Modeling of Hot-Mix Asphalt Compaction: A Thermodynamics-Based Compressible Viscoelastic Model

CHAPTER 8. SUMMARY AND CONCLUSIONS

The main reasons for the selection of the thermodynamics-based nonlinear viscoelastic model are as follows:

• The compaction process involves large strains, which can be modeled using the adopted thermodynamic framework.
• The asphalt mix experiences significant microstructure changes during the compaction process. These changes include aggregate reorientation, reduction in air voids, and increase in aggregate contacts. The continuum model was formulated to account for these phenomena through the specification of appropriate evolution functions of the Helmholtz potential and rate of dissipation.
• A few parameters are used in calibrating the model. These parameters are the ones used in determining the evolutions of Helmholtz potential and rate of dissipation.
• Continuum models similar to the one that was developed in this study have been used successfully in modeling asphalt-material behavior under complex loading and environmental conditions. The temperature of asphalt mix changes during compaction, and this phenomenon needs to be accounted for in the model in future research.

FE IMPLEMENTATION

The FE implementation of the material model enables numerical investigations into the behavior of the material when subjected to compaction. The following points summarize the main findings from the FE simulations:

• The model was implemented in CAPA-3D using the FE method. The implementation was then validated by comparing the FE solution for one-dimensional constant stress and constant strain loading with the corresponding solutions from analytical techniques (aided by calculations using MATLAB^®).
• The model exhibits a nonlinear response in shear through the exhibition of normal stress differences.
• The continuum model lends itself to implementation in the FE method to simulate the compaction process under various conditions. Therefore, it can be used to relate the laboratory compaction process to the field compaction process.
• The material model was implemented successfully in the FE package CAPA-3D and was used to simulate compaction in the SGC and in the field under various loading and boundary conditions.

SIMULATION OF GYRATORY COMPACTION

The FE method was used to simulate the laboratory compaction of asphalt mixtures. The main findings from the simulation of gyratory compaction are as follows:

• The parametric analysis conducted in this study showed that the compaction process is insensitive to changes in some of the model's parameters. Therefore, researchers used constant values for these parameters during all the FE simulations in order to simplify the process of finding the values of the remaining parameters. However, the model needs to be further examined for other processes in which these parameters might be significant.
• The parametric analysis demonstrated that some of the model's parameters are related mostly to the initial stage of compaction, when the material exhibits low-viscosity fluidlike behavior, while other parameters are related to the behavior of the mixture after it starts to exhibit a high-viscosity fluidlike behavior and the compaction rate decreases.
• A systematic method was developed to determine the model's parameters from the SGC curves. It was necessary to develop this method because of the difficulty of conducting conventional tests (e.g., triaxial or shear tests) to determine the model's parameters given the high compaction temperatures.
• The FE simulations demonstrated the capability of the constitutive model to simulate the Superpave^® gyratory compaction process at different compaction angles.
• The developed model and FE implementation will allow the prediction of mixture compactability in the field based on laboratory measurements. It will also allow the study of the influence of changes in material properties and mixture designs on mixture compactability under various laboratory and field conditions.

SIMULATION OF FIELD COMPACTION

The details of the FE model with regards to the mesh, geometry, loading methodology, and boundary conditions were presented earlier in this report. The primary findings from the simulation of field compaction are as follows:

• A loading algorithm was developed for use in CAPA-3D for pavement compaction simulations. The loading algorithm utilizes nonuniform pressure loads applied over the surface of the finite elements in contact with a roller.
• Moving loads were applied in a quasi-static manner by incrementally translating the necessary element surface loads during the simulation.
• The vibratory loads were also applied in a quasi-static manner. The inertial effects of the asphalt mix layer were not considered in these simulations.
• Interface-layer elements were utilized to resolve contact issues pertaining to the layer interactions.
• The FE model developed in this study is capable of capturing the influence of the boundary conditions (fixed/free) near the longitudinal joint on the compaction process. Consistent with the field experience, fixed longitudinal joints exhibited higher compaction than free edges.
• The FE model was validated against data from three field projects. The FE results agree well with the general trends of compaction observed in the field.
• The FE model is capable of providing insight into which mix design and rolling pattern combination provides the most compaction at different locations across the pavement.

RECOMMENDATIONS FOR FUTURE INVESTIGATIONS

This study can be extended in several ways. The model developed can be used to develop a database of parameters for different mix designs. This database of model parameters can be used for further numerical investigations to better understand the IC process.

The material model presented in this report has the capability to include the effect of temperature on the material behavior. However, due to the lack of experimental data on the influence of temperature on material properties within the range of compaction temperatures, this effect was not accounted for in the simulations conducted in this study. Conducting experimental measures is recommended to monitor temperature changes during compaction and to measure the effects of these changes on material properties. In addition, the model needs to be expanded to simulate nonisothermal conditions during the compaction process.

The material's model parameters were related qualitatively to the characteristics of the SGC curves. This approach was followed due to the lack of experimental methods to measure the asphalt mixture properties at the range of temperatures involved in the compaction process. Future research should focus on the development of a consistent experimental program to quantitatively assess the model's parameters.

The continuum model implemented in the FE method is a powerful approach for simulating the material and structure response under various loading conditions. The model's parameters are obtained from experiments on the asphalt mixtures. The relationships between the model's parameters and the properties of the mixture constituents can be estimated by observing changes in the model's parameters and material response due to changes in mixture constituents. However, it cannot be used to directly determine the influence of changes in material properties on compaction characteristics. Researchers at the Turner-Fairbank Highway Research Center have worked on the development of a micromechanical model for asphalt mixture compaction. Such micromechanical models are computationally intensive and do not lend themselves to simulations of field compaction at various compaction operations and field conditions. However, the advantage of the micromechanical approach is its ability to directly account for the characteristics and properties of mixture constituents in modeling compaction.

A combination of the continuum and micromechanical models could be used to directly estimate the continuum model's parameters using the micromechanical model's results. This can be achieved by reformulating the constitutive model developed in the framework of this project along the lines of multiplicative split "multislip" models that simulate, on a continuum level, the micromechanical interactions that occur at the constituent materials level.

As shown in figure 120, the new model will utilize features of the micromechanical response of an asphalt mix (e.g., dilation, aggregate rotation, etc.) that are controlled by the compositional characteristics of the mix (e.g., gradation, angularity, etc.) to determine the in-time development of material compaction as represented by the inelastic deformation gradient G of the continuum constitutive model.

Figure 120. Illustration. Micromechanical response of an asphalt mix.

The micromechanical model will be used to directly examine the influence of changes in materials (binder viscosity, aggregate shape characteristics, aggregate gradation) on compaction. Then, it will be possible to develop relationships that relate the continuum model's parameters to the compositional characteristics of the materials. This approach is illustrated in figure 121.

Figure 121. Chart. Representation of tasks involved in modeling asphalt mix compaction.