Analysis of Asphalt Pavement Materials and Systems: Emerging Methods

This paper presents some recent developments in using Finite Element Method (FEM) and the 3D microstructure of asphalt concrete reconstructed from x-ray tomography imaging to evaluate its performance. Asphalt concrete is modeled as a material composed of an effective solid and void. The stress concentration factor and its distributions are computed and correlated to the overall resistance of the mixtures against rutting and fatigue, and the internal structural quantities. The correlation of the stress concentration factor with the field performance of the WesTrack mixes indicates that mixes with large stress concentration factors, as expected, may be more prone to rutting and fatigue cracking than those with smaller stress concentration factors.

A number of researchers have studied aggregate characteristics including elongation, flatness, and other shapes that are believed to affect asphalt mixture properties such as internal resistance, rutting resistance, tensile strength, and complex modulus etc. However, the aggregate modulus also affects the asphalt mixture modulus significantly, which has not been taken into consideration. In this paper, the effect of aggregate particle-to-particle interaction was studied through numerical modeling using the discrete element modeling (DEM) approach. The individual material phases (e.g., aggregates and mastic) were modeled with clusters of discrete elements based upon laboratory testing of the individual phases. For a given set of material parameters for each phase, the degree of particle-to-particle contact in an asphalt mixture was found to have a profound influence on overall mixture modulus. A parametric investigation of aggregate modulus revealed that the contribution of aggregate modulus to overall mixture modulus was very significant. Pending further experimental verification on asphalt mixtures, this finding may shed new light on the importance of aggregate stiffness on overall mixture properties.

This paper presents a microstructure-based finite element model by incorporating elastic aggregates and viscoelastic sand mastic. The microstructure-based finite element (FE) approach was used to predict the linear and damage-coupled viscoelastic properties of reclaimed asphalt mixture. The two-dimensional (2D) microstructure of asphalt mixture was obtained from the scanned image of a smoothly sawn surface of a reclaimed asphalt pavement (RAP) mixture specimen. In the microstructure, the sketches of highly irregular aggregates were converted into polygons. The whole microstructure model was divided into highly irregular aggregate and sand mastic subdomains. The finite element mesh was generated within each subdomain. The deformation of the aggregate and mastic subdomains was connected through the sharing boundary nodes. Linear and damage-coupled viscoelastic finite element model was developed with displacement-based incremental formulation. The linear and damage-coupled viscoelastic simulation was conducted on the image sample of test specimen under different sinusoidal force loading frequencies. The uniaxial compression simulation results showed creep deformation constant cyclic force loading amplitude and damage-coupled viscoelastic responses have larger creep deformation. Simulations under different loading frequencies found compression strain deceases with loading frequencies due to less relaxation time.

Mastic properties are very important to the performance of asphalt concrete. Fine and coarse aggregates are usually not directly interacting with asphalt binder but with mastics. Permanent deformation and fatigue cracking tend to occur in the mastics and the interfaces between mastic and aggregate. Mastics demonstrate much more complicated behavior than asphalt binder including permanent deformation, non-linearity, and temperature sensitivity. Two important mastic properties include its shear modulus and permanent deformation. Too low stiffness may lead to high potential of rutting while too high stiffness may lead to cracking. This study targets at estimating the shear stiffness of mastics using the complex shear modulus of binder and the Hirsch model based on binder modulus and the volumetric composition of filler. Predictions of the mastics shear moduli using Hirsch model were compared with experimental measurements at different temperatures and frequencies and show very good agreement. The study also indicates that Time-Temperature superposition algorithms are applicable to mastics for different volume fractions of mineral fillers.

The compaction of asphalt concrete is the last step in controlling the quality of pavement. Tremendous information on the mixture's characteristics, segregation, and fundamental properties of asphalt concrete demonstrates itself in the compaction process. An understanding of the fundamental mechanisms of compaction will help better control the quality of asphalt concrete pavement. This paper presents the investigation of the compaction mechanics using both Finite Element Method (FEM) and Discrete Element Method (DEM). The FEM simulation adopted a porous viscoplasticity model, which considers the compaction in the view of its air void reduction resulted from the squeezing of aggregates and mastics. The DEM simulations provide a view of considering the compaction a process in which aggregate particles translate and rotate to positions forming denser packing. By this approach, particle shape and binder stiffness factors can be considered. The simulation results indicate that both approaches can describe the compaction phenomena consistently with field observations and empirical experience.

This study presents characterization of the nonlinear viscoelastic behavior of hot mix asphalt (HMA) at different temperatures and strain levels using the Schapery nonlinear viscoelastic model. This model is employed to describe experimental measurements of two asphalt mixes under several combined temperatures and strain levels. The master curve is created for each strain level using time temperature superposition principle (TTSP) with a reference temperature of 40°C. The measurements at strain levels higher than 0.01% are used to determine the nonlinear viscoelastic parameters in the model. The FE model with the calibrated time-dependent and nonlinear material parameters is used to simulate the creep experimental tests, and good predictions are shown.

An internal variable formulation for the introduction of the effects of temperature in viscoelastic models is presented. A new model, the temperature-dependent generalized Kuhn model of viscoelasticity, is developed by using this powerful and novel approach. An alternative formulation, through the use of the creep function and the time-temperature superposition principle, is also presented. The new model's predictions are compared to experimental data.

Various research efforts have proven that it is possible to interconvert the various linear viscoelastic material properties (i.e., the creep compliance, relaxation modulus, complex compliance, and dynamic modulus) of asphalt concrete materials. The interconversion makes it possible to predict one viscoelastic property from another and therefore eliminates the need to do more than one test to calculate all needed viscoelastic properties. This paper presents a laboratory validation of the interconversion between dynamic moduli and creep compliances obtained from two typical mixes used in the Commonwealth of Virginia. The measured dynamic modulus was successfully converted into creep compliance, and the measured creep compliance was successfully converted into dynamic modulus. Both converted properties were compared to the actual measured data. It was found that in most cases the converted properties were similar to the measured properties. However, the dynamic modulus predicted smaller creep compliance than was measured while the creep compliance predicted smaller dynamic modulus than was measured. This is attributed to the fact that the creep tests were performed at high, constant stress without confinement, which might have brought the material outside its linear viscoelastic range. More testing is therefore needed to confirm the promising results obtained with the existing conversion methods.

Surface cracking is one of major early distresses in newly constructed semi-rigid pavements in China. Such semi-rigid pavements usually consist of three layers of hot mix asphalt (HMA) mixtures on top of a cement-treated aggregate (CTA) base over lime treated subgrade. To control the CTA base cracks reflected to the surface, a HMA binder course mixture is desired to be not only anti-reflective cracking but also fatigue resistant. This study was in an effort to investigate the reflective cracking problem on semi-rigid pavements through a proper binder course mixture design and laboratory characterization of the fatigue and anti-cracking properties of binder course mixtures used in China.

Analysis of flexible pavements using the multilayered elastic theory involves the use of elastic moduli backcalculated from deflection basins measured utilizing the Falling Weight Deflectometer (FWD). Uncertainties associated with the thickness of the elastic layers, seed moduli, and the backcalculation algorithms have been studied to increase the reliability and accuracy of the backcalculated deflection basin by minimizing the relative root-mean-square error (RMSE). Therefore, the RMSE of the backcalculated deflection basin has been used always to assess the validity of the backcalculated set of elastic moduli of the pavement. In this study, the sensitivity of the pavement response to the RMSE is investigated by comparing the response of a pavement section using the exact set of moduli with that using the backcalculated set of pavement moduli with a RMSE less than 1%. The results showed that even when the RMSE in deflections is as low as 0.22%, the pavement response (strain, stress, and therefore fatigue and rutting predictions) using the backcalculated set of moduli can largely be different than that using the exact set of moduli, suggesting more uncertainties to the backcalculated set of moduli.

Flexible pavements are commonly used for low to high volume roads subjected to many wheel load applications and also for airfields subjected to rather heavy aircraft gear/wheel loads. As the demand for applied wheel loads and number of load applications increase, it becomes very important to properly characterize the behavior of unbound aggregate layers and subgrade soils as the pavement foundation geomaterials. Laboratory studies have shown that resilient responses for these geomaterials follow nonlinear, stress-dependent behavior under repeated loading. Therefore, a finite element (FE) type layered elastic analysis is needed to employ nonlinear resilient material models to predict accurate pavement responses for mechanistic based pavement design. In this study, modulus models well proven over the years to adequately describe the nonlinear pavement geomaterial behavior were programmed in a user material subroutine (UMAT) to perform axisymmetric and three-dimensional (3D) analyses using the general-purpose ABAQUS FE program. The results indicated that modulus characterizations of the nonlinear, stress-dependent base and subgrade layers were essentially needed to reliably predict accurate pavement responses both in axisymmetric and 3D analyses.

A series of monotonic and repeated load resilient modulus triaxial tests were conducted on unreinforced and geogrid reinforced crushed limestone samples to evaluate the effects of the stiffness, location, and number of geogrid layer/s on the strength properties and stress strain response parameters of those samples. Five different types of geogrids were used, and for each geogrid type, four different cases were investigated. For each monotonic triaxial test, three response parameters were selected to assess the benefits of geogrid improvement namely, E_s1%, E_s2%, and USS. Statistical analyses were conducted on the results using ANOVA and post ANOVA LSM. The results of these analyses indicated that generally geogrid with higher stiffness moduli exhibited greater improvement. In addition, double layers had always the maximum improvement, while the minimum benefit was observed for single geogrid layer placed at the sample mid-height. The cyclic triaxial test results were used to determine resilient moduli for the different samples. These results showed that the geogrid reinforcement did not enhance the resilient properties of the reinforced samples.

Few research projects have been conducted on the permanent deformation of subgrade soils in China. Based on a comprehensive literature review, a testing procedure to evaluate permanent deformation in subgrade soils is developed and presented in this paper. The parameters in the permanent deformation test include confining pressure, deviator stress, load shape, load duration, load rest period, moisture content and dry density. Using the procedure, the authors conducted repeated load tests for three subgrade soils. The test results are in good agreement with the relationships between the permanent strain and the number of load applications proposed by Tseng and Lytton. The regression models for sand, silty clay and clay have been developed from these results.