Microstructural Characterization of Material Properties and Damage in Asphalt Composites

Abstract

Asphalt composites are used to construct 90% of roads in the United States. These composites consist of asphalt binder, which is a product of the refinery process of oil, aggregates, and air voids. Fatigue cracking is one of the most important distresses that causes damage in asphalt pavements. However, there is still a gap in the understanding of the fatigue process of asphalt composites, such as the influence of material properties on this phenomenon and how the material microstructure changes as a result of fatigue damage.

This study focuses on the results of two experiments that were performed on asphalt composites to better understand phenomena related to fatigue cracking: nano-mechanical characterization of the properties of the asphalt composite material and X-ray Computed Tomography nondestructive imaging of damage in the microstructure. These experimental measurements were performed on specimens that are first damaged in the Dynamic Mechanical Analyzer (DMA). The DMA is a tool commonly used for the characterization of fatigue cracking. This test method applies cyclic loads on asphalt composites, damaging them, and in the process determines the viscoelastic properties of the composite throughout the test.

The nano-mechanical characterization experiment gives valuable results of the elastic modulus and hardness of the aggregate, binder, and the aggregate-binder interface that can be used to characterize different binder and aggregate combinations. The nanoindentation experiment successfully measured interface properties in the mix. The interface has elastic modulus and hardness values greater than the binder but smaller than the aggregate. This demonstrates that an interaction between these two phases creates a dissimilar phase between the two.

The second experiment using X-ray CT gives measurements that are indicative of the influences of fatigue damage on micro-level changes in the material microstructure. The results of this experiment revealed important changes regarding the nature of fatigue damage and its relationship to changes in the geometry of air voids and cracks in asphalt composites. The X-ray CT experiment measured size and shape parameters of air voids at 20 microns/pixel resolution at different damage levels. These results illustrated that reduction in bonding strength in the binder is involved in failure in the mix and thus fatigue cracking is not solely responsible for failure. This conclusion is made based on the results not showing a statistically significant change in air void shape and size parameters with increased damage. This is illustrated by viewing changes in the air void structure within the mix, there is no evidence of crack propagation, or drastic changes in the shape, size, or volume of air voids within the mix.