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Dynamic Mechanical Analysis of Fine Aggregate Matrix The research team developed a test method to evaluate the fatigue properties of fine aggregate matrix using the Dynamic Mechanical Analyzer (DMA). The DMA testing is used with the crack growth index developed at Texas A&M to define the fatigue cracking characteristics of different fine aggregate matrices. Detailed protocols for preparing specimens, conducting tests and analyzing the data using the DMA were documented as a standard procedure. The specimen preparation procedure yields approximately 15 test specimens from a single SuperPave gyratory compactor specimen. This not only reduces the specimen preparation time but also provides an adequate number of test replicates for conducting fatigue tests (using the DMA) and to make statistical inferences and comparisons. The crack growth index developed at Texas A&M is used in the DMA testing to define the fatigue cracking characteristics of different fine aggregate matrices. The crack growth index is based on the principles of fracture mechanics and dissipated energy. The index also incorporates the material properties determined from testing on the Scale 1 material (the components of the mixture: aggregate, binder, and filler). Moreover, results obtained thus far indicate that the crack growth index can be used to characterize fatigue cracking of mixtures independent of the mode of loading (controlled stress versus controlled stress), within certain limits. Test protocols and analytical tools were also developed to identify the maximum threshold stress or strain amplitude that results in a non-linear viscoelastic response without causing incremental damage to the specimen. The crack growth index derived from DMA testing has proven to be a reliable indicator of damage. It lowers the variability in the DMA fatigue tests to a point at which statistical inferences can be made about candidate fine aggregate mixtures (Scale 2 analysis). Fatigue testing at this scale makes it easier to identify the impact of not only mix components but the properties of these components, such as the surface energy of aggregate or binder or the adhesive or cohesive bond strength between aggregate and binder. Furthermore, different combinations of aggregate and binder can be efficiently evaluated using the DMA test. The same can be said for the impact of mineral filler on the rate of change in dissipated pseudo strain energy, which is charted in the DMA test.
Diffusion of Moisture in Asphalt Binders Moisture damage is the result of deterioration of mixture properties caused by one or more damage mechanisms triggered by moisture or water. Examples of damage processes include loss of adhesion at the binder–aggregate interface, weakening and disintegration of the binder or mastic from the intrusion of moisture, and spontaneous emulsification. One of the fundamental material properties that influence the rate of moisture damage in asphalt mixtures is the rate of moisture diffusion through asphalt binders. The Texas A&M research team has developed a test method using spectroscopic techniques to determine the diffusivity of water through thin films of asphalt binders. Work is underway to extend this test method to determine the rate of interfacial damage caused by displacement of the binder–solid interface by moisture. Gravimetric measurements are also being used to determine the rate of moisture diffusion through the Fine aggregate matrix as a function of temperature.
Healing Self-healing in asphalt binders during rest periods causes at minimum a reduction in the rate of damage and in some cases a reversal of crack growth. In all cases, healing results in an extension of the life of the asphalt mixture and the pavement. Recently, the ARC developed a model that defines the healing process in asphalt binders. The model is a convolution of two processes that represent the two fundamental actions that cause self-healing: wetting of crack surfaces and strength gain across the wetted interface. The model for wetting of crack surfaces is based on the approach developed by Schapery, which defines the rate of crack closure. This model takes into account fundamental material properties such as creep compliance of the binder and surface free energy. The model for strength gain across wetted interfaces (also referred to as intrinsic healing) is based on the Avrami equation used to model the kinetics of crack growth, or most growth processes for that matter, e.g. crystal growth. A simple test method using the dynamic shear rheometer (DSR) was developed to determine the intrinsic healing properties of different asphalt binders. By using simulations of molecular dynamics, the Texas A&M researchers determined that the molecular properties of asphalt binders play an important role in their healing characteristics. The molecular simulations are also being used to validate the mechanisms of healing that form the basis of the healing model. ARC
REFERENCES For more on the computation and use of the crack growth index, see: |
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Asphalt Research Correspondent |
