*2.2. Testing Asphalt Concrete Materials*

When AE asphalt concrete test samples are cooled down, thermally-induced stresses also develop in the test sample due to thermal contraction coefficient mismatch between the aggregates and the surrounding asphalt mastic. Different forms of thermal damage such as aggregate-asphalt mastic debondings, as well as thermal microcracks in the mastic, begin to occur in the sample when the local thermal stresses reach the local material strength. To visualize thermal damage in asphalt concrete test samples, X-ray computed micro-tomography (micro-CT) was employed using cubic asphalt concrete samples of one inch on-the-side, see Figure 10a. The X-ray micro-CT was conducted on asphalt concrete materials before and after two hours of conditioning the sample at −50 ◦C. Figure 10b shows X-ray tomographic images of the thermal damage in the sample with damaged areas circled in red. The following two types of damage can be identified by (1) microcracks within the mastic, and (2) debondings at the interface between aggregates and mastic. These thermally induced microdamages in the sample act as sources of acoustic activity during the AE test.

**Figure 10.** X-Ray micro-computer tomography imaging; (**a**) geometry and dimensions of used asphalt concrete sample; (**b**) images of undamaged (left) versus thermally damaged (right) asphalt concrete sample showing damaged regions.

Based upon many experimental observations, a typical schematic diagram of AE cumulative events versus temperature for asphalt concrete samples was observed as shown in Figure 11. Four distinct regions exist in the AE cumulative events vs. temperature plot, namely: pre-cracking, transition, stable cracking, and fully cracked regions. In the "pre-cracking region", thermal stresses begin to accumulate in the sample. However, thermal stresses are not yet high enough to cause any thermal damage in the material. As a result, no AE events are detected within this region. Progressively increasing thermal stresses in the specimen eventually result in the formation of thermal microcracks in the material, which are accompanied by the release of mechanical transient stress waves, i.e., AE events. The second region, i.e., the "transition region", is defined as the point in time when thermal micro-cracking in the specimen, as indicted by higher energy events, begins to occur. The temperature corresponding to the event with the energy level above a predetermined threshold has been termed the "*Embrittlement temperature (TEMB)*", as shown in Figure 12. For a given system amplification, the energy threshold is typically determined by calibrating the AE system by using samples made with a binder of a known cracking temperature, which is obtained by the traditional rheological-based methods (i.e., the BBR based methods).

**Figure 11.** Schematic diagram of AE cumulative events vs. temperature showing four different regions for asphalt concrete materials.

**Figure 12.** Typical plot of cumulative events and AE energy vs. temperature for asphalt mixtures.

The embrittlement temperature represents the onset of thermal damage in the asphalt concrete test sample. It is hypothesized that the embrittlement temperature represents a fundamental material property that is independent of material constraints, sample size (as long as a statistically representative volume or larger is used), and sample shape [3,32,33,38,39]. The "transition region" can be considered as the region where material behavior gradually changes from a quasi-brittle to a brittle state in which resistance to fracture is generally very low, allowing cracks to propagate readily. The "stable cracking region" usually initiates at a very low temperature, when the material is brittle and generates a significant amount of AE activity. The AE cumulative events versus temperature plot in this region usually has a steep slope that remains relatively constant. The "fully cracked region" starts right after the stable cracking region when the rate of AE activity of the samples begins to reduce until it reaches zero at the end of this region. Considering that the source of AE activities is the generation of new microdamage within the test sample, reduction in the rate of AE activity is an indication of the presence of plenty of microdamage in the sample. It should be noted that in AE-based tests, the fully cracked region is not usually observed unless the sample is cooled down to very cold temperatures, allowing all microdamage to fully develop within the test sample. Figure 12 shows a typical plot of cumulative event count and energy versus temperature for asphalt concrete test samples, where it is noted that the fully cracked region is not fully developed, mainly because only the embrittlement temperature is of interest.
