## 2 Acoustic Emission Based Evaluation of Embrittlement Temperatures
### Testing Asphalt Binders  
The performance of asphalt binders is extremely susceptible to changes in temperature. Binders are performance Graded (PG) with high- and low-temperature performance designations, which are based on the pavement's expected operating temperatures [11; 12]. This grading designation allows for appropriate binders to be chosen to prevent temperature-driven pavement failures, such as rutting at high temperatures and cracking at low temperatures. Performance graded binders are designated as _PG XX-YY_, in which _XX_ is the high-temperature designation, which corresponds to the average temperature the binder is expected to encounter over a seven day period, and _YY_ is the low-temperature designation, which corresponds to the lowest expected pavement temperature [11; 12]. Figure 5: AE testing of asphalt materials; (**a**) asphalt binder test sample after AE testing showing two AE sensors and one thermocouple; (**b**) test sample for asphalt concrete mixtures; (**c**) AE test system for evaluating binders and asphalt concrete mixtures test samples; (**d**) typical temperature vs. time cooling plot for asphalt concrete mixture test samples. In asphalt binder test samples, the thermal contraction coefficient of asphalt binders is several times greater than that of granite substrate. Therefore, as the binder specimen cools down, due to the differential thermal contraction between asphalt and granite substrate, thermal stresses build up in the restrained asphalt binder layer. At the same time, the tensile strength of the asphalt binder reduces with temperature. Eventually, when the thermal stress exceeds the strength of the binder material, thermal cracks develop, which are accompanied by the release of elastic strain energy in the form of transient mechanical stress waves, i.e., acoustic emissions. Both Figures 4b and 5a show the asphalt binder test sample after the AE test, illustrating several thermal cracks [2; 4; 31; 32; 33; 34]. The typical time domain response and the corresponding power spectral density curve of an AE event is shown in Figure 6. Analyses of AE activities of samples are performed on recorded AE signals and are associated testing temperature. The energy of the AE events was computed using Equation (1), in which E\({}_{\text{AE}}\) is the AE energy of an event (V\({}^{2}\)-\(\upmu\)s) with duration of time t (\(\upmu\)s) and recorded voltage of V(t) [31; 32; 33; 34]. \[\text{E}_{\text{AE}}=\int_{0}^{\text{t}}\!\!\text{V}^{2}(\text{t})\text{dt} \tag{1}\]  
A typical plot of AE cumulative events versus temperature for asphalt binders is shown in Figure 7, in which the energy of events versus temperature is also shown. In Figure 7, no events are recorded in the pre-cracking region, mainly because of the energy filtering process used. For each AE system gain, and using a binder of known cracking temperature, the energy threshold is selected (i.e., calibrated) to assure that the AE-obtained embrittlement temperature equals the known binder cracking temperature value grade, which is currently obtained using the binder rheological properties, i.e., the BBR methods. The AE-based embrittlement temperatures along the corresponding BBR-based critical cracking temperatures for various asphalt binders are provided in Table 1, in which, for comparison, the coefficient of variation (COV%) is also shown for both methods. Please note that while the coefficient of variation is temperature scale-dependent, for the present application in which the embrittlement temperatures are in a relatively narrow range and sufficiently below zero, the COV was deemed to be a useful statistical parameter to describe the repeatability of the measurements obtained via the two approaches. Results from both approaches are also presented in Figure 8. Comparison of results also indicate that T\({}_{\text{cracking}}\) (TANK) < T\({}_{\text{cracking}}\) (RFTO) < T\({}_{\text{cracking}}\) (PAV), in which RFTO and PAV stand for short-term and long-term aging, respectively, and TANK denotes virgin, i.e., unaged, binder.