PSI - Issue 64

Rebecca Grazzini et al. / Procedia Structural Integrity 64 (2024) 1532–1539 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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matrix. Different types of FRCMs have been tested, combining textiles of carbon, PBO, basalt, or glass with matrices of cement, lime, or gypsum. The failure mode of FRCMs is generally related to the debonding of the composite at the FRCM – substrate interface. Many experimental investigations have been carried out to understand FRCM bond behavior which appears to be the key factor in the effectiveness of FRCMs (Carloni et al., 2022; Fazzi et al., 2023a,b; Ferrara et al., 2021; Garcia-Ramonda et al., 2022; Jia et al., 2021; Rovero et al., 2020). The single-lap direct shear test is the most widely used test to investigate FRCM bond behavior as well as being recommended by national and international guidelines on acceptance criteria for FRCM composites (ACI Committee 549, n.d.; Centrale, 2019). Double-lap shear test, small-scale beam test, and pull- out test (D’Antino et al., 2017; Sneed et al., 2015, Misseri et al., 2019) have also been used to investigate the bond behavior. To analytically describe the behavior of the textile-matrix interface, pure Mode-II fracture mechanics was applied, considering relationships between the matrix – fiber shear stress and slip, i.e., interfacial cohesive material laws (CMLs). This formulation, in the shared hypothesis of non deformability of the matrix, leads to a one-dimensional differential problem in terms of slip between textile and matrix, which can be solved once the CML is known. The integration of the differential problem allows to study the stress transfer mechanism along the joint bonded length, investigating the trend of the slip and tangential interface stress and the textile strain (Bertolli & D’Antino, 2022; Carozzi et al., 2016; Colombi & D’Antino, 2019; Focacci et al., 2017; Grande & Milani, 2018). The CML can be calibrated directly from the global load-slip diagram using the local strains obtained instrumenting the specimen by strain- gauges along the composite jacket (D’Antino et al. 2018), although this procedure is not straightforward for FRCM systems due to the presence of the outer layer of mortar whose thickness is not negligible. Alternatively, in (Focacci et al., 2017) an indirect procedure of CLM back calibration is proposed, using the experimental results of shear single-lap tests, while in (Focacci et al., 2024) a simplified one is implemented. In (Carloni et al., 2015), the role of the different matrix layers in the stress transfer activated in the single-lap direct shear test is investigated proposing different bond-slip relationships for the two matrix-fiber interfaces. This proposal corresponds to the assumption that in the stress-transfer mechanism, locally different shear stress in the two matrix fiber interfaces corresponds to a textile slip. The two layers of the matrix are effectively connected in the areas left open by the mesh, and hence their behavior is mutually influenced. However, as a first approximation, the different constraint conditions to which the external and the internal layers of the matrix are subject may justify the existence of distinct CMLs, although referred to the interaction of the same materials. Indeed, the lower matrix layer, adhered to the substrate is hindered from unconstrained expansion in the direction of load application. Conversely, the external layer of mortar shows a free-from-stress face and can take in tensile stress through the bond interaction with the textile. In this study, six different FRCM systems were considered by combining the same glass-fiber textile, dry or epoxy coated, and three different matrices based on lime, gypsum, and cement binders. Then, assuming rigid mortar and two different shear stress slip laws for the upper and the lower mortar layer-textile interfaces, an analytical model is proposed to calibrate the CMLs and predict the corresponding global load slip diagram of the six systems. The experimental campaign was carried out on three different mortars (lime L, gypsum G, and cement C) coupled with glass fiber, either dry or epoxy coated. The textile consists of a heat-sealed balanced bi-directional mesh based on dry fiber bundles, with yarns spacing 12 mm and an equivalent thickness of 0.04 mm. The resin employed to coat the fiber is a bi-component epoxy resin with low viscosity and a Young Modulus in traction of 1800 MPa. The three employed mortars are: (1) a commercially available premixed dry lime mortar with a maximum dimension of grains of 1 mm and containing pure NHL 3.5 certified natural lime, natural river-washed fine (0.1-0.5 mm) and medium grained (0.1-1 mm) siliceous sand, dolomitic limestone (0-1.4 mm), white Carrara marble (0-0.2 mm) and mineral geo-binder (Kerakoll Spa, 2021); (2) a commercially available gypsum mortar obtained by firing gypsum stone of purity of more than 90%, with a maximum dimension of grains of 0.002 mm (Gessi Roccastrada, 2017); (3) a commercially available premixed and fiber-reinforced cement mortar with a maximum dimension of grains of 1.4 mm, it is based on hydraulic binders, with the addition of pozzolanic reagents, selected aggregates and special additives (Sika, 2022). 2. Experimental campaign 2.1. Materials and methods