U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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Publication Number: FHWA-HRT-10-065
Date: December 2010
Compaction is a process that reduces the volume of a mixture of hot asphalt binder, aggregates, filler materials, and air voids by the application of external forces to form a dense mass. This densification causes an increase in the unit weight of the material and improves the aggregate interlock. The goals of compacting an asphalt pavement are to achieve an optimum air-void content, to provide a smooth riding surface, and to increase the load-bearing capacity of the material under construction.(1) Improper compaction generally leads to poor performance from the asphalt pavement in spite of all other desirable mixture-design characteristics being met. This can lead to premature, irreparable damage to the built-in infrastructure. Such damage typically manifests in the form of rutting, permanent deformation, cracking, and moisture damage. At the elevated temperatures at which compaction is typically performed, hot-mix asphalt (HMA) is a nonlinear fluidlike material. Toward the end of the compaction process, the material slowly transforms into a nonlinear, highly viscous viscoelastic fluid. The challenge to modeling compaction is choosing an appropriate material model capable of representing such a transformation.
Compaction is an important process that has significant impact on the performance of asphalt pavements. Poor compaction of asphalt mixtures makes them susceptible to permeation of oxygen and moisture. As a result, these mixtures become more prone to failure mechanisms such as cracking and permanent deformation.
This study targeted the development of a theoretical and computational platform that can be adapted for the simulation of the typical compaction processes: laboratory compaction using a gyratory compactor and field compaction using a sequence of rolling passes. There are no effective methods currently available that are capable of addressing the simulation of these two processes. Efforts were undertaken in this study to fill this void and to provide a reasonable starting point to modeling compaction of HMA. The model developed and employed is an isothermal model that can be adapted in the future to account for nonisothermal phenomena and is capable of exhibiting a compressible viscoelastic fluidlike response when stimulated by external mechanical means. The model was implemented in the finite-element (FE) method to simulate the conditions most relevant to laboratory and field compaction processes.
This study aimed to provide a better understanding of the modeling aspects of HMA compaction and has the potential to lead to the development of more general integrated approaches to performance prediction and design improvement of pavements such as intelligent compaction (IC) systems. Once an appropriate computer-simulation environment is in place, users will be able to quickly adapt it to provide feedback to the controller of the compaction process in a highway construction project.
The primary objectives of this research project are as follows:
The modeling of asphalt mixtures is rather extensive and complex. It would be too ambitious to attempt to solve the issues of the whole field in a single research project. Therefore, restrictions were applied to limit the scope of the current research. This project is restricted to deducing material parameters for a single mixture type, HMA. A second restriction is that researchers have assumed isothermal conditions during laboratory and field compaction processes. The third restriction is that simulations for rolling compaction have only considered static steel rolling. Despite the limitations, the modeling and simulation framework developed here does work for HMA mixtures for static steel rollers. Researchers expect that the system can be adapted to work for other mixture-roller combinations as long as they have comparable characteristics.
The following is an outline of the remainder of this report:
The notations used in this report are similar to those used in standard continuum-mechanics texts. Vectors and tensors (second-order and fourth-order tensors) are represented as follows:
The standard gradient and divergence operators are employed as presented in figure 1 and figure 2.
Figure 1. Equation. Gradient and divergence operators with respect to reference configuration
Figure 2. Equation. Gradient and divergence operators with respect to current configuration.