Laith Tashman

Correlation of fine aggregate imaging shape indices with asphalt mixture performance

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.

X-RAY TOMOGRAPHY TO CHARACTERIZE AIR VOID DISTRIBUTION IN SUPERPAVE GYRATORY COMPACTED SPECIMENS

Air void distribution has considerable influence on the mechanical properties of asphalt mixtures. Several factors such as the compaction effort, method of compaction, aggregate gradation, and aggregate shape control the air void distribution. An X-ray computed tomography (CT) system along with image analysis techniques are used in this study for non-destructive characterization of air void distribution in gyratory specimens prepared using different gradations and compaction efforts. The air void distributions in gyratory specimens are quantified using parameters that describe the change in percent and volume of air voids along the horizontal and vertical directions. Air voids are shown to be more concentrated in the top and bottom regions that are in contact with the base plates, as well as in the outer region that is in contact with the mold. The non-uniformity of the distribution increases with an increase in compaction effort. The difference in aggregate gradations used in this study is shown to have just a slight influence on the air void distribution.

Microstructure Characterization for Modeling HMA Behaviour using Imaging Technology

ABSTRACT The properties of hot mix asphalt (HMA) highly depend on the microstructure. The ability to experimentally characterize the microstructure is fundamental in describing the macroscopic behavior of HMA. It improves our understanding of the influence of the microstructure distribution on the macroscopic response of the material, which is at the heart of constitutive modeling of granular materials. This paper is an attempt to summarize the recent advances in imaging technology and its applications to the characterization of HMA microstructure including aggregate orientation distribution, aggregate contact, aggregate shape properties, air void distribution, permeability, crack distribution, three-dimensional (3-D) microstructure reconstruction, and microstructure evolution. The microstructure of HMA is linked to the macroscopic behavior of the material within the framework of continuum modeling via microstructure tensors that are incorporated in the continuum modeling formulations. It is envisioned that this link will fill in the gap that has always been present between interpreting laboratory results and the reality of the interior behavior of HMA.

Spatial and Directional Distribution of Aggregates in Asphalt Mixes

This study presents experiments and analytical methods to capture and quantify the directional and spatial aggregate distributions in hot mix asphalt (HMA). The quantifying parameters were obtained by processing and analyzing images of HMA sections. An experimental technique in which HMA sections were treated with hydrofluoric acid was developed in order to improve the contrast between aggregates and asphalt prior to capturing an image. The developed methods were used to analyze the aggregate distribution in mixes compacted in the laboratory using the Superpave gyratory compactor (SGC) under different compaction variables, and in the field using different compaction patterns. The field cores exhibited similar aggregate distributions irrespective of the compaction pattern, whereas the angle of gyration and specimen height in the SGC were found to influence significantly the aggregate distribution. Laboratory specimens compacted to a height close to that of field cores, and using an angle of gyration of 1.25°, were better in simulating the aggregate distribution in the field.

Evaluation of Aggregate Characteristics Affecting HMA Concrete Performance

This report documents the outcomes of the ICAR study on the Evaluation of Aggregate Characteristics Affecting HMA Concrete Performance. This study was conducted with support from the Federal Highway Administration (FHWA) program on Simulation, Imaging, and Mechanics of Asphalt Pavements at Texas A&M University. The first outcome includes assessment of HMA sensitivity to aggregate shape characteristics. Aggregate shape is characterized through detailed measurements of angularity, form, and texture using the Aggregate Imaging System (AIMS). The shape characteristics are presented in terms of the distribution of the property in an aggregate sample rather than an average index of this property. The second outcome of this study is the development of a viscoplastic model for permanent deformation. The model accounts for the aggregate structure in the mix, which is related to the shape properties measured using AIMS. The model capabilities are demonstrated through matching the results of testing various mixes from the Accelerated Loading Facility (ALF) of the FHWA using the triaxial creep and strength tests. In addition, the model is used to predict the response of mixes that include aggregates with different shape characteristics in order to develop relationships between the model parameters and aggregate shape characteristics. As part of the model development, an experiment was conducted to capture and characterize damage evolution in HMA due to permanent deformation. HMA specimens were loaded using a triaxial compression setup to four predefined strain levels at three confining pressures. Consequently, image analysis techniques were used to analyze damage distribution. The results of the damage experiment supported the damage evolution function proposed in the viscoplastic model.

Simulation of fluid flow in granular microstructure using a non-staggered grid scheme

Abstract A finite difference numerical scheme has been utilized to simulate fluid flow in granular microstructures. The pixels of their digital images represent the granular microstructure in the finite difference grid. The scheme utilizes a non-staggered grid arrangement, which requires only one finite difference mesh to solve the governing fluid flow equations. As such, the scheme is more efficient when it comes to dealing with non-orthogonal coordinates and complex geometry of boundary conditions such as that of granular microstructure. The numerical scheme is verified by comparing the permeability values of a medium of packed columns to a closed form solution. It is then used to evaluate the permeability coefficients of idealized and natural granular microstructures. It has been found that as the directional aspect ratio increases, the resistance of a particle to fluid flow increases, which results in a decrease in the permeability coefficient. A medium of elliptical particles has higher permeability coefficient than a medium of rectangular particles for the same porosity because of its lower surface area. The permeability anisotropy has been found to increase with an increase in the aspect ratio or a decrease in porosity. Spherical glass beads have been found to have higher permeability coefficients than Ottawa sand and Silica.

Superpave® Laboratory Compaction Versus Field Compaction

Laboratory compaction is an important part of asphalt mix design. For the mix design process to be effective, laboratory compaction must adequately simulate field compaction. In this study mechanical properties measured with the Superpave® shear tester were used to evaluate field compaction and laboratory compaction. The field compaction consisted of three test sections with different compaction patterns. The laboratory compaction used the Superpave gyratory compactor with adjustments to several parameters. Results of this study indicate that current gyratory protocol produces specimens with significantly different mechanical properties than those of field cores produced with the same material and compacted to the same air voids. Results also show that adjustments to certain parameters of the gyratory can produce specimens that better simulate the mechanical properties of pavement cores.

Anisotropic Viscoplastic Continuum Damage Model for Asphalt Mixes

Permanent deformation is one of the most significant distresses that causes severe damage in asphalt concrete (AC) pavements. It is caused by high traffic loads associated with high field temperatures. It is believed that permanent deformation develops at a small rate and accelerates with the initiation of microcracks in asphalt pavements. An anisotropic viscoplastic continuum damage model is developed to describe the permanent deformation of asphalt pavements. The model is based on Perzynas formulation with Drucker-Prager yield function modified to account for the material inherent anisotropy. The material anisotropy parameter is measured through microstructural analysis of two-dimensional sections of asphalt mixes. Furthermore, a damage parameter is included in the model in order to quantify the nucleation of microcracks that develop to macrocracks at later stages. The models parameters are all determined from strength and static creep tests.