## 5 Restoring Original Low-Temperature Performance to Aged Asphalt Pavements
\[\epsilon_{i}=\frac{\partial f_{i}}{\partial x}\delta x+\frac{\partial f_{i}}{ \partial y}\delta y+\frac{\partial f_{i}}{\partial z}\delta z+\frac{\partial f _{i}}{\partial t}\delta t \tag{5}\]  
Gyratory compacted specimens with 5.6% of asphalt content were made using PG64-22 binder and nominal maximum aggregate size (NMAS) of 19 mm. Some specimens were short-term aged for two hours at 155 \({}^{\circ}\)C to simulate aging during plant production, and other specimens were aged for 36 h at 155 \({}^{\circ}\)C (in addition to the short term aging). The specimens' aging was performed on loose mixtures, which were hand-stirred every 12 h to ensure uniform aging. Each gyratory compacted specimen was then cut into two 5-cm tall cylindrical specimens with a diameter of 150 mm, for a total of eight test specimens. Figure 15 shows one of these specimens with four AE sensors coupled to the top and bottom plane surfaces of the cylindrical specimen. To avoid numerical instability, the sensors placed on the bottom of the specimens have a 45\({}^{\circ}\) offset angle with respect to the sensors coupled on the top surface. Figure 15: AE source location in asphalt mixture samples using eight piezoelectric sensors with four sensors mounted on each plane side of the specimen. To avoid numerical instabilities, the sensor pattern placed in one surface has a 45\({}^{\circ}\) offset angle with respect to the pattern of the opposite surface. Out of the eight 5-cm tall specimens, two long term specimens (aged for 36 h) and two short term aged, i.e., virgin specimens, were tested using AE, and used as the control samples. The other four specimens were treated by spreading a thin layer of rejuvenator (e.g., Reclamite) on the top surface of the specimens in the amount of 10% by weight of the binder. The four specimens were then stored for a prescribed dwell time of two, four, six, and eight weeks before conducting AE tests. After each dwell time, each specimen was then tested using the same AE source location procedure used to test the 36 h and two hours aged specimens, which allowed the estimation of the embrittlement temperatures throughout the specimen thickness. Figure 16 shows the combined embrittlement temperatures versus thickness results for all asphalt specimens using the [PERSON]'s iterative source location method. Figure 16 shows that the embrittlement temperature of 36-h aged samples (\(-\)13 \({}^{\circ}\)C) is warmer than that of virgin, i.e., short-term aged sample (\(-\)22 \({}^{\circ}\)C) due to oxidative aging. Figure 16 clearly shows that after two weeks of dwell time, all of the specimen embrittlement temperature material properties have been recuperated. After six and eight weeks, the achieved embrittlement temperatures far exceeded the embrittlement temperatures of the virgin specimens. In addition, Figure 16 also shows that for the dwell times of two and four weeks, the method also captured the gradation the embrittlement temperature properties throughout the thickness, mainly because the rejuvenator has had additional time to act on the top material layers. This is an indication that the used AE approach can be used to evaluate the graded embrittlement temperature properties of aged pavements in the field, see Figure 3b, Figure 8, and Table 1. These findings shows that this AE approach may be used to intelligently select the best maintenance strategies by assessing and optimizing the relative amount of milling and surface replacement, or the levels of rejuvenation needed to restore pavement to the original crack-resistant state. The AE results using source location are consistent with the results obtained using non-collinear ultrasonic wave mixing [48; 53; 54]. Figure 16: Average measured embrittlement temperatures of rejuvenator-treated oven-aged asphalt concrete samples (for 36 h at 135 \({}^{\circ}\)C) after dwell times of 2, 4, 6, and 8 weeks.