## 1 Introduction  
The sandwich design is optimal for pre-cast concrete structural elements (floor, roof, wall, facade insulated panels, etc. ), since it offers an excellent strength-to-weight ratio and minimum manufacturing costs [1]. Nowadays, the rigid skin phases are cast in textile-reinforced cement (TRC): cementitious mortar reinforced with continuous impregnated textiles that provides bending strength and stiffness [2]. In between the skins, an insulation layer of a low density (typically of foam nature), cast in high thickness contributes to thermal and acoustic isolation and provides flexural rigidity (increase in the moment of inertia) and, in some cases, shear and/or buckling resistance as well [3]. The concept imitates the well-established \"I-section\" on metal structures, as the skin phases act as the flanges and the core stands for the web [4], however an essential difference should be highlighted. The cementitious composite skin is adhesively bonded to or only cast and deposited on the core; therefore, it is evident that interfacial detachment can occur due to weak joining, a condition that detrimentally affects and potentially puts the sandwich element structural integrity at risk. The interfacial debonding issue has been investigated since the first ferrocement sandwich insulated panels were designed; at that moment, a wet-on-wet fabrication process was proposed to ensure a good adhesion between the aerated concrete and the ferrocement [5]. Technological advances based on the pioneering contributions of [PERSON] [6] and [PERSON] [7], among others, led to the establishment of continuous textile reinforcement at the sandwich skin layer [8; 9] m and nowadays demonstration projects and several commercial products have been launched and reported great structural performance. Still though, the interfacial bonding remains a bottleneck, as has been extensively discussed in [10]. In that direction, and in an attempt to enhance the mechanical response and ensure a firm coerskin connection, the use of metallic connectors was recommended [11; 12]. The nail-type connectors also address the creep issue by compensating for the viscoelastic behavior of the foam core [12]. Indicatively, a glass-fiber polymer connector is designed to interconnect the TRC skins and, under service loads, appears to control the mechanical response of the sandwich elements that eventually fail due to the local connector's pull-out and buckling phenomena [13]. In all study cases, it is evident that interfacial debonding detection and tracking is challenging, first of all because the phenomenon occurs internally, and therefore visual confirmation is not possible. Besides this, manufacturing defects can lead to pre-existing disbands that lead to skin/core debonding under compression [3]/bending [14] service loads. Based on empirical evidence, the surface treatment of the core material can enhance the interfacial adhesion and ensure the optimal response of the core under loads--namely, shear failure [15]. In a recent study, it was shown that the core material's nature dominates the sandwich response to flexure. In detail, EPS (expanded polystyrene) and XPS (extruded polystyrene) insulation cores were considered for the design of wall insulated panels, where progressive interfacial debonding was detected only in the case of the weaker XPS core [16]. The urgent need to assess the conditions under which this defect occurs has attracted the attention of the research community, since interfacial debonding can lead to a drop in the sandwich ultimate strength [16], but also it serves as a conduit that allows access to heat [17] or air entrapment [18]. Conventional methods for debonding detection, such as CT scans and X-ray scans, are cost-ineffective and limited to small-scale point-by-point inspections, therefore they cannot be considered for real-size sandwich-insulated panel evaluation [19]. Guided wave inspection is gaining popularity in the field: among others, a laser Doppler vibrometer was used to emit low-frequency guided waves [20].