## 3 Thermal Cracking Evaluation of Asphalt Roads Containing Recycled Materials  
The use of AE-based approach to characterize pavements is now applied for asphalt concrete pavements containing recycled asphalt pavement (RAP) materials [36; 37; 40; 41; 42] and/or pavements containing materials from recycled asphalt shingles (RAS) [43]. The use of these recycled materials in asphalt roads has gained significant popularity in recent years, mainly because of environment and sustainability concerns. Additionally, using recycled materials can result in sustainable designs and cost savings by reducing the amount of virgin materials required in the production of new asphalt pavements. The AE-based technique was implemented to assess the effects of using recycled materials on thermal cracking performance of these asphalt pavements. Results showed that the AE-based technique could not only successfully evaluate the effect of recycled materials on low-temperature cracking performance of asphalt concrete, but also it could accurately detect the important phenomena of partial blending of recycled and virgin asphalt binders in asphalt mixtures.  
Figure 13 presents the AE-obtained embrittlement temperature results of asphalt concrete samples of mixtures using the binder PG58-28 and containing the following different amounts of RAP: 0%, 10%, 30%, 40%, and 50%. Each data presented in the plot is the average of at least four test replicates for each mixture material. Two sets of results are demonstrated: Figure 13a shows the AE results of mixtures with partial blending of RAP and virgin binder, whereas Figure 13b presents AE results of asphalt mixtures with proper mixing of RAP and virgin materials at higher temperatures with a longer mixing time. In both cases, it is observed that adding RAP increases the embrittlement temperature of the material suggesting that RAP mixtures crack at warmer temperatures. In case of partial blending, surprisingly, there is not significant distinction between T\({}_{\text{EMB}}\) of mixtures containing different amounts of RAP. This can be explained by noting that the AE testing procedure measures material properties on a local scale. If AE tests are conducted on a composite material system consisting of two types of materials, the T\({}_{\text{EMB}}\) of the composite will be the temperature at which the weaker of the two materials starts to undergo damage and fracture. In the case of partial blending mixtures, regardless of the amount of RAP in the mixture, since the unblended RAP materials experience thermal cracking first, the measured T\({}_{\text{EMB}}\) of those mixtures is in fact the embrittlement temperatures of the RAP material, which is the same for different mixtures with different amounts of RAP. However, in case of proper blending, it seems that the higher the amount of RAP content in the mixture, the warmer the cracking temperature of the mixture. Similar observations were noted for asphalt mixtures containing RAS materials [43]. Clearly, partial blending of recycled materials is an important issue. Application of AE-based test can significantly help reduce the partial blending in asphalt concrete and make the pavement more resilient to cooler temperature environments.