*The Bending Beam Rheometer (BBR) Method*

For unmodified binders, the determination of cracking temperature is based on results from the bending beam rheometer (BBR) test in accordance with the standard test methods AASHTO TP1 [8]. Two asphalt binder parameters, i.e., the stiffness and the *m*-value, are determined based on the bending beam rheometer (BBR) test results at a loading time of 60 s. The cracking temperature is defined as the temperature at which the stiffness reaches the value of 300 MPa or the m-value reaches the value of 0.3, or, moreover, the temperature at which one of the specification thresholds is reached as temperature is decreased. The cracking temperatures determined using AASHTO TP1 have been found to be predictive of low-temperature cracking in asphalt pavements constructed using unmodified binders. However, AASHTO TP1 was found to grossly over predict low-temperature cracking performance for modified asphalt binders. As a result, for modified asphalt binders, additional testing using the direct tension test [3] (DTT) was proposed to address the case of binders with a stiffness higher than 300 MPa and with an m-value greater than 0.3. Bouldin et al. [13] presented methods for predicting cracking temperatures using both the BBR and the DTT test data in the so-called dual instrument method (DIM).

These methods involve the use of sophisticated computer software for computing thermal stress in asphalt binders using the bending beam rheometer (BBR) data and estimating binder strength using the direct tension test (DTT). The dual instrument method (DIM) has been proposed as a standard method (MP1A) [10] for evaluating both modified and unmodified asphalt binders. To be able to use the MP1A, sophisticated equipment (DTT and BBR) and analysis software are required. Concerns have already been raised about the expensive nature of the MP1A approach, because its use has become prohibitive to many practitioners. Different approaches to alleviate these issues have also been proposed by Kim [15], and by Kim et al. [16], by Roy and Hesp [17], and by Shenoy [18], with some but not enough success to evaluate the low-temperature performance of asphalt materials.

For the practical low-temperature evaluation of binders, binder blends and mixtures for the purpose of formulation, design, control, and forensics, there is still a need for a test which is rapid, simple, compact, portable, and applicable to all modern binder types. Acoustic emission (AE) is a promising technique for evaluating embrittlement in asphalt binders, since it has been used successfully in other materials.

In this review paper, an acoustic emission (AE) approach is used to accurately and rapidly evaluate the thermal cracking performance of asphalt concrete materials including virgin, short-term, and long-term aged asphalt binders, as well as different asphalt concrete mixtures. A review of this technique along with applications of this approach are presented and discussed. For more details regarding experimental setups, etc., the readers are encouraged to refer to the appropriate citations.

#### **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 ◦C, see Figure 4a. Prepared binder samples are allowed to cool down to room temperature for two hours before conducting the corresponding AE tests [31–37]. 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].

**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 ◦C) to assure good adhesion between the binder sample and the granite substrate.

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 ◦C to about −35 ◦C, or even to −50 ◦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 ◦C. Considering that the embrittlement temperature of almost all binders is below −10 ◦C, measuring the temperature at the specimen's surface appears to be a proper place [3,32,33]. Figure 5c shows an AE test system, while Figure 5d shows a temperature versus time plot while cooling the sample in a freezer.

**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.

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. The signals were then further amplified 21 dB (for a total of 41 dB) and filtered using a 20 kHz high-pass double-pole filter using the Fracture Wave Detector (FWD, Digital Wave Co., Denver, CO, USA signal condition unit. The signals were then digitized using a 16-bit analog to digital converter (ICS 645B-8) using a sampling frequency of 2 MHz and a length of 2048 points per channel per acquisition trigger. The outputs were stored for later processing using Digital Wave software (Wave-Explorer TM V7.2.6) [9,31–37]. The Embrittlement Temperature (TEMB) of asphalt materials is defined as the temperature corresponding to the first high-energy AE event above a chosen energy threshold.

#### *2.1. 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].

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 [2,4,31–34].

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 EAE is the AE energy of an event (V2-μs) with duration of time t (μs) and recorded voltage of V(t) [31–34].

$$\mathbf{E}\_{\rm AE} = \int\_0^\mathbf{t} \mathbf{V}^2(\mathbf{t}) \mathbf{d}\mathbf{t} \tag{1}$$

**Figure 6.** Typical acoustic emission event waveform (**a)** and corresponding spectral content (**b**).

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 Tcracking (TANK) < Tcracking (RFTO) < Tcracking (PAV), in which RFTO and PAV stand for short-term and long-term aging, respectively, and TANK denotes virgin, i.e., unaged, binder. In Table 1 and in Figure 8, the term Rolling Thin-Film Oven (RTFO) indicates the binder was submitted to short-term aging, the Term Pressure Aging Vessel (PAV) indicates the binder was submitted to long-term aging (or a 7 to 10 year period), and TANK indicates virgin, i.e., unaged binder.

**Figure 7.** Typical experimental plot of cumulative events and energy versus temperature for asphalt binder test samples.

**Figure 8.** Correlation between AE embrittlement temperature and BBR-based cracking temperature, illustrating the conservative nature of the BBR-based cracking temperatures, see Table 1.


**Table 1.** AE-based embrittlement temperature and BBR cracking temperatures of several different binders, each with three aging levels.

\* TANK, Rolling Thin-Film Oven (RTFO), and Pressure Aging Vessel (PAV) stand for unaged, i.e., virgin, short-term, and long-term aging, respectively. # A minimum of four replicas was used to estimate average values and corresponding Coefficients of Variation (COVs).

Results also indicate that AE-based embrittlement temperatures are lower than the corresponding BBR-based critical cracking temperatures. This is not surprising, mainly because the AE-based embrittlement temperatures denote the measured values, while the BBR-base critical temperatures are based upon the binder's rheological material properties and include an inherent factor of safety to avoid low-temperature pavement cracking. For additional information, the readers are referred to References [3,4,36]. The developed AE-based approach was successfully employed to estimate the low-temperature performance grade of virgin, short-term, and long term-aged asphalt binders.

In addition to the initial transverse cracks shown in Figure 9, which divide the binder test sample into several blocks, Figure 9 also shows three-dimensional spiral cracks, which became visible only after removal of the top material from the test sample [38]. These spiral cracks are a result of the three-dimensional state-of-stress field that develops in each block (created by the transverse cracks) by the constraints imposed by the granite block. These spiral cracks were modeled as three-dimensional logarithm spirals using three parameters, (a spiral tightness parameter "b", an apparent length scale parameter "A", and the pitch angle "ϕ") that control how tightly and in which direction the spiral is wrapped. In addition to observing that the spiral pattern represents the crack trajectory with maximum energy release, it was also observed that the AE obtained embrittlement temperatures and fracture energy (obtained using indirect tension tests) are related to the spiral crack tightness parameter [38]. For additional information regarding modeling spiral cracks as the mode of failure in asphalt materials, the readers are referred to Behnia et al. [38].

**Figure 9.** Asphalt binder showing the visible straight channeling cracks, i.e., mud cracks, and the spiral cracks that are only observed after the asphalt material is removed. The observed spiral cracks develop in each block due to the three-dimension state-of-stress induced by the thermal mismatch between the asphalt blocks and the granite substrate.

The promising AE results for low-temperature cracking performance of asphalt materials suggests that AE could be considered as a viable alternative for the ASHTO protocols, which estimate the low-temperature performance based upon the rheological binder properties. Furthermore, the AE approach has the advantage of being faster, having less variability, and capable of being used for all types of binders, including modified binders.
