## 4 Low-Temperature Characterization of Asphalt Mixtures Subjected to Cooling Cycles  
In cold regions, cooling cycles cause thermally-induced micro-damages in the microstructure of asphalt concrete, which further reduces the asphalt pavements' resistance to fracture. Here, AE tests were conducted on field cores, as well as laboratory gyratory compacted samples, and the corresponding embrittlement temperature of samples were determined [44]. During thermal cycling of asphalt concrete, it was observed that asphalt concrete specimens generated AE activities before the previous minimum temperature (i.e., maximum thermal loading level) was reached. This phenomenon is known as the \"Felicity effect\", and is characterized by the presence of detectable AE activity during reloading of the material before the load level reaches the previous maximum applied load [45]. The Felicity effect observed in asphalt materials is illustrated in Figure 14, which shows the cumulative AE events versus applied thermal load. The loading path from B to C (unloading) and C to D to E (reloading) clearly indicates the absence of AE activity up to a loading level (T\({}_{\text{D}}\)), which is below the level of the previous loading cycle (T\({}_{\text{B}}\)). It should also be noted that another AE loading phenomenon, known as the Kaiser effect, may also take place if the mixture loses the ability of self-healing. The Kaiser effects occurs when no AE activity occurs until the previous maximum load level is reached, which makes the Kaiser effect a particular case of the Felicity effect (i.e., points B and D coincide). The Kaiser effect can be observed in highly oxidized mixtures and/or when no time is allowed between cycles for the self-healing to take place. Figure 14: Typical AE test results of thermal cyclic loading of laboratory samples. Please note the presence of the Felicity effect as a result of self-healing. Figure 13: Embrittlement temperatures of PG58-28 mixtures containing different amounts of RAP: (a) mixtures with partial blending of RAP and virgin binders, (b) mixtures with proper blending of RAP and virgin binders. Observation of the Felicity effect instead of the Kaiser effect in asphalt pavement materials suggests partial healing of microcracks in the material. Due to the adhesive nature of asphalt concrete materials, some level of micro-crack self-healing may take place within material during thermal unloading. The amount of micro-crack self-healing may vary depending on the binder chemical composition and upon the level of oxidative aging of the asphalt mixture, as well as on the ambient temperature and presence of moisture. This means that in thermal cyclic loading of adhesive materials such as asphalt mixtures with high potential of micro-crack healing, the Felicity effect is the one that would occur rather than the Kaiser effect. Here, the Felicity Ratio (FR) is defined as the ratio of temperature at point D to the temperature of point B, the lowest temperature of the first cooling cycle. The average Felicity ratio for tested asphalt concrete materials was around 0.648 [44; 45]. To better quantify crack healing of asphalt material, a new parameter called the \"Healing Index\" was introduced and defined by the following equation. \[Healing\ Index\ (\%)=100(1-FR) \tag{2}\]  
The Healing Index was used as an indication of the amount of healing that may occur in asphalt concrete between the loading cycles over the period of thermal unloading. Depending on the sample temperature (i.e., thermal cycle), type of asphalt binder, and presence of moisture, the Healing Index of asphalt mixture ranges from 0% (no healing) to 100% (fully healed). For the mixtures tested, the average Healing Index of the tested samples was around 35%, which indicates partial healing of the asphalt concrete during the rest period between the first and second cooling cycles.