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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]

Stone-on-stone contact of the coarse-aggregate fraction is one of the main characteristics of permeable friction course (PFC) asphalt mixtures that is required to provide adequate resistance to both raveling and permanent deformation. Currently, stone-on-stone contact is determined by comparing the air voids content in the coarse aggregate (VCA), assessed in both the dry-rodded condition ([VCA.sub.DRC]) and the compacted PFC mixture ([VCA.sub.mix]). The underlying assumption is that the coarse aggregate of a compacted PFC mixture with [VCA.sub.mix] equal to [VCA.sub.DRC] would develop a stone-on-stone contact condition equivalent to that existing in the dry-rodded aggregate. This study focused on proposing enhancements for the quantitative determination of stone-on-stone contact of PFC mixtures. The assessment supported on both laboratory testing and application of the discrete element method and image analysis techniques, led to recommendation of a criterion to determine the breaking-sieve size. In addition, verification of stone-on-stone contact using a maximum VCA ratio of 0.9 was recommended to ensure the design and construction of PFC mixtures with fully developed stone-on- stone contact. DOI: 10.1061/(ASCE)MT.1943-5533.0000117 CE Database subject headings: Asphalt pavements; Porous media; Aggregates; Mixtures. Author keywords: Asphalt pavements; Porous pavements; Asphalts; Aggregates.

Moisture damage in asphalt mixtures is defined as the gradual loss of structural integrity caused by the presence of moisture. A simple experimental procedure was developed in this study to measure water vapor diffusion coefficients in coarse aggregates, fine aggregate mixture (blend of the fine portion of the aggregates with the asphalt binder), and hot-mix asphalt. The procedure is based on periodic weight measurements of specimen-container ensembles subjected to a controlled temperature and relative humidity environment. Fick's first law was used to estimate the diffusion coefficients of the materials. The results show that the proposed experimental method is an economic and efficient tool to quantify water vapor diffusion coefficients. Determining these material properties is fundamental to develop numerical models to study the deleterious effects of moisture vapor on the mechanical performance of asphalt mixtures. In order to exemplify the significance of the experimental measurements, a numerical simulation of transient moisture diffusion within the cross section microstructure of an asphalt mixture was conducted. The results of the simulations suggest that diffusion coefficients are fundamental for tracking the potential of a mixture to develop moisture-related degradation processes. Furthermore, it was observed that under the same environmental conditions, moisture damage in an asphalt mixture is highly influenced by the moisture diffusion coefficient of its constitutive phases. DOI: 10.1061/(ASCE)MT.1943-5533.0000059 CE Database subject headings: Water; Diffusion; Coefficients; Asphalt pavements; Aggregates; Finite element method; Simulation. Author keywords: Water; Diffusion; Coefficients; Asphalt pavements; Aggregates; Finite-element method.

Aggregates are the most widely used construction materials in the world in structures built from both asphaltic and portland cement concrete composites. The performance of these composites is affected by aggregate shape characteristics (e.g., angularity, texture, and dimensions). The aggregate imaging system (AIMS) is a computer automated system that was developed to measure aggregate shape characteristics using digital camera images of aggregates. This paper addresses four issues concerning AIMS measurements: (1) enhanced ways of handling and classifying the large data sets typically generated; (2) enhanced automation in processing fine aggregate images that appear to contain touching particles; (3) an improved consideration of measurement variability or repeatability between different operators, different AIMS units, and for aggregate placement and orientation; and (4) comparison of AIMS dimensional measurements with true three-dimensional measurements using X-ray computed tomography on a large coarse aggregate data set. The various aggregate sets used in this study covered a broad range of angularity and texture. The AIMS measurements of the ratios of dimensions were found to have excellent agreement with the more accurate X-ray computed tomography values, but the measurements of the individual dimensions were systematically low by about 10%. DOI: 10.1061/(ASCE)MT.1943-5533.0000033 CE Database subject headings: Aggregates; X rays; Material properties; Measurement; Imaging techniques. Author keywords: Aggregate; Texture; Angularity; Imaging; X-ray computed tomography; Material technology.

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. DOI: 10.1061/(ASCE)MT.1943-5533.0000031 CE Database subject headings: Moisture; Damage; Numerical models; Asphalts; Mixtures; Micromechanics. Author keywords: Moisture damage; Numerical modeling; Cohesive zone; Asphalt mixtures; Damage; Moisture diffusion; Micromechanics; Adhesion.

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. CE Database subject headings: Aggregates: Discrete elements; Asphalts; Mixtures; Cracking; Design. Author keywords: Aggregate; Discrete element method; Asphalt mixture: Fracture: Internal structure. DOI: 10.1061/(ASCE)MT.1943-5533.0000005

Different methods have been developed to assess fatigue cracking of asphalt mixtures based on dissipated energy. Most of these methods have been motivated by the need to develop a unified fatigue criterion that is independent of the mode of loading. This paper offers critical analyses of the energy methods based on their theoretical ability to: (1) unify the results from controlled-strain and controlled-stress modes of testing for the same material; and (2) accurately assess the fatigue cracking life of different materials. The efficacy of these methods is quantitatively compared using a common set of fatigue test data. The fatigue test data were obtained using the dynamic mechanical testing of three different mixtures that have been shown to exhibit different fatigue cracking resistance in the field. DOI: 10.1061/(ASCE)0899-1561(2009)21:2(83) CE Database subject headings: Energy methods; Fatigue; Viscoelasticity; Asphalts.

In the past few years, a number of full-depth or perpetual pavements have been designed and constructed in the State of Texas. A study was conducted to examine the quality of the compaction of the thick asphalt layers within these pavements using advanced forensic tools such as X-ray computed tomography (X-ray CT) and ground penetrating radar (GPR). The GPR is a nondestructive tool for evaluating the uniformity of density in pavements at highway speed. X-ray CT is a laboratory tool that is used to conduct detailed analysis of air void distribution and uniformity in asphalt pavement cores. This paper presents the results of analyzing one of the perpetual pavements constructed in State Highway 114 (SH-114). In this project, two different structural asphalt pavement sections were placed, one included a 1 in. (25.4 mm) stone filled (SF) Superpave mix and the other included a traditional dense graded Type B material. The dense graded Type B material was found to be uniformly compacted. However, major compaction problems were identified with the coarse SF Superpave mix. The poor compaction and associated high percent air vsoids were found to permit moisture infiltration, which could potentially lead to rapid pavement deterioration. The analysis showed very good agreement between the GPR and X-ray CT results and demonstrated the efficiency of using GPR and X-ray CT in the evaluation of asphalt pavement compaction. CE Database subject headings: Asphalt pavements; Compaction; Moisture; Radar; Tomography.

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. DOI: 10.1061/(ASCE)0899-1561(2007)19:11(977) CE Database subject headings: Aggregates; Imaging techniques; Degradation; Mixtures.

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. DOI: 10.1061/(ASCE)0899-1561(2007)19:10(912) CE Database subject headings: Asphalts; Viscoelasticity; Anisotropy; Creep.

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. DOI: 10.1061/(ASCE)0899-1561 (2007) 19:8(655) CE Database subject headings: Asphalt mixes; Voids; Fracture mechanics; Moisture; Damage assessment; Energy dissipation; Imaging techniques; X rays.

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. DOI: 10.1061/(ASCE)0733-9399(2006)132:2(158) CE Database subject headings: Asphalt mix; Micromechanics; Elasticity; Finite elements.

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. DOI: 10.1061/(ASCE)0733-9399(2005)131:1(48) CE Database subject headings: Viscoplasticity; Damage; Asphalt pavements; Asphalt mixes; Anisotropy; Deformation; Image analysis; Models.

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. CE Database subject headings: Asphalt mixes; Aggregates; Stability; Compaction.

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. DOI: 10.1061/(ASCE)0899-1561(2004)16:5(487) CE Database subject headings: Numerical models; Simulation; Asphalt mixes; Permeability; Agregates.

Achieving proper compaction of asphalt pavement is crucial to its longevity and acceptable performance. Laboratory compaction is an essential part of mix design. In order for the mix design process to be effective, laboratory compaction must adequately simulate field compaction. This study used mechanical properties measured with the Superpave shear tester 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 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. DOI: 10.1061/(ASCE)0899-1561(2004)16:1(78) CE Database subject headings: Asphalts; Compaction; Data analysis; Mechanical properties; Asphalt pavements.

This paper presents image analysis techniques by which to characterize the texture, angularity, and form of aggregate particles used in highway construction and geotechnical applications. For texture analysis, wavelet decomposition in gray scale images of particles is performed. The results demonstrate that multiscale wavelet representation is a powerful tool by which to capture the texture and to differentiate 'true' texture from 'false' texture caused by variations of natural color on a particle surface. Angularity and form analyses of particles are done using binary images. A gradient-based method is employed to describe angularity. This method is shown to differentiate between particles with different angularity characteristics. Form analysis of the particles includes computing the shape factor and sphericity index, which are based on measurements of the shortest, intermediate, and longest axis of the particle. Particle thickness is measured using the feature of an autofocus microscope. The width and length are calculated by an eigenvalue decomposition method of two-dimensional particle projections. Details of an interactive software developed to compute the different aggregate shape factors are discussed. The results indicate that these calculated values of the particle dimensions match very closely the values measured manually using a digital caliper. CE Database subject headings: Aggregates; Image analysis; Asphalt mixes; Highway construction.

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. CE Database keywords: Asphalt mixes; Anisotropy; Stiffness; Imaging techniques; Finite elements; Micromechanics.

A procedure for constructing resin impregnated sand specimens for fabric (microstructure) analysis is presented. The impregnation device provides excellent control of the impregnation rate and confinement for different types of soils. Such control is necessary to minimize disturbance to the soil fabric during impregnation. The proposed technique also allows monitoring of the uniformity of impregnation throughout the process. Image analysis techniques are used to examine the variation of porosity at the microscopic and macroscopic levels within specimens. The analysis shows that about twice the deviation in local porosity from average porosity is obtained when microscopic measurements are used, as compared with the macroscopic measurements. Soil fabric anisotropy is characterized using two directional functions. Each of these functions has two components: an average value that describes the isotropic condition and a deviatoric tensor that represents the deviation from the average. They are shown to capture the relative distribution of air voids and their connectivity in different directions. The functions are also used to simulate the three-dimensional anisotropic fabric of soils in a computer.