Evaluating the Role of Aggregate Gradation on Cracking Performance of Asphalt Concrete for Thin Overlays  
[PERSON]  
1 Department of Civil and Environmental Engineering, Universitat Politecnica de Catalunya-BarcelonaTech, 08034 Barcelona, Spain; [EMAIL_ADDRESS] (R.M. ); [EMAIL_ADDRESS] (F.E.P-J. )1 Department of Civil and Environmental Engineering, Universitat Politecnica de Catalunya-BarcelonaTech, 08034 Barcelona, Spain; [EMAIL_ADDRESS] (R.M. ); [EMAIL_ADDRESS] (F.E.P-J. Depending on users' needs, asphalt concrete mixtures for thin overlays range from dense graded mixtures to stone matrix asphalt, open-graded friction course, permeable friction course, or ultra-thin bonded wearing courses [6]. Since thin asphalt concrete overlay mixtures are used for the wearing course, their mix design is very similar to the employed practices for standard wearing mixtures. One of the main differences is their nominal maximum aggregate size (NMAS) due to the reduced layer thickness, ranging from 15 mm to 50 mm [7]. In order to ensure adequate compaction, the NMAS should not be longer than about one-third the compacted layer thickness. For thin layers, an NMAS between 12.5 mm and 4.75 mm is typically used [8]. Since fine aggregates make up the majority of aggregate used in thin asphalt concrete mixtures and the smaller NMAS, higher asphalt content is needed to properly coat and bind the aggregate. Asphalt grade must be selected according to climate and traffic level. The performance of thin asphalt concrete overlays will depend upon a number of factors including mix design approach and materials, underlying pavement type, surface preparation, traffic, climate and the construction quality. Based on the long-term pavement performance (LTPP) database, which is managed by the Federal Highway Administration, numerous studies have been conducted to evaluate the performance of different maintenance techniques, including thin overlays [9; 10; 11]. Indeed, there is a Specific Pavement Studies Experiment 5 (SPS-5) in the LTPP program that studies the effects of overlay thickness, overlay material, and pre-overlay treatment on the performance of asphalt overlay [12]. One of the main issues in thin asphalt concrete overlays is reflective cracking [13]. Reflective cracking occurs due to the propagation of discontinuities and cracks from the existing underlying pavement surface to the new overlay due to the vertical and horizontal movements of the overlay caused by temperature/seasonal variations and traffic loads. Reflective cracks can cause early deterioration and reduce the service life of asphalt concrete overlays. Researchers' efforts have been focused on enhancing cracking resistance of asphalt concrete overlays through increasing overlay thickness, density, employing polymer modified binders, geosynthetic interlayers or cut and seal techniques [13; 14; 15; 16; 17; 18]. Some authors studied the effect of using reclaimed asphalt pavement on the cracking resistance of asphalt concrete overlays [19]. But extensive laboratory studies that examine the effects of material selection on cracking performance of asphalt mixtures for thin overlays have not been conducted. Since thin asphalt concrete overlays can address functional and safety issues and last over 10 years, the proper selection of materials, the mix design approach, and the proper conditioning of the underlying pavement are crucial to develop well-performing and long-lasting pavements. The purpose of this study is to gain insight on how a key mix design variable such as aggregate skeleton gradation affects the cracking performance of asphalt concrete mixtures used for thin overlays. Because aggregate gradation can strongly influence aggregate interlock and mortar performance, there is a need for an in-depth study of the relationship between aggregate gradation and cracking response of mixtures.