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1. INTRODUCTION
Transportation infrastructures play a fundamental role in the economic growth of nations. Over the last three decades, the volume of traffic in highway networks has notably increased (1). Nonetheless, in the field of construction engineering, sustainability issues have been of increasing concern (2). Therefore, the optimization of the maintenance activity of highway and road networks can be considered as a critical aspect to be solved. Clearly, the main aim of rehabilitation and maintenance programs is to extend the service life of these infrastructures. Thus, highway agencies worldwide are attempting to find the most cost-effective and efficient solutions to achieve this goal (3). Traditionally, dense-graded mixture overlays (with a thickness around 50 mm) have been used as a rehabilitation action for asphalt pavements. Later, in order to diminish the consumption of natural resources and due to the development of modified bitumen, other types of gap-graded mixtures with a reduced thickness (around 30-40 mm) such as BBTM or SMA, have successfully substituted the traditional dense-graded mixtures. In recent years, whilst there are other alternative rehabilitation methods (seals, micro-surfacings, etc.), budget limitations and environmental considerations have encouraged highway agencies to use Very Thin Asphalt Overlay (VTAO) as a new rehabilitation strategy (4-6). According to available scientific references, there is no maximum determined thickness for VTAOs, but based on previous studies it appears that a thin overlay refers to an overlay with a maximum thickness of 38 mm (4-7). Additionally, World Road Association (PIARC) has defined thin asphalt surfacing and very thin asphalt surfacing as layers which have an average laying thickness of 30 to 50 mm and 20 to 25 mm respectively (5). Usually, VTAO can be efficiently utilized as a rehabilitation method for pavements with an adequate structure and in which initial symptoms of distress (e.g. block cracking, longitudinal cracking in the wheel path, shallow rutting, raveling, oxidation, and loss of skid resistance) have appeared (8).
As reported by some researchers, VTAOs provide certain environmental benefits such as reducing rolling noise level (4, 5) or diminishing the rolling resistance, which might decrease CO2 emissions (5). Furthermore, some investigations have revealed the strong anti-spray properties, efficient light reflection, and good skid resistance of VTAOs (4, 5). The cost-effectiveness of VTAOs has also been reported...