## 1 Introduction  
Asphalt pavement is exposed directly to the environment and continuous temperature cycling, and the sharp drop in temperature can cause huge shrinkage stress. Because it is closely bonded with the base, the asphalt surface will not be free to bend, which, as a result, can create different stress at the surface and the bottom of the asphalt surface layer. Combined with other stresses, larger tensile stress and cracks will appear in the surface. Under the repeated action of traffic loads and temperature stresses, micro-cracks in the surface and base will expand, density and connect to eventually form macro-cracks, then cause damage to the pavement structure [1; 2; 3]. Thus, to control the road surface cracks, effective preventive maintenance measures are necessary. Hot mix asphalt (HMA) thin overlay technology, as a preventive maintenance method, has been widely used around the world. In the 1970s, France was the first country to use a thin asphalt overlay called Beton Bitumineux Mince (BBM) on high-graded asphalt pavement [4]. In the early 1990s, the United Kingdom began to construct ultra-thin asphalt concrete (VTHMAL) test roads [5]. The United States introduced and promoted Stone Mastic Asphalt (SMA) technology in 1991. NovaChip is a pavement maintenance treatment developed by the Colas USA company [6]. In 1998, Wroclaw in Poland built a thin overlay test road, which, after being in service for several years, still has good performance. Stockholm in Sweden paved an ultra-thin asphalt concrete test road in 1999 [5]. Indonesia developed an ultra-thin overlay (20-40 mm) called LASTIKA, a hot mix asphalt concrete overlay with Superflex modified asphalt [7]. Gradually, a fine-grained SMA mixture, the nominal maximum particle sizes of which are 4.75 mm and 9.5 mm, were developed for the preventive maintenance of high-grade highway [8]. Application experience shows that thin asphalt overlay is an effective technology for delaying pavement distresses, restoring pavement function, improving pavement service level and extending the service life of the pavement [9; 10]. For example, some research has demonstrated that thin overlays can efficiently improve the skid resistance of pavements [11; 12; 13; 14; 15]. Besides, asphalt overlays can also delay and restore various kinds of cracks in asphalt pavements. Some researchers tried to improve the crack resistance of asphalt mixture by developing new materials. For example, [PERSON] et al. studied the resistance to reflective cracks of interlayer materials in asphalt overlay; the test results showed that Atactic Polypropylene (APP) asphalt felt > Fiberglass geogrid > Geotextile > Without interlayer material [16]. [PERSON] et al. investigated the effects of grid stiffness, tensile strength, coating type, amount of tack coat, overlay thickness, crack width, and stiffnesses of asphalt overlay and existing asphalt concrete on reinforced asphalt overlay performance with a geogrid interlayer [17]. [PERSON] et al. used a kind of geosynthetic called glass-grid composite (GGC) to improve the crack resistance of asphalt overlays because GGC can mobilize high tensile strength at a low strain value [18]. Some researchers tried to improve crack resistance with material composition. For example, [PERSON] et al. analyzed the effect of notch depth, gradation, asphalt properties and temperature on the crack resistance of the asphalt mixture based on semicircular bending (SCB) tests [19]. [PERSON] et al. studied the effects of nominal maximum aggregate size (NMAS) on the performance of SMA and found that a decrease of NMAS showed better cracking and raveling resistance [20]. [PERSON] et al. studied the effect of aggregate gradation on the cracking performance of asphalt concrete for thin overlays and the results showed there is a significant correlation between the fine fraction content and the maximum nominal aggregate size and the cracking performance of the asphalt concrete mixtures [21].