PSI - Issue 64

Chiara Pepi et al. / Procedia Structural Integrity 64 (2024) 1896–1903 Maria Eleonora Pipistrelli / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Research efforts aimed at developing reinforcing materials for retrofitting of masonry buildings have increasingly focused on enhancing the efficiency and performance of biocomposite materials as sustainable and environmentally friendly alternatives. When dealing with architectural heritage reinforcing interventions, retrofitting materials are chosen looking at the compatibility with masonry in terms of mechanical and chemical properties (Corradi et Al. 2024). Fiber Reinforced Cementitious Matrix (F.R.C.M) materials have been widely studied and valued as an optimal solution for historical buildings preservation, due to their ease of application and specific properties (Thomoglou et Al. 2023). To increase compatibility with masonry and to favour circular economy, natural fibers have been widely studied in literature for their optimal characteristics toward their use in F.R.C.M biocomposites (Gioffré et Al. 2023, Saini et Al. 2024, Greco et Al. 2024, Majumder et Al. 2024). While traditional synthetic fibers, such as carbon and glass fibers, exhibit superior mechanical properties, they often result in reinforcement materials that are incompatible with historical masonry, leading to different behaviors under external forces, such as seismic actions (Abbass et Al. 2020). Moreover, natural fibers meet the growing demand for environmentally friendly materials to be used in the building industry, reducing carbon emissions and pollution (Kerni et Al. 2020). Among natural fibers, research has focused on vegetable fibers, derived from plants, due to their cost-effectiveness and ease of availability compared to animal-derived alternatives (Sabir at Al., 2022). As highlighted by several studies, bast fibers are the strongest vegetal fibers, and, thus, are preferred for developing reinforcing composite materials (Santos et Al. 2024, Syduzzaman et Al. 2023, Summerscales et Al. 2010). Despite these advantages, natural fibers exhibit high variability in mechanical performance due to the irregularities in their physical and chemical composition (Madueke et Al. 2023). Additionally, their organic origin presents durability challenges when embedded in composite material matrices, potentially compromising the interaction between matrix and fibers, leading to poor structural performance. Indeed, when natural fibers are embedded in mortar based matrices, cracks of the mortar can occur at early load stage before the natural fibers begin to fulfill their reinforcing role. Consequently, consistent number of contributions in literature have explored various coatings and surface treatments to improve the performance of vegetal fibers (Wei et Al. 2014, Abbass et Al. 2023). Sodium hydroxide (NaOH) treatments (i.e. alkali treatments or mercerization) are commonly used to remove impurities from vegetal fibers in an easy and affordable way (Luchese et Al. 2024). The effectiveness of the treatment relies in weakening the hemicellulosic structure of the vegetal fibers, to have more reactive surfaces. Consequently, mercerization improves the interaction between vegetal fibers and other materials they are embedded in, such as matrices in composites or coatings. Several recent studies in the literature explore the efficacy of graphene-based coatings on vegetal fibers to enhance their mechanical, thermal and conductivity properties (Liu et Al. 2023, Gadakh et Al. 2019). Additionally, coatings made by graphene and its derived products (i.e. graphene flakes, graphene oxide, reduced graphene oxide) increase the interaction between fibers and matrix, resulting in enhanced load transferring processes (Gadakh et Al. 2019). It has to be pointed out that the chemical and microstructural of graphene derivatives can be tailored by the chemical conditions of the treatments (Protopapa et Al. 2022, Burresi et Al. 2020). While many studies in literature explore the use of alkali treatments and graphene coatings, most of them focus on the study of different concentrations rather than treatment duration (Silveira et Al. 2022, Abbass et Al. 2023). In this paper, two types of bast vegetal fibers, namely hemp and Spanish broom fibers, have been selected and subjected to tensile tests before and after the application of alkali treatments and subsequent graphene coating. While numerous studies explore the properties of jute, rami and kenaf fibers, less attention is given to the study of hemp and Spanish broom fibers, which have traditionally been used, particularly within the European countries. The use of these fibers could significantly reduce production costs in composite materials manufacturing. Treatment times are here extended up to 24 hours, while maintaining consistent concentration levels. While some studies have observed improvements in properties with certain treatment durations, a clear consensus regarding the optimal treatment time related to different types of vegetal fibers is lacking, especially concerning their effects on tensile properties.