**1. Introduction**

The majority of the roads in United States are surfaced with asphalt material, indicating the importance of this material in U.S. transportation infrastructure. In asphalt roads located in cold regions or in milder climate regions with large daily temperature fluctuations, a major form of deterioration is associated with the formation of low-temperature cracks, a.k.a., thermal cracks, in the pavement. Thermal cracks are categorized into two groups based on their formation mechanism: (1) "single-event thermal cracks", which occur in cold climates due to fast cooling rates; and (2) "thermal fatigue cracks", which develop after several cooling cycles in regions with a milder climate and large daily temperature fluctuations [1–4]. Thermal cracking phenomenon demonstrates itself as a group of parallel, evenly-spaced, transversely-oriented, surface-initiated cracks in pavements. A typical thermal cracking pattern in asphalt pavements is depicted in Figure 1a. As the temperature decreases, thermal stresses build up in the pavement due to the tendency of the "restrained" continuous layer of asphalt pavement to contract. The distribution of thermal stresses through the pavement thickness is

non-uniform, with the highest stresses occurring at pavement surface, see Figure 1b. Thermal cracks happen when the thermally-induced stresses exceed the material fracture strength.

**Figure 1.** Thermal cracks as a result of oxidation; (**a**) typical thermal cracking pattern in asphalt pavement, (**b**) thermal cracking formation and non-uniform distribution of thermal stresses through the pavement thickness.

On the local scale, another source of thermal stresses in asphalt pavements is the thermal contraction mismatch between aggregates, i.e., crashed stone, and the surrounding asphalt mastic material. As the pavement temperature drops, asphalt mastic tends to contract more (often more than ten times) than the aggregate particles, causing increasing thermally-induced stresses in the pavement structure. At the same time as the temperature drops, the asphalt mastic becomes increasingly more brittle with less resistance against fracture. As a result, microcracks develop within the asphalt mastic when the thermally-induced tensile stresses overcome the fracture resistance of mastic material. In addition to mastic cracking, another type of damage, i.e., debondings, also occurs at low temperatures at the interfaces between asphalt mastic and aggregates, see Figure 2. Repair and rehabilitation of low-temperature cracks in pavements costs millions of dollars every year. In addition, damaged pavements also develop higher surface roughness, which leads to vehicle damage. A study by Islam and Buttlar showed that the presence of cracks in pavements would add over \$300 per vehicle per 19,000 km (12,000 mi) driven in user costs [5].

**Figure 2.** Schematic depiction of thermally-induced stresses within the asphalt mastic causing microcracks in the mastic and mastic-aggregate debondings.

Asphalt roads also suffer from oxidative aging [6,7]. Figure 3a shows two images of the same pavement section: one taken right after the construction and the other 19 months later when the pavement has already encountered some level of oxidative aging. Figure 3b shows a schematic diagram illustrating the typical steep gradation of material properties (e.g., complex modulus) caused by oxidative aging at the pavement top material layer. Computer models have already been developed to estimate the change in material properties due to oxidative aging for given climatic conditions [7]. In addition to changing the asphaltenes to maltenes ratio [6], oxidative aging also increases the stiffness of the top material layers, see Figure 3b, which increases the pavement vulnerability to cracking in cold

environments. Oxidative aging is an important issue in asphalt pavements that negatively affects low temperature cracking of asphalt materials. The higher the oxidative aging level of asphalt pavements, the higher the extent of thermal cracking damage in the material.

**Figure 3.** Oxidation of asphalt concrete pavements: (**a**) pavement section right after the construction (left), and the same pavement 19 months after construction (right); and (**b**) Schematic diagram illustrating the steep gradation of material properties (e.g., complex modulus) caused by oxidative aging at the pavement top material layer.

A great deal of research efforts has been directed towards the characterization and prevention of thermal cracks in pavement. The Superpave tests developed under the Strategic Highway Research Program (SHRP) in the 1990s have significantly improved the performance tests to assess the behavior of asphalt pavements by providing fundamental material tests over a broad range of production and service temperatures. However, the Superpave tests were not developed for (1) the characterization of highly modified binders, (2) the characterization of asphalt pavements containing recycled materials such as Reclaimed Asphalt Pavement (RAP) or Reclaimed Asphalt Shingles (RAS), and (3) the characterization of warm-mix materials. In addition to Superpave performance tests [8–12], studies conducted by several researchers [7,13–29] also provide valuable and significant insight into the estimation of asphalt materials low-temperature performance.

Acoustic Emission (AE) has been used extensively for damage detection and assessment of several materials including concrete, steel, wood, and rock. However, there has been limited application of AE for evaluating damage mechanisms in asphalt materials. Khosla and Goetz [21] used AE techniques to detect crack initiation and propagation in indirect tensile (IDT) specimens at −23 ◦C. The study found that failure by fracture is indicated by a sharp increase in total AE counts, and that significant AE counts occur at about 80% of the peak load. Valkering and Jongeneel [22] used AE to monitor temperature cycling tests with restrained asphalt concrete specimens at low temperatures (−10 ◦C to −40 ◦C). They observed that the repeatability of AE measurements is good, that the AE activity (i.e., number of events) correlates with the thermal fracture temperatures, and that the AE activity in restrained specimens at low temperatures is caused by defect initiation in the binder. Hesp et al. [23] used AE measurements to detect crack initiation and propagation in restrained specimens at low temperatures (−32 ◦C to −20 ◦C). They concluded that the Styrene-Butadiene-Styrene (SBS)-modified

mixes produced less AE activity than unmodified mixes. Li et al. [24–28] used AE techniques to characterize fracture in semi-circular bend asphalt specimens at low temperatures (−20 ◦C). They also concluded that large amounts of accumulated AE events occur at 70% of material strength, that the maximum intensity of AE peaks correlates with the development of macro-cracks, and that the location of AE events suggests that a several centimeter-sized process zone forms before the peak load. Nesvijski and Marasteanu [29,30] used an AE spectral analysis approach to characterize fracture in semi-circular bend asphalt specimens at low-temperatures, and concluded that an AE approach could be used for evaluation of asphalt pavements. All of this research work led to the development of standards by the American Association of State and Highway Transportation Officials (AASHTO MP1, 1998; AASHTO TP1, 1999; AASHTO MP1A, 2001) [8–10] that allowed the estimation of the low-temperature performance of binders based upon their rheological properties.
