## 2 Acoustic Emission Based Evaluation of Embrittlement Temperatures  
The AE-based technique was implemented for low-temperature fracture assessment of asphalt materials. During the course of developing this testing approach, a wide range of asphalt binders and asphalt concrete materials with significantly different low-temperature cracking characteristics, as well as different oxidative aging levels (i.e., unaged, short-term aged, and long-term aged), were evaluated. Figure 4a schematically shows the geometry of the developed test specimen for virgin (i.e., unaged) or aged asphalt binders, which consists of a thin layer of asphalt binder (with nominal dimensions of 125 mm long, 12 mm wide, and thickness of 6 mm) bonded to a granite substrate. These asphalt binder specimens are prepared using aluminum molds, and they have the same dimensions as the standard Bending Beam Rheometer (BBR) specimens [9]. To make the binder specimen, the asphalt binder is poured into the aluminum mold wrapped in Teflon tape and placed on the granite slab, which is heated to the temperature of 135 \({}^{\circ}\)C, see Figure 4a. Prepared binder samples are allowed to cool down to room temperature for two hours before conducting the corresponding AE tests [31; 32; 33; 34; 35; 36; 37]. Figure 4b shows the specimen after being tested using AE. Figure 5a shows the binder test specimen along with the position of the AE sensors and the thermocouple. The geometry of AE specimens utilized for testing compacted asphalt concrete mixtures is shown in Figure 5b, in which the position of the AE sensor and the thermocouple is also shown. These specimens are typically semicircular, measuring 150 mm in diameter and 50 mm in thickness. This geometry was selected after examining several shapes and different sample thicknesses. In addition, the semicircular geometry is very practical and easy to make from cylindrical extracted field cores or laboratory gyratory compacted samples [36; 37]. Apart from different test sample geometries, the same testing set up and procedures are used for testing both asphalt binders and asphalt concrete specimens. To conduct the AE test, prepared specimens are placed inside the freezer and are cooled down from 20 \({}^{\circ}\)C to about \(-\)35 \({}^{\circ}\)C, or even to \(-\)50 \({}^{\circ}\)C, if necessary. A K-type thermocouple is placed on the specimens' surface to record the temperature of the samples. Because of the finite size of the test sample, there is a thermal lag at the beginning of the test, which becomes negligible (almost zero) at temperature lower than \(-\)10 \({}^{\circ}\)C. Considering that the embrittlement temperature of almost all binders is below \(-\)10 \({}^{\circ}\)C, measuring the temperature at the specimen's surface appears to be a proper place [32; 33; 3]. Figure 5c shows an AE test system, while Figure 5d shows a temperature versus time plot while cooling the sample in a freezer. Figure 4: Fabrication of binder test samples: (**a**) top: molds for fabricating binder test samples; bottom: completed binder test sample in the mold; (**b**) typical visible crack patterns in asphalt binder test sample after AE testing. The binder samples have the same nominal dimensions as the specimens in the Bending Beam Rheometer (BBR) test (125 mm long, 12 mm wide, and 6 mm thick). The binder is poured into the mold seating on the granite (which has been heated to 135 \({}^{\circ}\)C) to assure good adhesion between the binder sample and the granite substrate. Wideband AE piezoelectric sensors (Model B1025, Digital Wave Co., Denver, CO, USA) with a relatively flat sensitivity in the frequency band of 50 kHz to 1.5 MHz were utilized to monitor and record the acoustic activities of the samples during the test. AE stress waves were detected using the AE piezoelectric sensors, amplified, filtered, and then recorded. High-vacuum grease was used to couple the AE sensors to the test sample. AE signals were pre-amplified 20 dB using broad-band pre-amplifiers to reduce extraneous noise.