Journal of Materials in Civil Engineering

Discrete element modeling has been used to simulate constant strain rate compressive tests for an idealized asphalt mixture comprising approximately single-sized sand particles. A range of constant strain-rate compressive tests to failure have been undertaken in the laboratory and the axial stress-strain response has been carefully measured. The peak stress (compressive strength) of the material was found to be as a power-law function of the equivalent (temperature compensated) strain rate in the viscoelastic region of behavior. The internal geometry of the idealized asphalt mixture has been reproduced in PFC-3D and internal damage (cracking) in the material was modeled by allowing bond breakage between adjacent particles. Elastic contact properties have been used to investigate the effect of random variations in internal sample geometry, the distribution of bond strengths between adjacent particles and the coefficient of friction between particles where the bond has broken. A simple viscoelastic Burger’s model was used to introduce time dependent shear and normal (tensile) contact stiffnesses and an elastic contact has been assumed for the compressive normal contact stiffness. To reduce the computation time, both viscosities in the Burger’s model were scaled which has been shown to have the same effect as scaling the loading velocity (strain rate) by the same factor. A strain rate dependent bond breakage criterion has been developed and model results were found to compare well with the experimental data.

Bonding characteristics between pavement layers have an important influence on responses of pavement structures. This paper deals with this subject by analyzing the interlayer behavior of double-layered asphalt (DLA) specimens taken from an in-service motorway using a coaxial shear test (CAST) and a layer-parallel direct shear (LPDS) test. To analyze and model the influence of the interlayer condition, finite-element simulations of CAST DLA specimens were conducted. In the models, both idealized fully bonded and no-bonding assumptions were used to characterize the interlayer behavior of DLA specimens. Experimental results presented for CAST and LPDS tests demonstrate that there is a strong influence of temperature in the interlayer bonding mechanism. At lower temperatures, there is an increase on the bonding strength produced by the binder stiffening and aggregate interlocking. Further, it was observed that comparisons of CAST experimental and numerical results indicate that the interlayer can be close to the fully bonded condition at low temperatures below $20°\text{C}$ while the interlayer, at higher temperatures, behaves between fully bonded and no-bonding idealized conditions.

Indirect tensile (IDT) test is widely adopted to evaluate the anticracking performance of hot-mixture asphalt (HMA) at intermediate and low temperatures. This paper presented an approach to simulate the IDT test based on the microstructure of mixture. The X-ray computed tomography and digital image processing technologies were employed to characterize the microstructure information of the HMA specimen. The viscoelastic numerical simulation results agreed well to the laboratory IDT tests. Two indices, mixture stiffness obtained by numerical simulation and local stress state, were adopted to study the effects on the IDT tests of many factors such as loading orientation, air voids, loading rate, and the aggregate/mastic modulus ratio. Loading orientation will influence the IDT performance of heterogeneous HMA specimens. The multiple IDT tests were suggested to be conducted to reduce the effects of distribution variation of aggregates and mastic in mixture specimens. Loading rate will induce more remarkable influence on the IDT tests at a higher temperature ( $15°\text{C}$ other than $-10°\text{C}$ ). The mixture stiffness without air voids can increase about 8% according to the numerical simulation. The aggregate/mastic modulus ratio has significant impact on the strain state of aggregates and mastic. The strain incoordination of aggregate and mastic may induce the distress of mixture under loading.

Cracking is a main source of structural distress in asphalt materials and asphaltic pavements. To predict crack-associated fracture damage in asphalt mixtures, this study presents a model using the finite-element method and a cohesive zone fracture model. The approach allows advanced characterization of the microstructural damage evolution in a more realistic length scale, the mixture heterogeneity, the inelastic material behavior, and the interactions among mixture constituents. The model presented herein accounts for (1) actual mixture heterogeneity by using digital image techniques; (2) inelastic material behavior based on elastic-viscoelastic constitutive relations; and (3) microscale fracture damage represented by the cohesive zone fracture model. A computational modeling framework is presented, and the applicability of the model is demonstrated through simulations. Model simulations are further analyzed by comparing numerical predictions to laboratory test results and by conducting parametric analyses of fracture properties. It is expected that the successfully developed computational model can provide better insights into the effect of mixture constituents on overall mixture performance, while minimizing modeling efforts and producing more accurate simulations than traditional approaches, with significant savings in experimental costs and time.

Capability to effectively discretize a problem domain makes the finite-element method an attractive simulation technique for modeling complicated boundary value problems such as asphalt concrete pavements with material non-homogeneities. Specialized “graded elements” have been shown to provide an efficient and accurate tool for the simulation of functionally graded materials. Most of the previous research on numerical simulation of functionally graded materials has been limited to elastic material behavior. Thus, the current work focuses on finite-element analysis of functionally graded viscoelastic materials. The analysis is performed using the elastic-viscoelastic correspondence principle, and viscoelastic material gradation is accounted for within the elements by means of the generalized iso-parametric formulation. This paper emphasizes viscoelastic behavior of asphalt concrete pavements and several examples, ranging from verification problems to field scale applications, are presented to demonstrate the features of the present approach.

An asphalt mixture is a composite material consisting of three components: asphalt binder, aggregate, and air. The mechanical properties of asphalt mixtures are mostly evaluated from empirical approaches that are usually limited to measurement conditions. This paper takes a mechanistic approach by using micromechanics theory for composite materials to develop self-consistent micromechanics models for an asphalt mixture. The mixture analysis method described in this paper is applied to measured properties of an asphalt concrete mixture that is commonly used in Texas. These models are programmed in MATLAB using the system identification method and are applied to the analysis of the frequency-dependent magnitudes of the viscoelastic properties of an asphalt mixture at different aging periods. The inverse micromechanics model takes as input the volumetric composition of the mixture and the measured frequency-dependent bulk and shear properties of the asphalt mixture and the binder and extracts from them the bulk and shear properties of the aggregate. The forward micromechanics model takes as input the frequency-dependent bulk and shear properties of the aggregate and binder and produces the frequency-dependent properties of the asphalt mixture. It has been demonstrated that the inverse and forward micromechanics models are in fact the inverse of each other and that the inferred aggregate properties are realistic. These models provide a technique to catalog the properties of aggregates and use them in a computerized determination of the combinations of binders, aggregates, and air to produce the desired properties of asphalt mixtures.

The writers recently developed a nonlinear viscoelastic-viscoplastic constitutive model, in order to represent the response of asphalt mixtures under different temperatures and rates of loading. This model has been implemented in the finite-element (FE) code Abaqus via the user material subroutine UMAT, and it was verified through comparisons with experimental data of asphalt mixtures at various stress levels and temperatures. This research develops a three-dimensional FE model using Abaqus to represent a three-layer pavement structure and to simulate the viscoelastic and viscoplastic responses under repeated loading at different temperatures. The results demonstrate the capability of the model in simulating the influence of temperature on permanent deformation and in predicting viscoelastic and viscoplastic strain distributions in the asphalt layer. The simulations show that tensile viscoplastic strain accumulates at the pavement surface, a phenomenon that could be associated with cracking of asphalt pavements. In addition, the results show that at high pavement temperature $\left(40°\text{C}\right)$ , tensile viscoplastic strain develops at the sides of the applied load due to asphalt mixture heave associated with permanent deformation and dilation.

An empirical $\left|{\mathit{E}}^{\ast }\right|$ predictive equation, known as Witczak’s $\left|{\mathit{E}}^{\ast }\right|$ predictive model, has been adopted in the Mechanistic-Empirical Pavement Design Guide. However, empirical approaches have significant drawbacks, particularly with application of the model for materials that vary from those used in the calibration of the model and their reliance on large calibration data sets. In order to overcome the limitations of an empirical approach, a fundamental micromechanics modeling framework based on the differential scheme effective medium theory has been developed and introduced in this paper. The proposed modeling framework herein takes a sequential approach, using a series of two-phase differential particulate composite models to predict the effective tensile complex modulus $\left(\left|{\mathit{E}}^{\ast }\right|\right)$ of a three-phase HMA mixture, which consists of the asphalt binder, aggregate particles, and air voids. The model prediction results were compared with experimentally measured $\left|{\mathit{E}}^{\ast }\right|$ of five dense graded HMA mixtures across a range of low temperatures. In general, the model was found to predict $\left|{\mathit{E}}^{\ast }\right|$ with a very reasonable level of accuracy. Additional work to extend the applicability of the modeling framework to higher temperature regimes and the need to develop a more general and systematic approach for practical implementation are discussed.

Microstructural-based discrete element (DE) models have been used for a better understanding of asphalt pavement concrete since the late 1990s. Most current studies have been done with two-dimensional (2D) models. Moreover, elastic models are primarily employed for simulation of an asphalt matrix’s time-dependent behaviors. A 2D model is too simple to capture the complex microstructure of asphalt concrete, and an elastic model is not sufficient for simulating an asphalt matrix’s viscoelastic behaviors. Therefore, it is necessary to consider a three-dimensional (3D) viscoelastic model for microstructural-based DE simulation of asphalt mixture behaviors. Currently, it is easy to build such a 3D microstructural-based DE viscoelastic model using the existing techniques presented in the previous studies. A major challenge, however, is to reduce the computation time to run the 3D microstructural-based DE viscoelastic modeling process which is extremely time-consuming. The primary objective of this paper is to simulate and analyze creep responses of an asphalt mixture with a 3D microstructural-based DE viscoelastic model. A key task in this study is to develop an approach to reduce the computation time with the time-temperature superposition principle. Using this developed approach, creep compliance tests of an asphalt mixture under temperatures of 0, −10, and $-20°\text{C}$ , were simulated with a microstructural-based DE viscoelastic model. Additionally, a DE elastic model was also employed and compared with the viscoelastic model. It was observed that (1) the computation time was reduced to several hours from several decades and (2) the DE viscoelastic model, the DE elastic model, and the experimental measurements yielded similar results.