# Limitations on the Use of Recycled Asphalt Pavement in Structural Concrete
## 3 Quality and Homogeneity of RAP Aggregates  
### Definition and Origin/Production of RAP Aggregates  
Recycled asphalt pavement is the name given to crushed, milled, pulverized, processed or/and unprocessed pavement materials containing asphalt and aggregates. These materials are generated when asphalt pavements are removed for reconstruction by milling from the road/motorway surface. A reclaimed asphalt pavement consists of two components [34]:  
* RAP aggregate, which is the aggregate part of the reclaimed asphalt;
* RAP binder that consists of the asphalt cement of the reclaimed asphalt. As both components have a great impact on the performance and the mixture of concrete, the amount of RAP content is still limited by the maximum amount of aggregate and binder allowed in the mixture. The properties of RAP are largely dependent on the properties of both the original aggregate and the binder. Based on the studies investigated, the chemical composition of RAP derives from the type of pavement, which in turn depends on the type of parent rock of the aggregate. For  
Figure 1: SITEB analysis on RAP in Europe: (**a**) RAP available in each country; (**b**) Percentage of reused RAP [50]. this reason, a petrographic analysis is required. [PERSON] [3] just refers to a petrographic analysis without indicating the relative results, while [PERSON] et al. [33] reported a chemical composition of the RAP aggregate studied, which is shown in Table 2 as a result of the X-ray fluorescence tests. RAP contains mineral aggregates up to 97% by weight and the remainder is made of hardened asphalt cement. RAP has a similar chemical composition in comparison to that of natural aggregates. It was observed that the major element compounds of RAP, in percentage by weight, are SiO\({}_{2}\), Fe\({}_{2}\)O\({}_{3}\), CaO [33]. The asphalt in RAP is a viscoelastic material, whose properties depend on temperature and the loading conditions. As the RAP is exposed to the environment, oxidation causes two conversion processes in the aged asphalt: (i) oils in resins; and (ii) resins in asphaltenes. The viscosity increases with the RAP binder content and it plays a major role in terms of workability. A decrease in the viscosity of the bitumen allows a better workability of the mixes. Consequently, when the RAP is included as an aggregate in the concrete, the properties of the latter, such as creep and shrinkage, get affected. The mechanical properties of RAP also depend on the original asphalt pavement type. There can be significant differences between asphalt concrete mixes, since the aggregates in surface course must have high resistance to abrasion to contribute to acceptable friction resistance properties: these aggregates may be of higher quality than the aggregates in binder course applications, where polishing resistance is not fundamental. Furthermore, both milling and crushing can cause some aggregate degradation [51]. The particle size distribution of milled or crushed RAP varies in accordance with the type of equipment used to produce the RAP, the type of aggregate in the pavement, and whether any underlying base or subbase aggregate has been mixed in with the RAP during the pavement removal [51]. Asphalt pavement is generally removed either by milling (which can remove up to 50 mm thickness in a single pass) or by full-depth removal (using a rhino horn on a bulldozer and/or pneumatic pavement breakers) [51]. The crushed material is conveyed to a facility for processing. At this facility, the RAP undergoes a series of operations, including crushing, screening, transporting and stacking. Especially during the stacking phase, an aspect that is generally investigated regards the potential environmental risks related to a possible leaching of pollutants of RAP. In the time frame between transportation and storage, precipitation could fall on the RAP stocked on landfills, resulting in contaminants leaching from the RAP itself. Water-carrying contaminants could pollute surface waters or reach groundwater by infiltration.