# Flexural Behavior of a New Precast Insulation Mortar Sandwich Panel
## 1 Introduction  
Precast concrete sandwich panel (PCSP) was first prefabricated as a non-bearing system called \"cladding panel\", which comprised two thick internal and external concrete wythes designed as load- and non-bearing walls, respectively. PCSP is commonly used to construct the outer shell of several typical buildings, such as residential, commercial, and warehouse infrastructures [1; 2; 3]. As shown in Figure 1, a typical PCSP consists of two precast reinforced concrete layers (called wythes) separated by a layer of insulation and joined with connectors that penetrate the insulation layer [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. To increase structural efficiency, various types of connectors (sometimes made of steel) can be used to promote composite behavior [15]. [PERSON] and [PERSON] [16; 17] carried out experiments on small-scale concrete sandwich panels to study their flexural behavior. Different types of wythe reinforcement, shear connectors, and core thickness were used in their experiments. They concluded that the presence of edge ribs and the type of shear connectors significantly affect the failure load and the flexural behavior, respectively, of the panels. There are many different types of sandwich panel insulation layer, such as polystyrene particles, foam concrete, and thermal insulation mortar. Insulation mortar is the term used to describe an insulation mortar composed mainly of fibers, expanded vitrified microspheres, and Portland cement [18]. The heat transfer coefficient usually varies between 0.07 and 0.085 W/m\({}^{2}\cdot\)K after curing. Although it is less efficient in terms of heat transfer than particles made of polystyrene (between 0.03 and 0.05 W/m\({}^{2}\cdot\)K), it is less expensive and easier to build. Since different companies develop different formulas for insulation mortar, there is not a single, universal formula. The insulation mortar used in this paper was composed of fibers, fly ash, Portland cement, expanded vitrified microspheres, and polystyrene particles. PCSPs were used in this research as prefabricated floor slabs. Shear forces are transmitted along the vertical direction and are primarily impacted by bending loads in terms of structural stress. For shear force transfer, shear connectors were used. The quantity and quality of these connectors affect the composite connection effect they create, since they transfer shear force between the insulation layer and concrete wythes. [PERSON] [19] arranged shear connectors on a two-dimensional plane and found that higher shear stiffness of the connector can provide better composite behavior, and arranging shear connectors at the maximum slip position has a better effect on improving the structural response. [PERSON] [20] conducted a large-scale flexible test and found that more shear connectors can provide better composite action. The truss-shaped shear connector used in this article is made up of two bent web bars that join the upper and lower chord bars to form a triangular cross-section, as seen in Figure 2[21]. The height for each bent web bar was 100 mm, and the distance was 100 mm. The stiffness and strength of the connector play a major role in the panel's strength, and the insulation layer's thermal resistance affects the panel's general thermal resistance value. Due to the varying properties of the sandwich material, the degree of connection between the various composite layers of the sandwich panel will have a significant impact on the final internal force distribution under bending loads. Depending on the degree of composite action achieved, a PCSP may be regarded as a fully composite, semi-composite or non-composite panel [22; 23; 24; 25]. Up until they sustain damage, sandwich panels should be used and designed in accordance with the principle of maintaining integrity. The fully composite panel's steel bars and shear connectors will not sustain damage prior to the concrete experiencing compression failure. As seen in Figure 3a, its strain essentially stays linear along the thickness direction.