## 2 Methodology  
### Database Development  
The aim of this research was to categorize any asphalt mixture for thin wearing courses according to their mechanical, functional, and environmental performance through a labeling scheme that evaluates their attributes. To do so, it was necessary to develop a wide database on bituminous mixture performance related issues. This database development was divided into three stages: (1) data collection, (2) laboratory testing, and (3) pavement monitoring. The first stage started with the involvement of different IMs at national, regional, and local levels. Data of the quality controls carried out during the construction of national and local roads were collected by the provided access to the IMs' database. The sample size included more than 1500 quality control samples from 73 roads. The obtained data included mix design parameters (i.e., binder type and content, nature of the aggregate, aggregate gradation, filler content, air void content, and density), as well as mechanical and functional properties (i.e., stability, water sensibility, disaggregation resistance, macrotexture, and skid resistance). All roads had an average daily heavy traffic between 200 and 799 heavy vehicles per day. As expected, the collected data from the quality controls neither included results of a key property such as cracking resistance, because it is not specifically mentioned in the Spanish technical standard [20], nor any result related to their environmental performance (i.e., tire-pavement noise, recycled products, etc.). For this reason, it was decided to carry out a laboratory testing to analyze in depth the cracking performance of asphalt mixtures for thin overlay and a monitoring phase to evaluate other properties related to the surface texture. Laboratory testing was executed in two stages. At the first phase, the influence of binder type on the cracking performance of asphalt mixture was studied [21]. The second phase covered the influence aggregate gradation; in particular, the nominal maximum aggregate size and the fine fraction content were analyzed [22]. The Fenix test was applied to evaluate the cracking performance of the specimens. This test, developed by the Road Research Laboratory of the Technical University of Catalonia, is a direct tensile strength test that evaluates the cracking resistance of asphalt mixtures by calculating the dissipated energy during mixture cracking [23]. Asphalt mixtures were Fenix-tested and, based on their stress-displacement curve, two main parameters were considered to explain the cracking response of the tested mixtures (see Figure 1):  
* The tensile strength (TS) is defined as the maximum load divided by the fracture surface and gives a notion of the cohesion provided by the asphalt mastic to the mixture;
* The displacement at 50% post-peak load (\(\text{d}_{0.5\text{PostFmax}}\)) is directly related to the ductility of the mixture. Figure 1: Fénix test set-up and stress–displacement output curve. During the measurement campaign, different pavements with significantly different wearing courses (ultrathin asphalt concrete, gap-graded mixtures, open graded friction courses, and asphalt-concrete mixtures) were selected and monitored in order to obtain information about their \"splash and spray\" response and their tire-pavement noise emissions. Moreover, macrotexture and skid resistance measurements were taken to enlarge the collected data. Splash and spray measurements were recorded by a test trailer developed by Euroconsult, a specialized auscultation company, and the Technical University of Catalonia, which was based on the traffic speed drainometer measuring principle and pulled by a SCRIM vehicle (sideway-force coefficient routine investigation machine) (see Figure 2) [24]. The obtained results measure the number of splashed raindrops that cross a reference plane at a given time. Noise measurements were performed according to the close-proximity (CPX) method.