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The soils across Lake Texcoco (LT) basin are notorious for the presence of highly compressible salt-affected lacustrine clays. The geological formation of the area accompanied by the unmonitored flow of water from Mexico City region has resulted in a unique sodium-rich clay that is highly plastic (plasticity index~200), and impermeable ( k~1.5 × 10−9 m/s) and possesses low compressive strength (unconfined compressive strength

Lime stabilization is a common technique used to improve the engineering properties of clayey soils. The process of lime stabilization can be split into two parts. First, the mobilization and crowding of [Formula: see text] ions or [Formula: see text]molecules from hydrated lime at net negative surface charge sites on expansive clay colloids. Second, the formation of pozzolanic products including calcium-silicate-hydrate (C-S-H) because of reactions within lime-soil mixtures. The pozzolanic reaction is generally considered to be more durable, while the [Formula: see text] adsorption has been associated with more easily reversible consistency changes. This study offers a protocol to assess whether the stabilization process is dominated by durable C-S-H (pozzolanic) reactions or a combination of cation exchange and pozzolanic reactions. Expansive clays with plasticity indices >45% from a major highway project in Texas are the focus of lime treatment in this study. The protocol consists of subjecting lime-soil mixtures to a reasonable curing period followed by a rigorous but realistic durability test and investigating the quality and quantity of the pozzolanic reaction product. Mineralogical analyses using quantitative X-ray diffraction (XRD) and thermogravimetric analysis (TGA) indicates the formation of different forms of C-S-H. In addition, geochemical modeling is used to simulate the lime-soil reactions and evaluate the effect of pH on the stability of C-S-H. The results indicate C-S-H with Ca/Si ratio of 0.66 as most the stable form of C-S-H among other forms with Ca/Si ratio ranging from 0.66 to 2.25. The effect of reducing equilibrium pH on C-S-H is also evaluated. A reduction in pH favored dissolution of all forms of C-S-H indicating the need to maintain a pH ≥ 10.

The slopes along the Friant-Kern Canal were last treated in the 1970s with 4% quick lime to mitigate issues related to slope failure caused by expansive Porterville soils. The immediate benefits of lime treatment were well documented by the Bureau of Reclamation. However, questions remain over the long-term durability of lime-treated materials. In this study, we compare the engineering properties and changes in the soil mineralogy of treated and untreated sections to establish the effectiveness of lime after more than 40 years of performance. A geochemical model was developed using the GEM-Selektor program to simulate the geochemical reactions in the soil-lime system and predict stable pozzolanic products. The experimental results show a reduction in the plasticity index from 23 to 6 after lime treatment together with a tenfold increase in strength. Lime addition lowers the risk of volumetric expansion and erosion in soils from moderately high to very low. Further, a pH increase from 6.30 to 8.90 in lime-treated sections indicates that lime treatment continues to be effective. X-ray fluorescence analysis shows the presence of Ca2+ ions in quantities similar to the initial treatment dosage indicating negligible leaching of lime. The geochemical model provides evidence of the formation of pozzolanic products in the soil-lime system which was validated using thermogravimetry analysis. The performance history of the Friant-Kern Canal together with the findings of this study affirm the long-term durability of lime treatment on this project and strengthens the case for using lime in the repair of hydraulic structures.

The prediction of stress, strain, and permanent deformation (rutting) in granular layers is a challenging task and requires sophisticated constitutive relationships. This paper uses a mechanistic-based model to predict the rutting of granular layers in airfield pavements when subjected to different tire pressures. This model incorporates Drucker–Prager (D-P) plasticity and includes an evolving hardening function that accounts for the changes in the hardening function in cyclic loading. The developed model is implemented in Pavement Analysis Using the Nonlinear Damage Approach—Airfield Pavements software developed for performance prediction of airfield pavements. Using this model, it was possible to predict the permanent deformation of airfield pavements when subjected to C-17 and C-130 aircraft wheels. The model was validated against the test sections tested at the Engineering Research and Development Center for two different wheel loads.

This paper proposes a straightforward procedure to characterize the nonlinear viscoelastic response of asphalt concrete materials. Furthermore, a model is proposed to estimate the nonlinear viscoelastic parameters as a function of the triaxiality ratio, which accounts for both confinement and deviatoric stress levels. The simplified procedure allows for easy characterization of linear viscoelastic (LVE) and nonlinear viscoelastic (NVE) responses. First, Schapery’s nonlinear viscoelastic model is used to represent the viscoelastic behavior. Dynamic modulus tests are performed to calibrate LVE properties. Repeated creep-recovery tests at variable deviatoric stress levels (RCRT-VS) were designed and conducted to calibrate the nonlinear viscoelastic properties of four types of mixtures used in the Federal Aviation Administration’s National Airport Pavement and Materials Research Center test sections. The RCRT-VS were conducted at 55°C, 140 kPa initial confinement pressure, and wide range of deviatoric stress levels; mimicking the stress levels induced in a pavement structure under traffic. Once calibrated, the model was validated by comparing the model predictions and experimental measurements at different deviatoric stress levels. The predictions indicate that the proposed method is capable of characterizing NVE response of asphalt concrete materials.

Reliability has been incorporated in many pavement design procedures to account for the effects of inputs variabilities and uncertainties on predicted performance. The American Association of State Highway and Transportation Officials (AASHTO) mechanistic empirical pavement design guide (MEPDG) computes the reliability of pavement sections with the assumption that the variability of predicted distresses follows normal distribution. This approach does not account for the systematic contribution of each design input variability on the overall output variance. This paper evaluates a two-component reliability analysis methodology for pavement application. The two-component reliability analysis methodology uses a response surface method (RSM) for a surrogate model generation and the first order reliability method (FORM) for reliability computation. Three different response surface methods (central composite, Box–Behnken and Doehlert designs) were implemented and statistically verified for their suitability for surrogate model generation. The two-component reliability analysis methodology was further utilized for the generation of partial safety factors for the development of a load and resistance factor design (LRFD) procedure for pavement applications. Field pavement sections with a wide range in design inputs and target reliabilities were used to evaluate the proposed reliability analysis methodology. The results have shown that the three RSM can be used effectively for pavement reliability problems.

Asphalt concrete is a highly heterogeneous mixture with complicated microstructures. This heterogeneity strongly affects the overall three-dimensional (3D) mechanical behavior and damage-associated performance of asphaltic mixtures, which makes accurate modeling a challenge. This paper presents a computational microstructure model using multiple two-dimensional (2D) finite element microstructures as an alternative and efficient approach to predict the actual 3D fracture-associated response of asphaltic mixtures. To simulate crack initiation and propagation, an autonomous algorithm was developed to efficiently generate multiple 2D microstructures using image processing of scanned microstructures, a phase-based segmentation module to separate particles and the surrounding matrix phase, and a finite element meshing module that allows cohesive zone elements to be embedded within the mixture microstructure. Aggregates were considered elastic, but viscoelastic and fracture properties were used to model the binding matrix phase. The validity of the multiple 2D microstructures approach was statistically investigated by comparing the model simulation results with the corresponding experimental results from two cases: (1) a three-point bending beam testing of a gap-graded mixture; and (2) a semicircular bending beam testing of a conventional dense-graded mixture. The 2D simulation results of multiple microstructures could generally capture 3D viscoelastic fracture behavior, but the prediction power of the modeling was reduced when volume fraction, distribution of coarse aggregates became high and complex. With some limitations to be further resolved, this study implies that multiple 2D microstructures can appropriately represent the complex viscoelastic-fracture behavior of 3D mixtures, which can significantly reduce the experimental and computational costs for laboratory mixture tests and 3D microstructure simulations with fracture, respectively.

Rapid traffic loading induces pore water pressure inside partially or fully saturated interconnected cracks and voids of asphalt concrete. The induced pore pressure contributes extra stresses within the pavement and accelerates crack evolution and propagation. Crack propagation facilitates diffusion of moisture through the solid phase and accelerates the degradation of the time-dependent stiffness and strength of asphalt concrete. Therefore, it is imperative to consider the coupled effects of pore water pressure, moisture diffusion, and mechanical loading on asphalt concrete pavement. The effect of pore water pressure was considered by using the effective stress concept inside deformable media. Biot’s approach was used and coupled to the nonlinear viscoelastic and viscodamage (moisture and mechanical) constitutive relationships for asphalt concrete. The models were implemented in PANDA, a finite element code developed at Texas A&M University. Capabilities of the proposed framework and constitutive relationships were demonstrated through the simulation of several realistic microstructural representations of asphalt concrete. The results of numerical simulations demonstrated how the effect of pore water pressure can intensify damage within asphalt concrete and reduce its strength. Therefore, this outcome emphasizes the importance of incorporating the effect of pore water pressure in investigating the response of asphalt concrete.

The fracture resistance of a chemically stabilized base or subbase layer is important to the durability and sustainability of a pavement structure. Thus, an appropriate test protocol to characterize the fracture resistance of stabilized bases, subbases, and subgrade soils is essential to the design of pavement materials and structures. This paper proposes a protocol developed on the basis of the semicircular bending test to measure fracture resistance (i.e., fracture energy and fracture toughness) of chemically stabilized material. The effects of three test variables, including specimen thickness, notch length, and loading rate, on fracture properties were investigated, and appropriate values for these test variables were selected for the semicircular bending test protocol. The proposed semicircular bending test method was successful in characterizing the fracture resistance of three chemically stabilized materials. To address fracture properties of the chemically stabilized material more definitively, three-dimensional zone modeling was used and the simulations agreed very well with the experimental results. Both the fracture properties obtained from the experiment and the cohesive zone modeling indicated that polymer-stabilized limestone exhibited a much higher fracture resistance than cement-stabilized limestone and cement-stabilized sand. However, the polymer used demonstrated susceptibility to degradation in the presence of water. Correction of this limitation is the focus of ongoing research on this type of polymer.

Rutting performance of airfield pavements is predicted using mechanistic-based constitutive relationships for both asphalt and granular layers. Pavement analysis using nonlinear damage appr...

It is well known that the accurate prediction of long term performance of asphalt concrete pavement requires modeling to account for viscoelasticity within the mastic. However, accounting for viscoelasticity can be costly when the material properties are measured at the scale of asphalt concrete. This is due to the fact that the material testing protocols must be performed recursively for each mixture considered for use in the final design. In this paper, a four level multiscale computational micromechanics methodology is utilized to determine the accuracy of micromechanics versus directly measured viscoelastic properties of asphalt concrete pavement. This is accomplished by first measuring the viscoelastic dynamic modulus of asphalt binder, as well as the elastic properties of the constituents, and this comprised the first scale analysis. In the second scale analysis, the finite element method is utilized to predict the effect of mineral fillers on the dynamic modulus. In the third scale analysis, the finite element method is again utilized to predict the effect of fine aggregates on the dynamic modulus. In the fourth and final scale analysis, the finite element method is utilized to predict the effect of large aggregates on the dynamic modulus of asphalt concrete. This final predicted result is then compared to the experimentally measured dynamic modulus of two different asphalt concretes for various volume fractions of the constituents. Results reveal that the errors in predictions are on the order of 60 %, while the ranking of the mixtures was consistent with experimental results. It should be noted that differences between the “final predicted results” and the experimental results can provide fruitful ground for understanding the effect of interactions not considered in the multiscale approach, most importantly, chemical interactions.

This paper presents findings from a study conducted to evaluate changes in the microstructure of asphalt binder resulting from aging as well as the effect of these changes on the evolution of damage resulting from tensile deformations. Two types of asphalt binders were aged with the rolling thin film oven and pressure aging vessel aging techniques. The microstructure and the microrheology of the six binders were obtained with atomic force microscopy (AFM) imaging and creep indentation experiments. This information was then used to perform numerical simulations to examine the effect of tensile strains on the internal stress distribution in the binder. Experimentally, a microloading apparatus was used to induce tensile strains in the samples of the asphalt binder while the binder was observed for damage with the use of AFM. The damage and changes in the binder microstructure resulting from the tensile load observed experimentally were compared with the results from the numerical simulations. Results suggest that localized regions of high stress intensity between different domains act as damage nucleation sites. Results also suggest that the differences in the rheological properties of the microdomains reduce with aging. As a result, the internal distribution of stresses becomes less heterogeneous, the magnitude of stress localization decreases, and there are fewer sites where damage nucleates.

Mechanical behavior and fatigue damage characteristics of chemically stabilized soils under cyclic loading conditions are complex and require better understanding due to its large demand in many critical infrastructures including roadways. Fatigue tests are usually not easy to conduct and time-consuming, and the challenges increase when materials become more complex due to chemical stabilization. This study proposed a simple monotonic fracture test that is integrated with computational fracture modeling to identify fracture characteristics for predicting fatigue damage behavior of complex materials such as chemically stabilized soils. Toward that end, an extrinsic inelastic cohesive zone model (CZM) was used. The fracture parameters of the CZM were first obtained from a monotonic fracture test that is integrated with its model simulation. With the fracture parameters, the same CZM was used to predict fatigue behavior and ultimate failure. The simulation results indicate that the predicted fatigue behavior and performance are comparable to the fatigue test results. With limitations remained for further improvements, the proposed experimental-computational approach incorporated with the inelastic fracture model such as the CZM in this study presented its benefits for predicting time-consuming and labor-intensive fatigue behavior of complex materials.

Rheological properties of asphalt binders significantly affect distress development and performance of asphalt concrete materials. This article presents the effect of Evotherm-M1 modifications on rheological properties of asphalt binders used in the construction of test sections at the Federal Aviation Administration’s National Airport Pavement & Materials Research Center. Four different binders (i.e., polymer styrene butadiene styrene [SBS]-modified PG 76-22, PG 64-22, SBS-modified PG 76-22 plus Evotherm-M1, and PG 64-22 plus Evotherm-M1) are studied. Multiple stress creep recovery (MSCR) and strain-controlled frequency sweep (FS) test results are analyzed to construct the master curves for the binders. Results indicate high sensitivity of SBS-modified PG 76-22 to Evotherm-M1 modifications as compared with PG 64-22. Subsequently, the results of dynamic modulus tests conducted on asphalt mixture specimens (prepared using job mix formula and different binders) are analyzed to investigate the effect of binder type and modification on rheological properties and rutting performance of asphalt mixtures. It is shown that the rutting resistance and rheological properties of asphalt mixtures can be ranked based on the results of MSCR and FS tests conducted on asphalt binders. It is shown that the rutting resistivity of traffic test sections and lab-tested asphalt mixtures can be ranked as follows: SBS-modified PG 76-22, SBS-modified PG 76-22 plus Evotherm-M1, PG 64-22, and PG 64-22 plus Evotherm-M1. This is consistent with the results obtained for tested asphalt binders.

This paper presents a computational micromechanics modeling approach to predict the dynamic modulus of asphalt concrete mixtures. The modeling uses a finite element method combined with the micromechanical representative volume element (RVE) of mixtures and laboratory tests that characterize the properties of individual mixture constituents. The model treats asphalt concrete mixtures as heterogeneous with two primary phases: a linear viscoelastic fine aggregate matrix (FAM) phase and a linear elastic aggregate phase. The mechanical properties of each phase were experimentally obtained by conducting constitutive tests: oscillatory torsion tests for the viscoelastic FAM phase and quasistatic nanoindentation tests for the elastic aggregate particles. Material properties of each mixture phase were then used in the finite element simulation of two-dimensional mixture microstructures obtained from digital image processes of asphalt concrete mixtures. Model simulations were compared with the experimental dynamic moduli of asphalt concrete mixtures. Simulation results indicated that the micromechanical approach based on the mixture microstructure and phase properties could fairly predict the overall mixture properties that are typically obtained from laboratory mixture tests. Furthermore, the RVE dimension of 60 mm might be used to predict the undamaged viscoelastic stiffness characteristics of asphalt concrete mixtures with reduced computing efforts.

Warm-mix asphalt (WMA) produced by foaming is widely used in the asphalt paving industry and is the largest segment of the WMA market in the United States. To produce foaming, a small quantity of cold water is injected into hot asphalt to cause a reduction in the asphalt viscosity and improve the coating of the aggregate particles. The primary objectives of the study reported here were to compare the foaming characteristics of asphalts of different sources and grades and to evaluate the effect of certain additives on the characteristics of the foamed asphalt. To characterize the foamed asphalt, a laser distance meter (LDM) was used in the laboratory. The LDM was used as an alternative to the dipstick (traditionally used in foaming measurements) because of its better repeatability and accuracy. The LDM results indicated that the foaming characteristics depended on the source and performance grade of the asphalt. The addition of a foaming-enhancing additive had a significant effect on the foaming characteristics of most asphalt. However, no obvious difference in foaming characteristics was observed when warm-mix additives were used as part of the foaming process.

The coupled effect of moisture diffusion and mechanical loading on the microstructure of asphalt concrete is studied. The traditional Continuum Damage Mechanics (CDM) framework is modified to model detrimental effects of moisture and mechanical loading. Adhesive/cohesive moisture-induced damage constitutive relationships are proposed to describe the time-dependent degradation of material properties due to moisture. X-ray two-dimensional (2D) computed tomography-imaging technique is used to construct finite element (FE) microstructural representation of a typical dense-graded asphalt concrete. After being calibrated against pull-off experiments, the proposed moisture-induced damage constitutive relationship, which is coupled to thermo-viscoelastic–viscoplastic–viscodamage mechanisms, is used to simulate the microstructure of asphalt concrete. Simulation results demonstrate that the generated 2D FE microstructural representation along with the coupled moisture–mechanical constitutive relationship can be effectively used to model the overall thermo-hygro-mechanical response of asphalt concrete.

In this chapter, several different computational methods are demonstrated as they apply to modeling asphalt mixtures and flexible pavements. Specifically, the following computational methodologies are deployed herein: (1) micromechanics; (2) expanding multi-scaling; (3) contracting multi-scaling; and (4) two-way coupled multi-scaling. As demonstrated herein, each of these approaches has its place in the modeling of flexible pavements.

This study utilizes a computational-experimental method to determine the effective oxygen diffusivity of asphalt concrete based on diffusivities of its constituents, i.e., air void, aggrega...

This study modeled structural performance of bituminous pavements with inelastic deformation and fatigue damage. Two major distresses, namely rutting and fatigue damage, of typical bitumino...

A nonlinear thermoviscoelastic, thermoviscoplastic, and thermo-visco damage constitutive model was developed recently to predict the response of asphalt mixtures under loading. This paper presents a systematic approach to determine the parameters of the model. The paper also demonstrates the efficacy of this approach through the calibration and validation of the model through various tests conducted on fine aggregate mixtures (FAM) and full asphalt mixtures. The dynamic modulus test was conducted to obtain linear viscoelastic model parameters while repeated creep-recovery tests were performed to obtain nonlinear visco elastic and viscoplastic parameters. Viscodamage model parameters were obtained through the constant strain rate test. Once the model was calibrated, it was validated through a comparison of the predicted results with measurements from repeated creep-recovery tests conducted at various combinations of recovery times and temperatures. The predicted responses for FAM and full asphalt mixtures on the basis of the model were found to be in good agreement with the experimental data. Model parameters obtained on the basis of laboratory testing provided useful insights into the relationship between FAM and full asphalt mixture responses and the material properties that affect these responses.

The study reported in this paper presented a multiscale computational model, along with its validation and calibration, to predict the damage-dependent behavior of asphalt mixtures subjected to viscoelastic deformation and cracking. Asphalt mixture is a classic example of a multiphase composite that represents different lengths of scales. The understanding of the mechanical behavior of asphaltic materials has been a challenge to the pavement mechanics community because of the multiple complexities involved: heterogeneity, anisotropy, nonlinear inelasticity, and damage growth in multiple forms. To account for this issue in an accurate and efficient way, the study reported here presented a two-way linked multiscale computational modeling approach. The two-way linked multiscale model had its basis in continuum thermomechanics and was implemented with a finite element formulation. With the unique multiscale linking between scales and the use of the finite element technique, this model could take into account the effects of material heterogeneity, viscoelasticity, and anisotropic damage growth in small-scale mixtures on the overall performance of larger-scale structures. Along with the brief theoretical model formulation, the multiscale model was validated and calibrated through the comparison of the numerical, analytical, and experimental results of three-point bending beam tests of asphalt mixture samples that involved viscoelasticity, mixture heterogeneity, and cohesive zone fracture.

A cyclic hardening-relaxation model is proposed that significantly enhances the prediction of the viscoplastic (VP) strain of asphalt concrete under cyclic compressive-loading conditions at high temperatures. The hardening-relaxation mechanism is physically tied to the changes in the material’s microstructure during the rest period. A memory surface that memorizes the viscoplastic deformation history is defined in the viscoplastic strain space as the general initiation and evolution criteria for the hardening-relaxation mechanism. The proposed model is coupled to the classical Perzyna-type viscoplastic model and Schapery’s nonlinear viscoelastic model, and the associated numerical algorithms are implemented in the finite element software ABAQUS through the user-defined material subroutine UMAT. Model predictions show that the proposed model predicts well both the axial and radial viscoplastic responses of asphalt concrete subjected to the cyclic creep tests at various loading times, unloading time...

The fatigue damage response of asphalt concrete is modeled with a proposed damage evolution function in the context of continuum damage healing mechanics. The uniaxial constant strain rate test is used to identify damage density experimentally during this test and to propose a form for the evolution of the damage density variable in the model. In addition, the model is formulated to include the microdamage healing effect. The proposed damage model is incorporated in the pavement analysis with a nonlinear damage approach model that includes Schapery's viscoelastic model, Perzyna's viscoplastic model, and the microdamage healing model, which is used to simulate the fatigue damage response of asphalt concrete. The damage model is validated against extensive experimental data, including constant strain rate, cyclic displacement-controlled tests, and cyclic stress-controlled tests over a range of temperatures, strain rates, loading frequencies, and stress–strain levels and amplitudes. Model predictions show that the proposed damage model is capable of predicting the fatigue damage response of asphalt concrete subjected to different loading conditions.

This paper presents a framework that combines experimental techniques and computational methods for modeling the microscopic response of asphalt mixtures subjected to various loading conditions. The basis of this framework is capturing the three-dimensional microstructure of asphalt mixtures with X-ray computed tomography and a sequence of image-processing methods to identify the microstructure components or mixture phases. This microstructure is then converted to a finite element model in which the various phases are represented with constitutive models that describe their mechanical behavior. In this study, the coarse aggregate phase was modeled as a linear elastic material, and the matrix phase (asphalt, fine particles, and air voids) was represented as a thermoviscoelastic, viscoplastic, and damage model. The analysis results showed that the model captured the effects of temperature, rate of loading, repeated loads, and mixture design on the microstructure response. These results demonstrate that the developed framework will help engineers and researchers to understand the effects of mixture design and material properties on performance and to establish the link between microscopic response and macroscopic behavior.

Rutting (permanent deformation) is the most important distress that asphalt concrete pavements are prone to at high temperatures. However, available classical viscoplastic (VP) theories are incapable of properly predicting permanent deformation of asphalt concrete materials at high temperatures subjected to cyclic loading conditions. A physically based VP hardening relaxation model is proposed to overcome this issue. This model considers the changes in the microstructure that occur during the rest period and cause the induced hardening stress to relax and recover. This mechanism affects the VP properties of asphalt concrete before and after the rest period is applied. A memory surface is introduced as the general criterion for the evolution of the hardening relaxation mechanism. The memory surface possesses a state variable memorizing the history of the VP deformation during the loading history. The proposed VP hardening relaxation model is coupled with Schapery's viscoelastic model and the classical Perzyna's VP model and is validated against extensive experimental data, including repeated creep and recovery tests at different stress levels, loading times, and rest periods. It is shown that the proposed model reasonably predicts the cyclic VP response of asphalt concrete materials at high temperatures.

Pavement friction is one of the primary factors that affect highway safety. Pavements with adequate surface friction reduce the number of wet skidding crashes. The objective of this study was to develop a predictive model for friction loss on pavement surfaces. The model incorporates parameters that describe aggregate shape characteristics, aggregate resistance to abrasion and polishing, aggregate gradation, and polishing cycles. This model was developed on the basis of the results of a comprehensive experimental program. Square-shaped slabs of different asphalt mixtures were prepared in the laboratory by using a linear kneading compactor and polished with a wheel-polishing device. The frictional characteristics of the surface of the test slabs were measured after different intervals of polishing, and statistical analysis was performed to relate friction to mixture and aggregate characteristics.

Despite the detrimental effects of moisture damage in asphalt pavements, few macroscale models are capable of modeling this important phenomenon. Existing models have limitations in accounting for the irreversibility and time dependency of moisture-induced damage. This study presents a moisture damage model based on continuum damage mechanics. Adhesive and cohesive moisture damage phenomena are modeled independently; this procedure allows for the introduction of fundamental mechanical properties for each process and for modeling the transition between adhesive and cohesive damage. Two- and three-dimensional simulations are performed, and the results of the simulations are presented to demonstrate the applicability and utility of these micromechanical computational models. It is shown that the proposed moisture damage model can simulate the effect of moisture damage on the mechanical response of asphalt concrete subjected to different loading conditions. The model also provides useful insight into the effect of mixture design and material properties on resistance to moisture damage.

When subjected to cyclic creep (ratcheting) loading with rest periods between the loading cycles, the viscoplastic behavior of asphaltic materials changes such the rate of accumulation of the viscoplastic strain at the beginning of the subsequent loading cycle increases comparing to that at the end of the preceding loading cycle. This phenomenon is referred to as the hardening-relaxation (or viscoplastic-softening) and is a key element in predicting the permanent deformation (rutting) of asphalt pavements which is one of the most important distresses in asphalt pavements. This paper presents a phenomenological-based rate-dependent hardening-relaxation model to significantly enhance the prediction of the permanent deformation in asphaltic materials subjected to cyclic-compression loadings at high temperatures. A hardening-relaxation memory surface is defined in the viscoplastic strain space as the general condition for the initiation and evolution of the hardening-relaxation (or viscoplastic-softening). The memory surface is formulated to be a function of an internal state variable memorizing the maximum viscoplastic strain for which the softening has been occurred during the deformation history. The evolution function for the hardening-relaxation model is then defined as a function of the hardening-relaxation internal state variable. The proposed viscoplastic-softening model is coupled to the nonlinear Schapery’s viscoelastic and Perzyna’s viscoplastic models. The numerical algorithms for the proposed model are implemented in the well-known finite element code Abaqus via the user material subroutine UMAT. The model is then calibrated and verified by comparing the model predictions and experimental data that includes cyclic creep-recovery loadings at different stress levels, loading times, rest periods, and confinement levels. Model predictions show that the proposed approach provides a promising tool for constitutive modeling of cyclic hardening-relaxation in asphaltic materials and in general in time- and rate-dependent materials.

In this paper, we present within the finite element context the numerical algorithm for the integration of the thermodynamically consistent thermo-viscoelastic, thermo-viscoplastic, thermo-viscodamage, and thermo-healing constitutive equations derived in the first part of this paper. The nonlinear viscoelastic model is implemented using a recursive-iterative algorithm, whereas an extension of the classical rate-independent return mapping algorithm to the rate-dependent problems is used for numerical implementation of the viscoplasticity model. Moreover, the healing natural configuration along with the power transformation equivalence hypothesis, proposed in the first part of the paper, are used for the implementation of the viscodamage and micro-damage healing models. Hence, the thermo-viscoelastic and thermo-viscoplastic models are also implemented in the healing configuration. These numerical algorithms are implemented in the well-known finite element code Abaqus via the user material subroutine UMAT. A systematic procedure for identification of model parameters is presented. The model is then used to simulate the time-, temperature-, and rate-dependent response of asphalt concrete over an extensive set of experimental measurements including creep-recovery, creep, triaxial, constant strain rate, and repeated creep-recovery tests in both tension and compression. Comparisons of the model predictions and the experimental measurements show that the model is capable of predicting the nonlinear behavior of asphalt concrete subjected to different loading conditions.

A novel continuum damage mechanics-based framework is proposed to model the micro-damage healing phenomenon in the materials that tend to self-heal. This framework extends the well-known Kachanov’s (1958) effective configuration and the concept of the effective stress space to self-healing materials by introducing the healing natural configuration in order to incorporate the micro-damage healing effects. Analytical relations are derived to relate strain tensors and tangent stiffness moduli in the nominal and healing configurations for each postulated transformation hypothesis (i.e. strain, elastic strain energy, and power equivalence hypotheses). The ability of the proposed model to explain micro-damage healing is demonstrated by presenting several examples. Also, a general thermodynamic framework for constitutive modeling of damage and micro-damage healing mechanisms is presented.

Hot-mix asphalt (HMA) laboratory mix design is intended to determine the proportion of aggregate and binder that, when mixed and compacted under a specified effort, will withstand anticipated loading conditions. Current mix design procedures that use the Superpave® gyratory compactor rely on the engineering properties and volumetrics of the compacted mixture to ensure reliable performance; however, a definitive performance test does not exist. The asphalt pavement analyzer (APA) was evaluated as a tool for assessing HMA mixtures designed to perform under high tire pressure aircraft following FAA specifications. The APA used in this study was specially designed to test simulated high tire pressures of 250 psi, which are becoming more common for aircraft. Thirty-three HMA mixtures were included in the study. Each was designed with the Superpave gyratory compactor, according to preliminary criteria being developed by FAA. The study included some mixtures that contain excessive percentages of natural sand and that do not meet FAA criteria. These mixtures were included to provide relative performance for mixtures expected to exhibit premature rutting. APA testing with the high tire pressure APA resulted in rapid failure of HMA specimens compared with traditional APA testing at lower pressures. Data were analyzed, with a focus on the provision of acceptance recommendations for mixtures to support high tire pressures. A preliminary 10-mm rut depth criterion after 4,000 load cycles is recommended.

The surface free energy of a solid is a material property that influences its interaction with other solids, liquids, and gases. Mineral aggregates are mixed with bitumen in the production of asphalt mixtures, and the interaction of aggregates with bitumen, water, or chemical modifiers affects the overall performance of the mixtures and, in turn, the pavement structure. A comprehensive characterization of the minerals that make up these aggregates can be used to explain the interactions between the aggregates and components used to bind the aggregates including bitumen, water, and chemical additives. This paper combines several techniques to quantify and describe or catalog mineral properties. These properties primarily include surface free energy, specific surface area, and surface carbon content of some of the most common minerals found in aggregates. The paper presents data that exemplify how this information can be used to characterize the moisture-induced stripping potential for bitumen–mineral combinations. Results indicate that a simple energy ratio calculated on the basis of adhesive bond energies from surface free energy measurements of the minerals and the bitumen can predict performance in asphalt mixtures and that a few minerals can form thermodynamically stable bonds with some bitumens that are inherently resistant to stripping.

The importance of healing and its impact on the fatigue cracking performance of asphalt mixtures are well recognized. Microdamage healing in an asphalt mixture is a function of several factors, such as the physical and chemical properties of the binder, the properties of the mixture, the level of damage before the rest period during which healing occurs, the duration of the rest period, temperature, and pressure. An experimental and analytical method characterizes the healing in an asphalt composite (fine aggregate matrix) as a function of the level of damage before the rest period and the duration of the rest period. The proposed method builds on the viscoelastic continuum damage theory that has been used successfully by other researchers to characterize fatigue cracking in asphalt mixtures. The method is applied to evaluate the healing characteristics of four fine aggregate matrix mixes and is able to quantify the influence of the duration of the rest period and the level of damage preceding the rest period on overall healing. Two verification tests were used to demonstrate that the amount of healing measured by the proposed method was independent of the sequence of loading or rest periods. Results from the initial tests support the hypothesis that the healing characteristics determined with the proposed test method at isothermal conditions can be treated as a characteristic material property.

It is well recognized that asphalt binders have the ability to self-heal or reverse microdamage during rest periods between load cycles. The ability of an asphalt mixture to self-heal depends on the physical and chemical properties of the asphalt binder, mixture properties, magnitude of damage before the rest period, and external conditions such as temperature. A thorough understanding of the healing mechanism is required to develop a comprehensive damage model that accounts for both fracture and healing on the basis of material properties and external factors such as temperature. Previous studies have hypothesized that healing was a two-step process comprising microcrack closing, referred to as crack wetting, and strength gain, referred to as intrinsic healing. Intrinsic healing or strength gain was further demonstrated to be the sum of two components: instantaneous strength gain immediately on wetting and time-dependent strength gain. A test method using the dynamic shear rheometer was developed to measure the rate of intrinsic healing, which was modeled with a modified form of the Avrami equation. The influence of temperature and aging on the rate of intrinsic healing of asphalt binders was investigated. The intrinsic healing of three asphalt binders was measured at three temperatures and two aging conditions. Results support the hypothesis that time-dependent intrinsic healing increases with an increase in temperature and that the overall capacity of the asphalt binder to heal decreases with aging of the asphalt binder.

Field observations suggest that ettringite-induced swell in lime-treated soils may manifest rapidly after placement and compaction or after months or even years after lime treatment. In either case, forensic investigations have identified the presence of the mineral ettringite in distressed sections. However, the time window between the observation of distress and the subsequent forensic evaluations has left room for doubt as to whether the ettringite caused the observed distress or was formed between the time of the observed distress and the time of the forensic investigation. The study focuses on identifying alternative, probable mechanisms of swelling when sulfate-laden soils are stabilized with lime. The research addresses the hypothesis that swelling in sulfate-bearing fine-grained soils results from one or a combination of three mechanisms: (a) volumetric expansion during ettringite formation, (b) water movement triggered by a high osmotic suction caused by sulfate salts, and (c) the ability of the ettringite mineral to absorb water and contribute to the swelling process. The data validate the conclusion that a synergy of the mechanisms discussed contributes to swelling distress. The study also addresses the impact of matrix strength of the stabilized soil on damage potential.

The focus of the current paper is on the development and validation of a micro-damage healing model that improves the ability of an integrated nonlinear viscoelastic, viscoplastic, and viscodamage constitutive model based on continuum damage mechanics for predicting the fatigue life of asphalt paving mixtures. The model parameters of the continuum-based healing model are related to fundamental material properties. Recursive–iterative and radial return algorithms are used for the numerical implementation of viscoelasticity and viscoplasticity models respectively, whereas the viscodamage and micro-damage healing models are implemented using the concept of the effective undamaged-healed natural configuration. Numerical algorithms are implemented into the well-known finite element code Abaqus via the user material subroutine UMAT. Finally, the model is validated by comparing its predictions with experimental data on an asphalt mix that include repeated creep-recovery tests for different loading times and rest periods in both tension and compression. The significant enhancement of the ability of the constitutive model to predict fatigue life due to inclusion of the micro-damage healing is clearly demonstrated.

The presence of moisture in asphalt mixtures deteriorates their structural integrity. Moisture also acts as a catalyst to promote other forms of pavement distresses. Two critical factors influence the rate and intensity of moisture-induced damage: (a) the speed of moisture transport within the asphalt mixture and binder and (b) the influence of moisture on the cohesive and adhesive properties of the constituent materials. Quantifying these factors is essential to understand, model, and mitigate moisture-induced damage in asphalt mixtures. The experimental and analytical procedures used to measure diffusivity of the asphalt binder are presented. Fourier transform infrared–attenuated total reflectance spectroscopy was used to monitor the diffusion of water into thin films of asphalt binder. The diffusion process was characterized by changes in the portions of the spectra that correspond to the presence of water. Two models were used to fit the data obtained: a diffusion model that followed Fick's second law and a dual mode diffusion model with two diffusion coefficients (D1 and D2) and weighting factor X1. Four asphalt binders were evaluated (AAB, AAD, AAF, and ABD), and all presented better fitting under the dual mode diffusion. Diffusivities of asphalt binders AAB, AAD, and AAF were statistically similar, but diffusivity of asphalt binder ABD was different from that of the others.

Dynamic modulus is the key property used to characterize stiffness of asphaltic mixtures in pavement performance evaluation programs such as the Guide for Mechanistic–Empirical Design of New and Rehabilitated Pavement Structures. This paper investigates various models for predicting the dynamic modulus of asphalt mixtures and compares model predictions with experimental test results. The predictions of two semi-empirical models (Witczak's model, modified Hirsch model), an analytical micromechanics model (Hashin's model), and the computational micromechanics model are compared with the dynamic modulus test results obtained from cylindrical asphalt concrete specimens. For the computational micromechanics approach, the finite element method was incorporated with laboratory tests that characterize the properties of individual mixture constituents and with a digital image analysis technique to represent detailed microstructure characteristics of asphalt concrete mixtures. All predicting models investigated in this paper are in fair agreement with the test results. Witczak's equation simulates dynamic moduli somewhat greater than laboratory test results, whereas the modified Hirsch model generally underpredicts moduli. The computational micromechanics model presents a relatively higher deviation at lower loading frequencies, but it shows better predictions because the loading frequency is higher. Hashin's analytical micromechanics model is limited to accurately predicting the dynamic modulus of the asphalt mixtures because of geometric simplifications and assumptions. With further improvements, the computational micromechanics method incorporated with the testing protocol seems attractive, because it can directly account for geometric complexity due to aggregates and inelastic mixture component properties with fewer of the required laboratory tests.

Damage in sulfate-bearing soils and aggregate systems stabilized with additives containing lime, including lime and portland cement, has drawn considerable attention over the past two decades. Researchers and practitioners have made considerable contributions to the understanding of the problem, including the mechanisms involved in the formation of the two minerals, ettringite and thaumasite, that are most often associated with this damage. This paper provides a case history analysis of the expansion history compared with the ettringite growth history of three controlled low-strength mixtures containing fly ash with relatively high sulfate contents. Samples were subjected to three curing conditions: a dry cure, in which only mixing water was available for curing; a moist cure, in which an external source of water was available for curing; and a sulfate cure, in which an external source of sulfate-bearing water was made available for the duration of cure. Ettringite was quantified by using both differential scanning calorimetry and X-ray diffraction; the resulting volume changes in the samples were measured. Results suggested that sorption of water by the ettringite molecule was at least part of the reason for expansion. The importance of sorption was based on the fact that although expansion increased in moist cure compared to dry cure systems, the quantities of ettringite formed under each regime were about the same and remained about the same throughout the experiments. As expected, samples exposed to sulfate cure responded with the greatest expansion, which was concomitant with continued ettringite crystal growth due to a supply of the limiting reagent, sulfate.

A self-healing material has the inherent ability to partially reverse damage. There is consensus that the healing of damage has a substantial effect on the performance of asphalt pavements. It is important to understand the mechanisms responsible for healing in asphalt binders, to develop a model to predict healing on the basis of these mechanisms, and to develop test methods to determine the properties required for use with such a model. A healing model for asphalt materials comprises a convolution of wetting and intrinsic healing processes that occur across a crack interface. According to the proposed convolution process, intrinsic healing or strength gain across a crack surface occurs in two steps: an instantaneous strength gain resulting from the interfacial cohesion between the crack faces, and a time-dependent strength gain resulting from intermolecular diffusion and randomization across the crack interface. A modified form of the Avrami equation, which includes a parameter for instantaneous strength gain, is used to describe the intrinsic healing in asphalt binders. A test method is described that is based on the use of a dynamic shear rheometer for estimating the parameters of the intrinsic healing function. Five asphalt binders with considerably different compositions were studied. Limited validation of this test method and the intrinsic healing model are presented by comparing the parameter related to instantaneous healing with the thermodynamic work of cohesion for the selected binders.

Considerable attention has been given to sulfate-induced heave in soils stabilized with calcium-based stabilizers over the past two decades. Empirical evidence has supported threshold values of water-soluble sulfates that are used as guidelines to define the risk of distress. This study uses two approaches to evaluate these sulfate threshold values as well as to compare the sensitivity of soils with different mineralogies to sulfate-induced damage. Five soils, each belonging to a different soil series and with significantly different chemistries and mineralogies, are evaluated. Differential scanning calorimeter and stoichiometric mass balance calculations based on a thermodynamics-based phase diagram approach are used to evaluate the influence of mineralogy on soil behavior. The variation in sensitivity of different soils to ettringite formation was substantiated and was found to be dependent on reactive clay content in the soil. Although the sensitivity of soils to ettringite formation was observed to change with the availability of reactive clay fractions, no definitive relationship was established between clay content and weight percentage of ettringite formed. The empirical threshold of about 3,000 ppm is generally substantiated as is the efficacy of the concept of extended mellowing periods with homogeneous mixing while adding as much mixing water as possible. Finally, a general agreement between the mass balance predictive approach and the direct measurement by using the differential scanning calorimeter was established.

Moisture damage in asphalt mixtures is a complex phenomenon involving thermodynamic, chemical, physical, and mechanical processes that contribute to pavement deterioration. The empirical nature of the majority of test methods and the inherent variability of the results are the two primary challenges impeding the reliable characterization and assessment of moisture damage. This paper addresses these two challenges by offering a framework that (a) is based on probabilistic analysis that accounts for the uncertainty of the deterioration process and the inherent variability associated with the experimental data, (b) is based on a fracture mechanics damage model for viscoelastic materials that has successfully been applied to the characterization of moisture damage in asphalt mixtures, and (c) combines the fundamental material properties of the asphalt mixture that can be obtained from reliable tests (e.g., surface energy measurements of the material components and dynamic shear test and relaxation modulus data from the dynamic mechanical analyzer). This framework was applied to analysis of the fine matrices of four asphalt mixtures containing different aggregate materials and fillers. The results are compared with the results obtained by assessment of moisture susceptibility for the full asphalt mixtures by the saturation aging tensile stiffness test.

The directional dependency of material properties of aggregate systems have been the subject of study over the last 10 years. A procedure was established to determine the level of anisotropy of aggregate systems using simple aggregate properties and anisotropy level as an input to predict performance of aggregate bases. Stress-induced directional dependency of material properties were evaluated on the basis of multiple variable dynamic confining pressure stress path tests for 10 aggregate sources. Various gradations and saturation levels were considered for each aggregate source. Particle geometry was characterized using the Aggregate Imaging System. The cumulative Weibull distribution function was used to describe aggregate size and aggregate geometrical characteristics. The fine portion of the gradation was characterized by the Rigden voids test and methylene blue test to account for fine particle shape properties and the deleterious effect of plastic fines on the volumetric stability of aggregate layers. The cross-anisotropic modular ratios were used as indicators of the level of anisotropy. Aggregate characteristics (form, angularity, texture, and gradation) are in turn related to Weibull cumulative distribution parameters. A sensitivity analysis revealed that the level of anisotropy is very sensitive to aggregate form, angularity, and textural properties. It is also sensitive to the level of bulk stress and shear stress as reflected by the k2 and k3 parameters. Cross-anisotropic shear strength ratios for four pavement sections at the top of the base layer were calculated using a finite element program. The results indicated that the level of anisotropy significantly affected the shear strength ratio.

Fatigue cracking is a common form of distress in asphalt pavements. During the last four decades, significant research has been conducted to characterize fatigue damage in asphalt mixtures. This paper evaluates an analytical method that is independent of the mode of loading (controlled strain or controlled stress) to quantify the fatigue resistance of asphalt mixtures. The evaluation was based on fatigue tests conducted on the fine aggregate matrix (FAM) portion of an asphalt mixture by using a dynamic mechanical analyzer. A number of tests were performed by applying oscillatory torque under controlled-strain and controlled-stress conditions at a frequency of 10 Hz and a temperature of 25°C. The data from these tests were analyzed by using a fracture model for viscoelastic materials to calculate a fatigue damage parameter, R(N), which represents the influence of the numbers of cracks with equivalent crack radii on the material fracture behavior and which quantifies crack growth in FAM. This damage parameter has a coefficient of variation lower than the coefficients of variation of conventional parameters, such as the number of load cycles to failure and the cumulative dissipated energy. In addition, this parameter provides comparable results for a given material independent of the mode of loading.

Conventional methods to quantify the moisture sensitivity of asphalt mixtures are based on the comparison of mechanical properties of the mix before and after a moisture-conditioning process. Although this approach consolidates the effect of material and mixture properties on moisture sensitivity, it does not identify the causes responsible for the poor or good performance of the mixture. In this study, surface free energy of asphalt binders and aggregates was used to derive energy parameters that quantify the moisture sensitivity of various combinations of materials. The moisture sensitivity of 12 asphalt mixtures carefully designed to represent a wide range of asphalt-aggregate interactions was measured in the laboratory under controlled conditions. Test results indicate that the moisture sensitivity of these mixtures correlates well with the energy parameters, which are based on the surface energy properties of the constituent materials. Incorporating the specific surface area of the aggregate into the energy parameters improved this correlation. The proposed energy parameters have the potential to serve as an effective tool by which to select material combinations that result in asphalt mixtures that are more resistant to moisture-induced damage.

Performance of asphalt pavements depends on the properties of its constituent materials, mixture volumetrics, and external factors such as load and environment. An important material property that influences the performance of an asphalt mixture is the surface free energy of the asphalt binder and the aggregate. Surface free energy governs the adhesive bond strength between the asphalt binder and the aggregate as well as the cohesive bond strength of the asphalt binder. These bond energies in turn influence the resistance of the asphalt mixture to distresses such as fatigue cracking and moisture induced damage. Asphalt binders undergo several types of engineering and natural modifications that influence their chemical and mechanical properties. Three common examples of modifications are the addition of polymers, addition of additives (e.g., antistrip agents), and oxidative aging of the asphalt binder. This paper is part of a study conducted to examine the effect of different types of modifications on the surface free energy components of the asphalt binder. The change in surface free energy was used to calculate parameters that are related to the performance of the asphalt mixtures. Results from this study demonstrate that the magnitude and nature of change to the surface free energy and concomitant performance-related parameters varied significantly among different asphalt binders. For example, certain modifications improved the performance of some binders while adversely affecting the performance of others. The experiment and analysis methods presented in this study can be used to examine the influence of modifications on performance and optimize the engineering properties of binders and asphalt mixtures.

The impact of increasing fines content on the performance of unbound (unstabilized) and lightly stabilized aggregate systems was evaluated. The aggregate systems analyzed varied in amount of mineral fines, the moisture state during curing and at the time of testing, and the amount of portland cement used to stabilize the blend. The evaluation was based on measurements of anisotropic resilient properties, permanent deformation, and unconfined compressive strengths of aggregate systems. In addition, the nonlinear anisotropic resilient properties of the aggregate blends were used in a finite element program to determine critical pavement responses. Aggregate systems with higher fines content were, as expected, more sensitive to moisture than control systems with standard fines content. The increase in the fines content in the unbound systems when molding moisture was wet of optimum dramatically diminished the quality of performance. However, the aggregate systems with higher fines benefited considerably from low percentages of cement stabilizer. It was found that with the proper design of fines content, cement content, and moisture, the performance of the stabilized systems with high fines content can perform equivalent to or even better than the systems with standard fines content. This was clearly evinced by enhancing the resilient properties (increase in stiffness and decrease in anisotropy), decreasing the rate and magnitude of permanent deformation, and increasing compressive strength. The beneficial use of mineral fines will result in benefit to the aggregate industry.

In 2005, the Idaho Transportation Department constructed a field project on State Highway 67 (SH-67) to evaluate the impact of hydrated lime and a liquid antistrip agent on the mechanical properties of an Idaho hot-mix asphalt (HMA) mixture. This paper documents the results of a laboratory evaluation on field-produced HMA mixtures sampled from lime and liquid antistrip test sections on SH-67. As the two mixtures were subjected to multiple freeze-thaw (F-T) cycles, the mixture treated with hydrated lime (lime mixture) maintained good resilient-modulus properties over the entire 21 F-T cycles. The mixture treated with the liquid antistrip agent (liquid mixture) fully disintegrated after 22 F-T cycles. The results of a mechanistic analysis showed that, as a result of multiple F-T cycling, the liquid mixture will incur a 220% increase in potential rutting compared with the lime mixture, which will incur only a 65% potential increase in rutting. The dynamic moduli in compression demonstrated that both dry and moisture-conditioned lime mixtures can be considered less susceptible to rutting compared with liquid mixtures. Furthermore, the lime mixtures demonstrated better resistance to moisture damage than the liquid mixtures on the basis of the ratio of dynamic modulus in tension of moisture-conditioned to dry mixtures. The additional stiffness of the lime mixtures did not contribute to premature fatigue cracking when mixtures were subjected to dynamic creep testing in the tensile mode of loading.