## 2 Recycled Material Analysis  
### Recycled Asphalt Pavement  
The first instance of asphalt pavement recycling dates back to 1915 with the Warren Brothers portable asphalt plant [6]. However, it was the oil crisis in the early 1970s that increased asphalt price and energy costs, which truly incentivised increased RAP use [7]. RAP is currently ranked as a highly desirable material for use as an economic, technical and environmentally advantageous waste material in pavement construction. It is preferred over other waste materials including fly ash, rubber tires, steel slag, bottom ash, waste glass, building rubble and incinerator residue [5]. Presently, 99% of RAP generated in the USA is reused; 460 million tons of crushed aggregate is used for paving works each year [8; 9]. Likewise, in Japan, 99% of all RAP is reused in new pavement construction [10]. RAP is reused at an impeccable 100% rate in Denmark and Sweden [9]. However, Flemish road standard SB250 in Belgium disallows the use of RAP in the surface course of road pavement because of the environmental impact of potential leachates [11]. The incorporation of RAP as an alternative material in asphalt base and sub-base layer construction offers the potential for reducing global warming (20%), energy consumption (16%), water consumption (11%), life cycle costs (21%) and hazardous waste generation (11%) [12]. The primary technical advantage of asphalt mixes containing RAP is higher stiffness resulting from the combination of a virgin asphalt binder with an aged (hardened) asphalt binder. The resulting blended binder has a higher stiffness than the virgin binder because of higher activation energy [13; 14; 15]. In other words, the aged asphalt binder increases the stiffness of the blended asphalt binder [16], which results in an increase in the dynamic modulus and indirect tensile strength of the asphalt mixture [17; 18] (Figure 1). The slope of indirect tensile strength (ITS) indicates that each additionalpercent of RAP increases the ITS of the mixture by 13 kPa. Moreover, the binder content in the RAP reduces the optimum new binder content (OBC). Figure 1 demonstrates the inverse relation between the OBC and RAP concentration. Therefore, incorporation of RAP not only increases the structural capacity of the asphalt mixture in terms of dynamic modulus and ITS, it also reduces the required volume of the new binder, resulting in greater sustainability benefits. Although higher stiffness is an advantage for mix performance at higher temperatures, this attribute is a disadvantage at lower temperatures because of the increased potential for cracking and intermediate temperature fatigue (Figure 2). The construction temperature of mixes incorporating RAP is typically higher than that of traditional hot asphalt without RAP, which results in greater degrees of ageing, fatigue and susceptibility to low temperature cracking. However, the crack-resistance of asphalt mixes can be improved using fibreglass, which bridges the microcracks formed in the mix [20]. To avoid high stiffness, using lower quantities of RAP is recommended. Furthermore, some asphalt production plants now incorporate the RAP after the new aggregates are heated, thereby reducing the additional ageing induced by the RAP binder. Therefore, optimal RAP content is determined based on the rheological properties of the recovered asphalt binder and the engineering properties of the mixture and additives (such as antistripping agents or asphalt modifiers). For example, Figure 2 illustrates that approximately 10% RAP is an ideal concentration in terms of fatigue for both binder types. A softer binder can be used to compensate for the increased stiffness caused by the inclusion of RAP materials and to improve resilience to fatigue and low-temperature cracking. Figure 2 demonstrates that the fatigue life of mixtures produced using PG 52-34 and 10% RAP is approximately 11 times higher than a similar mix produced using the harder PG 64-22 binder.