## 1 Introduction  
As urbanization continues, the urban surface is gradually covered by closed, hardened pavements such as asphalt and cement concrete. The increased coverage of the road network has greatly improved traffic conditions, but the negative impacts have been increasing as the proportion of closed hardened pavements has gradually increased, such as the urban heat island effect (UHI) [1]. Permeable pavement is considered to be one of the effective measures to improve this series of problems. On the one hand, permeable pavement can effectively improve urban drainage capacity, alleviate urban flooding pressure, and regulate the urban hydrological cycle. On the other hand, permeable pavement can realize physical filtration and pollutant removal, maintain the moisture of the pavement structure, reduce the temperature of the pavement, alleviate the heat island effect, and maintain the urban ecological balance [2; 3; 4]. According to the different pavement materials, the common permeable pavement can be divided into asphalt permeable pavement, cement permeable pavement, resin permeable pavement, and so on [5; 6; 7; 8]. However, the characteristics of traditional asphalt-based permeable pavements and cement-based permeable pavements lie in the low structural strength of their pavements, and their large pore structure will be clogged after a certain number of years, leading to a shorter service life [9; 10; 11; 12]. More importantly, when asphalt is used as a binder, the water stability of asphalt mixtures that are insufficient will make asphalt pavements undergo a series of water damages, such as loosening, spalling, and so on. This is due to the fact that water is more likely to infiltrate the aggregate surface than asphalt, reducing the adhesion properties between asphalt and aggregate, which leads to the constant spalling of asphalt from the aggregate surface. Compared with traditional permeable materials, resin-based materials have the advantages of faster strength formation, excellent water permeability, and better adhesion properties. Therefore, resin-based materials are gradually being used as binders in permeable mixtures, such as epoxy resin (EP), polyurethane (PU), unsaturated polyester (UP), etc. [13; 14; 15; 16]. PU refers to polymers with urethane-containing groups, which are generally made from isocyanates and polyols through polycondensation reactions. In recent years, PU-based materials have been gradually used in the field of road engineering due to their good bond strength, heat resistance, and corrosion resistance [17; 18; 19]. At present, PU has been widely used in the modification of traditional binders, such as PU-modified asphalt mixtures, which have more excellent high- and low-temperature performance and aging resistance than traditional asphalt mixtures [20; 21; 22; 23; 24]. In the latest research, PU can also be used as a new binder, completely replacing the original asphalt, and directly prepared into PU mixtures. Compared with asphalt mixtures, PU mixtures can be mixed at room temperature, which has the advantages of low energy consumption and reduced environmental pollution [25; 26; 27]. In fact, the strength of the interfacial phase formed between the binder and the aggregate is the key factor affecting the water stability of the mixture. The methods for evaluating the adhesion properties of the binder-aggregate interface mainly include the water boiling method, the water immersion method, the pull-out test, etc. However, such methods are more based on experience for the qualitative evaluation of the adhesion properties and are not capable of evaluating them quantitatively [28]. Therefore, the surface free energy theory began to be used to quantitatively evaluate the adhesion between asphalt and aggregates in the 1990s [29]. The surface free energy theory is an important concept for describing the interaction forces between atoms/molecules on liquid or solid surfaces.