# A Review of Sustainable pavement Aggregates
## 3 Waste Materials  
### Tire Recycling  
The polluting waste of tires at the end of their useful life reaches 3.9 million tons per year in Europe [42]. This growing problem requires studies aimed at recycling rubber through sustainable methods [43]. The application of crumb rubber (CR) in the manufacture of asphalt pavements offers an opportunity to reduce its negative impact on the environment while improving the properties of asphalt mixtures [44]. Rubber is obtained from tires with a consumed service life by reducing it by crushing it to fragments whose size allows them to be incorporated into an asphalt mixture as aggregates [45]. [PERSON] et al., (2019)  
Figure 1: Plastic waste treatment, adapted from [27]. mention two processes for adding crumb rubber to an asphalt mixture: the wet one that integrates the rubber fragments into the asphalt binder and the dry one that replaces crumb rubber a part of the aggregate destined for the mixture with the advantage that the latter does not require additional equipment concerning that used for conventional mixtures [46]. In addition, acoustic properties are improved for an AC11 asphalt concrete tread (Asphalt Cement 11) with the addition of crumb rubber using the dry process compared to an SMA 11 mix tread (Stone Mastic Asphalt 11) [47]. Incorporating modified crumb rubber (CRM) in the hot asphalt mixtures (HMA), the cost of road maintenance, and the improvement of abrasion resistance with a partial elastomeric recovery will be a function of the type and size of the particle, the mixing process and the rubber content in the mixture [48]. The asphalt mixtures with contents of 5%, 7%, and 10% of crumb rubber within a temperature range between 46 \({}^{\circ}\)C and 82 \({}^{\circ}\)C applying the response surface methodology (RSM), suggests that the content of crumb rubber impacts more than the temperature on the complex modulus (G*) and phase angle (\(\delta\)), both rheological properties of the asphalt mixtures [49]. In addition, it was found that for contents of 12% and 15% of crumb rubber, it was necessary to modify it with an increase in temperature above 200 \({}^{\circ}\)C in their respective samples. The degradation and expansion of the rubber particles allows the agile absorption of the binder of the mixture reducing the formation of cracks in the asphalt concrete modified with this material [50]. However, through the Analytic Hierarchy Process (AHP), two types of asphalt cement were classified by their performance grade (PG), and a third cement was classified by its viscosity (AC), with particle sizes of the crumb rubber at room temperature defined by the meshes no. 20 and no. 40, it was observed that the recommended amount of rubber for the mix with PG64-16 asphalt cement is 15% with particles passing mesh no. 20. Furthermore, for mixtures with asphalt cement PG58-28 and AC20 are 10% and 15%, respectively, with particles passing the mesh no. 40 [51]. In contrast, blends with a maximum of 15% crumb rubber under temperature ranges between-15 to 40 \({}^{\circ}\)C have a lower stiffness modulus than blends that do not include this material, where the void content is significant for establishing differences, even changing the trend described [52]. Consequently, the higher the content of crumb rubber in the asphalt mixture, the lesser the risk of cracking and fracture in the asphalt mixture; therefore, a high fatigue strength implies a lower deformation and, thus, a lower phase angle [49]. Total X-ray reflection spectrometry (TXRF) quantifies the rubber content in asphalt mixtures and represents an alternative with a greater sustainable range, guaranteeing good rheological properties that inhibit the formation of rust and reflection cracks [53]. The Indirect Tensile Strength (ITS) test at 60 \({}^{\circ}\)C for SMA blends shows that higher rubber content by wet process reduces chafing and presents a high percentage of voids per mineral aggregate favoring resistance to cracking [54].