It has long been accepted that cracking of hot-mix asphalt pavements is a major mode of premature failure. Many state agencies have verified that pavement cracking occurred not only in fatigue cracking, in which a crack initiates from the bottom of the asphalt layer, but also in other modes such as low-temperature cracking and the more recently identified top-down cracking. To improve current pavement designs and the cracking resistance of mixtures, it is necessary to understand the mechanisms associated with crack initiation and crack growth in hot-mix asphalt mixtures. However, the complexity of the problem and the lack of simple-to-use analysis tools have been obstacles to a better understanding of hot-mix asphalt fracture mechanics and the development of better hot-mix asphalt fracture models. Until today, the well-known finite element method has been the primary tool used for modeling cracks and their effects in mixtures and pavements. Unfortunately, it is both complex and numerically intensive for fracture mechanics applications. The displacement discontinuity boundary element method is presented, which is a numerical method that has been very successful in many other engineering fields, as a potential method for modeling cracking in hot-mix asphalt mixtures and pavements. A series of examples are provided to illustrate the effectiveness of the method in dealing with cracks, crack propagation, and visco-elasticity in hot-mix asphalt. It was concluded that the method was easy to use, resulted in accurate solutions, required minimal computation time, and significantly simplified the modeling of crack-related problems.

It has long been accepted that cracking of hot-mix asphalt (HMA) pavements is a major mode of premature failure. Many state departments of transportation have verified that pavement cracking occurred not only in fatigue cracking in which a crack initiates from the bottom of the asphalt layer but also in other modes such as low-temperature cracking and the more recently identified top-down cracking. Recent work at the University of Florida has led to the development of a crack growth law based on viscoelastic fracture mechanics that is capable of fully describing both initiation and propagation of cracks in asphalt mixtures. The model requires the determination of only four fundamental mixture parameters, which can be obtained from less than 1 h of testing using the Superpave® indirect tensile test (IDT). These parameters can account for microdamage, crack propagation, and healing for stated loading conditions, temperatures, and rest periods. The generalization of the HMA crack growth law needed for its successful implementation into a displacement discontinuity boundary element method is described. The resulting HMA boundary element approach is shown to predict the crack propagation of two coarse-graded mixtures under cyclic IDT loading conditions.

Cracking in hot-mix asphalt (HMA) pavements is a major mode of premature failure. Recent work at the University of Florida has led to the development of a new viscoelastic fracture mechanics-based crackgrowth law called the HMA fracture mechanics law, which is capable of fully describing both initiation and propagation of cracks in asphalt mixtures. The successful simulations of crack growth for generalized pavement conditions depend largely on how well the state of stress can be predicted in and around existing cracks in pavements. Previous work has focused on the adaptation of a displacement-discontinuity boundary-element method for predicting stresses in the Superpave® indirect tensile test (IDT), which then were subsequently used to predict the crack initiation and crack growth in simulated IDT tests that used HMA fracture mechanics. The previous displacement-discontinuity boundary-element formulation is here extended into layered materials. Homogeneous layers are stitched together numerically in "welded" contact. The ability of the new numerical formulation to model the effects of temperature-induced stiffness gradients on tensile stresses at the top of two cracked pavement sections in Florida is demonstrated. These pavement sections were modeled with and without temperature-induced stiffness gradients. The introduction of stiffness gradients into the HMA layer is shown to increase the magnitude of tensile stresses at the top of the pavement, which is consistent with previous observations.

Plastic waste has been incorporated with asphalt to improve the physical properties of asphalt and alleviate the increasing trend of plastic waste being introduced into the environment. However, plastic waste comes in different types such as thermoplastic or thermoset, which results in varied properties of polymer modified asphalt (PMA). In this work, four thermoplastic vulcanizates (TPVs) were prepared using different peroxide concentrations to produce four formulations of gel content (with varying extent of crosslinked part) in order to imitate the variation of plastic waste. All four TPVs were then mixed with asphalt at 5 wt% thus producing four formulations of PMA, which went through physical, rheological, and storage stability assessments. PMA with higher gel content possessed lower penetration and higher softening temperature, indicating physically harder appearance of PMA. Superpave parameters remained unchanged among different gel content PMA at temperatures of 64, 70, and 76 °C. PMA with any level of gel content had lower Brookfield viscosity than PMA without gel content at a temperature of 135 °C. Higher gel content resulted in shorter storage stability measured with greater different softening temperatures between top and bottom layers of PMA after 5 days of 163 °C storage. This study shows that asphalt with thermoset plastic waste is harder and easier to pave, thus making the non-recycling thermoset plastic waste more useful and friendly to the environment.

Crumb Rubber Modified (CRM) is one of techniques for improving asphalt mixture performance. There are two alternatives for applying crumb rubber (CR) to Hot Mix Asphalt (HMA), "Wet" and "Dry". In the wet process, CR is added into hot asphalt cement in a factory, then CR-asphalt cement infusion is transported to a HMA plant for blending with hot aggregates. In the Dry process, CR is blended with hot aggregates and asphalt cement directly in a HMA plant. Although the dry process is considered less efficient due to lower infusion with asphalt cement, it offers some advantages that CR content can be independently controlled and higher amount of CR can be added into the HMA. This study focuses on dry process that uses CR modification by replacing some aggregate particles in mixture. HMA is blended with CR in 3 different sizes: 1.18-2.36mm, 0.6-1.18mm and smaller than 0.6mm. The x-sized CR is added to the mixture in substitution of the same x-sized aggregate particles at the amount of 1% and 2% by the whole aggregate volume in the mixture. The mixtures' performance in deformation resistance is represented by Wheel Tracking Slope (WTS). The results of Wheel Tracking tests on the specimens are enlightening. The mixture with CR particles smaller than 0.6mm shows excellent performance on deformation resistance, indicated by significantly lower WTS than others. Secondly, mixtures with higher amounts of CR have better performance than those with lower amounts. The mixture with 2% CR with smaller than 0.6mm provides optimal performance at 2.1 times better than conventional HMA.