# A Review of Nanomaterials' Effect on Mechanical Performance and Aging of Asphal Mixtures
## 5 Aging Resistance
### Asphalt Mixtures  
#### 5.2.1 Methods  
The evaluation of the asphalt mixture aging is complex and, currently, there is no test method to assess it directly. A possible approach is to characterize the asphalt mixture's mechanical performance at early age (fresh mixture) and under aged conditions and then, determine the difference between them. Alternatively, some researchers compute the percent variation (principle similar to the VAI, Equation (3)) or the ratio between aged and fresh (principle similar to a retained performance, Equation (1)), commonly designating them as aging sensitivity and aging index, respectively. In this review, the effects of the nanomaterials in the aging resistance were analyzed by calculating the aging sensitivity (Equation (4)) of the evaluated mechanical performance parameters.  
\[\text{Aging sensitivity}=\frac{\text{Aged value}-\text{Unaged value}}{\text{ Unaged value}}\times 100 \tag{4}\]  
Apparently, the higher the absolute value of the selected indicator (difference aged-unaged, aging sensitivity, or aging index) the most aging sensitive is the mixture. A drawback of this approach is that in the case of a mechanical property which value increases with aging, e.g. stiffness or tensile strength, in some cases, a poor aging resistance performance may be wrongly understood as a good aging resistance. As example, Figure 14 presents the case of a mixture (mixture B) with higher aging sensitivity than the control mixture (mixture A) but presenting lower aging index in all evaluations after age one. Thus, to draw conclusions about aging resistance, one should have a good overview of the performance under aged conditions by conducting several performance tests. In this regard,due to the brittleness effect caused by aging, testing for fatigue resistance is particularly important. The production of aged specimens consumes a significant amount of time and resources, thus, generally, it is more practical to perform different tests than to simulate several aging times.  
To obtain aged asphalt mixture specimens few laboratory aging methods were developed. Currently, the methods used more often worldwide by the researchers are the described by the specification AASHTO R30-02 [133], initially proposed by the Strategic Highway Research Program [134]. In brief, the specification presents two methods, the short-term oven aging (STOA) and the long-term oven aging (LTOA). The STOA consists in conditioning the loose asphalt mixture in a draft oven for four hours at 135 \({}^{\circ}\)C and, aims to simulate the aging caused the plant mixing and compaction processes. The LTOA consists in conditioning the compacted asphalt specimens for five days at 85 \({}^{\circ}\)C and, aims to reproduce the aging of the pavement in service for approximately five to ten years. Although STOA and LTOA are practical methods requiring only simple equipment, they do not simulate all the actions present during field aging, e.g. solar radiation and moisture damage. Thus, researchers proposed few alternative approaches to address aging simulation, such as the saturation aging tensile stiffness (SATS) developed by [PERSON] [135], the Viennese Aging Procedure (VARro) developed by [PERSON] et al. [136], and the Tecnico accelerated aging (TEAGE) developed by [PERSON] et al. [132].