**3. Polymer modifiers**

The binder characteristics strongly influence the mechanical properties of asphalt mixture. To this point, binder should have a certain mechanical and rheological requirements as follows in order to fulfill the pavement criteria:


Accordingly, it can be concluded that obtaining bitumen to work well under all aforementioned conditions can be difficult. To surmount this problem, many researchers have tried to develop the asphalt pavement performance by improving the asphalt binder behavior using different modifiers. There are a large varieties of materials, which are often used for modifying the binder, of which polymers are widely known to be easy to use and cost effective. Referring to available literature (for example, see [6–9]), polymer addition may result in both a more flexible binder at low in-service temperature and enhanced properties at high in-service temperature, which significantly prevent the pavement from being deformed. They also improve the adhesive bonding to aggregate particles [10]. Polymers can exist in two different morphologies while in a solid phase:


The degree of crystallinity in a polymer is affected by different factors such as polymer type, additives, and cooling rate. The morphology and degree of crystallinity significantly influence the polymers' properties. Polymers with high degree of crystallinity have a higher glass transition temperature and higher modulus, toughness, stiffness, tensile strength, and hardness. In addition, they have more resistance to solvents but are less resistant to impact strength [11].

Today, there are a large varieties of polymers that are often used for modifying the binder. These polymers can be mainly classified into the following categories:


Although modifying the binders will result in the enhancement of binder's properties, using virgin additives as modifier will increase the road construction cost. Therefore, in recent years, many investigations have been conducted on modifying binders using waste materials as additives. Among these waste materials, application of waste plastics and rubbers in certain amount as binder modifier can substantially enhance the stability, strength, fatigue life, and generally the asphalt performance in one hand [14], and on the other hand, it would be an ideal solution for reducing the environmental pollution associated with these non-biodegradable wastes. In light of this, according to the characteristics of elastomers and plastomers, this chapter analyses the characteristics of high-density polyethylene (HDPE) and crumb rubber as potential binder modifier. The tests used to evaluate the polymer properties were thermal analysis by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), and microstructure analysis by scanning electron microscopy (SEM).
