SWUTC Research Project Description
Title of Project: Evaluating the Fundamental Mechanisms of Fatigue Crack-growth in Hot and Warm Mix Asphalt
Project Number: 161022
Principal Investigator:
Amit Bhasin
(512) 471-3667
P.I. Affiliation: University of Texas at Austin
Project Monitor:
Dr. Yong-Rak Kim
Asst. Professor
Dept. of Civil Engineering
University of Nebraska
Lincoln, NE
(402) 472-1727
Project Status: Active
Date Started: 9/1/09
Estimation Completion Date: 8/31/10
Estimated Cost - Current Fiscal: $39,000
Estimated Cost - Total Planned: $39,000
Project Summary:
Project Abstract:
The main objective of this project is to quantify the micro-structure of asphalt mixtures and establish its relationship to fatigue-cracking characteristics of the mix. This will be achieved by using a combination of mechanical tests and imaging tools such as Micro X-Ray CT imaging. In a recent study sponsored by the SWUTC, an image analysis tool was developed to characterize the 3D internal micros-structure of an asphalt mixture. The CT images will be used to determine the distribution of binder or mastic film within the asphalt mixture. The distribution of the binder or mastic micro-structure will be expressed in the form of parameters that can be related to the fatigue-crack growth behavior in the material when combined with its mechanical properties. The materials used in this study will include sand-asphalt mixtures that will be used to produce conventional hot mix asphalt as well as warm mix asphalt. The variables of interest will include binder content, aggregate shape, and aggregate gradation. The influence of these variables on the micro-structure and expected performance of the mixture will be determined. Researchers envision that, ultimately, this information can be fed back into the conventional mixture design procedures to adjust binder contents and gradations in order to achieve improved levels of performance.
Project Objectives:
The main objective of this study is to quantify the micro-structure of asphalt mixtures as a function of mixture characteristics such as aggregate gradation, shape, and binder content and establish its relationship to fatigue-cracking characteristics of the mix. This will be achieved by using a combination of mechanical tests and imaging tools such as Micro X-Ray CT imaging. The following section presents in more detail the tasks that will be used to accomplish this objective.
Task Descriptions:
Task 1. Quantifying the microstructure of sand-asphalt mixtures
The microstructure of a sand-asphalt mixture can be broadly defined as the spatial distribution of the aggregates and asphalt binder within a representative volume element. The main objective of this task will be review, identify, and if required, develop quantitative measures for the microstructure of sand-asphalt mixtures. Statistical tools and measures to characterize the microstructure include Star Volume Distribution (SVD), Star Length Distribution (SLD), Mean Intercept Length (MIL), Auto correlation function, and Principal Component Analysis. This information will be obtained by conducting 3-D image analysis on high-resolution X-Ray tomography images of sand asphalt mixtures. This task will primarily build on the findings from a recently accomplished research study sponsored by the Southwestern University Transportation Center. The protocols to fabricate sand-asphalt mixtures and analyze X-Ray CT data will follow this previous study. High resolution CT imaging will be carried out at the UTCT imaging center.
These analyses will be conducted on sand-asphalt mixtures by varying different mixture properties. The variables that will be included are:
Task 2. Quantifying aggregate shape and gradation
The performance of an asphalt mixture is dictated by its microstructure. However, in practice it is not feasible to fabricate an asphalt mixture and use imaging techniques to obtain information about its 3-D microstructure. A more practical solution is to find the interrelationship between properties such as binder content, aggregate shape and gradation, mixture microstructure, and mixture performance. Unlike microstructure of the asphalt mixture, aggregate shape and gradation are always known as a priori to mixture design. Therefore, these properties can be quantified in the form of an index that can be used to predict the microstructure, and ultimately, the mixture performance.
The main objective of this task will be to review, identify, and if required, develop quantitative measures for aggregate shape and gradation. These quantitative measures, typically some type of an index, will have to be sensitive to the range of values commonly encountered with typical mixtures. The Aggregate Imaging System (AIMS) will be used to measure the shape characteristics (form and angularity) of the aggregate. The binder content and gradation are variables that are specified by the user.
Task 3. Relationship of mixture properties to the microstructure of sand-asphalt mixtures
The main objective of this task will be to map the relationship between the three key properties of the sand-asphalt mixture, i.e. aggregate shape, gradation, and binder to its microstructure. Another variable in this context is the method of compaction and compaction effort. However, for this study the compaction effort and method will not be varied. Information from tasks 1 and 2 will be used to identify the relationship between the mixture properties and the parameters (eg. SVD and SLD) that are used to characterize the microstructure.
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.
Index Terms: