# Some Key Aspects in the Mechanics of Stress Transfer Between SRG and Masony
## 1 Introduction  
The use of composite materials for strengthening existing masonry structures is a topic of great interest among civil engineers. Fiber-reinforced polymer (FRP) systems, which belong to the family of composite materials, comprise fibers of different kinds, such as glass, steel, aramid, and carbon impregnated with a polymeric matrix (typically epoxy resin). Although FRPs have been widely used over the past decades to strengthen concrete and masonry structures [1; 2; 3], there are some disadvantages, mainly associated with the use of epoxy matrices, such as poor performance at high temperatures and poor vapor permeability, high costs, inapplicability on wet surfaces, hazardous fumes for workers, and incompatibility with the substrate. Therefore, a new type of composite with the same type of fibers organized in an open-mesh textile embedded in an inorganic matrix, instead of a polymeric matrix, has been recently employed. Due to their advantages, such as compatibility with different types of substrate and high vapor permeability, these new materials, usually referred to as fiber-reinforced cementitious matrix (FRCM) composites [4], are considered an effective system to strengthen historical masonry structures [5]. When a unidirectional textile made of high strength steel fibers is employed, FRCMs are named steel-reinforced grout (SRG). Numerous experimental campaigns were conducted in the last 15 years to investigate the contribution of externally bonded FRCMs andSRGs to the load-carrying capacity of concrete and masonry members. These studies showed an increase in the bending [6; 7; 8], shear [9; 10; 11], and axial capacity [12; 13; 14] of FRCM/SRG-strengthened reinforced concrete (RC) members that are comparable with the counterparts strengthened with FRPs. Analogously, FRCMs and SRGs were proven to be effective to increase the in-plane and out-of-plane strength of masonry walls (see Figure 1a,b, respectively) [15; 16; 17], axial strength of masonry columns [18; 19; 20], and the load-carrying capacity of masonry arches [21; 22]. Inorganic-matrix composite materials, which are externally bonded to a concrete or masonry member, usually fail due to composite debonding [4]. Depending on several parameters, such as the matrix and textile type and layout and the properties of substrate and matrix, debonding may occur at different interfaces [23]. For FRCM that feature fibers that are not coated (i.e., not impregnated/sized with organic resin), debonding at the matrix-fiber interface has been frequently reported [24; 25; 26; 27]. In the case of SRG composites, the debonding of the external matrix layer (that covers the fibers) together with the fibers from the internal layer (bonded to the substrate), referred to as matrix interlaminar failure, has been often observed [28; 29]. However, both the matrix-fiber debonding and matrix interlaminar failure are affected by the specific strengthening layout, and even changing the number of textile layers may affect the debonding failure observed [30; 31]. Furthermore, the test set-up adopted may influence the bond behavior observed. A comparison between the single-lap shear test and modified beam test showed that the composite bond behavior is equally captured by both tests, although in the beam test, two interfaces are involved, and, therefore, the bond of the weakest interface is activated [32]. A similar outcome was found by the authors when a comparison was carried out for FRCMs applied to concrete by conducting single-lap and double-lap tests [33]. The experimental campaign presented in this paper focuses on an important aspect of masonry elements strengthened with SRG composite, i.e., the bond between the composite itself and the substrate. Single-lap direct shear tests are performed on SRG composites bonded to masonry blocks. For SRG, debonding generally occurs with matrix interlaminar failure, but there are some factors that might influence this type of debonding.