# Impact of Fiber Type on Chloride Ingers in Concrete: A MacroXRF Imaging Analysis
## 1 Introduction  
A central aim in concrete research, encompassing both material properties and microstructure, is the enhancement and preservation of its service life (SL). Various types of concrete have been characterized in pursuit of this objective, among which fiber-reinforced concrete (FRCs) stand out due to their mechanical and durability benefits. FRC has been used since the 1960s, initially incorporating asbestos fibers mixed with cement. Over time, fibers such as steel, sisal, polypropylene, carbon, and glass, amongothers, have been incorporated into concrete mixtures. FRC is widely used in industrial flooring structures, bridge decks, viaducts, and port yards, typically in environments exposed to varying loads, high tensile stresses, temperature fluctuations, aggressive agents, and wetting and drying cycles. Fibers can improve various mechanical properties of concrete, such as crack resistance, by acting as a tensile bridge for microcracks and re-distributing stresses. Additionally, fibers improve toughness, fatigue resistance, impact resistance, and other material properties [1; 2; 3]. Among these fibers, polypropylene fibers (PFs) are notable for their micrometric reinforcement, enhancing strength, toughness, and impermeability [4]. These synthetic polymer filaments, originally from the textile industry, are relatively lighter than the cement matrix and exhibit a low modulus of elasticity and tensile strength. However, their higher deformation at break gives them superior reinforcement capacity during microcracking of the concrete [5]. Glass fibers (GFs), in turn, are filaments composed of an amorphous silicon-oxygen network [6], with small diameters on the order of micrometers. They exhibit a modulus of elasticity approximately twice that of the matrix and possess one of the highest tensile strengths compared to other fibers, making them a promising alternative for inclusion in concrete [4; 7]. Despite the mechanical advantages, both polypropylene and glass fibers tend to form agglomerates when added to the concrete matrix. These filaments do not disperse individually, remaining clustered in the matrix, which can compromise their distribution and lead to poor dispersion. This phenomenon may negatively affect compressive strength [8], increase porosity, and cause surface finishing issues, as reported by [9]. Conversely, several studies have shown that, when properly dispersed, these fibers deliver positive effects in terms of durability, such as reducing electrical conductivity [8], water absorption [10], and porosity [11]. The performance of fiber in concrete depends on factors such as the properties of the matrix, fiber type, geometry, mechanical properties of the fibers, dosage, dispersion, and orientation in the concrete [12]. EFNARC (European Federation of Specialist Construction Chemicals and Concrete Systems) provides guidelines for the use of fibers in structural concrete, indicating that the amount of polypropylene fibers typically varies between 0.1% and 0.5% of the total volume of concrete. For glass fibers, a quantity between 0.5% and 2% is recommended. Nevertheless, a clear understanding of how fibers influence concrete durability, particularly under chloride exposure, remains elusive in the current literature, with both positive [13; 14; 15] and negative [16; 17] results reported. The incorporation of fibers may increase the matrix porosity if not properly controlled, although it can also reduce the porosity when it results in a homogeneous mix. Furthermore, studies in the literature indicate that there have not yet been tests allowing for a more precise analysis of the microstructure of fiber-reinforced concretes with synthetic fibers. Accurate quantification of chlorides can enhance prevention techniques against such agents and improve the durability predictions of the reinforced concrete in question. In this regard, employing more detailed tests contributes to a better understanding of the effectiveness of fiber reinforcements in the cementitious matrix.