SWUTC Research Project Description
Title of Project: Determining the Microstructure of Sand-Asphalt and Full Asphalt Mixtures Used to Evaluate Fatigue Cracking
Project Number: 476660-00070
Principal Investigator:
Amit Bhasin
(512) 471-3667
P.I. Affiliation: University of Texas at Austin
a-bhasin@mail.utexas.edu
Project Monitor:
Mikhail Magdy
TxDOT Cedar Park
Austin, Texas
Phone: 512-506-5838
mmikhail@dot.state.tx.us
Project Status: Active
Date Started: 2/1/09
Estimation Completion Date: 1/31/10
Estimated Cost - Current Fiscal: $47,570
Estimated Cost - Total Planned: $47,570
Project Summary:
Project Abstract:
The beam fatigue test or overlay test are examples of test methods that are used to evaluate the fatigue cracking resistance of full asphalt mixtures. Although these tests are easy to perform and interpret, the high variability in the test results makes it difficult to evaluate the influence of mixture design variables on its fatigue cracking resistance. More recently, the Dynamic Mechanical Analyzer (DMA) was used to identify the fatigue cracking resistance of sand-asphalt mixtures with improved precision. The findings from these studies have generated interest in the research and transportation agencies to evaluate the performance of sand-asphalt mixtures and propagate this information to predict the fatigue cracking resistance of full asphalt mixtures. However, there is lack of evidence to support the premise that sand-asphalt mixtures replicate the microstructure of the same phase within the full asphalt mixture. The internal microstructure or distribution of air voids and film thickness dictates the fatigue cracking characteristics of the mix.
The main objectives of this research are: i) to compare the internal micro-structure of sand-asphalt mixtures prepared using current procedures to the internal micro-structure of the representative portion of the full asphalt mixture, ii) refine procedures used to produce sand-asphalt mixture specimens to achieve a micro-structure representative of the full asphalt mixture, iii) determine variability in the internal micro-structure of replicate specimens of full asphalt mixtures, and iv) provide a semi-quantitative assessment on the contribution of the variability in internal micro-structure to the variability in the results from fatigue tests conducted on full asphalt mixtures. The internal microstructures of full asphalt mixtures and sand-asphalt mixtures will be determined using the newly acquired high resolution X-ray tomography equipment at the University of Texas at Austin.
Project Objectives:
The following are the main objectives of this research:
The internal microstructure of full asphalt mixtures and sand-asphalt mixtures will be determined using the newly acquired high resolution X-ray tomography equipment at the University of Texas at Austin.
Task Descriptions:
Task 1: Developing a method to quantitatively determine the microstructure
This is the most critical task that constitutes the bulk of this study. The objective of this task will be to develop a procedure to determine the internal microstructure of the sand-asphalt mixtures as well as the full asphalt mixtures. The procedure will constitute of two parts. In the first part a method to scan the specimens using the ultra high resolution X-Ray tomography equipment will be developed. Issues such as sampling and selection of a representative volume for microstructure scanning will be addressed. In the second part, a method will be developed to quantitatively determine the internal microstructure from the 3D digital images. The size and spatial distribution of the air voids and the asphalt binder film will be used to determine the internal microstructure of the specimens.
One of the challenges that will need to be addressed in developing these procedures is as follows. Typically a threshold gray scale value for the 3D image needs to be identified in order to distinguish between the film of asphalt binder, air void, and the aggregate. The classical approach to determine this threshold value is by iteratively adjusting the value until the measured volumetric properties becomes equal to the computed volumetric properties from the 3D images. However, in this study since only small representative volumes of the test specimen will be scanned in order to obtain higher resolution images. Therefore, it will be not be feasible to directly measure the volumetric properties of the representative volume specimen and use it for threshold selection. As an alternative, the use of absolute values to identify the thresholds will be evaluated in this task.
Task 2: Determining the microstructure of select materials
At least three different full asphalt mixtures will be selected for this task. The mixtures will be fabricated using standard procedures prescribed by the Superpave design guide. Sand-asphalt mixtures that represent the fine aggregate matrix portion of these full asphalt mixtures will be fabricated using the current design practice [5]. The procedures developed in Task 1 will be used in this task to determine the microstructure of the selected full asphalt and sand-asphalt mixtures. The distribution of the air void and film thickness in the sand-asphalt mixtures will be compared to similar distributions from the respective fine aggregate matrix portion of the full asphalt mixture.
Task 3: Refining the procedure to fabricate the sand-asphalt mixture
The results from Task 2 will be used to assess whether or not there is a significant difference between the microstructure of the sand-asphalt mixture and the microstructure of the fine aggregate matrix from a full asphalt mixture. In this task, researchers will try to modify the fabrication procedures used for the sand-asphalt mixtures to minimize differences between the microstructures of the sand-asphalt mixtures and the full asphalt mixtures. Parameters such as the normal load and number of gyrations used to compact the specimen will be used to adjust the procedure to fabricate sand-asphalt mixtures.
Task 4: Contribution of microstructure of asphalt mixtures to the variability in fatigue test results
It is often speculated that variability in the microstructure of test specimens is one of the contributors to the variability observed in the results obtained by conducting fatigue tests in full asphalt mixtures. At the end of Task 3, researchers will have data pertaining to the internal microstructures of replicate specimens of asphalt mixtures. The objective of this task will be to determine the contribution of variability in internal microstructure of replicate specimens on the variability in fatigue test results. This will be achieved making semi-quantitative comparisons between the variability in the microstructure of asphalt mixtures to the variability in the fatigue test results. The comparison is semi-quantitative because both procedures (determining the internal microstructure and conducting fatigue tests) are destructive in nature.
Task 5: Preparation of the final report
The final report of this project will document the findings from this research study. In particular, the procedures to quantitatively determine the internal microstructures of asphalt mixtures and sand-asphalt mixtures will be documented. The final report will also include recommendations for future work to improvise the fatigue cracking characterization of asphalt mixtures.
Index Terms:
Asphalt mixtures, Microstructure, Overlays (Pavements), Cracking, Failure, Dynamic mechanical analyzer, Sand, X ray computed tomography, Air voids, Texas, Research projects