# Bonding Evaluation of Asphalt Emulsions used as Tack Coats through Shear Testing
## 1 Introduction  
Asphalt pavements are widely used due to their characteristics, advantages, and versatility [1]. In fact, as a way of making pavements more economical, less polluting, and with less energy consumption, emulsions can be used; these are dispersed systems made up of two immiscible liquids [2] united by an emulsifier. Additive components such as stabilizers, adhesion enhancers, coating enhancers, or breakage control agents could also be part of these [3]. Emulsions are classified according to the emulsifier and type of setting [4]. If the emulsifier has a negative charge, it is anionic, while a cationic emulsifier has a positive charge. There are rapid setting (RS) emulsions, designed to react quickly with the aggregate [3,5]; these are used for irrigation as they are highly viscous to prevent runoff [2]. On the other hand, those with slow setting (SS) properties are designed to achieve stable mixtures, using closed granularometries with a high percentage of fines [3,5]; these are commonly used for pavement priming [2]. An emulsion's viscosity and stability depend on several variables. Viscosity is controlled by the water/oil ratio, droplet size, and surfactant, where the water/oil ratio is very important, i.e., when the water/oil ratio decreases (at higher asphalt concentrations), the viscosity increases [6]. [PERSON] [7] affirmed that an emulsion was more stable when the droplet coalescence rate was slower. On the other hand, asphalt-solid adhesion depends on the electrical charge of the emulsion (anionic or cationic) and the pH of the aggregate; for example, if an anionic emulsion is mixed with an alkaline aggregate, it can produce a reaction that improves adhesion. In contrast, ifthe anionic emulsion is mixed with an acid aggregate, the adhesion is compromised. On the other hand, cationic emulsions have improved adhesion when mixed with either alkaline or acid aggregates, so they have been preferred [6]. Because pavements are formed of multiple layers subjected to combinations of traffic load and climatic changes [1], they are subject to distress as a consequence of inadequate selection of materials. It has been interesting to analyze the principal types of distress, such as slippage, delamination, and top-down cracking, due to the asphalt emulsion quality and its capacity to bond two layers [8]. When the adhesion between coats is low, the binder of bearings tends to crack early with increased rutting due to the internal energy consumption of the material, resulting in fatigue problems and top-down cracks [9]. Rutting is a permanent deformation due to traffic and is more important at high temperatures, while fatigue manifests itself at medium and low temperatures, causing the evolution of micro-cracks and micro-voids [10]. Due to the above, several researchers have studied the behavior of asphalt mixtures through different tests; for example, [PERSON] et al. [11] evaluated the geomechanical parameters of the asphalt concrete-sand interface using direct shear testing and a nonlinear analysis. [PERSON], et al. [12] evaluated the behavior of low-temperature mixtures through bending beam rheometer (BBR), direct tension (DT), and semi-circular bending (SCB) fracture testing, in order to obtain the normalized tensile stress (NTS). [PERSON], [PERSON], [PERSON] and [PERSON] [1] added carbon black (CB) as a conductive component to increase the electromagnetic sensibility of asphalt in order to accelerate the heating and healing of the material. They used indirect tensile (IDT), uniaxial compressive, and SCB testing. [PERSON], et al. [13] used activated carbon (AC) as a binder-based conductive additive to increase the electromagnetic radiation absorption of the asphalt binder; they used SCB and uniaxial testing.