## 1 Introduction  
The ever-increasing safety requirements demanded in the engineering and construction of structures to protect people from terrorist threats or industrial accidents are fostering the study and development of new materials and processes. Polyurea are one of these versatile engineering materials, which has been commercially available since the early 1990s. They have the processing conditions and performance properties which allow the application of this type of material as a coating in a wide range of industries, like defense, security, construction, and automotive [1, 2, 3]. According to the Polyurea Development Association (PDA), polyureas result from the combination of two components: isocyanate and synthetic resin blend, such as amine-terminated polymer resins and (or) amine-terminated chain extenders. The isocyanate component of the polyurea can be aromatic or aliphatic in nature. It can be a monomer, a polymer, or a variant reaction of isocyanates, a quasi-prepolymer, or a prepolymer. The prepolymer, or a quasi-prepolymer, can be made of an amine-terminated polymer resin or a hydroxyl-terminated polymer resin. The resin blend is an amine-terminated polymer resin and (or) amine-terminated chain extenders [4]. Polyureas have great abrasion resistant capacity, that combined with its exceptional physical properties, such as high hardness, flexibility, tear strength, tensile strength, and chemical and water resistance, that makes this material ideal as a coating in applications that have very demanding functional requirements. The reaction that forms the polyurea is rapid with gel times measured in seconds, which means the reaction proceeds largely independently of ambient temperature and humidity, facilitating application of polyurea under diverse conditions. An important concern associated with polyurea coatings is the requirement of rapid mixing of the reactants, an issue which has been overcome using a suitable mixing module by impingement at high pressures. The viscosity of both components (isocyanate and resin) needs to be almost equal (difference less than 100 mPa\(\cdot\)s), which mandates a heating arrangement, with higher viscosity reactants requiring higher pressures for spraying. It is to be noted that the spraying pressure and reaction temperature of the reactants greatly affects the properties of the product formed. The micro-structure polyureas are comprised of two distinct domains, the hard domains, formed by hydrogen bonded polar urea linkages (-NH-CO-NH-), and the soft domains that consist of well-mixed hard and soft long chain aliphatic chains. Discrete hard domains are formed only when the molecular weight of the soft segment cross a particular threshold [5]. Polyureas are ideal coatings in new engineering projects of the infrastructure industry or the refurbished and maintenance of the existing ones. The high durability and strong resistance polyureas make them a very interesting choice for situations where the impact energy of earthquakes or blast phenomena must be dissipated [4]. The worldwide increase in terrorist activity and its lethality has exposed the weaknesses of the existing infrastructure to blast loading, particularly when large amounts of explosives are used (Figure 1). According to data collected from 1970-2017 by the National Consortium for the study of Terrorism and Response to Terrorism, bombings and (or) explosions compromise approximately 49% (88,600 incidents) of the total numbers of terrorist attacks (182,300 incidents) [6]. Thus, significant research projects have been developed within the last years to retrofit building and vehicle structures to resist blast loading. Retrofitting systems are inexpensive, easy to install, and protect the occupants of the facilities [7]. In the case of buildings typically, the main structure withstands the blast, but the facades are destroyed as well as non-structural masonry partitions, elevator and stairway shafts, furniture, drop ceilings, and light fixtures, which are propelled at high velocities hitting the occupants.