PSI - Issue 64

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Pier Francesco Greco et al. / Procedia Structural Integrity 64 (2024) 1888–1895 Pier Francesco Greco/ Structural Integrity Procedia 00 (2019) 000 – 000

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2. MATERIALS The biocomposite materials proposed in this study include a natural hydraulic lime mortar (NHL) and two types of vegetal fibers: Spanish broom and hemp. Lime mortar is chosen as the matrix due to its compatibility with masonry strengthening without compromising historical and technical integrity. While lime mortar exhibits good compressive strength, its tensile strength is comparatively low due to inherent fragility. Spanish broom and hemp fibers are incorporated into the mixture to improve the mechanical performance of the mortar, thereby enhancing the overall quality of the composite Formisano et al (2017), Juradin et al (2021), Greco et al (2024). Mechanical properties of Spanish broom and hemp fibers are investigated in two previous articles Greco et al (2024), Pepi et al (2024), Pepi et al (2024). The specimens were prepared in the Materials Laboratory of the Department of Civil and Environmental Engineering (DICA) at the University of Perugia. Following the manufacturer's guidelines, the mortar mixture was manually prepared, with the water content adjusted to be 22% of the mortar powder weight. Subsequently, the mortar mixture was carefully poured into prismatic molds measuring 40 × 40 × 160 mm, specifically designated for three point bending tests. For the fiber’s length and concentration parameters, 30 mm and 1% by weight were selected based on previous studies, as they prove to be effective in enhancing the mechanical characteristics of the composite materials Juradin et al (2021), Greco et al (2024). Three specimens for each combination were prepared. The labels used for the specimens follow a specific pattern: letter ‘‘M’’ is used for unreinforced mortar specimens; letter ‘‘H’’ is used for hemp fibers reinforced specimens; letters ‘‘S’’ is used for the samples reinforced with Spanish broom fibers. 3. THREE POINT BENDING TEST The tensile strength of the specimens was evaluated through a three-point bending test following the UNI-EN 772 – 1 and UNI-EN 771 – 1 standards Normazione, UNI-Ente Italiano (2015). Each specimen was positioned on two support points, and a concentrated load was applied at the midpoint. Testing was carried out at a controlled displacement rate of 0.5 mm/min, aiming to achieve failure within 300 seconds, using a 30 kN load cell (Fig. 1- a-h). After the first crack occurrence, the application of the load was continued to capture the softening in the force – displacement curve. Throughout the test, both the load and the corresponding displacement were meticulously recorded. Stress and strain were then derived from these recorded values of force and displacement, following the methodology outlined in M. Gioffré et al. (2021). The resulting stress – strain curves for all tested specimens are reported in Fig. 2-a, b, c, for all the tested specimens. The unreinforced specimens exhibited a brittle fracture, characterized by rapid crack propagation without significant plastic deformation (Fig. 2-a). Conversely, specimens with hemp and Spanish broom fibers reinforcement exhibited a distinct behavior (Fig. 2- b, c), demonstrating an initial increase in stress and strain, reaching a peak before softening and final failure. This behavior can be due to the bridging effect provided by the fibers near the crack position, as reported in the literature Juradin et al (2021), Greco et al (2024). Mechanical properties are visually compared in Fig. 2- d, e, f and summarized in Table 1:  peak , ε peak , ε max denote the maximum tensile strength, the strain corresponding to  peak and the maximum strain, respectively; E is used to indicate the elastic modulus. To assess the post-crack energy absorption ability induced by both Spanish broom and hemp fibers, the results obtained from the three-point bending tests (i.e. the stress – displacement curves) are used to calculate the fracture energy G f and the fracture energy ratio rG f , following the same procedure reported in Greco et al. (2024). These two metrics are used to quantify the capacity of the specimens to absorb energy during crack propagation and the energy absorbed after first crack with respect to the total one, respectively. The obtained G f and rG f are reported in Table 1. As it was expected, results show that presence of fibers in composite do not affect peak’s values,  peak and ε peak , but they affect the maximum strain, ε max, and the fracture energy parameters, G f and rG f , giving ductility properties to the biocomposite material, differently from the typical brittle behavior of mortar. Work is in progress to assess the performance of these biocomposites, where the vegetal fibers are coated with nanomaterials.