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Publication Number: FHWA-HRT-10-065
Date: December 2010

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:

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:

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:

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:

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.

Illustration. Micromechanical response of an asphalt mix. Click here for more information.

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.

Chart. Representation of tasks involved in modeling asphalt mix compaction. Click here for more information.

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

 


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The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). Provide leadership and technology for the delivery of long life pavements that meet our customers needs and are safe, cost effective, and can be effectively maintained. Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
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