PSI - Issue 64

Agostino Walter Bruno et al. / Procedia Structural Integrity 64 (2024) 1411–1418 Bruno et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction The construction sector needs to drastically change current building practices to meet the environmental challenges posed by global warming. In this endeavor, the replacement of energy-intensive building materials (such as concrete and fired earth) with more sustainable alternatives is attracting the interest of the civil engineering community, including academic researchers, professional stakeholders and building end-users (Fachinotti et al., 2023; Ramón Álvarez et al., 2023). Among various material alternatives, raw earth has the potential to reduce the environmental footprint of the residential construction sector with advantages spanning over the full life cycle of buildings. Firstly, earth can be sourced nearby the construction site, which limits energy consumption and costs associated with transportation (Morel et al., 2001). Earth materials are also manufactured with less transformation than concrete and fired earth, which instead require intensive thermal treatments and inclusion of aggregates with consequent depletion of natural resources (Minke, 2013). Secondly, earth materials are excellent hygrothermal regulators because of their capacity to exchange moisture with the surrounding environment, as extensively demonstrated by both experimental and numerical studies (McGregor et al., 2014; Losini et al., 2023). Finally, earth materials, especially if unstabilised, are fully recyclable at the end of their useful life as they can be converted into new building resources or safely disposed with no harm to the environment (Muguda et al., 2019; Bruno et al., 2020). Despite the above advantages, earth materials are still not adopted in mainstream construction practice because of their relatively weak mechanical performance (especially under tensile and flexural loads) and poor durability against water erosion. As for the energy performance, earth materials have a relatively high thermal conductivity, which may favor heat exchanges and hence reduce indoor comfort (Soudani et al., 2016). This negative feature is, however, balanced by the high thermal inertia generated by both the hygroscopic activity of the earth and the large thickness of perimetral load-bearing walls. An improvement of the mechanical and thermal properties of raw earth may be potentially achieved by embedding natural fibers within the material’s volume (Laborel -Préneron et al., 2016). To this end, the present work explores the mechanical and thermal behavior of two different earth materials (namely a sandy silt and a clayey silt) incorporating three flax fibers percentages of 0% (unreinforced earth), 0.5% and 1% by volume. Both unreinforced and fiber reinforced earth samples were subjected to three-point bending and unconfined compression tests as well as thermal conductivity tests. Results show that the inclusion of the flax fibers markedly enhances the mechanical performance while the relatively small fiber content tested in this work is insufficient to significantly improve thermal properties. 2. Materials and methods The earth materials tested in this work were provided by the two brickwork factories Nagen and Bouisset , both from Toulouse (France). The grain size distribution of both materials was measured by wet sieving and sedimentation in agreement with the norms XP P94-041 (AFNOR, 1995) and NF P 94-057 (AFNOR, 1992). Figure 1 shows the grain size distributions of the two materials together with the recommendations by MOPT (1992), CRATerre-EAG (1998) and the norm XP P13-901(AFNOR, 2001) for manufacturing compressed earth bricks. Figure 1 indicates that the Nagen earth is a sandy silt with a grain size distribution close to the finer limit of the recommended regions. The Bouisset earth is instead a clayey silt which is slightly finer than the recommended limit, though this is still acceptable for the purpose of the present work. The fine fraction of both earth materials, i.e. the fraction passing through the 0.4 mm sieve, was subjected to plasticity tests for measuring both the liquid and plastic limits according to the norm NF P94-051 (AFNOR, 1993), with the plasticity index being calculated as the difference between the previous two limits. These results are plotted in the plasticity chart of Figure 2, which indicates that the fine fraction of both Nagen and Bouisset earths is composed by an inorganic clay of low to medium plasticity, which fits the recommendations by Houben and Guillaud (1994), CRATerre-EAG (1998) and the norm XP P13-901 (AFNOR, 2001). The clay activity (i.e. the ratio between the plasticity index and the earth fraction smaller than 2 μ m) is equal to 0.79 for the Nagen earth and 0.60 for the Bouisset earth, which classify the clay fraction of the former material as normally active and that of the latter material as inactive. These results are consistent with XRD analyses performed by Bruno et al. (2018) and Cuccurullo et al. (2022) on both materials, which indicated a predominant illitic content with a small fraction of montmorillonite for the Nagen earth and a predominant kaolinitic fraction for the Bouisset earth. The specific gravity