## 1 Introduction
The most accepted mechanisms include the following:  
* Shock-wave-induced hard-domain compaction and ordering, where the extent of this blast-mitigation effect is expected to be directly proportional to the hard-domain volume fraction;
* Shock-wave-induced hard domain crystallization/densification, where the hard domains of the polyurea experience irreversible compaction and densification with an associated increase in their degree of order upon being subjected to shock-wave loading;
* Shock wave induced hydrogen bond cleavage and formation, where the exposure to shock loadings leads to the cleavage of bi-dentate H-bond between the urea linkages, which subsequently rearrange to form more numerous H-bonds within the hard domains, thereby leading to the absorption and dispersion of shock energy;
* Shockwave-capture-and-neutralization, shock waves travel as a single wave in homogeneous materials. However, upon loading of a layered heterogeneous material system, e.g., polyurea, a two-wave structure is obtained: a leading shock front followed by a complex pattern that varies with time. This dual shock-wave pattern is attributed to the material architecture through which shock wave propagates, i.e., the impedance (and geometric) mismatch present at various length scales, and nonlinearities arisingfrom material inelasticity and failure. This secondary trailing shock-wave (release wave), reportedly catches up with and attenuates the leading shockwave, thereby leading to shock attenuation [13]. In work presented by [PERSON] et al. [14], a polyurea layer reinforced clay brick masonry resulted in a considerable improvement of the blast resistance of the structure. An adjustment in the proportion between the isocyanate and amino groups of the polyurea led to increased NCO content in the prepolymer. The tensile strength at break and the breaking elongation ratio obtained were 23 MPa and 510%, respectively. Consequently, the energy absorption capability in the polyurea increased, reinforcing the structure. [PERSON] et al. [15] reported an optimal ratio of chain extender to optimize the H-bonding, which improves the mechanical properties. In the work of [PERSON] et al., a commercial isocyanate prepolymer, Suprasec(r)2054, NCO = 15.5% was used with Poly (propylene oxide) (PPO) with the commercial name JEFFAMINE(r)D-230 and JEFFAMINE(r)D-200. The presence of a chain extender, diethyl-toluene diamine, increased the vicinity of urea linkages, which in turn favored the formation of H-bonds in the hard segment. The mitigation of the explosion increased with the coating thickness. A polyurea coating with six mm thickness could withstand fragmentation pressures higher than 90 psi [15]. [PERSON] et. al. [12] studied the positive effects of using polymers for retrofitting reinforced concrete structures subjected to blast loads by conducting a numerical investigation on the behavior of an unretrofitted reinforced concrete panel subjected to the blast load from a 2 kg charge at 1.6 m stand-off distance, and subsequently comparing its performance with several polymer coated panels. The increment of the thickness of the polyurea coating did not significantly influence the reduction of the displacement. The application of the polyurea coating on the non-blast-facing side of the panel tends to increase the effectiveness of displacement control. [PERSON] et al. [3] studied the mechanical behavior and equivalent configuration of a polyurea under wide strain rate range. The polyurea with the commercial name of Paxon(r)PX-3350, developed by LINE-X LLC, had a three mm thickness. The isocyanate and resin components of the polyurea were preheated at 60 \({}^{\circ}\)C, then mixed under high pressure (2000-2500 psi) with specific pressure equipment before applying the polyurea by spray coating. The polyurea's stress-strain curves shared a similar mechanical and strain hardening coefficient trend under different strain rate loads (between 0.001-2800 s\({}^{-1}\)). [PERSON] et al.