# Impact of Fiber Type on Chloride Ingers in Concrete: A MacroXRF Imaging Analysis
## 4 Discussion
Regarding the permeability of concrete, [PERSON] [48] reports that there are three types of possible paths in the microstructure: continuous conductive path (CCP), discontinuous conductive path (DCP), and insulating conductive path (ICP). CCPs are those where the micropores are directly connected to each other, allowing the electric current to pass through. DCPs have parts of the path discontinued by barriers of other materials, such as cement particles, also called discontinuity points, and they are therefore non-permeable. ICPs, on the other hand, are mostly made up of particles of cementitious material, in effect forming a path in the concrete, albeit an insulating one. Thus, chloride ion transport mechanisms are generally influenced by pore size, volume, connectivity, and crack penetration. When the pores are disconnected or filled with other elements, the mechanism is impaired and there is less chloride penetration. The compressive strength of concrete is affected by voids, whether they are interconnected or isolated. In this context, the fiber content plays a critical role in defining the microstructure. The addition of fibers, especially longer ones, restricts the movement of particles, particularly coarse aggregates. Since it is recommended that fibers be at least twice the maximum aggregate size, they hinder the mobility of aggregates and reduce the mixture's flowability. Moreover, fiber rigidity influences concrete workability, as it is determined by the shape factor and, most importantly, the fiber's modulus of elasticity. The higher the fiber rigidity, the greater the restriction on the movement of concrete components [45]. This explains why the glass fibers, being more rigid than the polypropylene fibers, resulted in more expressive chloride peaks in the GF samples than in the PF samples. On the other hand, some studies indicate that fine fibers, such as glass fiber, can act as nucleation points for hydration products, especially when they have coatings such as zirconia, which is used in alkali-resistant fibers. Mills [49] demonstrated that, in certain conditions, glass fibers contribute to the development of a denser microstructure by providing surfaces where hydration products can precipitate, potentially mitigating some of the negative effects of increased porosity. These results align with previous permeability studies, such as those by [PERSON] [10], which also found that concretes reinforced with 0.4% to 0.6% PF exhibited lower water absorption than unreinforced concretes, a factor directly linked to the reduced porosity. However, previous studies have also reported a slight increase in porosity when fibers are added. [PERSON] and [PERSON] [16] and [PERSON] et al. [50] found that the porosity of fiber-reinforced concrete mixtures increased from 2.9% (no fibers) to 5.0% with 0.1% glass fiber and 4.2% with 0.1% polypropylene fiber. Similar trends were observed by [PERSON] and [PERSON] [44] and [PERSON], [PERSON], and [PERSON] [40; 51], who attributed this behavior to the formation of an interfacial transition zone around the fibers. In this context, the lower chloride penetrability observed in the present study (Figure 10) highlights the positive effect of fiber reinforcement in mitigating chloride-induced deterioration in concrete structures. When analyzing the mechanical strength and resistance to chloride penetration together, it is observed that although fibers do not contribute to an increase in compressive strength--since their primary benefit lies in tensile strength--they still help mitigate chloride ion ingress. A difference was noted between the influence of the glass and polypropylene fibers, with the latter demonstrating superior performance. Furthermore, the samples containing 0.25% fiber exhibited better results compared to those with 0.50%, suggesting that higher fiber contents may compromise the concrete's microstructure.