**6. Conclusions**

An acoustic emission based testing approach was developed to address the shortage for accurate and reliable techniques to evaluate the low-temperature cracking performance of asphalt pavements. The developed AE approach was successfully implemented to evaluate and characterize both virgin, short-term, and long-term asphalt binders and asphalt concrete materials. The extension of AE-based method to asphalt concrete materials opened the door for the use of the technique for asphalt pavement condition assessment. The AE technique was also successfully employed in different areas such as evaluating asphalt pavements containing recycled materials such as RAP or RAS, assessing the effect of cooling cycles upon the structural integrity of pavements, and evaluating the thermal cracking performance of graded, i.e., aged asphalt pavements.

The developed acoustic emission-based testing technique appears to be a viable approach for the characterization of low-temperature cracking of asphalt pavements, and it could be a powerful tool for enhancing pavement sustainability when used for preventive maintenance and rehabilitation. This technique could yield a significant payoff to practice for both up-stream and down-stream suppliers and producers of asphalt concrete binders. Up-stream supplies of polymer, chemical, and other additives (warm-mix additives, antistrip agents) could use the proposed technology to rapidly assess the low-temperature characteristics of trial formulations, and could quickly assess the compatibility of blended additive systems. Asphalt mixture designers could also use the technology to verify binder grade selection, optimize the amount of recycled materials, and/or select the appropriate binder grade to use in RAP and RAS mixtures, assess and design warm-mixtures, or verify compatibility when multiple additives are used. Pavement owners may be able to use the AE approach for quality assurance of binders and mixtures, for the periodic assessment of pavement conditions, and for the scheduling of preventive maintenance and rehabilitation, when pavement cracking is of concern.

**Acknowledgments:** The authors are very grateful for the support provided by the NCHRP Innovations Deserving Exploratory Analysis (IDEA) Program (managed by Inam Jawed) under Project 144 and Project 170 [36,37], which led to several of the authors' articles cited in this paper. Without this support, the current review paper would not have been possible. The authors are also grateful for the partial support (Contract Number W912HZ-16-C-0006) of the US Airforce Civil Engineering Center (AFCEC), including the technical support of our technical contacts George Vansteenburg and Jeb S. Tingle. Their support and many technical discussions was invaluable. The authors are also very grateful to the Missouri Department of Transportation for their support of current work.

**Author Contributions:** The authors contributed equally to the manuscript.

**Conflicts of Interest:** The authors declare no conflicts of interest.
