This project pursues the idea of creating model asphalt mixtures, comprised of only 5-10 compounds, that replicate several physical properties of SHRP core asphalts while possessing chemical functionalities consistent with those of real asphalts, as based on experimental characterizations available from the literature. Such mixtures would not replace asphalts in engineering applications. Instead, they provide inputs for fundamental studies in order to elucidate why different asphalts exhibit different physical properties, and how those properties could be tuned to more desirable values.
Highway
Asphalts are complicated, poorly defined, and inexpensive mixtures of hundreds of chemical compounds. Even with well-documented samples, such as the Strategic Highway Research Program-sponsored "core", determining specific and effective strategies for attaining targeted properties is a difficult process. This project pursues the idea of creating model asphalt mixtures, comprised of only 5-10 compounds, that replicate several physical properties of SHRP core asphalts while possessing chemical functionalities consistent with those of real asphalts, as based on experimental characterizations available from the literature. Such mixtures would not replace asphalts in engineering applications. Instead, they would provide inputs for fundamental studies in order to elucidate why different asphalts exhibit different physical properties, and how those properties could be tuned to more desirable values. Analogous techniques based on solution thermodynamics have been developed in fuel science. The fuel research results were mixtures (also called "recipes") of 5-10 chemicals that exhibit physical properties, such as temperature-dependent volatility, that resemble those of real fuels. The direct outcome of this project will be sets of mixture compositions that are predicted to exhibit physical properties comparable to those of the core asphalts. Such mixtures will create a future opportunity to test new additive strategies using modeling tools in advance of experiments. Their molecular-level detail will enable assessing why particular additive strategies succeed or fail, allowing for further science- and engineering-based improvements.
This project will develop appropriate methods for asphalts using "molecular simulations": statistical mechanics-based tools for predicting microscopic and macroscopic properties based on the details of molecule-molecule interactions. These tools statistically track the mechanics and dynamics of interacting atoms and molecules. The output of such large-scale computer simulations are predictions of macroscopic-level properties and activities. Required inputs are the chemical identity and relative concentrations of the simulated molecules. Parameters are based on molecular-level chemical identity and, if well-chosen, can lead to predictive methods.
Two common molecular simulation approaches - Monte Carlo and molecular dynamics - will be applied in the proposed work. In Monte Carlo simulation, random motions and configuration changes are applied to molecules in order to compute averages across the different arrangements and molecular organizations that are consistent with a specified energy, temperature, and applied stress. Proposed changes are accepted or rejected by comparing the corresponding probability to a random number, and the name "Monte Carlo" refers to an analogy in which games of chance are won or lost based on a random result (such as in a Lotto game). Each individual molecular configuration provides a single estimate of a physical property, and macroscopic predictions arise from millions of such individual contributions.
In molecular dynamics simulation, individual molecular arrangements and orientations are probed by following how molecules move in response to intermolecular forces through a numerical solution to Newton’s equations of motion, F = m a. The forces arise from the potential energy between molecules, which depends on the local molecular arrangements. The resulting accelerations cause changes to the molecule velocities, which then lead to new arrangements. As in Monte Carlo, each set of molecular arrangements leads to a physical property estimate, and the macroscopic prediction emerges from a large number of contributions. The Newtonian equations of motion are integrated numerically via discrete time steps, each of order 1 fs (10-15 s). Trajectories of order 1-10 ns (10^-9 -10^-8 s) are possible using significant computing resources (days to weeks of run time). Appropriate facilities are available in the principal investigator’s research lab.
One future application of creating a model asphalt mixture is developing and screening different proposed modification strategies. Why do certain additives affect high or low temperature properties? What kinds of polymers, copolymers, or plasticizers might be most compatible? Do time-temperature superposition concepts developed in polymer science apply to original or modified asphalt systems? This initial research project will generate base-case model asphalt mixtures for addressing such questions. Follow-up studies would target examples from the experimental asphalt literature in order to check the simulation predictions and explain the underlying reasons for successful additive effects. An intermediate-term step (beyond the 12-month scope of this project) is to establish collaborations with a laboratory experiment-based transportation research group. Both groups would jointly propose strategies and improvements, assess the proposed improvements using the modeling tools developed, and finally test them experimentally. The long-term application is thus creating modified asphalts that exhibit superior physical property characteristics for highway use.
Task 1. 1/1/04-3/31/04 Literature Search and Initial Formulation
Several studies used the SHRP "core asphalts" investigate relationships between asphalt structure and properties. We will conduct a comprehensive review with two aims: (1) to collect experimental asphalt property data that can be estimated from molecular simulations, (2) to learn which asphalt molecular structure theories are most appropriate for interpreting molecular-scale results. In addition, molecules will be identified whose presence in asphalt is consistent with NMR asphalt speciation data and for which experimental glass transition data are available in the literature.
Task 2. 1/1/04-3/31/04 Simulation Validation for Initial Compounds
The next task is to validate the molecular simulation approach using pure compounds that are similar in size, shape, and chemical functionality to those found in asphalts. An example is predicting the glass transition of moderate sized aromatic compounds, such as alkyl-substituted naphthalene molecules. The intent is to demonstrate that molecular simulation approaches using parameterized force fields are sufficiently accurate for moderate molecular-weight compounds, the sizes present in asphalts. Accurate calculations for pure molecules are a prerequisite for accurate mixture calculations.
Task 3. 1/1/04-6/30/04 Initial Mixture Simulations
Mixtures comprising 5-10 of the molecules identified above will be selected at relative concentrations consistent with those in asphalt. Simulations will be conducted using these compositions to determine equilibrium density, expansion coefficient, glass transition range, and moduli for each proposed asphalt mixture. Interpretations of property changes with composition will address the “whys” of asphalt function.
Task 4. 4/1/04-12/31/04 Mixture Composition Iteration.
The model asphalt mixture compositions will be modified in successive simulations in order to achieve different predicted physical properties. The results from Task 3 will be compared to literature data for an SHRP core asphalt. Differences will be quantified between the predicted results for the model composition and the experimental (literature) results for the core asphalt. Mixture concentrations will be changed and new simulations will be conducted. The goal is to obtain predicted results from the model system that are comparable to those of the desired experimental system. This process will iterate until convergence is as good as possible. It is expected that many iterations will be required. Compositions representative of as many SHRP core asphalts as possible will be identified during this period. The order of core asphalts addressed will be decided in consultation with RIDOT.
Task 5. 10/1/04-12/31/04 Final Report
A comprehensive report will document the simulation methods, findings, and conclusions. The report will include all final model asphalt compositions that are predicted to reproduce properties of SHRP core asphalts. These compositions could then be used in subsequent property prediction studies, using both modeling and experiments.
Task 1. 1/1/04-3/31/04 Literature Search and Initial Formulation
Task 2. 1/1/04-3/31/04 Simulation Validation for Initial Compounds
Task 3. 1/1/04-6/30/04 Initial Mixture Simulations
Task 4. 4/1/04-12/31/04 Mixture Composition Iteration.
Task 5. 10/1/04-12/31/04 Final Report
$84,241.20 ($84,241.00 Yearly)
Research Assistantship (Academic year and summer) for Liqun Zhang, URI Chemical Engineering Department.
Overlap in molecular simulation techniques used with non-transportation research in the Greenfield lab (molecular simulation of ultraviolet absorber additives in polymers).
Seminar presentations at a national asphalt meeting, at a transportation meeting in Washington, and at AIChE.
More complete understanding of asphalt fundamentals and modification strategies.
asphalt, asphalt chemistry, asphalt composition, asphalt model, glass transition, molecular simulation, molecular dynamics, Monte Carlo, asphalt speciation