## 2 Recycled Material Analysis
### Recycled Glass  
Glass production technology is resource- and energy-intensive. Waste glass or recycled glass can be used as either a coarse or fine aggregate and filler in asphalt mixtures [39; 40; 41]. Moreover, glass particles are hydrophobic with low water absorption (0.3% to 0.4%) [39]. Consequently, asphalt binders are not absorbed by glass particles. The OBC of asphalt mixtures containing glass is less than for comparable traditional asphalt mixtures. This discrepancy means that binder contents must be reduced, or else air void volume may be inadvertently impacted. Furthermore, [PERSON] et al. [42] reported that stability decreases and air voids increase as recycled glass concentration increases. The low absorption capacity of recycled glass reduces aggregate-binder adhesion by limiting bituminous binder absorption. This unabsorbed binder attracts compaction energy, resulting in less efficient absorption by the aggregate materials. As a result, less interlock is achieved between aggregate particles, reducing the load-bearing capacity of the mixture (which becomes more dependent on the stiffness of the binder rather than on the friction between the aggregate particles). It should be noted that asphalt surface interface strength is also a function of adhesion, friction and aggregate embedment or interlock [39] and may be adversely affected by high recycled glass content.  
It should be noted that OBC decreases as the glass content increases. However, a laboratory study conducted by [PERSON] [43] showed that glass contents up to 15% had no adverse effect on the ITS and resilient modulus, which are factors influencing the load-bearing capacity of the mixture. Also, incorporation of 15% waste glass actually improved the fatigue life of the asphalt mixture. Therefore, 15% can be considered the optimum content of recycled glass in asphalt mixtures.  
Recycled glass was also used as a coarse and fine aggregate in concrete pavements. Experimental results indicated that the use of waste glass as a course aggregate reduced the concrete's compressive strength [44; 45; 46]. In contrast, fine-graded glass for sand replacement increased the compressive strength [47; 48; 49]. This reflects the strength gain of concrete as a hydration reaction. Since the surface area of fine-graded glass particles is higher than that of course glass particles, the reaction potential of the glass particles increases during hydration. Consequently, hydration products increase and the strength of the concrete increases.  
Safety must also be considered when recycled glass asphalt pavement utility is assessed. Vehicles can dislodge glass particles from segregated pavement surfaces that can damage vehicles and injure pedestrians [39]. Therefore, recycled glass is only recommended for surfacing of low volume and low speed roads, for road base, and recreational pavements such as footpaths. Recycled glass is not recommended for the surface layer of highways or for airport pavements because the high shear stresses may dislodge glass particles and cause damage to vehicles and aircraft. Also, shear-related distress may result in delamination at the surface layer, or excessive shear creep within the asphalt surface layer as a result of the reduced asphalt internal friction and adhesion [50].  
In addition, safety is an important consideration during the construction phase because sharp glass particles may injure paving crew members. Moreover, members of the paving crew might inhale glass dust. However, [PERSON] et al. [51] reported that larger (9.5-mm diameter) glass particles do not present a safety concern, based on experimental studies and field investigations, which agrees with previous research by [PERSON] [52]. Furthermore, [PERSON] [53] reported that even particles smaller than 9.5 mm did not present a safety risk to asphalt paving crews as long as goggles and gloves are worn.