**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 [36,37,40–42] 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 TEMB 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 TEMB 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 TEMB 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.

**Figure 13.** Embrittlement temperatures of PG58-28 mixtures containing different amounts of RAP: (**a**) mixtures with partial blending of RAP and virgin binders, (**b**) mixtures with proper blending of RAP and virgin binders.

#### **4. Low-Temperature Characterization of Asphalt Mixtures Subjected to Cooling Cycles**

In cold regions, cooling cycles cause thermally-induced micro-damages in the microstructure of asphalt concrete, which further reduces the asphalt pavements' resistance to fracture. Here, AE tests were conducted on field cores, as well as laboratory gyratory compacted samples, and the corresponding embrittlement temperature of samples were determined [44].

During thermal cycling of asphalt concrete, it was observed that asphalt concrete specimens generated AE activities before the previous minimum temperature (i.e., maximum thermal loading level) was reached. This phenomenon is known as the "Felicity effect", and is characterized by the presence of detectable AE activity during reloading of the material before the load level reaches the previous maximum applied load [45]. The Felicity effect observed in asphalt materials is illustrated in Figure 14, which shows the cumulative AE events versus applied thermal load. The loading path from B to C (unloading) and C to D to E (reloading) clearly indicates the absence of AE activity up to a loading level (TD), which is below the level of the previous loading cycle (TB). It should also be noted that another AE loading phenomenon, known as the Kaiser effect, may also take place if the mixture loses the ability of self-healing. The Kaiser effects occurs when no AE activity occurs until the previous maximum load level is reached, which makes the Kaiser effect a particular case of the Felicity effect (i.e., points B and D coincide). The Kaiser effect can be observed in highly oxidized mixtures and/or when no time is allowed between cycles for the self-healing to take place.

**Figure 14.** Typical AE test results of thermal cyclic loading of laboratory samples. Please note the presence of the Felicity effect as a result of self-healing.

Observation of the Felicity effect instead of the Kaiser effect in asphalt pavement materials suggests partial healing of microcracks in the material. Due to the adhesive nature of asphalt concrete materials, some level of micro-crack self-healing may take place within material during thermal unloading. The amount of micro-crack self-healing may vary depending on the binder chemical composition and upon the level of oxidative aging of the asphalt mixture, as well as on the ambient temperature and presence of moisture. This means that in thermal cyclic loading of adhesive materials such as asphalt mixtures with high potential of micro-crack healing, the Felicity effect is the one that would occur rather than the Kaiser effect. Here, the Felicity Ratio (FR) is defined as the ratio of temperature at point D to the temperature of point B, the lowest temperature of the first cooling cycle. The average Felicity ratio for tested asphalt concrete materials was around 0.648 [44,45]. To better quantify crack healing of asphalt material, a new parameter called the "Healing Index" was introduced and defined by the following equation.

$$\text{Healthing Index} \left( \% \right) = 100(1 - FR) \tag{2}$$

The Healing Index was used as an indication of the amount of healing that may occur in asphalt concrete between the loading cycles over the period of thermal unloading. Depending on the sample temperature (i.e., thermal cycle), type of asphalt binder, and presence of moisture, the Healing Index of asphalt mixture ranges from 0% (no healing) to 100% (fully healed). For the mixtures tested, the average Healing Index of the tested samples was around 35%, which indicates partial healing of the asphalt concrete during the rest period between the first and second cooling cycles. Results also showed that the oxidative aging adversely affected the thermal cracking healing capability of asphalt pavements, as the asphalt materials with higher level of oxidative aging exhibited lower Healing Index. For additional information, the readers are referred to Reference [44].

#### **5. Restoring Original Low-Temperature Performance to Aged Asphalt Pavements**

Oxidative aging leads to an increase in stiffness, loss of ductility and cohesion of binders, and warmer embrittlement temperatures, see Table 1 and Figure 8. As a result, oxidative aging lowers the resistance to fracture of oxidized mixtures as compared to their virgin state [37,46–48]. To restore the crack-resistant state of oxidized asphalt concrete pavements, measures such as pavement surface milling and/or the application of rejuvenators are taken. Asphalt rejuvenators are asphalt additives and modifiers that are used to revitalize, provide sealing, and restore the physical and chemical properties of the aged asphalt concrete [49–51]. Rejuvenators address the issue of oxidative hardening by softening the aged asphalt binder through restoration of the asphaltenes to maltenes ratio [6]. After applying a thin layer of rejuvenator over the top surface of pavement, the rejuvenator penetrates the asphalt concrete using the pore and tortuosity structure via gravity and capillary action, and diffuses through the asphalt concrete to chemically react with the asphalt binder. The rejuvenator/binder reaction restores the binder's material properties to its original state, i.e., the material properties of the virgin material. As the asphalt binder is softened, it also increases its adhesive properties and reduces the susceptibility of the asphalt pavement to thermal cracking.

As described by Brown [49], with the exception of visual inspection, there is no standardized method to evaluate the performance of rejuvenators when applied in the field. Currently, the ability of rejuvenators to improve pavements' durability is typically evaluated by (1) estimating the penetration in samples at 25 ◦C using asphalt binder extracted from untreated and treated cores, (2) comparing the viscosity at 60 ◦C of asphalt binder samples extracted from untreated and treated cores, and (3) comparing the percentage of aggregate loss when untreated and treated samples are subjected to a pellet abrasion test. Mainly because these tests are cumbersome and time consuming, they are not often used. Here, AE source location is used to characterize and evaluate the rejuvenators' ability to restore mixtures to their original low-temperature performance grading, i.e., their original crack resistant state.

*Appl. Sci.* **2018**, *8*, 306

Because of the granular nature of asphalt concrete mixtures, the Geiger's iterative source location technique [51,52] was used to accurately locate the source of each event. The Geiger's method is the application of the Gauss-Newton algorithm, which requires data from at least four AE sensors to build the following arrival time function of the *i*th sensor:

$$f\_i(\mathbf{x}, y, z, t) = T\_s + \frac{1}{\upsilon} \sqrt{(\mathbf{x}\_i - \mathbf{X}\_s)^2 + (y\_i - \mathbf{Y}\_s)^2 + (z\_i - Z\_s)^2} \tag{3}$$

in which (*Xs*, *Ys*, *Zs*) represent the spatial coordinates of the source, (*xi*, *yi*, *zi*) represent the coordinates of the ith sensor, *v* is the known wave velocity, and *Ts* and *ti* are the unknown source event occurring time and the known receiving time by the *i*th sensor, respectively. Equation (3) can be expanded using Taylor series at a point (*x*0, *y*0, *z*0), near the actual source, resulting in Equation (4):

$$f\_i(x, y, z, t) = f\_i(x\_0, y\_0, z\_0, t\_0) + \epsilon\_i \tag{4}$$

in which *i*, the residual term, a.k.a. the correction vector, is the difference between the calculated arrival time and the observed arrival time with respect to the *i*-th sensor. It can be calculated using the first order derivatives of the arrival time function. By going through several iterations of Equation (5), the Geiger's method tries to minimize the correction vector.

$$\epsilon\_{\bar{i}} = \frac{\partial f\_i}{\partial x} \delta x + \frac{\partial f\_i}{\partial y} \delta y + \frac{\partial f\_i}{\partial z} \delta z + \frac{\partial f\_i}{\partial t} \delta t \tag{5}$$

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 ◦C to simulate aging during plant production, and other specimens were aged for 36 h at 155 ◦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◦ 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◦ 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 Geiger's iterative source location method. Figure 16 shows that the embrittlement temperature of 36-h aged samples (−13 ◦C) is warmer than that of virgin, i.e., short-term aged sample (−22 ◦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 ◦C) after dwell times of 2, 4, 6, and 8 weeks. For comparison, the embrittlement temperatures of the short-term aged (i.e., virgin) samples and of oven-aged samples for 36 h at 135 ◦C of −22.0 ◦C and −13 ◦C, respectively, are also noted (see dotted and dashed lines respectively). Note that after two weeks of dwell time, the unaged mixture embrittlement temperature was already restored. Also, at 2 and 4 weeks of dwell time, the top material layers have a cooler embrittlement temperature, because the rejuvenator had more time to act upon the binder.
