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Generation of virtual asphalt concrete in a physics engine
Abstract
The precise, quick and efficient simulation of asphalt concrete is an important step to select asphalt mixtures for a range of applications. A method to create virtual aggregates based on aggregates' topological properties and aggregate skeletons like those of asphalt concrete using a physics engine is proposed. It can produce hundreds of virtual aggregates with shape properties equivalent to these of real ones in a matter of seconds, pack them to a level of compaction like that of the Marshall compactor, and measure properties such as the expected air voids content and number of contacts between the aggregates. In the paper, only aggregates bigger than 2 mm have been considered due to computing efficiency. Besides, it was found that the main factors influencing asphalt compaction are the number of aggregates in the mixture and the amount of dust, or aggregates smaller than 2 mm, which correspond to the parts of the solid skeleton that have not been simulated.
... DE models of asphalt mixture were also established using the PFC2D software to predict the stress-strain characteristics of the coarse aggregate (Ding et al., 2017a;Ma et al., 2016aMa et al., , 2018c. The particle shape Zhou et al., , 2019bZhou et al., , 2019c and grain size (Garcia-Hernandez et al., 2021;Liu et al., 2019d) of aggregates in asphalt mixtures have been characterized using the DEM method. Barrasso et al. (2015) introduced a mechanistic model for a wet granulation process, combining the techniques of population balance modeling and DEM to predict the porosity and size distribution of the granule. ...
Sustainable and resilient pavement infrastructure is critical for current economic and environmental challenges. In the past 10 years, the pavement infrastructure strongly supports the rapid development of the global social economy. New theories, new methods, new technologies and new materials related to pavement engineering are emerging. Deterioration of pavement infrastructure is a typical multi-physics problem. Because of actual coupled behaviors of traffic and environmental conditions, predictions of pavement service life become more and more complicated and require a deep knowledge of pavement material analysis. In order to summarize the current and determine the future research of pavement engineering, Journal of Traffic and Transportation Engineering (English Edition) has launched a review paper on the topic of “New innovations in pavement materials and engineering: A review on pavement engineering research 2021”. Based on the joint-effort of 43 scholars from 24 well-known universities in highway engineering, this review paper systematically analyzes the research status and future development direction of 5 major fields of pavement engineering in the world. The content includes asphalt binder performance and modeling, mixture performance and modeling of pavement materials, multi-scale mechanics, green and sustainable pavement, and intelligent pavement. Overall, this review paper is able to provide references and insights for researchers and engineers in the field of pavement engineering.
This chapter deals with the materials and design aspects of asphalt pavements. In the first part of the chapter, some of the upcoming technologies arising out of recent research, since the last decade or so, are highlighted. The immediate research challenges are also identified. In the second part, possible future directions are discussed briefly.
The geometric and distributional characteristic of aggregate contacts in an asphalt mixture significantly affect its load-bearing capacity. An approach to the virtual construction of cylindrical samples with customized aggregate contacts is presented to simulate the internal structure of an asphalt mixture. To obtain virtual samples, coarse aggregates are prepared from a digital library of realistic aggregates, allocated coordination numbers, and placed in a cylindrical container with the expected relations constructed. Next, the air voids and the asphalt mortar are generated in sequence to obtain the microstructure of a virtual specimen. Three AC-13 samples were constructed using different parameters. The difference between actual and expected average coordination numbers is around 6% for AC-13 Marshall samples virtually constructed with appropriate parameters, showing the reliability of the control effect of the proposed approach on the aggregate contact in asphalt mixtures. Furthermore, compared with a realistic AC-13 sample, virtual samples constructed have a close quantity, area, and orientation of aggregate contacts. Moreover, simulation results of the displacement-controlled test indicate the virtual sample with most aggregate contacts has the largest strain energy, demonstrating the correlation between contact properties and deformation resistance.
In this paper we present an algorithm to produce realistic virtual asphalt concretes. The inputs are the geometry of aggregates, and the composition of the asphalt concrete. The outputs are the air voids content and, a 3-dimensional representation of the aggregate skeleton. The air voids that are predicted by the algorithm have been compared to those measured from asphalt made in the laboratory. The key parameters that influence the compaction of an asphalt concrete are the shape of the aggregates, number of aggregates and, volume of mastic, i.e. bitumen, filler, and aggregates smaller than 2 mm. An equation based on a Weibull function, with independent variables the number of aggregates and force required to compact the virtual aggregates, has been developed. This equation can predict the force that the algorithm needs to compact the virtual aggregates to a level equivalent to that reached by a laboratory compactor.
Various asphalt mixtures have been widely used in transportation, and their importance to road construction is self-evident. The stability of the asphalt mixture is the physical property that deserves the most attention, and it has a significant impact on the road performance of the material in practical applications. A new virtual asphalt mixture simulation method is introduced to obtain the micro performance data of asphalt mixture. By combining the microdata of the virtual mixture and the macro test results of the equivalent test mixture, the researcher found a high correlation between the dust and porosity and the RLAT results of the asphalt mixture through statistical analysis. Through regression analysis and model screening, an equation that can predict the repeated load axial test results within the effective range through the dust content and air voids is determined. The research also found that the virtual asphalt mixture's dust and virtual force in the compaction stage will affect the stability of the asphalt mixture by affecting the compaction effect of the asphalt mixture. Through regression analysis and model screening, an equation that can predict the repeated load axial test results within the effective range through the dust content and virtual force is determined. These equations may be used to optimize the stability of asphalt mixtures in future studies. The research method using virtual design can provide some suggestions and directions for optimizing macroscopic performance by adjusting microscopic variables in the material design and development stage in the future.
In this paper, the role of crushing operation on the cubical controlling features of coarse aggregates, and its influence on the shape distribution characteristics of aggregates (i.e. angularity, sphericity, and flatness & elongation ratio) were investigated through the second version of Aggregate Image Measuring System (AIMS II). Three types of aggregate from different origins were chosen and crushed by the vertical shaft impactor (VSI) with fixed rotational speed. The results indicate that the rotation speed of VSI is influenced by the input power of the machine to the particle sizes, and higher speed leads to more cracks on aggregates during the crushing process. By using X-ray diffraction (XRD), the aggregate samples from different origins and mineral compositions were analysed under the same working operation of crusher. Besides that, the correlation between particle shape characteristics influenced by the mineralogical composition, and their collision behaviours have been discussed according to the tested results. A recommendation for optimal operation of VSI on crushing granite aggregates is given based on this study. Whereas, the cubical condition of aggregates and energy saving factors were taken into account. Furthermore, petrographic analyses were imposed to correlate particle mineralogy to its collision behaviour during crushing.
During asphalt mixture compaction, loads in the material are mainly transferred through contact between the stones and the interaction between the stones and the binder. The behaviour of such materials is suitable to model using the Discrete Element Method (DEM). In this study, a new DEM modelling approach has been developed for studying the asphalt compaction process , incorporating contact and damage laws based on granular mechanics. In the simulations, aggregate fracture is handled by a recently developed method of incorporating particle fracture in DEM, based on previously performed fracture experiments on granite specimens. The binder phase is modelled by adding a viscoelastic film around each DEM particle. This surface layer has a thickness that obtains the correct volume of the binder phase and has mechanical properties representative for the binder at different temperatures. The ability of the model to capture the influence of mixture parameters on the compactability and the eventual stone damage during compaction is examined for the cases of compaction flow test and gyratory compaction. Explicitly, the influence of different aggregate gradations, mixture temperatures and binder properties are studied. The results show that the proposed DEM approach is able to capture qualitatively and quantitatively responses in both cases and also provide predictions of aggregate damage. One large benefit with the developed modelling approach is that the influence of different asphalt mixture parameters could be studied without re-calibration of model parameters. Furthermore, based on comparative DEM simulations, it is shown that the proposed approach provides more realistic force distribution networks in the material.
The current evaluation of Superpave design aggregate structure is deterministic. The design involves evaluation of selected trial blends based on volumetric, compaction, and dust proportion requirements. This paper incorporates the uncertainties of all 17 variables involved in the process, measured by the coefficient of variation (CV), and develops a revised procedure for comparing mixture properties with the performance-based criteria. The uncertainties of the 8 measured properties are propagated through the calculation and make the uncertainty of some variables very large. Issues related to the reliability of some variables (CV > 25%) are discussed and criteria to resolve them are established. The developed mathematical formulas of uncertainty were verified using Monte Carlo simulation. The results show that the potentially unreliable variables are volume of absorbed asphalt, percent of absorbed asphalt, and percent voids in total mix at the design compaction level. A tool is provided to help the designer trace the uncertainty of the unreliable variables back to the measured properties so that their precisions may be revised. The current NCHRP recommended precisions for specific gravities were found to be generally satisfactory. However, further research should continue to improve the precision of specific gravity measurements as some variables may still become unreliable.
The objectives of this study are: (1) to create aggregate representations based on their realistic shapes and establish an aggregate shape library by storing aggregate numerical representations of various shapes and angularities; and (2) to prepare mesostructure models for stone based materials using aggregate representations from the library. The numerical representations based on realistic shapes are created through three steps: (1) scan individual aggregate particles using an X-ray CT machine to obtain a series of cross sectional images at different heights; (2) process the sectional images and stack them to obtain the 3D aggregate boundary; and (3) apply a sphere growth model to fill the interior space of the boundary. The sphere growth includes a first growth in 26 divergent directions and a second growth in 8 divergent directions which start from each sphere in the first growth. Four aggregate particles were selected for the numerical representation construction using the sphere growth model. The modeling results showed that the four aggregates had filling ratios of 97.8, 95.3, 91.3 and 98.5 %, respectively. Afterward, the four numerical representations were stored as aggregate templates to establish a preliminary representation library, which can be invoked to generate numerical aggregate particles in the model reconstruction of stone based materials. A discrete element model and a finite element model of an asphalt concrete beam were reconstructed as examples of application of the numerical representation library. The study overall showed that: (1) the sphere growth model is an effective approach to construct aggregate numerical representations with a high accuracy; (2) the sphere growth model can significantly reduce the sphere amount of the numerical representations compared to previous related studies; and (3) these numerical representations can be successfully used to reconstruct the mesostructure models of stone based materials, which can potentially bring time and cost savings while keeping the model precision.
This research targeted two primary purposes: to estimate current aggregate angularity test methods and to evaluate current aggregate angularity requirements in the Nebraska asphalt mixture/pavement specification. To meet the first research objective, various aggregate angularity tests were estimated with the same sets of aggregates and were compared by investigating their characteristics on testing repeatability, cost, testing time, workability, and sensitivity of test results. For the second objective, the effect of aggregate angularity on mixture performance was investigated by conducting laboratory performance tests (the uniaxial static creep test and the indirect tensile fracture energy test) of five mixes designed with different combinations of coarse and fine aggregate angularity, and statistical analyses of five-year asphalt pavement analyzer test results of field mixtures. Results from the indirect tensile fracture energy test were then incorporated with finite element simulations of virtual specimens, which attempted to explore the detailed mechanisms of cracking related to the aggregate angularity. Results from the estimation of various angularity test methods implied that for the coarse aggregate angularity measurement, the AASHTO T326 method was an improvement over the current Superpave method, ASTM D5821, in that it was more objective and was very simple to perform with much less testing time. For the fine aggregate angularity measurement, the current Superpave testing method, AASHTO T304, was considered reasonable in a practical sense. Rutting performance test results indicated that higher angularity in the mixture improved rut resistance due to better aggregate interlocking. The overall effect of angularity on the mixtures’ resistance to fatigue damage was positive because aggregate blends with higher angularity require more binder to meet mix design criteria, which mitigates cracking due to increased viscoelastic energy dissipation from the binder, while angular particles produce a higher stress concentration that results in potential cracks. Finite element simulations of virtual specimens supported findings from experimental tests. Outcomes from this research are expected to potentially improve current Nebraska asphalt specifications, particularly for aggregate angularity requirements and test methods to characterize local aggregate angularity.
One of the most important properties of flexible pavements in terms of tire-pavement interface is surface texture. The texture of a pavement surface and its ability to resist polishing effect of traffic is of prime importance in providing skidding resistance. Pavement surface texture greatly contributes to tire-pavement skid resistance which has a direct effect on traffic operation and safety particularly at high speeds. Doubtless, there exists a close relationship between pavement surface texture and aggregate angularity within the wearing course. This paper is aimed to determine the effect of aggregate shape on the surface properties of Hot Mix Asphalt (HMA). Two different mineralogical types of aggregate (basalt and limestone) have been crushed with impact, jaw and roll type of crushers. Various types of aggregate with different shapes have been mixed with 50/70 penetration grade bitumen to form dense graded mixtures. Test methods related with the evaluation of shape and texture characteristics have been utilized to characterize the geometrical properties of aggregates. The texture and friction properties of asphalt slabs have been evaluated by means of sand patch test, 3D laser scanner and dynamic friction tester respectively. The results indicated that a relationship exists between the shape characteristics of aggregate and the surface properties of HMA.
Heterogeneous air void distribution is a common phenomenon in asphalt mixtures and is intimately related to the behavior of the asphalt mixture. Air void distribution within an asphalt mixture specimen is related to several factors, such as compaction effort, method of compaction (equipment), and aggregate gradation. In this study, virtual digital specimens generated by an open source discrete element method (DEM) program were utilized to investigate the effect of compaction factors on air void distribution. The compaction processes of a Superpave gyratory compactor (SGC) and a vibratory compactor were simulated by DEM. Effects of aggregate gradation, specimen height, mold size, mold shape, and compaction method on the inhomogeneous distributions of air voids were studied through statistical analyses. The results from DEM simulation were in good agreements with laboratory test results as well as those found in the literature. This research demonstrated that DEM simulation could be a potentially helpful tool for analyzing asphalt mix compaction and selecting appropriate aggregates for asphalt mix design.
The greater part of asphalt mixtures is composed of aggregates. This means that particular features, such as shape and angularity, are the primary factors that affect the development of the mechanical performance of asphalt pavements. In order to investigate the combined effect of grain shape and angularity on the packing and stability of an aggregate's assembly for asphalt mixes, the authors have performed an experimental program using 3D Discrete Element Method. The results obtained from triaxial tests and from statistical analysis of the distribution of particle-particle contact forces show that the grain shape and angularity significantly affect the assembly behaviour.
Rutting has been identified as one of the primary distresses in asphalt pavements. Rutting in hot-mix asphalt (HMA) mixtures can be attributed to either the lack of interlocking of aggregate structure or insufficient bonding between aggregate and asphalt binder, or both. In the present study, efforts have been made to identify the contributions of aggregate structure and asphalt binder to the rutting characteristics of a dense-graded surface HMA mixture. Coarse gravels at five different angularity levels (100, 85, 70, 50 and 35% of aggregate with two or more fractured surfaces) were used to produce mixtures with similar aggregate gradations. Three different asphalt binders (PG 64-22, PG 76-22 and PG 82-22) were used to make mixtures for laboratory rut evaluations. The aggregate imaging system (AIMS), uncompacted voids in coarse aggregate (VCA) and tri-axial shear tests were conducted to evaluate the coarse aggregate angularity (CAA). The US Army Corps of Engineers' gyratory testing machine (GTM), creep and the asphalt pavement analyser (APA) tests were selected to characterise the rut resistance of asphalt mixtures.The results from this study indicated that coarse aggregate AIMS, VCA and tri-axial tests were related to the CAA and laboratory-measured rutting indices. At temperatures close to the binder's upper grade limit, aggregate structures played a critical role in the rut resistance of HMA mixtures; whereas, at temperatures below the binder's upper grade limit, the stiffness of the asphalt binder played a more important role in the rut resistance of asphalt mixtures evaluated in this study.
The mechanical properties of concrete composites mainly depend on its ingredients and meso-structure, especially the shape, distribution, and grading of aggregates. Therefore, it is very important to establish realistic meso-scale models for concrete composites performance prediction and structural optimization design. However, there is only a few methods to generate models with high volume fraction in current studies. In this paper, a novel idea for numerical modeling, namely the aggregate expansion method, is used to establish more realistic 3D models with different volume fractions of convex and concave aggregates. The shrunken aggregates are first placed in an empty container, and then these aggregates are expanded to the predetermined sizes to obtain a numerical model. Using this approach can efficiently obtain a high volume fraction model with randomly distributed aggregates. In addition, realistic aggregates are generated according to the statistical data of actual aggregates shapes. A series of statistical data show that the randomness of aggregates in the space before and after expansion is still maintained. At last, the simulation results of 3D numerical models with different volume fractions under uniaxial tension are calculated. The consistency between the simulation results and experimental data verifies the good accuracy of the models.
This paper investigates the effects of air void topology on hydraulic conductivity in asphalt mixtures with porosity in the range 14%–31%. Virtual asphalt pore networks were generated using the Intersected Stacked Air voids (ISA) method, with its parameters being automatically adjusted by the means of a differential evolution optimisation algorithm, and then 3D printed using transparent resin. Permeability tests were conducted on the resin samples to understand the effects of pore topology on hydraulic conductivity. Moreover, the pore networks generated virtually were compared to real asphalt pore networks captured via X-ray Computed Tomography (CT) scans. The optimised ISA method was able to generate realistic 3D pore networks corresponding to those seen in asphalt mixtures in term of visual, topological, statistical and air void shape properties. It was found that, in the range of porous asphalt materials investigated in this research, the high dispersion in hydraulic conductivity at constant air void content is a function of the average air void diameter. Finally, the relationship between average void diameter and the maximum aggregate size and gradation in porous asphalt materials was investigated.
Aggregate angularity plays a critical role in the performance of asphalt mixtures. Current asphalt mix design methods do not specify compaction effort according to aggregate angularity or locking points. This study utilised two methods of compaction, gyratory (Superpave Gyratory Compactor) and impact (Marshall Hammer), to investigate the influence of aggregate angularity on the locking point of asphalt mixtures. For gyratory compaction, densification curves were used to determine the gyratory locking point. For impact compaction, an accelerometer was connected to the Marshall Hammer to register asphalt response, by which the impact locking point was determined. Five mixtures were designed and produced in the laboratory with different aggregate angularities. The mixtures were tested for their locking points and then related to aggregate angularities. Results show that locking point could be determined for most of the mixtures in the study by the gyration and impact methods, except for the mixture composed of uncrushed, round aggregates compacted with the Marshall Hammer. The locking point of asphalt mixtures was highly related to aggregate angularity. A higher aggregate angularity resulted in a lower locking point. Mixtures produced with only crushed aggregates showed locking points much lower than mixtures made with both crushed and uncrushed aggregates, while mixtures made with only uncrushed aggregates had much higher locking points than those made with both aggregates. The findings from this study indicate that compaction effort in asphalt mix design should be specified according to the locking point of the mixtures.
With the rapid growth of traffic, asphalt pavement with well performance conditions is urgently needed. Maintenance and renovation consume large amounts of natural aggregates and generate large amounts of solid waste which are negative to environment. For improving the performance of asphalt pavement, this study identified the effect of single morphological variable on the adhesion of aggregate and the mechanical properties of asphalt mixture. In order to obtain aggregates of single morphological variable, 9.5–13.2 grade of limestone and basalt were studied and manually divided into 5 groups respectively according to aggregate angularity in the Aggregate Imaging Measurement System (AIMS) specification. AIMS was used to accurately quantify the morphological characteristics of aggregates. Aggregates adhered by asphalt and AC-13 asphalt mixture with different aggregates morphological characteristic were prepared for the measurement of road performance. Results show that there are close linear relationships between angularity or sphericity of aggregate and the asphalt-aggregate adhesive property. It was also found that the relationship between angularity or sphericity of aggregate and mechanical properties such as Marshall stability and flexural strength tends to rise first and then decrease, demonstrating that there are “optimal angularity and sphericity” for Marshall stability and flexural strength. Accurately understanding morphological characteristics of aggregates is of great significance to the initial design stage of pavement. Therefore, it is suggested that the digital aggregate morphology test could be included in the experimental specifications of aggregates for achieving better road performance.
Road pavement friction is a main physical phenomenon of interaction between tires and the road surface; it provides skid resistance during accelerations and decelerations. The paper focuses on the effects, at laboratory scale, of geometry (in terms of fine aggregates - minimum/maximum size of 0.063/2 mm - content), mineralogy (basalt or metamorphic - minimum/maximum size of 8/16 mm) and shape (in terms on Flakiness and Shape index) on friction performance of asphalt wearing courses for road pavements, in terms of British Pendulum Number and Mean Texture Depth. Laboratory results were statistically treated and used to define predictive formulas which correlate friction characteristics and the explanatory variables considered by the study. The amount of fine particles in the mix and the shape of coarse aggregates demonstrated to affect friction properties and macro texture.
The tools and technique used in the Design of Experiments (DOE) have been proved successful in meeting the challenge of continuous improvement over the last 15 years. However, research has shown that applications of these techniques in small and medium-sized manufacturing companies are limited due to a lack of statistical knowledge required for their effective implementation. Although many books have been written in this subject, they are mainly by statisticians, for statisticians and not appropriate for engineers.Design of Experiments for Engineers and Scientists overcomes the problem of statistics by taking a unique approach using graphical tools. The same outcomes and conclusions are reached as by those using statistical methods and readers will find the concepts in this book both familiar and easy to understand. The book treats Planning, Communication, Engineering, Teamwork and Statistical Skills in separate chapters and then combines these skills through the use of many industrial case studies. Design of Experiments forms part of the suite of tools used in Six Sigma.Key features: Provides essential DOE techniques for process improvement initiatives Introduces simple graphical techniques as an alternative to advanced statistical methodsâ reducing time taken to design and develop prototypes, reducing time to reach the market Case studies place DOE techniques in the context of different industry sectors An excellent resource for the Six Sigma training program This book will be useful to engineers and scientists from all disciplines tackling all kinds of manufacturing, product and process quality problems and will be an ideal resource for students of this topic.Dr Jiju Anthony is Senior Teaching Fellow at the International Manufacturing Unit at Warwick University. He is also a trainer and consultant in DOE and has worked as such for a number of companies including Motorola, Vickers, Procter and Gamble, Nokia, Bosch and a large number of SMEs.
This study aims to investigate the relationship between different roller compaction modes and resulting mechanical performance. In order to accomplish these objectives, a trial asphalt pavement section was observed and monitored both during the paving and compaction processes. Asphalt mix was sampled from the paver to be used later in laboratory and evaluate the differences between compaction in-field and laboratory. Asphalt concrete mixes were prepared and compacted in the laboratory using a steel roller compactor, with varying factors that influence compaction in terms of compaction modes (static, vibratory and a combination of both), as well as effort and temperatures in order to attempt to simulate field compaction conditions. The identification of a harmonised compaction procedure for specimen fabrication able to produce mixtures with stiffness properties resembling field compacted mixtures was also investigated based on the performance-based test results.
The need to understand and predict the long- and short term behavior of railway ballast calls for extensive experimental programs as well as the development of reliable numerical models. From the modeling point of view, the granular nature of ballast in connection with the high onset speed of dynamic loading makes it ideal for the application of the Discrete Element Method (DEM). The paper describes the employment of the DEM to simulate ballast behavior in large-scale oedometric testing. Ballast grains are represented by convex polyhedral particles whose shapes are randomly generated via Voronoi tessellation. The effect of the aspect ratio of particles is investigated. A novel algorithm to compute repulsive contact force based on the intersecting volume of polyhedrons is presented. Crushing of grains is included via splitting the particles into smaller polyhedrons when a certain stress-based criterion is fulfilled. The results obtained from the model are compared with the published experimental investigations.
The field measurement and laboratory tests were conducted to investigate potential causes of in-place rutting. The results indicate that the major rutting is attributed to the decrease in thickness of middle and lower layer, and the driving lane shows a severer rutting. Inadequate compaction is a major cause for the final rutting depth. Also the aggregate gradation has a major contribution to rutting. The gradation between 1.18 and 4.75mm in sieve size becomes finer for the three layers. The upper and middle layer show an increase in asphalt content, but the lower layer presents a decrease in asphalt content.
Various degrees of degradation of aggregate particles can occur in hot-mix asphalt (HMA) production during the mixing and compaction process. This breakdown may significantly increase the dust fraction of the aggregate gradation relative to that of laboratory specimens. This increased dust fraction could cause a loss of voids in the mineral aggregate (VMA) and air voids. After evaluation of 22 variations of HMA mixtures during production and construction, it was determined that aggregate degradation did result from plant mixing and field compaction activities. Aggregate degradation varied somewhat with respect to where the mixture was sampled within the production and construction processes. Aggregate degradation resulting from binder extraction that uses the ignition method, relative to the solvent extraction method, was also examined. Mixture volumetric properties (i.e., air voids and VMA) were significantly affected by aggregate degradation. On the basis of this study, before a volumetrics specification for mixture acceptance is investigated for future implementation in Georgia, a change in the Georgia Department of Transportation's mix design specification is recommended. The minimum VMA values for Superpave® mix design should be increased by 1 % for all dense-graded mixes to compensate for the amount of aggregate degradation and loss of VMA during HMA production and construction. It is believed that this research-based policy change for mix design will better maintain the balance between proper durability and rut resistance of asphalt pavements in Georgia.
The microfabric discrete element modeling (MDEM) approach is used herein to predict the asphalt mixture complex modulus in extension/compression across a range of test temperatures and load frequencies. The method allows various constitutive models to be employed to describe particle and interface properties, such as normal and shear stiffness and strength. An uncalibrated two-dimensional (2-D) model was developed, and complex modulus predictions were compared to theoretical bounds on moduli. As expected, the uncalibrated 2-D model underestimates the significant stiffening effects of the coarse aggregate skeletal structure and predictions are found to be near the lower theoretical bounds, well below experimentally determined moduli. A technique was developed to calibrate the MDEM model to experimental results by dilating aggregates to create additional aggregate contact, which is believed to be more representative of the actual three-dimensional behavior. This method is shown to provide better modulus estimates across a range of test temperatures and load frequencies compared to more traditional calibration methods. As future modeling efforts are extended to three dimensions, the degree of model calibration required should be greatly reduced.
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