Introducing a Vision for a Virtual  Asphalt Concrete Testing Laboratory 

To realize the vision of a fundamental predictive model of asphalt concrete properties requires a multiscale, multiphysics approach involving the cooperative involvement of many research entities.  In short, a chemo-mechanical model for asphaltic concrete is a tall order.  The Asphalt Research Consortium is taking it on.

The chemo-mechanical model will be useful for predicting the performance of asphaltic materials in roadway construction applications. It will have enormous importance to researchers, state DOTs, and the paving industry who are seeking to improve pavement performance.

The time for developing the model, tentatively called the Virtual Asphaltic Concrete Testing Laboratory (VACTL), is now.  Much has been learned about the fundamental chemistry and physics of bitumen over the last 20 years, and especially over the last decade.  Also, during that time, computers have become exponentially more powerful. 

A wealth of experimental evidence indicates that asphalt binders, which are composed of complex petrol-organic molecules, are capable of dynamic structural changes, including phase separation, flocculation, crystallization, and melting. In the past two decades, several powerful computer modeling methods have been developed that are capable of simulating/predicting these kinds of phenomena, and we expect that these models can be used to help predict material softening and embrittlement in pavement performance.

As Mehta et al. (2004a) have shown, there are well-established ties between phase field model parameters and the properties of the system that may be obtained by upscaling thermodynamic and kinetic properties from molecular mechanics models of the asphalt binder, such as viscosity, composition-dependent free energies, interfacial free energies, and elastic moduli.

At the largest length scales, asphalt concrete is a suspension of randomly shaped aggregate particles in a complex viscoelastic suspension. Modeling a material as complex as asphalt binder will rely heavily on cooperative theoretical approaches grounded in molecular mechanics/dynamics simulations, non-equilibrium statistical mechanics (Glansdorff and Prigogine 1971) (i.e., rate dependent thermodynamic phase-field models [Cahn 1959, 1965; Cahn and Hilliard 1958, 1959]) and continuum mechanics conducted at multiple scales, including molecular/ nano, micro, meso and macro scales.

This kind of integrated project is already being carried out for Portland cement and concrete in the Virtual Cement and Concrete Testing Laboratory (VCCTL) (Bullard et al. 2004; Bentz et al. 2006).  That project began in 2001 and is approaching completion.

The goal for the Virtual Asphaltic Concrete Testing Laboratory is the same as that for the VACCTL: to simulate microstructure and properties with known chemistry and physics so that a range of tests can be carried out in the computer rather than the laboratory. 

Asphalt Research Consortium (ARC) researchers will use the materials, data, and performance evaluations available from the pavement material comparison field sites to help validate and calibrate performance predictive test methods and models being developed in the Moisture Damage, Fatigue, and Engineered Materials program areas.

The tasks and expertise necessary to develop the Virtual Asphaltic Concrete Testing Laboratory are disparate and highly specialized.  WRI is coordinating a team of scientists and engineers from the University of Rhode Island, Virginia Technological University, the National Institute of Standards and Technology, and Delft University of Technology, Netherlands, and WRI to get the job done. ARC 

Historical Note:

In asphalt pavement performance investigations conducted in the mid 1900s (Hveem 1943), failure modes were often attributed to the “chemical action” of the binder (Robertson et al. 2001).  Four properties of the binder materials were deemed critical to pavement quality (Hveem 1943). These were (1) consistency, (2) durability, (3) curing rate, and (4) resistance to water. 

Consistency relates to the flow properties of asphalt.  Durability relates to changes in the consistency of asphalt materials, usually as a function of temperature and time.  An example is the change in material properties caused by oxidative age hardening.  Cure rate relates to how resistant an asphalt–aggregate mixture is to “setting” during construction. Mixtures that shove during compactive rolling, for example, are considered “tender” and are immediately subject to permanent deformation as soon as they are laid down.  Finally, asphalt pavement quality was and is thought to rely heavily on its resistance to water, which directly relates to moisture damage.  

These failure modes are still being vigorously investigated today.  Sixty-five years after Hveem, there is still no comprehensive understanding of the material properties and fundamental mechanisms that relate to asphalt pavement longevity and failure.  That is why the Asphalt Research Consortium is initiating a cooperative approach to asphalt composition “microstructure” modeling. ARC

REFERENCES

 

Bentz, D. P., E. J. Garboczi, J. W. Bullard, C. F. Ferraris, and N. S. Martys, 2006, “Virtual testing of cement and concrete,” Significance of Tests and Properties of Concrete and Concrete-Making Materials, Joseph Lamond and James Pielert, eds., ASTM STP 169D.

Bullard, J. W., C. F. Ferraris, E. J. Garboczi, N. S. Martys, and P. A. Stutzman, 2004, “Virtual Cement and Concrete,” Innovations in Portland Cement Manufacturing, Chapter 10.3, J. I. Bhatty, F. M. Miller and S. H. Kosmatka, eds., Portland Cement Association, pp. 1311-1331.

Cahn, J.W., 1959, ”Free Energy of a Nonuniform System. II. Thermodynamic Basis.”  J. Chem. Phys., 30 (5), 1121-1124.

Cahn, J. W., 1965, “Phase Separation of Spinodal Decomposition in Isotropic Systems,” J. Chem. Phys., 42 (1), 93-99.

Cahn, J. W., and J. E. Hilliard, 1958, “Free Energy of a Nonuniform System. I. Interfacial Free Energy.”  J. Chem. Phys., 28 (2), 258-267.

Cahn, J. W., and J. E. Hilliard, 1959, “Free Energy of a Nonuniform System. III. Nucleation in a Two-Component Incompressible Fluid.”  J. Chem. Phys., 31 (3), 688-699.

Glansdorff, P., and I. Prigogine, 1971, Thermodynamic Theory of Structure, Stability and Fluctuations, Wiley-Interscience, division of John Wiley & Sons, Ltd., London.

Hveem, F. N., 1943, “Quality Tests for Asphalts – A Progress Report.”  Proceedings, Association of Asphalts Paving Technologists, 15: 111-152.

Mehta, R., W Keawwattana, and T. Kyu, 2004, “Growth Dynamics of Isotactic Polypropylene Single Crystals During Isothermal Crystallization from a Miscible Polymeric Solvent.”  J. Chem. Phys., 120(8): 4024-4031.

Robertson, R. E., J. F. Branthaver, P. M. Harnsberger, J. C. Petersen, S. M. Dorrence, J. F. McKay, T. F. Turner, A. T. Pauli, S.-C. Huang, J.-D. Huh, J. E. Tauer, K. P. Thomas, D. A. Netzel, F. P. Miknis, T. Williams, J. J. Duvall, F. A. Barbour, C. Wright, 2001, “Fundamental Properties of Asphalts and Modified Asphalts, Volume I:  Interpretive Report, FHWA-RD-99-212.”  U. S. Department of Transportation, Federal Highway Administration, McLean, VA.

Asphalt Research Correspondent