**1. Introduction**

As an important component of the transportation infrastructure, asphalt pavement is composed of multi-layer complex system of different materials subjected to various combinations of traffic as well as environmental loadings. During their lifetime asphalt pavements experience various forms of distresses as they undergo oxidative aging, freeze-thaw cycles, and traffic repetitions. One of the most widespread type of deterioration in asphalt roads which shortens the pavement life and results in premature failure of the pavement structure is cracking. An accurate and realistic assessment of cracking performance of asphalt pavements has remained as a challenging task for civil engineers.

The AE technique has become very popular in recent decades due to its unique ability in detecting and locating microstructural failures in different types of materials. This method has been successfully used for damage detection of various materials such as steel, concrete, wood, and rock. However, for the case of asphalt roads, there has been only a limited application of this technology in damage assessment of asphalt pavements. In one of the studies Khosla and Goetz [1] used the acoustic emission approach at low temperatures to locate crack initiation and propagation in indirect tensile (IDT) asphalt concrete specimens. They found that the material failure due to fracture was accompanied by a sudden increase in total AE counts where a big portion of AE counts occurred at about 80% of the peak load. In another study conducted by Valkering and Jongeneel [2], AE technique was implemented to carefully monitor the thermally-induced cracking in asphalt concrete materials subjected to low temperature cooling cycles (−10°C to −40°C). Results showed that the AE activity of the material such as number of events were strongly correlated with the extent of thermal fracture in the specimens. Results also demonstrated that at low temperatures the source of AE activities in restrained specimens were crack initiation originated from defects exists in the material. In the research study performed by Hesp et al. [3] the AE method was employed for restrained asphalt concrete specimens at low temperatures (−32°C to −20°C) to measure and to detect crack initiation and propagation in the restrained samples. They compared the total amount of AE activities in different mixtures and found that the styrenebutadiene-styrene (SBS)-modified asphalt concrete materials exhibited less AE activities as compared to that of for the unmodified asphalt concrete mixes. The AE approach was implemented by Li et al. [4–8] to evaluate and to quantify fracture in semi-circular bending (SCB) asphalt specimens at −20°C. They concluded that most of the AE activities in the material happens at about 70% of the material strength. Their results also showed that the maximum intensity of AE amplitudes correlated well with the extent of macrocracking damage in the specimen. They also found that the location of AE events is the good indicator of approximate size of the fracture process zone (FPZ). Nesvijski and Marasteanu [9, 10] in another research study, used the AE spectral analysis approach at low temperatures in order to investigate and assess fracture in semi-circular bending (SCB) asphalt samples. They were able to successfully demonstrate that the AE approach could be applied for accurate characterization of cracking in asphalt concrete materials.

This chapter will focus on various applications of the acoustic emission technique in asphalt pavements including: (1) assessing the low-temperature cracking performance of asphalt binders and asphalt pavement materials (2) use of acoustic emission technique for quantitative evaluation of restoring aged asphalt pavements with rejuvenators.
