This study describes experimental and analytical methods to quantify the structure of air voids in asphalt mixes. An x-ray computed tomography system along with image analysis techniques were used to measure air void sizes at different depths within asphalt mix specimens. The statistical analyses performed validated the applicability of the Weibull model for describing the air void distribution. Consequently, the Weibull model was used to quantify the effect of the compaction effort, method of compaction, and aggregate size distribution on air voids. The air void size distribution in Superpave gyratory compacted specimens was found to exhibit a “bathtub” shape, whereby larger voids were present at the top and bottom parts of a specimen. This shape was more pronounced at higher compaction efforts. The method of compaction was significant in influencing the air void size distribution. Specimens prepared using the Superpave gyratory compactor with different aggregate sizes were found to have noticeably diffe...

Characterization of air void distribution in asphalt mixes using x-ray computed tomography

This study addresses the relationship between fine aggregate shape properties and the performance of hot-mix asphalt. Aggregate shape was expressed as three independent properties: form, angularity, and texture. Image analysis procedures and indices were developed to capture these properties. The indices were measured for 22 aggregate samples. The samples were related to the hot-mix asphalt rutting resistance measured under wet and dry conditions in the Purdue wheel-tracking device. Among the different aggregate shape properties, texture had the strongest correlation with rutting resistance. Resistance to rutting increased exponentially with an increase in aggregate texture.

Correlation of fine aggregate imaging shape indices with asphalt mixture performance

This paper is a review of the use of particulate discrete element modeling (DEM) in geomechanics. The overall objective of the paper is to serve as an introduction to researchers and practitioners in geomechanics who are considering adopting DEM in their work or using the results of DEM simulations to guide other studies, for example, the development of constitutive models for continuum-based numerical analysis. It is hoped that prior converts to the use of DEM will also benefit from a relatively objective overview of current DEM use in geomechanics. The introductory sections present the background to the method and give an overview of the evolution of the use of particulate DEM in recent geotechnical research. The general principals of the algorithm are then presented, considering the types of particles typically used, the calculation of contact forces, and formulation of simulation boundary conditions. Some techniques available to interpret and postprocess of DEM results and provide the information to l...

Particle-Based Discrete Element Modeling: Geomechanics Perspective

This paper presents a viscoelastic model of asphalt mixtures with the discrete element method, where the viscoelastic behaviors of asphalt mastics (fine aggregates, fines, and asphalt binder) are represented by a Burger's model. Aggregates are simulated with irregular shape particles consisting of balls bonded together by elastic contact models, and the interplaces between aggregates are filled with balls bonded with viscoelastic Burger's model to represent asphalt mastic. Digital samples were prepared with the image analysis technique. The micromechanical model was developed with four constitutive laws to represent the interactions at contacts of discrete elements (balls) within an aggregate, within mastic, between an aggregate and mastic, and between two adjacent aggregates. Each of these constitutive laws consists of three parts: a stiffness model, a slip model, and a bonding model in order to provide a relationship between the contact force and relative displacement and also in order to describe slipping and tensile strength at a particular contact. The relationship between the microscale model input and macroscale material properties was derived, and an iterative procedure was developed to fit the dynamic modulus test data of asphalt mastic with Burger's model. The favorable agreement between the discrete element prediction and the lab results on dynamic moduli and phase angles indicates that the viscoelastic discrete element model developed in this study is very capable of simulating constitutive behavior of asphalt mixtures.

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Viscoelastic Model for Discrete Element Simulation of Asphalt Mixtures

The objective of this study is to experimentally and theoretically estimate the strain distribution in an asphalt binder as it exists in hot mix asphalt (HMA). This is important to evaluate whether the strain magnitudes can cause nonlinear behavior for the binder and HMA. The experimental procedure relies on capturing images of the surface of an HMA specimen during deformation. A computer algorithm is then used to calculate the strain values within an image. Finite-element analysis of the internal structure of HMA is also used to estimate the strain distribution. The results are shown to have good correlation with the experimental measurements. However, due to limitations imposed by the image resolution, the strain distribution is computed within areas that include a combination of binder and fine particles (mastic) rather than the binder phase. Consequently, micromechanics analysis of the mastic is used to calculate the binder strain. The results demonstrate that the binder strain magnitudes could reach high values well into the range of nonlinear behavior of the binder.

Modeling and experimental measurements of strain distribution in asphalt mixes

The performance of asphalt concrete (AC) mixtures is influenced by the arrangement of aggregates and their associated air voids. Parameters to measure aggregate orientation, aggregate gradation, and air void distribution in AC mixes are proposed. Computer automated image analysis procedures were used to measure these parameters. The air void distribution was characterized using X-ray tomography images. The new parameters were used to study the evolution of the internal structure of AC mixes during laboratory compaction by the Superpave Gyratory Compactor and in the field. The preferred orientation of the aggregate structure in the laboratory was found to increase with compaction up to a certain compaction effort. Thereafter, the aggregate structure tended to have more random orientation. Percent voids measured on X-ray tomography images compared well with percent voids measured in the laboratory. The void distribution in the specimens was found to be nonuniform. More internal voids were concentrated at the top and the bottom portions of the gyratory compacted specimen. The gyratory compacted specimens reached the initial aggregate orientation of the field cores at a higher number of gyrations whereas they reached the percent air voids in cores at a lower number of gyrations. Coarse aggregate gradation of gyratory compacted specimens was well captured using the image analysis techniques. There was no change in gradation with compaction.

Internal Structure Characterization of Asphalt Concrete Using Image Analysis

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. Ho...

Micromechanical Modeling of the Viscoelastic Behavior of Asphalt Mixtures Using the Discrete-Element Method

The coefficient of thermal expansion (CTE) is a fundamental property of concrete. It has long been known to have an effect on joint opening and closing in jointed plain concrete pavement, crack formation and opening and closing in continuously reinforced concrete pavement, and curling stresses and thermal deformations in both types of pavements. However, it has not been included as a variable either in materials specifications or in the structural design of concrete pavements. Hundreds of cores were taken from Long-Term Pavement Performance sections throughout the United States and were tested by FHWA's Turner-Fairbank Highway Research Center laboratory, using the AASHTO TP 60 test procedure. The CTE values were then assimilated into groups on the basis of aggregate types, and the mean and range of CTE were calculated. These results were then used in the new mechanistic-empirical pavement design guide to determine the significance of the measured range of CTE on concrete pavement performance. The CTE of t...

Measurement and Significance of the Coefficient of Thermal Expansion of Concrete in Rigid Pavement Design

Asphalt pavement is a complex material that consists of aggregates, air voids and asphalt. The performance of asphalt concrete mixtures is influenced by its internal structure, which refers to the arrangement of aggregates and their associated voids. This paper focuses on developing computer automated image analysis procedures and parameters to quantify the aggregate orientation and segregation in asphalt concrete mixes. Aggregate orientation is described by a directional function with its components determined experimentally. Aggregate segregation is quantified by means of spatial statistics. The new parameters of orientation and segregation are used to study the variation of aggregate orientation and segregation in an asphalt concrete mix during compaction using the Superpave Gyratory Compactor (SGC). The measured parameters of the SGC compacted specimens are compared with those of field cores

Aggregate orientation and segregation in asphalt concrete

External pressures applied to a saturated pavement pore structure are often dynamic due to the repeated tire loading. Therefore, a dynamic permeability constant is more realistic representation of the response of a pavement pore structure than the Darcy’s permeability. To investigate the unsteady (dynamic) fluid flow in asphalt pavements, a 3D fluid flow model was developed using the lattice Boltzmann (LB) method. The model was validated using the wellknown closed form solution of oscillating flow through a circular tube. A number of simulations were carried out to calculate the dynamic permeabilities of different asphalt specimens exposed to pulsatile pressures. It was shown that the dynamic permeability of an asphalt pore structure collapse on a single curve for a given frequency, confirming its universal behavior.

Tom Harman

Papers

Internal Structure Characterization of Asphalt Concrete Using Image Analysis

Micromechanical Modeling of the Viscoelastic Behavior of Asphalt Mixtures Using the Discrete-Element Method

Measurement and Significance of the Coefficient of Thermal Expansion of Concrete in Rigid Pavement Design

Aggregate orientation and segregation in asphalt concrete

Dynamic hydraulic conductivity (permeability) of asphalt pavements