The Use of Atomic Force Microscopy to Evaluate Warm Mix Asphalt

The main objective of this study was to use the Atomic Force Microscopy (AFM) to examine the moisture susceptibility and healing characteristics of Warm Mix Asphalt (WMA) and compare it with those of conventional Hot Mix Asphalt (HMA). To this end, different AFM techniques such as the tapping mode imaging and force spectroscopy experiments were conducted on two types of asphalt binders produced using various WMA technologies as well as a conventional HMA. The considered WMA technologies included: Advera, Evotherm M1, Sasobit, and foamed WMA. Dynamic Shear Rheometer (DSR) tests were conducted on the evaluated binders, and AASHTO T283 test was performed on mixtures prepared using those binders. The results of the AFM imaging showed that while the Sasobit additive has reduced the dimensions of the so called ‘bee-like’ structures within the neat and polymer modified asphalt binders, the other WMA technologies did not have any significant effect on these structures. In addition, the Sasobit resulted in increasing the relative stiffness of dispersed domains containing the ‘bee-like’ structure in comparison with the flat asphalt matrix for both types of binders, which explained the higher shear modulus values obtained in the DSR test for binders with this WMA additive. The results of the AFM force spectroscopy experiments indicated that all WMA technologies resulted in increasing the nano-scale adhesive forces for both types of asphalt binders prior to moisture conditioning. Advera and foamed WMA had the highest improvement to these forces, while the Sasobit had the least. This might be the cause for the lower indirect tensile strength value that was obtained for the Sasobit mixtures in comparison to other WMA mixtures. The AFM results also showed that the adhesive forces were significantly reduced due to moisture conditioning of the control and WMA 64-22 binders. However, the control and Evotherm WMA binders exhibited the least reduction, while the Advera WMA binder had the highest decrease, which may have contributed to reducing the tensile strength ratio values of the Advera 64-22 mixture. The Sasobit and Advera was also found to reduce the nano-scale cohesive forces within the considered asphalt binders upon moisture conditioning, indicating that it might adversely affect the cohesive bonds within the asphalt binder. The results of the AFM force spectroscopy experiments also suggested that the AASHTO T283 test results primarily depend on the adhesive forces between the aggregate and the binder. The AFM healing experiments indicated that all WMA technologies except the Sasobit resulted in improving the micro-crack closure rate in both types of asphalt binders considered in this study. In addition, only the Sasobit resulted in significant decrease in the cohesive bonding energy; indicating that it might adversely affect the intrinsic healing of the considered asphalt binders. On the contrary, the other WMA technologies improved the –OH cohesive bonding energy and did not significantly influence the –COOH cohesive bonding energy for both asphalt binders. Finally, the results of this study indicated that the AFM is a viable device to study the moisture damage and healing phenomena in asphalt materials.