**3. Use of acoustic emission technique for quantitative evaluation of restoring aged asphalt pavements with rejuvenators**

Oxidative aging is a common problem in asphalt pavements which leads to an increase in stiffness and loss of ductility and cohesion of binders. It negatively

affects the fracture resistance of pavements. Certain chemical properties of the asphalt binders such as asphaltenes to maltenes ratio changes in the oxidation process. The oxidation rate of asphalt materials is accelerated at high temperatures and/or high exposure to ultraviolet light and air [21–26]. Different methods such as pavement surface milling and the application of rejuvenators are employed to restore asphalt pavements to their crack-resistant state. Application of rejuvenators is one of the popular techniques to restore the physical and chemical properties of aged asphalt materials. Rejuvenators change the asphaltenes to maltenes ratio to its original state leading to softening the aged asphalt materials [21–26]. Rejuvenators are generally sprayed on the surface of aged pavements. It is very important that rejuvenator could penetrate the surface via capillary action and gravity and diffuse through the aged asphalt.

Currently there is no standardized method to assess the performance of rejuvenators when applied in the field. The efficiency of rejuvenators is evaluated by the following three methods which are cumbersome and time consuming and they are not often used.: (1) estimating the penetration of rejuvenator in the pavement by comparing the penetration value of the binder at 25°C in the asphalt binder extracted from untreated and treated sample; (2) comparing the asphalt binders' viscosity at 60°C obtained from untreated and treated cores; and (3) comparing the amount of loss in aggregates in the abrasion test in untreated vs. treated samples [26].

The AE source location approach has recently been employed to assess the efficiency of rejuvenators in restoring aged asphalt materials to their original crack resistant condition. The Geiger's iterative source location method was used to accurately detect the source of AE activities in the material [26, 27]. This iterative technique is based on the Gauss-Newton algorithm. To build the arrival time function of the ith sensor, see Eq. (2), data from at least four sensors is required for the Geiger's method:

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

where (*XYZ ss s* , , ) represent the spatial coordinates of the AE source, ( *xyz ii i* , , ) is the coordinates of the ith sensor, *v* is the velocity of wave in the material, *it* and *Ts* represent the known receiving time and unknown AE source event occurring time by the ith sensor, respectively. Taylor series is used to expand Eq. (2) at a point ( *xyz* <sup>000</sup> , , ,) close to the actual source leading to Eq. (3):

$$f\_i(\mathbf{x}, y, z, t) = f\_i(\mathbf{x}\_0, y\_0, z\_0, t\_0) + \epsilon\_i \tag{3}$$

where *<sup>i</sup>* is the residual term, a.k.a. the correction vector, which is the difference between the calculated arrival time and the observed arrival time with respect to the ith sensor. The correction vector can be determined using the first order derivatives of the arrival time function. The Geiger's method tries to minimize the correction vector by going through several iterations of Eq. (4).

$$\epsilon\_i = \frac{\partial f\_i}{\partial \mathbf{x}} \delta \mathbf{x} + \frac{\partial f\_i}{\partial y} \delta y + \frac{\partial f\_i}{\partial \mathbf{z}} \delta \mathbf{z} + \frac{\partial f\_i}{\partial t} \delta t \tag{4}$$

*Application of Acoustic Emissions Technique in Assessment of Cracking Performance of Asphalt… DOI: http://dx.doi.org/10.5772/intechopen.101541*

In this chapter results from one of the studies on evaluation of rejuvenators on aged asphalt materials are presented where PG64-22 was used as the based binder. The asphalt content of the mixture was 5.6% by weight and the gyratory compacted specimens were made using a maximum aggregate size of 19 mm. Some specimens were aged in the oven for 2 h at 155°C to simulate the aging level during plant production. Part of the specimens were aged in the oven for 36 h at 155°C (in addition to the short term aging) to mimic the long term aged asphalt pavement materials. The oxidative aging process was done on loose mixtures in order to obtain uniformly-aged compacted samples. **Figure 7** shows one of specimens with eight AE sensors mounted on the top and bottom surfaces of the specimen, four sensors on each side. To avoid numerical instability, AE sensors pattern at the bottom of the specimen has a 45° offset angle with respect to the pattern of sensors coupled on the top surface.

Some aged specimens were treated by spraying a thin layer of rejuvenator on the top surface of the sample. The amount of rejuvenator used was 10% by weight

**Figure 7.**

*(a) Oxidative aged AE testing sample with eight piezoelectric sensors, four sensors on each side. (b) AE testing setup used for source location [21].*

of the asphalt binder. The rejuvenator-treated specimens were then stored for a prescribed dwell time of 2, 4, 6, and 8 weeks before performing the AE tests. After each dwell time, specimens were tested using the same AE source location procedure used to test the 36 h and 2 h aged specimens, allowing the estimation of the embrittlement temperatures throughout the sample thickness.

To characterize the efficiency of rejuvenator on the aged asphalt materials, the embrittlement temperatures of the material were determined throughout the thickness of asphalt concrete samples by implementing the AE source location method. **Figure 8** illustrates the embrittlement temperatures results vs. sample thickness for different aged asphalt concrete materials. The effect of oxidative aging on the embrittlement temperature is clearly noticeable as the embrittlement temperature of the short-term aged sample (−22°C) is lower than that of the 36 h aged samples (−13°C). It is also observed that for all specimens the embrittlement temperatures of oxidative aged materials after 2 weeks of dwell time of rejuvenator have been recuperated. The test results obtained from samples after 6 and 8 weeks of dwell time were quite surprising as the embrittlement temperatures of the aforementioned samples far exceeded the embrittlement temperatures of the virgin materials. Moreover, the method was also able to successfully capture the embrittlement temperature gradation throughout the sample thickness for the dwell times of 2 and 4 weeks. This could be attributed to the fact that the rejuvenator has had enough time to penetrate and act on the top material layers. Results suggest that the AE method can be employed to accurately evaluate the graded embrittlement temperature properties of oxidative aged asphalt pavements. One important outcome of this study is that the AE approach can be used to intelligently select the best maintenance strategies for oxidative aged asphalt roads through optimizing the amounts of rejuvenators required to restore pavement to the original crack-resistant condition, or by optimizing the relative amount of milling and surface replacement of asphalt roads. In addition, the AE results obtained from source location approach were found to be consistent with those of obtained from non-collinear ultrasonic wave mixing method [28–31].

#### **Figure 8.**

*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 [21, 25, 26].*

*Application of Acoustic Emissions Technique in Assessment of Cracking Performance of Asphalt… DOI: http://dx.doi.org/10.5772/intechopen.101541*
