Toward a Mechanistic Approach for Analysis and Design of Asphalt Pavements

Most current analysis and design methodologies of asphalt pavements rely on experience and empirical relationships. These relationships were developed on the basis of results of laboratory tests and by monitoring the performance of actual pavements and sections in accelerated loading experiments. The empirical relationships are limited in accounting for the fundamental properties of the materials, loading configuration, distribution of loads, and variable environmental conditions. In spite of these limitations, fairly good pavements could be constructed until recently, primarily because the lack of appropriate models could be backed up by practical experience.

Relying on experience is, however, not acceptable any more because of the rapidly changing conditions of the road network. First, the significant increase in the capital investment in road construction and the need for maintenance-free roads encouraged the design of improved pavement materials (modified binders, aggregate treatment, layer stabilization, and new designs of asphalt mixes). In addition, economical and environmental factors have put more emphasis on the use of local and secondary recycled building materials. Advancing the material properties and the use of locally available materials should be accompanied by developing experimental and analytical approaches that can accurately determine the effect of these materials on the design of long-lasting pavements and life-cycle cost. Second, the number and loads of trucks have increased beyond expectation as a consequence of the need to increase road transport efficiency. Furthermore, the expected introduction of aramide-reinforced truck tires in combination with foreseeable changes in wheel configurations will result in extremely high wheel pressures.

Recently, a significant change in design philosophy has occurred through the development of the empirical-mechanistic design guide. The mechanistic part refers to the use of layered elastic analysis and viscoelastic material properties in some parts of the guide to determine the pavement response. Empirical relationships are used to link the calculated pavement elastic response at critical points within the structure and damage mechanisms such as permanent deformation and cracking. The empirical-mechanistic design guide includes approaches to account for traffic distribution over time and for the influence of environmental conditions on material properties.

Although the empirical-mechanistic approach is considered a major step, the constitutive material models in this approach are still considered to be oversimplified because they are not able to accurately determine all aspects of the inelastic triaxial and temperature-sensitive response of pavement materials.

Because the interactions among the previously mentioned material, environmental, and loading factors are complex, a need exists to develop proper design and evaluation models that are based on sound mechanics-based principles. For this reason, interest in mechanics-based constitutive models and numerical implementation of these models has recently grown considerably and internationally. These efforts in pavement engineering have greatly benefited from many of the constitutive theories, like viscoelasticity, viscoplasticity, and damage and fracture mechanics, all of which have been successfully applied to such materials as concrete, rocks, and soils. Mechanistic models of pavement materials should be supported with experimental methods for determining model parameters, verifying model predictions and investigating, and the influence of the internal structure’s directional distribution, heterogeneity, and wide range of length scales on damage anisotropy and localization.

The papers in this special issue contribute to different aspects of mechanistic modeling of asphalt pavements by building on well-established approaches that have been used successfully for modeling geotechnical and structural systems. However, this task is not a simple one, since the complexity of the traffic distribution, internal structure, constitutive behavior of asphalt pavement materials, and environmental conditions require modeling efforts that extend beyond what has been done in other systems. A number of papers in this special issue report on the development of constitutive models (e.g., linear viscoelastic, elastoviscoplastic) and their numerical implementation for asphalt pavement materials, with emphasis on asphalt mixes. This issue includes a paper that treats the asphalt mix as a particulate composite material. Another paper focuses on analyzing nonlinear viscoelastic damage and fracture by using finite-element analysis of the internal structure of asphalt mixes. Discrete element modeling of asphalt mixes is also present in this special issue through modeling the internal structure of asphalt mixes and their response to dynamic loading. All papers discuss, in different levels of detail, microscopic and macroscopic experimental methods that have been used to determine model parameters, verify predictions, and monitor damage localization.

The contributions in this special issue demonstrate the ability of mechanistic methods to simulate the response of pavement materials and systems. They also reveal that the mechanistic approach is powerful in quantifying the interaction between the materials and the geometric characteristics of the pavement. Consequently, one can speak of a new generation of integral pavement analyses and design techniques, known as mechanistic techniques, addressing the pavement as a whole and aiming to optimize the overall performance rather than the response of individual components or materials.

We would like to thank Dr. Desai, the editor-in-chief of the International Journal of Geomechanics, for his support of this special issue. We also thank the reviewers for their valuable comments which helped improve the quality and presentation of this issue. On behalf of the authors, we would also like to extend our gratitude to the agencies that funded the research that led to the results published in this special issue.