microstructure properties such as anisotropy and damage on permanent deformation. The model incorporates a yield surface based on the Drucker-Prager function that is modified to capture the influence of stress state on the material response. Also, parameters that reflect the directional distribution of aggregates and damage density in the microstructure are included in this yield surface model. The elastoviscoplastic model is converted into a numerical formulation and is implemented in finite element (FE). The FE model is used in this study to simulate experimental measurements under different confining pressures and strain rates. [ABSTRACT FROM AUTHOR]

This paper presents a methodology for analyzing the viscoelastic response of asphalt mixtures using the discrete-element method (DEM). Two unmodified (neat) and seven modified binders were mixed with the same aggregate blend in order to prepare the nine hot mix asphalt (HMA) mixtures used in this study. The HMA microstructure was captured using images of vertically cut sections of specimens. The captured grayscale images were processed into black and white images representing the mastic and the aggregate phases, respectively. These microstructure images were used to represent the DEM model geometry. Rheological data for the nine binders were obtained using the dynamic shear rheometer. These data were used to estimate the parameters of the viscoelastic contact models that define the interaction among the mix constituents. The DEM models were subjected to sinusoidal loads similar to those applied in the simple performance test (SPT). The DEM model predictions compared favorably with the SPT measurements. However, the simulation results tended to overpredict the dynamic modulus, E*, for mixtures made with neat binders and underpredict E* for those that consisted of modified binders. The DEM models gave mix phase angles, [lowercase_phi_synonym]mix, higher than the experimental measurements. [ABSTRACT FROM AUTHOR]

This paper presents the development of a methodology for microstructure analysis and modeling of hot mix asphalt (HMA). This methodology relies on using effective local material properties and strain gradient theory in the finite element analysis of HMA microstructure. The effective local properties are calculated using an analytical micromechanical model that captures the influence of percent of particles on the microscopic response of HMA. Strain gradient elasticity is used in order to account for the effect of particle size in the finite element analysis. The autocorrelation function and the moving window technique are used to determine the microstructure characteristic length scales that are used in strain gradient elasticity. A number of asphalt mixes with different aggregate types and size distributions are analyzed in this paper. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents an approach for constitutive modeling of the viscoplastic behavior of asphalt mixes. This approach utilizes an anisotropic non-associated flow rule based on the Drucker–Prager yield surface. The selection of this yield surface is motivated by the field stress paths and material properties associated with permanent deformation at high temperatures. The efficacy of the model is demonstrated by analyzing data from compressive triaxial tests conducted at different confining pressures and strain rates for three different mixes. The model parameters are related to the experimental measurements of aggregate shape characteristics, aggregate surface energy, inherent anisotropic distribution of aggregates, and microstructure damage measured using X-ray computed tomography and image analysis techniques. Establishing the relationship between the model parameters and material properties is important in order to optimize the mix properties, and achieve desirable mix performance. [Copyright &y& Elsevier]

Abstract: The description of particle surface characteristics is of interest in many disciplines in science and engineering such as powder technology, geology, soil science, and infrastructure materials. Consequently, there is a need to develop methods to quantify particle surface characteristics rapidly and accurately. In this study, a number of computational methods were utilized to describe the surface characteristics (form, angularity and texture) of three different granular materials. These computational methods are the radius method, gradient method, form index, Fourier series, spherical harmonic series, and sphericity index. To illustrate the validity of these methods, they were first used to analyze the shape of standard images that were used in the past by geologists for visual classification of particles. Subsequently, the methods were used to analyze images captured using nondestructive X-ray computed tomography (CT), and an automated computer controlled system known as the Aggregate Imaging System (AIMS). The results demonstrated the capabilities of the analysis methods to quantify the multi-scale nature of form, angularity, and texture. In general, the images captured using AIMS can be used to develop a particle shape classification system, while the X-ray CT images, in conjunction with spherical harmonic analysis, a powerful technique to represent and reconstruct three-dimensional images of particles, can be used to mathematically represent the shape of particles in computational models. [Copyright &y& Elsevier]

Abstract: A microstructure-based viscoplastic continuum model is developed for the permanent deformation of asphalt concrete (AC). The model accounts for several phenomena that influence the permanent deformation of AC at high temperatures. These phenomena include strain rate dependency, confining pressure dependency, dilation, aggregate friction, anisotropy, and damage. The material anisotropy was included in the model by replacing the stress invariants in the yield function with invariants of both the stress and a microstructure tensor, which describes the aggregate orientation distribution. The components of the microstructure tensor were determined using image analysis techniques (IAT) conducted on digital images taken from two-dimensional cut sections of AC. Furthermore, damage was included in the model based on the effective stress theory to account for crack and air void growth that significantly reduces the load-carrying capacity of the material. Experimental data from triaxial compressive strength tests conducted at five strain rates and three confining pressures were used to develop a methodology to determine the material parameters that characterize AC permanent deformation. The model predictions were in a good agreement with the experimental measurements. [Copyright &y& Elsevier]

This study uses the discrete element method (DEM) to simulate the dynamic mechanical behaviour of asphalt mastics. Asphalt mastics are defined as dispersions of aggregate fillers within a medium of asphalt binder. The fillers refer to the fraction of mineral aggregate passing the 200-mesh sieve, (i.e. smaller than 75  μm). Mastic measurements obtained using the dynamic shear rheometer were compared to DEM predictions and available micromechanics-based models. Three asphalt binders (ABD-1, AAM-1 and ABM-1) and four mineral fillers (glacial gravel, granite, limestone and greywacke) were used to prepare the mastics in this study. The stiffening effect of the mineral fillers was investigated at different filler volume fractions. The DEM results captured the stiffening behaviour of asphalt mastics as a function of the volumetric concentration of mineral fillers. The DEM results exhibited a high rate of stiffening that is typically observed in experimental measurements of mastics at relatively low volume concentrations of fillers. Compared to the DEM results, the micromechanics-based models were not sensitive to the dynamic shear modulus of the asphalt binder, and they underestimated the stiffening effect of the mineral fillers. [ABSTRACT FROM AUTHOR]

Permanent deformation is one of the major distresses in asphalt pavements. It is caused mainly by high traffic loads associated with high field temperatures. An anisotropic viscoplastic continuum damage model is developed in this study to describe permanent deformation of asphalt pavements. The model is based on Perzyna’s formulation with Drucker–Prager yield function modified to account for material anisotropy and microstructure damage. The material anisotropy is captured through microstructural analysis of aggregate distribution on two-dimensional sections of hot mix asphalt. A damage parameter is included in the model to quantify the nucleation of cracks and growth of air voids and cracks. A parametric study was conducted to demonstrate the sensitivity of the model to strain rate, aggregate distribution, and microstructure damage. Triaxial strength and static creep measurements obtained from the Federal Highway Administration Accelerated Loading Facility were used to determine the model parameters. [ABSTRACT FROM AUTHOR]

The resilient properties of unbound aggregate bases are important parameters in the design of asphalt pavements. Previous studies have shown that these resilient properties exhibit nonlinear and transverse anisotropic characteristics. The paper in hand presents a micromechanics-based approach to model the nonlinear and anisotropic properties of unbound aggregate bases. The anisotropic behavior is captured using two microstructure parameters representing the preferred orientation of aggregate particles, and the ratio of the normal contact stiffness to shear contact stiffness among particles. The nonlinear response is modeled using a relationship that relates the shear modulus to particle packing, material properties, particle size, and confining pressure. The micromechanics model is used to represent the resilient properties for a total of 18 different combinations of material conditions with different aggregate types, moisture contents, and gradation characteristics. Anisotropic and nonlinear resilient properties were measured at ten different stress states for each of the material conditions. The results presented in this paper show that the micromechanics model is capable of successfully representing the experimental measurements. [ABSTRACT FROM AUTHOR]

A method is developed to assess hot mix asphalt (HMA) stability during compaction in the mix design process, and prior to conducting further expensive and time consuming testing. The method consists of a mathematical derivation, and an experimental procedure to determine a new stability index, referred to as the contact energy index (CEI). The CEI reflects the ability of the aggregate structure to develop contacts among the particles when subjected to shear stresses. The stability of HMA increases with an increase in CEI. The sensitivity of the CEI to changes in mix components is evaluated by analyzing asphalt mixes that differ in their nominal maximum aggregate size, aggregate gradation, source of aggregate, percent of natural sand, and asphalt content. In addition, the CEI is compared with mechanical properties of a number of asphalt mixes. The results show that the CEI captures the influence of changes in mix constituents on mix stability, and has very good correlation with the mechanical properties. [ABSTRACT FROM AUTHOR]

Permeability is an important property that influences the performance of hot-mix asphalt (HMA). It is a function of compaction effort, and several properties of HMA such as asphalt content, and the shape and size distribution of aggregates. Due to the different laboratory and field methods for measuring permeability, and the interaction among the factors that influence its value, it would be difficult to develop an analytical equation that accurately relates permeability to all factors contributing to HMA permeability. This paper presents a simple equation for approximating the permeability of asphalt mixes. It utilizes the percent air voids and surface area of aggregates. The equation is empirical but it is derived based on the well-known Kozeny–Carman equation for calculating the permeability of granular materials. The developed equation was used successfully to fit permeability data collected from several studies that carried field and laboratory measurements of HMA permeability. A finite element model was developed to investigate the influence of the gradient of percent air voids in HMA on water flow patterns. The x-ray computed tomography was used to measure the percent air void gradients among sublayers of the asphalt mix. The permeability of these sublayers was calculated using the developed equation, and used as an input to the finite element model. The simulation results show that air void gradients in HMA encourage lateral flow in the horizontal direction and reduce the flow in the vertical direction. [ABSTRACT FROM AUTHOR]

This paper presents a methodology to evaluate the aggregate structure stability in hot mix asphalt (HMA). The HMA response in the Superpave Gyratory Compactor (SGC) is used to derive an index, referred to as the Contact Energy Index (CEI), that reflects the energy utilized in establishing contacts among aggregate particles. Then, a three-dimensional (3-D) finite element model of the SGC is presented, and the CEI calculated from the finite element model is compared with experimental measurements. The 3-D finite element model is shown to give CEI results similar to measurements. Image analysis techniques are used to measure the evolution of the aggregate structure in terms of orientation and contacts during compaction. The image analysis results are compared with experimental measurements of CEI. The comparison shows strong correlation between the number of contacts, the resistance of the aggregate structure to re-orientation and the CEI value. [ABSTRACT FROM AUTHOR]

Hot mix asphalt (HMA) is a composite material that consists of mineral aggregates, asphalt binders, and air voids. A finite element model of the HMA microstructure is developed to study the influence of localized strain distribution on the HMA mechanical response. Image analysis techniques are used to capture the HMA microstructure. Due to limitations on the image resolution, the microstructure is divided into two phases: aggregates larger than 0.3 mm and mastic (binder and aggregates smaller than 0.3 mm). A viscoelastic constitutive relationship is used to represent the mastic phase of the HMA microstructure. The mastic viscoelastic properties are obtained from the results of testing asphalt binders in a dynamic shear rheometer and microstructure analysis of idealized mastic. A step-wise finite element procedure is employed in order to account for the influence of the localized high strains on the mastic viscoelastic properties, and HMA mechanical response. The mastic and binder elements of the microstructure are shown to exhibit high strain values within the nonlinear viscoelastic range. The HMA viscoelastic properties are calculated at different strain levels, and the results are compared with experimental data obtained from the frequency sweep shear test. [ABSTRACT FROM AUTHOR]

The mechanical behavior of many bound granular materials such as asphalt mixtures is anisotropic in nature. However, the majority of the current mechanical tests and analytical models for asphalt mixtures are based on the assumption of isotropic material properties. This study investigates the stiffness anisotropy of asphalt mixtures using micromechanics-based models. The models’ parameters are obtained by quantifying the internal structure anisotropy in terms of the preferred orientation of longest axes and contact normals of aggregates. Image analysis techniques are used to conduct the internal structure measurements. The orientations of the longest axes are found to be easier to measure, and better descriptors of anisotropy, than the contact normals. Finite-element analyses of the internal structure are also used to provide insight into the mixture stiffness anisotropy. The mixture properties are selected to represent a wide range of temperatures. The stiffness in the horizontal direction is shown to be as high as 30% more than the stiffness in the vertical direction. The stiffness anisotropy decreases with a decrease in the mixture temperature. The finite-element results are shown to have very good correlation with the results of the micromechanics model derived based on the orientation of the longest axes of aggregates. [ABSTRACT FROM AUTHOR]

Estimates the strain distribution in an asphalt binder as it exists in hot mix asphalt . Description of image correlation techniques; Results of experimental measurements of strain distribution; Description of finite-element models;

The effect of fine aggregate morphologic properties on asphalt mixtures' behavior is well recognized. However, the current procedures for measuring fine aggregate properties are at best indirect indicators of these properties. The indirect nature of the current measurements has led to inconsistency in predicting the extent to which the measured properties influence pavement performance. Two independent methods that integrate several aspects of image-analysis techniques are presented for quantifying angularity and texture of fine aggregates. The first technique relies on the change of aggregate shape as it is subjected to a number of erosion-dilation morphologic operations. The second technique uses the change of an object perimeter with its shape. The analysis shows that the two methods are able to capture aggregate shape at two different scales. Angularity is captured using low-resolution images, whereas surface texture is captured using highresolution images. The presented image-analysis techniques have potential benefit in quantifying the effects of texture and angularity on asphalt pavement performance. [ABSTRACT FROM AUTHOR]

Presents a procedure for constructing resin impregnated sand specimens for fabric microstructure analysis. Construction of soil samples for image analysis; Components of the resin impregnation apparatus; Results of uniformity and anisotropy analysis of resin impregnated specimens.

Presents a finite-element study of the effects of temperature variation on plain-jointed concrete pavements. Capability to solve for partial contract between curled slabs and the ground; Effect of superposition of both stresses; Arithmetic addition of positive curling stresses and thermal-expansion stresses; Linear distribution of temperature.

This study presents a novel methodology to generate computational replicates of nanostructures of multiphase materials, such as asphalt binders, by integrating image analysis techniques with stochastic random field (RF) modeling. Image analysis techniques are used to identify and segment nanostructure images obtained by atomic force microscopy, while RF is used to model the spatial distribution of their material properties. The results of this process are images showing probable arrangements of nanostructures with stochastic material properties that replicate the experimentally obtained images. The computationally generated nanostructures are then used as inputs in a finite element model to evaluate the effect of heterogeneity on their mechanical response. The efficacy of the developed approach is demonstrated through simulations of asphalt binders' nanostructures, which reveal novel insights regarding their nanoscale mechanical behavior and response. The FE simulations provided the link between the distribution of nanoscale properties of asphalt binders and variations in their mechanical response. The application of this methodology expands the body of knowledge beyond the deterministic analysis of asphalt binders toward probabilistic analysis and uncertainty quantification that considers their heterogeneous, multiphase structures. Consequently, the methodology can be used to design multiphase materials, such as asphaltic blends, with tailored properties and enhanced performance. [ABSTRACT FROM AUTHOR]

This study addresses the utility of polyelectrolytes, i.e., cationic poly(diallyldimethylammonium chloride) (PDADMAC) and anionic polystyrene sulfonate (PSS), as additives to improve properties of the polymer-stabilized soil. This paper specifically focuses on the resistance of polymer-stabilized soils to degradation and/or damage during and following multiple wetting–drying cycles (zero, one, two, three, five, and seven cycles). Each cycle consisted of 24 h of moisture conditioning using capillary rise followed by 24 h of drying. Then, these specimens were evaluated for their unconfined compressive strength (UCS). The microstructure and composition of the soils were investigated using scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), and X-ray fluorescence analysis (XRF). Based on the results, the soils used in this study for polymer treatment were primarily composed of carbonates and silicates with a small amount of clay minerals. The polyelectrolyte stabilizers (PDADMAC and PSS) and polyelectrolyte complexes (PECs) were added to the soils at dosages ranging from 0.2% to 1.6% by weight of dry polymer to dry soil. Treated soils demonstrated increased UCS compared with untreated counterparts. The untreated soils exhibited rapid degradation of UCS and mechanical collapse within three to four wetting–drying cycles. On the other hand, the polymer-treated soils exhibited a strength reduction of between 10% and 50% following the first cycle and then maintained the UCS of about 3–6 MPa after completion of all wetting–drying cycles. Furthermore, the stabilized soil demonstrated significant improvement in toughness compared with their untreated and cement-treated counterparts. The ability of the polymer-stabilized soils to stand up to wetting–drying cycles is a key finding and contribution of this study. [ABSTRACT FROM AUTHOR]