# Physical and Mechanical Properties of Reclaimed Asphalt
## 2 Review of Related Studies  
The main concern of the research community is to determine the optimal dosage of RAP in new mixtures without compromising pavement performance. Since RAP is investigated in this study as a component of the unbound layers, it is important to evaluate not only the physical properties, but also (i) the bearing capacity and (ii) the permanent deformations that may cause rutting in the pavement. Therefore, a brief literature review of the existing evidence on these two critical aspects is presented below. After the initial study by [PERSON] [31], RAP was investigated as a component of RAP-VA mixtures. [PERSON] et al. [32] were among the first researchers to evaluate the performance of unbound aggregates containing RAP in terms of Resilient Modulus (M\({}_{\text{R}}\)), which is supposed to better reflect the strength of the unbound materials compared to the Young modulus (E). They concluded that a mixture of VA including RAP resulted in a higher M\({}_{\rm R}\) compared to pure VA. [PERSON] et al. [16] investigated and evaluated the use of RAP as an unbound aggregate in terms of California Bearing Ratio (CBR). It was found that the CBR value decreased as the percentage of RAP was increased from 20% to 100% and none of the samples achieved the desired minimum CBR value, which was 80% in the case of a base course. Therefore, in their conclusions, they suggested that RAP should be used in a proportion lower than 60% in the case of subbase courses, while this proportion was limited to 10% in the case of base courses. In the 2000s, many researchers continued to evaluate RAP as a base course material and conducted M\({}_{\rm R}\) tests. The conclusions of some of these studies [33; 34] stated that the M\({}_{\rm R}\) of 100% RAP samples is greater than that of a natural aggregate. On the other hand, [PERSON] [35] concluded that samples with 100% RAP did not have the same bearing capacity as virgin aggregates, while [PERSON] et al. [23] concluded that a mixture of 50% aggregate and 50% RAP had higher stiffness than samples with 100% aggregate at higher confining pressures. Finally, other researchers [36; 37; 24; 38] confirmed that RAP materials in a certain ratio were a suitable and sustainable solution for unbound base and subbase courses since they were reported to have equivalent or higher M\({}_{\rm R}\) and higher stiffness than a mixture of 100% virgin aggregates. [PERSON] et al. [37] emphasized that a proportion of RAP of up to 50% increased the stiffness of the overall base course material, while [PERSON] [38] concluded that the RAP aggregates underwent lower stresses compared to conventional limestone aggregates. A recent study [39] examined RAP-VA blends of four different types of RAP and three different types of VA with respect to M\({}_{\rm R}\) and found that a RAP-VA blend had a higher M\({}_{\rm R}\) value than VA under the same loading conditions. Regarding the deformation properties of RAP, some researchers have pointed out that specimens containing RAP appear to develop higher permanent deformation than natural aggregates [23; 24; 33]. This was confirmed by [PERSON] and [PERSON] [40], who evaluated and compared the properties of RAP, crushed limestone and crushed gravel with similar gradation and degree of compaction. In contrast, [PERSON] et al. [26] conducted multistage permanent deformation tests of 100% RAP and 100% VA and found that RAP exhibited higher deformation than VA at low stresses, while the opposite was true at higher stresses. [PERSON] [41] also performed triaxial repeated loading tests on three different specimens, namely 100% RAP, 50% RAP \(-\)50% VA and 100% VA, and reported that a mixture of RAP and VA exhibited lower permanent deformation than a 100% VA specimen. [PERSON] and [PERSON] [27] came to similar conclusions in that the increase in residual cumulative strains was insignificant when the RAP contents varied from 0 to 50%.