## 1 Introduction
#### Significance of Current Work  
By itself, concrete represents a material with high compressive strength, but characterized by high fragility, low tensile strength, and low cracking resistance. For this reason, reinforcing materials using natural fibers represent the oldest form of composite materials; in fact, they have always been obtained, at low cost, from materials present in nature using local labor and technology and with reduced energy consumption. Due to these characteristics, the use of natural fibers as a form of concrete reinforcement is more widespread, mostly in less-developed regions, where high-cost construction materials are present and high-cost skilled labor is available.  
The aim of this study was to examine a new type of fiber, extracted from the stem of _Arundo donax_ L., as a potential reinforcement in concrete blocks. Specifically, the present study investigated the effect of the aspect ratio of _Arundo donax_ fibers on the tensile properties of concrete blocks. In this regard, _Arundo donax_ fibers of different aspect ratios (30, 50 and 90) and fixed content (1% by weight) were incorporated into concrete blocks to study their influence on tensile strength.  
The _Arundo donax_--harvested annually, preferably during the winter period--produces a large quantity of biomass that can be used for thermal energy production or for the pro duction of second-generation ethanol [69]. In addition, this natural fiber has a variety of applications, including as a supply of fiber for printing paper [70] and as a biomass source for energy production [56]. It has also been utilized in the manufacturing of alternatives to wood-based chipboard [71; 72], insulating panels [73], or ecological starch-bonded panels [74], among other things, due to its non-woody qualities.  
The stems of the _Arundo donax_ are often used for the production of supports for horticultural plants and fishing rods, in the musical field for the creation of reeds for wind instruments, in phyto-purification plants, and in the interventions of stabilization of land at hydrogeological risk. The most significant studies on the mechanical properties of _Arundo donax_ were conducted by [PERSON] et al. [75] in 1997, who focused on the variability of mechanical properties through bending, compression, and torsion tests at different points of the plant, highlighting that the central part of the culm presented the most homogeneous mechanical characteristics. Other experimental tests carried out more recently by [PERSON] et al. [76], through three-point bending tests, concerned the influence of moisture content on the flexural elastic modulus of _Arundo donax_. [PERSON] et al. [77] in 2021 studied the potential of this natural fiber as sustainable reinforcement in building materials. The results they obtained showed good mechanical properties, very similar to those of many bamboo species, which are already used in construction. [PERSON] et al. [78] instead investigated the mechanical properties of _Arundo donax_ to provide technical data in order to propose the use of this green material in sustainable construction. [PERSON] et al. [79] in 2014 studied the possibility of using _Arundo donax_ natural fibers as reinforcement in polymer composites. Using tensile tests on single fibers and a mathematical model they investigated the relationship between the transverse size of the fibers and the mechanical properties. Other authors [80] have previously highlighted the high tensile strength of this natural fiber, especially when compared to other natural fibers such as Spanish broom [81].