# Interface Shear Performance between Porous Polyurethane Mixture and Asphalt Sublayer
## 1 Introduction  
The open graded friction course (OGFC) was mainly used as road surface layer placed on dense graded asphalt layer to improve traffic safety at rainy conditions [1, 2]. With the high air void content (18%-22%), OGFC offers safety advantages in terms of high skid resistance and no splash or spray in addition to its benefits in noise reduction and water runoff quality enhancement [3, 4, 5, 6]. Despite of the functionality benefits of OGFC, engineering practices have found that the application of OGFC faces three challenges during the service life of pavement, including clogging of air voids [7, 8], raveling [9, 10, 11], and freezing-induced damage in winter [12, 13]. These problems apparently reduce pavement durability and affect the functional benefits of OGFC. Due to the challenges related to OGFC, its use has been limited through the years. A survey conducted in 2016 revealed that 25 states in U.S. do not use OGFC, among which 19 states have used it before but did not continue due to freezing damaged in winter [14].  
Recently, porous polyurethane mixture (PPM) has been proposed for road surface layer for noise reduction and better durability [15; 16; 17; 18]. Although the cost of PPM is higher, PPM has three advantages over OGFC. The first is that it can be mixed at room temperature, which can significantly save heating energy and eliminate the pollution of asphalt smoke during high temperature mixing [19; 20]. The second is that the PPM performs better to resist particle-related clogging due to its high air void (30%), because the pores at the greater porosity are more difficult to be clogged by small particles [18]. Lastly, the polyurethane is hard after curing and can maintain high stiffness during hot days. This makes PPM have better ability to resist deformation-related clogging and produce sufficient bonding with aggregates, which minimize the possibility of raveling [21; 22]. The recent work also found that PPM can significantly retard ice-formation time and reduces the adhesive strength with ice, which shows superior anti-icing and deicing performance than OGFC [23].  
Since the functional benefit and durability of PPM have been documented, the application of PPM as road surface layer is most beneficial. In this case, the interface bonding between the PPM surface layer and asphalt sublayer is important. However, previous researchers have found that early failure could be caused by the delamination of PPM with its sublayer, especially at tire braking or acceleration conditions [24]. The Japanese Public Works Research Institute (PWRI) found that adhesion of polyurethane mixture was not very durable with dense asphalt mixture but its adhesion with cement concrete was better. Therefore, they developed a semi-flexible layer (porous asphalt with the pores filled with cement) as sublayer of polyurethane mixture to solve the adhesion issue [25]. In the period 2002-2005, full-scale experiments on Japanese highways with eight different types of PPM on three locations all failed within a relative short time after construction (less than one year) owing to raveling, adhesion, and friction problems. The similar interface failure between polyurethane mixture and dense asphalt concrete layer was observed in Sweden only after four months of construction [26]. The reason causing the delamination of PPM could be insufficient bonding between PPM and underlying layer. Therefore, adhesive materials may be needed to improve the interface bonding between PPM and the traditional dense asphalt layer underneath.  
Traditionally, asphalt emulsions have been used as tack coat between asphalt layers. It was found that the interface shear strength between asphalt layers was affected by temperature, tack coat type and dosage, surface roughness, and mixture type [27; 28]. However, few studies have investigated the effectiveness of adhesive material on interface bonding between PPM and asphalt sublayers.