SWUTC Research Project Description
Workability of Asphalt Binders at Mixing Temperatures for Hot and Warm Mix Asphalt
University: University of Texas at Austin
Principal Investigator:
Amit Bhasin
Department of Civil and Environmental Engineering
(512) 471-3667
Project Monitor:
Yong-Rak Kim, Associate Professor
362M Whittler Research Center
Department of Civil Engineering
University of Nebraska
Lincoln, Lincoln, NE 68583
Funding Source: USDOT and State of Texas General Revenue Funds
Total Project Cost: $79,285
Project Number: 600451-00062
Date Started: 5/1/12
Estimated Completion Date: 6/30/13
Project Summary
Project Abstract:
Warm Mix Asphalt (WMA) is a relatively new technology that is used produce asphalt mixtures for construction and maintenance of our highways. Benefits associated with the use of this technology include reduced harmful gas emissions and energy consumption during production and placement, increasing serviceable life of the pavement due to reduced aging when properly designed and increased opportunities to use recycled materials. Although there are several different technologies that are available to produce WMA mixtures, the fundamental mechanism by which these technologies allow the production of asphalt mixtures at reduced temperature is not well understood. In addition, the influence of production temperatures on the mixing and interfacial bonding characteristics of recycled asphalt binder with new asphalt is very poorly understood. The main objective of this study is to evaluate the wetting and coating ability of asphalt binders at the mixing temperatures associated with HMA and WMA technologies. This will be achieved by using the maximum bubble pressure method to directly measure the surface properties of several different asphalt binders with and without modifications. In addition, the influence of additives on the viscosity of the binders at different temperatures and shear rates will also be determined. This information will be used to explain the role of the additives in improving binder workability at the mixing temperatures. Finally, we will also attempt to use a combination of different spectroscopic techniques and mechanical tests to evaluate the role of temperature on the interfacial bonding and mixing that occurs between the fresh binder and the recycled binder.
Project Objectives:
The main objective of this study is to evaluate the wetting and coating ability of asphalt binders at the mixing temperatures associated with HMA and WMA technologies. The PI has specialized in measuring the surface properties of aggregates and asphalt binders, including surface free energy. However, to the best of the PIs knowledge there has not been a study to measure these properties at mixing temperatures with and without chemical additives used for WMA production. In this study, we will use the maximum bubble pressure method to directly measure the surface properties of several different asphalt binders with and without modifications. We will also measure the viscosity of the binders as a function of different shear rates at these temperatures. This information will be used to explain the role of the additives in improving binder workability at the mixing temperatures. More importantly, this information will highlight the relative contributions of the surfactant effect and viscosity reduction on the improved workability of asphalt binders at reduced mixing and compaction temperatures associated with the WMA.
The second objective of this study is to evaluate the role of temperature on the interfacial bonding between aged asphalt binders (such as that from recycled asphalt pavement) to fresh asphalt binder. Two-dimensional FTIR mapping will be used across interfaces of fresh and aged asphalt binders to determine the extent of co-mingling between the two binders as a function of temperature. The poker chip geometry will be used to evaluate the mechanical properties of the aged – fresh asphalt binder composite as a function of temperature.
Task Descriptions:
Task 1. Literature Review
A detailed literature review will be conducted with emphasis on the following three areas.
- Methods to measure the surface tension of asphalt binders at mixing temperatures.
- Methods to measure the mechanical properties of composite in a manner that is sensitive to the interface.
Task 2. Workability of asphalt binders
The workability of different asphalt binders at the mixing temperatures will be determined based on its surface tension and viscosity. Different asphalt binders will be selected for this task. The asphalt binders will be modified using different types of WMA additives. The WMA-asphalt binder combinations will then be used to determine the influence of additives and binder types on the viscosity of the binder at elevated temperatures. The viscosity measurements will be conducted using a Brookfield viscometer at elevated temperatures and different shear rates. The influence of binder type and additive on the surface tension of the binder will also be determined using the maximum bubble pressure method. This is a relatively simple technique that can provide direct measurements of the surface tension of the asphalt binder at mixing temperatures. Both the equilibrium surface tension as well as the dynamic surface tension will be measured using this technique.
Task 3. Interaction with aged binder
The ability of an asphalt binder to adhere or co-mingle with aged asphalt binder will be determined as a function of temperature. A spatial FTIR mapping technique will be used. In addition, mechanical tests on a two-piece composite specimen comprising of aged asphalt binder and fresh asphalt binder will also be conducted. The mechanical tests will be used to determine the effect of temperature, binder type and additive on the rate and quality of the formation of the interfacial bond between the RAP and the fresh binder. The poker chip geometry will be used for theses tests.
Task 4. Documentation of the research findings
A final report documenting the detailed procedures, results, conclusions and recommendations will be prepared at the end of the project. In addition, the results and findings from this research study will also be documented and submitted for consideration as a journal publication.
Implementation of Research Outcomes:
Asphalt mixtures are produced by heating the asphalt binder until it turns to a low viscosity liquid and then using the liquid binder to coat mineral aggregate particles (stones of various sizes). Warm mix asphalt (WMA) is a new technology that allows the production of asphalt mixtures at temperatures that are lower than those used to produce conventional hot mix asphalt (HMA). The benefits of using WMA include energy savings, reduced emissions and extended service life of the pavement. However, chemical additives are needed in the asphalt binder in order for the binder to flow and coat the aggregate particles at these lower temperatures. These chemical additives claim to lower the surface tension of the binder and allow mixture production at reduced temperatures. Although there are several commercially available chemical additives, there is no study that documents the reduced surface tension achieved using these additives. This study, perhaps for the first time, measured the surface tension of asphalt binders with and without these additives at typical mixing and compaction temperatures.
Products developed by this research:
Planned Presentations: Tentative presentation title: Surface Tension of Asphalt Binders with and without Warm Mix Additives at Typical Mixing and Compaction Temperatures, Amit Bhasin, to be presented to producers of asphalt binders and additive manufacturers, State DOT and FHWA personnel.
Journal Article in Preparation: Surface Tension of Asphalt Binders with and without Warm Mix Additives at Typical Mixing and Compaction Temperatures, Amit Bhasin, to be submitted to the International Journal of Pavement Engineering.
Impacts/Benefits of Implementation:
The knowledge from this study can be used to explain mechanisms that allow the production of WMA and assess the efficacy of different chemical additives. The findings from this study have already been used to address a specific problem in an ongoing NCHRP project. The research methodology was also solicited for incorporation for another research study in Michigan.
The findings from this research are interesting in the sense that they present the mechanism of aggregate coating in an entirely new light. We hope that DOT engineers can use this knowledge to make more informed decisions while approving chemical additives for use in WMA. Finally, the results from this study have also provided indirect evidence for a model of the asphalt binder that has been hypothesized for several years now.
Web Links:
Final Technical Report