One key application and use of creating model asphalts is developing and screening different proposed modification strategies. The overall objective will be to identify the molecular-scale mechanisms that underlie differences in measured physical properties, such as glass transition or modulus, between unmodified and modified asphalts. Specifically,
1. What underlying intermolecular interactions, forces, and orientations lead to the properties that specify which asphalts are better for cold or hot environments?
2. Why do certain additives affect high or low temperature properties?
3. What asphaltene, resin, or maltene interactions make different kinds of polymers or copolymers most compatible with asphalts?
4. Do time-temperature superposition concepts tested in SHRP for asphalts transfer directly to molecular simulations of model and modified model asphalt systems? What deviations are seen, and what are the implications for unmodified and modified asphalts?
The initial research projects generated base-case model asphalt mixtures for addressing such questions, and this continued follow-up study will expand on examples from the experimental asphalt literature in order to check additional simulation predictions and to explain the underlying reasons for successful additive effects. The result of this follow-up project will be a demonstration of how modifications to the physical properties of model asphalt compositions, when combined with model additives such as polymers or cross-linked rubber, compare to the physical property modifications found in real asphalt/modifier samples compared to pure asphalts.
Highway
Asphalts are complicated materials formulated so they meet specified criteria for physical properties under extreme pavement temperature conditions. Polymer modifiers have the capability to improve properties so an asphalt meets a more challenging specification (e.g. specified creep compliance at yet higher temperatures and failure strain and stiffness at yet lower temperatures). The link between real-world benefits and laboratory test improvements via polymer modifications is still a matter of some debate, however.
In this work, it is proposed that molecular-scale testing of asphalt additive strategies be continued, again using modeling tools in advance of experiments. Prior studies have developed model asphalt mixtures for use in computer simulations and have tested effects of one type of polymer modifier. Good correlation was found for some properties, and the available molecular configurations provided packing details (related to force transmission) and insights that are not experimentally accessible for real asphalts. In the proposed work, molecular dynamics simulations will lead to predictions of stress correlation functions in these modified model asphalt mixtures, which can be transformed into low-strain compliance and modulus predictions comparable to those specified by Superpave for true binders. Molecular configurations and orientations will quantify how packing and force transmittal change in the presence of different modifiers. The underlying hypothesis is that understanding molecular-level mechanisms in model asphalt / model additive combinations will facilitate assessing additive strategies in true asphalt systems. Potential long term applications include interpreting screening tests that relate asphalts in fundamental and applied settings.
Task 1. Asphalt Modifier Selection
* Expand the range of asphalt modifiers beyond those under study in the current project.
Modifiers of most interest to the Rhode Island Department of Transportation and its contractors will receive preference, to the extent that simulations can be reasonably conducted, with a special focus on additives appropriate for unmodified asphalts similar to those modeled using the simplified asphalt mixtures. The availability of literature data for modified asphalt mechanical properties will also be taken into account and documented during this selection process.
Task 2. Simulations of Asphalt/Modifier Mixtures and Comparisons with Literature Data
* Conduct molecular simulations on the selected model asphalt/modifier mixtures.
The predicted results will be compared to experimental data from the literature for real asphalts with the same modifiers. The simulation results are expected to reproduce the expected additive effects. Composition models will be improved as necessary to include suggestions from other practitioners. Polymer modifiers to be simulated include individual chains of polystyrene, polybutadiene, and polyethylene. Molecular simulations of chemically modified crumb rubber will also be conducted, if sufficient chemical details can be inferred. Precisely defined mixtures related to asphalts that can be attained experimentally (e.g. well defined and commercially available compounds only) can also be simulated, upon specific request by RIDOT. Corresponding experimental measurements will need to be conducted by a collaborator or external lab.
Task 3. Chemical Effects on Asphalt Systems
* Probe chemical effects by simulating altered model asphalts and polymer-modified asphalts.
The molecular-level changes will be chosen with an intent to replicate the reactions that occur in the presence of naturally occurring catalysts (e.g. aging processes) and engineered reactions that occur during some modification processes.
Task 4. The "Whys" of Polymer Modifier Function
* Deepen the interpretation of asphalt/modifier simulations and identify distinctions that occur in the presence of different modifiers.
In particular, the specific molecular-level interactions that change overall modified asphalt properties will be sought out. This will enable exploring and interpreting the "whys" of asphalt function. New simulation interpretation techniques will be developed as necessary to assess the impact of modifiers on physical properties. Such results and consequent new knowledge will be used to improve and optimize model asphalt and asphalt/modifier compositions.
April 2006 Select set of target asphalts and additional modifier combinations of interest. Find appropriate literature data that describe modification effects.
June 2006 Revise initial model asphaltene mixtures. Attain next model asphalt/modifier combination that replicates, in molecular simulations, physical property modifications for a modified asphalt of interest.
November 2006 Present current work stage at American Institute of Chemical Engineers (AIChE).
December 2006 Complete studies of next mixtures. Develop next level of "why" questions.
January 2007 Present unmodified and modified asphalt modeling work at TRB.
Summer 2007 Complete answers to next set of "why" questions. Complete simulations designed with experimentalist collaborators and complete next mixture simulations. Present findings at Petersen Asphalt Conference. Prepare publications and final report.
Fall, Dec. 2007 Participate in URITC symposium; publish final report.
$146,858.86 ($146,859.00 Yearly)
Graduate student assistantship for a Ph.D. student, including summer employment; paid employment for an undergraduate researcher
Extends joint URITC/RIDOT projects 216 and 506. Those projects developed model asphalts and first tested modifier effects.
The principal investigator's research focuses on molecular simulations of polymers, liquids, and additives. One graduate student in the group is working on the first phase of this project. The other graduate student in the group is using Monte Carlo, molecular dynamics simulations, and geometric analysis calculations to study packing and function of polythiophene polymers. Independently, the principal investigator is conducting simulations of molecular tribology and of diffusion through polymers. While the research content is separate from these other projects, the methodologies are similar. This has already allowed the graduate student conducting asphalt simulations (in the previous phase of the project) to benefit from intra-group mentoring, discussions, and interactions.
Results will be presented to a chemical engineering audience at the Annual Meeting of the American Institute of Chemical Engineers (AIChE) and a chemistry audience at the American Chemical Society. Results will be communicated to a transportation audience at the Transportation Research Board (TRB) Annual Meeting and/or at the Petersen Asphalt Conference, as well as via URITC and RIDOT. Project reports will be converted into journal articles for widespread long-term accessibility.
One key application of creating model asphalt and modified asphalt mixtures is developing and screening different proposed modification strategies. Why do certain additives affect high or low temperature properties? Is it possible to provide low-temperature flexibility and high-temperature rutting resistance simultaneously? What kinds of polymers, copolymers, or plasticizers might be most compatible? What are the limits and deficiencies of time-temperature superposition concepts when applied to modified vs unmodified asphalt systems? This follow-up study will begin with base cases from the initial research projects and will target examples from the experimental asphalt literature in order to check the simulation predictions and explain the underlying reasons for successful additive effects.
The results will ultimately be useful to the materials area within the Research and Technology section of the RIDOT Transportation Development section. Research progress will be shared with Colin Franco and Francis Manning, RIDOT Research and Technology Development, on an ongoing basis. Franco and Manning will help to identify the most promising findings that can move towards direct highway tests via continued laboratory testing.
Asphalt, asphalt chemistry, asphalt composition, asphalt model, glass transition, molecular simulation, molecular dynamics, Monte Carlo