# Bonding Evaluation of Asphalt Emulsions used as Tack Coats through Shear Testing
## 2 Background  
As stated by [PERSON] [24], the application of a tack coat is considered a simple and cheap procedure during pavement construction; however, it should be seen as a requisite to guarantee the correct performance of the pavement. As stated in [PERSON], et al. [25], the tack coat achieves its principal purpose by bonding a previous surface with the new layer to prevent deterioration, as mentioned before. We proposed that tack coats are not sufficiently taken into consideration in most pavement procedures, and acceptable bonding is not always achieved, as affirmed in reference [26]. Thus, slippage between layers appears to reduce the service life and shear distress becomes excessive at some particular points of the road where the horizontal loads increase. This process can occur in curves, intersections, and zones with ascendant or descendant gradients, causing surface distress.  
Currently, the Mexican regulation for the quality control of asphalt emulsion consists basically of the procedures stated in N-CMT-4-05-001/05 [27]. However, as shown in Ontiveros [28], these tests presented by the Secretary of Communications and Transportation are only indicative of the quality of the material and do not describe the mechanical behavior of the binder performing as a tack coat when bonding the pavement top layers.  
To evaluate the mechanical behavior of a tack coat, researchers have considered shear, tensile, and torsion models to try to represent the stresses presented in the pavement during its service life. Some of the most representative models are the LCB (Laboratorio de Caminos de Barcelona) test implemented by the Polytechnic University of Cataluna, the ASTRA (Ancona Shear Testing Research and Analysis) [22] test from Polytechnic University of Marche, the SHSTM (Sapienza Horizontal Shear Testing Research and Analysis) test from Sapienza University of Rome, the FDOT (Florida Department Of Transportation) test, the UTPP Pull Off Test by the University of Texas El Paso, and the Torque Bond Test and LPDS (Layer-Parallel Direct Shear) [29].  
[PERSON], et al. [30] presented some of the research where the LCB shear test was implemented. As a necessity to study the performance of tack coats, a shear resistance test was developed by the Polytechnic University of Cataluna in Spain. This procedure is known as the LCB shear test, and a comparison between conventional, term-adherent, and modified emulsions was developed to evaluate the mechanical response of the tack coat.  
Figure 1: Force and stress diagram acting on the specimen in the LCB (Laboratorio de Caminos de Barcelona) shear test.  
The LCB test was developed by the Laboratorio de Caminos de Barcelona with the objective of assessing the tangential stresses caused by the application of a shear force (Figure 1). The maximum load is the highest point of the load-displacement curve; therefore, higher resistance increases the adhesion between asphalt layers. On the other hand, the fracture energy is the area under the load-displacement curve, implying that the greater the area under the curve, the more energy that will be needed for disengaging between layers [31].