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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.

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... 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. ...
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Chapter
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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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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