2. Asphalt thermal cracking behavior

The low-temperature cracking of asphalt concrete pavements is a major pavement distress mechanism in cold regions costing hundreds of millions of dollars in rehabilitation costs to various agencies. It usually occurs in the form of regularly spaced transverse cracks, initiating at the surface of the asphalt layer and further propagating downward. Consequences of thermal cracking are an immediate increase of the roughness of the pavement surface (i.e., a reduction of the comfort and safety of the ride) and the loss of the sealing function of the pavement for the underlying layers. However, predictions of this distress have not been accurate enough, often resulting in premature road failures. It is believed that the excessive brittleness due to the increase in stiffness and decrease in the ability to relax stress leads to the buildup of thermally induced stress and ultimately cracking of mixtures in pavements.

The prediction of asphalt pavement thermal cracking has been the subject of numerous studies that date back to the early 1960's [26]. In many of these studies, attempts were made to introduce a procedure to predict pavement cracking based on the stress-strain-time—temperature relationship. A number of methods have been introduced throughout the years to model the viscoelastic behavior of asphalt binders and mixtures to estimate the accumulation of thermal stress during cooling cycles and predict the temperature at which cracking occurs [27–29].

testing at a low stress levels (0.1 and 3.2 kPa) is the best way to characterize the rutting resistance of an asphalt binder. The stresses and strains in the binder can be high, much higher than the linear limit for the material. Permanent deformation in asphalt binder is highly dependent on the stress level. Determining the stress level at which the binder is exposed in the mixture is an important matter [19, 20]. Permanent deformation is not a linear viscoelastic phenomenon and, therefore, measurement of linear viscoelastic binder properties are not likely to correlate with it [21]. The selection of two stress levels is not necessarily based on the stresses that asphalt binder experiences inside the pavement. The number of cycles and the time of loading do not provide full picture of characterizing long term deformation in the material. The recovery time may need to be longer to fully capture delayed elastic behavior of modified binders; some binders are still recovering after 9 (seconds) of recovery [22]. Golalipour [23] investigated these factors and provided some

In the latest version of AASHTO standard T 350 standard "Standard Method of Test for Multiple Stress Creep Recovery (MSCR) Test of Asphalt Binder Using a Dynamic Shear Rheometer (DSR)" or American Society for Testing and Materials standard (ASTM) 7405, the test consists of 20 cycles of 0.1 kPa stress creep and recovery, followed immediately by another 10 cycles of 3.2 kPa stress creep and recovery. Each cycle consists of 1 second of loading and 9 seconds of recovery upon

The non-recoverable creep compliance, Jnr, and the percent recovery, R%, are two of the parameters calculated from the measured strain under different stress cycles [9, 11]. The Jnr parameter was suggested as a measure of the binder contribution to mixture permanent deformation. Different factors can have significant impact on Jnr value such as the duration of the creep interval, the duration of the recovery interval, the number of loading cycles and, of course, the entity of the applied shear stress. In other words, Jnr depends on the mechanical history of the

The low-temperature cracking of asphalt concrete pavements is a major pavement distress mechanism in cold regions costing hundreds of millions of dollars in rehabilitation costs to various agencies. It usually occurs in the form of regularly spaced transverse cracks, initiating at the surface of the asphalt layer and further propagating downward. Consequences of thermal cracking are an immediate increase of the roughness of the pavement surface (i.e., a reduction of the comfort and safety of the ride) and the loss of the sealing function of the pavement for the underlying layers. However, predictions of this distress have not been accurate enough, often resulting in premature road failures. It is believed that the excessive brittleness due to the increase in stiffness and decrease in the ability to relax stress leads to the buildup of thermally induced stress and ultimately cracking of mixtures

The prediction of asphalt pavement thermal cracking has been the subject of

numerous studies that date back to the early 1960's [26]. In many of these studies, attempts were made to introduce a procedure to predict pavement cracking based on the stress-strain-time—temperature relationship. A number of methods have been introduced throughout the years to model the viscoelastic behavior of asphalt binders and mixtures to estimate the accumulation of thermal stress during cooling cycles and predict the temperature at which cracking

improvements for the MSCR test protocol.

Creep Characteristics of Engineering Materials

2. Asphalt thermal cracking behavior

instantaneous unloading [24, 25].

experiment.

in pavements.

occurs [27–29].

26
